The physics of airplane flight(10maurycy10.github.io)
10maurycy10.github.io
The physics of airplane flight
https://10maurycy10.github.io/misc/the_physics_of_flight/
175 comments
A lot of the difficulty of explaining lift of airfoils is that generally explanations try to follow a neat chain of cause an effect. But with the wing there isn't really a clear one. All these statements:
- There is an upwards force on the wing
- The pressure above the wing is lower than the pressure below it
- The air around the wing follows a curved path downwards
- The air above the wing travels faster than the air below it (NB: not in equal time!)
- The air behind the wing has a downward momentum
are related to all the others, but not straightforwardly: they all imply each other to some extent, both caused by and causing some of the others. So basically all explanations try to follow some path through the tangled web, but by doing so they always cause some oversimplification. The only top level chain is: shape of wing and angle of attack -> ????? (tangled mess of fluid dynamics few people fully understand) -> lift!
- There is an upwards force on the wing
- The pressure above the wing is lower than the pressure below it
- The air around the wing follows a curved path downwards
- The air above the wing travels faster than the air below it (NB: not in equal time!)
- The air behind the wing has a downward momentum
are related to all the others, but not straightforwardly: they all imply each other to some extent, both caused by and causing some of the others. So basically all explanations try to follow some path through the tangled web, but by doing so they always cause some oversimplification. The only top level chain is: shape of wing and angle of attack -> ????? (tangled mess of fluid dynamics few people fully understand) -> lift!
Exactly, this is why understanding fluid dynamics is so difficult. You can't look at some physical laws and assume that the right hand side "causes" the left hand side. They all represent relations and it so happens that the fluid configuration that fulfill all the relations (and that the world adopts) is the one that causes lift. Just trying to talk about cause and effect is a misunderstanding.
My favorite example of this was in the air-data computer I was working on for a fighter trainer. I was just on the software side rather than the aerodynamics, but it was notable that the corrections to angle-of-attack and angle-of-sideslip measured by the multifunction probes (which are way up at the nose of the plane) included terms related to the position of the flaps (which are way back at the trailing edges of the wings).
That's awesome!
I'm not surprised about the angle-of-attack needing correction. The angle-of-attack is defined as the angle between the average chord (an imaginary line running from the leading edge of the wing to the trailing edge of the wing) and the relative wind. Since changing the flap position changes the position of the trailing edge, the angle-of-attack will also change.
I'm not surprised about the angle-of-attack needing correction. The angle-of-attack is defined as the angle between the average chord (an imaginary line running from the leading edge of the wing to the trailing edge of the wing) and the relative wind. Since changing the flap position changes the position of the trailing edge, the angle-of-attack will also change.
Honest question: if wing's cross section has a big curve in front, how do determine which point exactly constitutes the "leading edge"?
Usually you draw a line from the trailing edge (which is sharp so unambiguous) to the point on the leading edge that makes the line the longest.
The definition that makes the most sense, though, is to disregard the geometry of the wing and define zero angle of attack as the zero-lift angle, because then lift is proportional to AoA.
The definition that makes the most sense, though, is to disregard the geometry of the wing and define zero angle of attack as the zero-lift angle, because then lift is proportional to AoA.
> explanations try to follow a neat chain of cause an effect. But [...] there isn't really a clear one. All these [...] are related to all the others, but not straightforwardly
There seems a pattern of misattributing pervasive failures of science education content design, to physical system complexity and student deficiency. A favorite of mine was a PhD thesis "We taught grade G young students common incoherent nonsense about atoms. Surprisingly, that's didn't work out well. We draw the obvious conclusion: students in G are developmentally incapable of understanding atoms." Which might even be valid... for a "regurgitate incoherence" definition of "understand atoms".
Here, I wonder if an atomistic explanation might work better? Could one craft a nicely accessible, coherent, transferably powerful, molecular superball mosh pit story of wings? The confusion and disagreements here sound a bit like "It's net molecular motion! No, surface impacts! No, differential surface impacts!". An abstraction/model fail, rather than underlying irreducible system complexity.
There seems a pattern of misattributing pervasive failures of science education content design, to physical system complexity and student deficiency. A favorite of mine was a PhD thesis "We taught grade G young students common incoherent nonsense about atoms. Surprisingly, that's didn't work out well. We draw the obvious conclusion: students in G are developmentally incapable of understanding atoms." Which might even be valid... for a "regurgitate incoherence" definition of "understand atoms".
Here, I wonder if an atomistic explanation might work better? Could one craft a nicely accessible, coherent, transferably powerful, molecular superball mosh pit story of wings? The confusion and disagreements here sound a bit like "It's net molecular motion! No, surface impacts! No, differential surface impacts!". An abstraction/model fail, rather than underlying irreducible system complexity.
It's easiest to understand as a black box, or a "control volume". Consider the air coming in the front (horizontal) and the air going out the back (velocity is deflected downwards). Momentum change, needs a force to keep things in balance. Simple! Fluid mechanics is all about that kind of thinking.
This is exactly one of the common pathways through that middle section, which is nice and simple but doesn't really explain anything (why is the air deflected downards?).
Because the plane has speed and trading that speed for lift
OK, but that's just saying "the aerodynamic force exists, because we can observe the plane goes up and the air goes down", isn't it?
If I'm not wrong, it seems dead simple when you put it like this:
- Imagine the jet moves the wing forwards some small distance in some small amount of time.
- Due to the shape of the wing, there is now a temporary vacuum above the wing as air particles have yet to rush in and occupy the space where the wing used to be.
- There is now an unbalanced pressure around the wing sufficient to overcome gravity and give lift.
No Bernoulli, no math, just visualizing a bunch of particles getting pushed around.
If you think about air this way it also becomes obvious why a helium balloon moves in the direction of acceleration inside a car. Car moves forward, air in the rear of the cabin is now squished while the air in front is stretched out as it hasn't caught up to the car yet, pressure gradient sends the balloon forwards.
- Imagine the jet moves the wing forwards some small distance in some small amount of time.
- Due to the shape of the wing, there is now a temporary vacuum above the wing as air particles have yet to rush in and occupy the space where the wing used to be.
- There is now an unbalanced pressure around the wing sufficient to overcome gravity and give lift.
No Bernoulli, no math, just visualizing a bunch of particles getting pushed around.
If you think about air this way it also becomes obvious why a helium balloon moves in the direction of acceleration inside a car. Car moves forward, air in the rear of the cabin is now squished while the air in front is stretched out as it hasn't caught up to the car yet, pressure gradient sends the balloon forwards.
As a purely conceptual illustration of the fact that the air must be deflected downwards by the wing, sure. It doesn't really work, though. For example, there's no reason for the air to move faster over the top of the wing in your scenario, and without that you'd underestimate the amount of lift a wing actually generates.
> there's no reason for the air to move faster over the top of the wing in your scenario
I suppose the compressed air at the top of the wing will find its way into a vacuum and travel a bit faster than the air at the bottom that's encountering normal pressure, but I'm honestly out of my depth at this point. Not sure if the air coming in on the left cancels that out either, I'd have to run the equations. https://webwhiteboard.com/board/KhTCsoDvhyGTy0uJtmPpQNhvldF1...
I suppose the compressed air at the top of the wing will find its way into a vacuum and travel a bit faster than the air at the bottom that's encountering normal pressure, but I'm honestly out of my depth at this point. Not sure if the air coming in on the left cancels that out either, I'd have to run the equations. https://webwhiteboard.com/board/KhTCsoDvhyGTy0uJtmPpQNhvldF1...
Published in 1944, Stick and Rudder[0] by Wolfgang Langeweische has this to say:
>Forget Bernoulli's Theorem
>When you studied theory of flight in ground school, you were probably taught a good deal of fancy stuff concerning an airplane's wing and just how it creates lift. As a practical pilot you may forget much of it. Perhaps you remember Bernoulli's Theorem: how the air, in shooting around the long way over the top of the wing, has to speed up, and how in speeding up it drops some of its pressure, and how it hence exerts a suction on the top surface of the wing. Forget it. In the first place, Bernoulli's Theorem does not really explain- the explanation is more puzzling than the puzzle! In the second place, Bernoulli's Theorem doesn't help you in the least bit in flying. While it is no doubt true, it usually merely serves to obscure to the pilot certain simpler, much more important, much more helpful facts.
>The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down.
>It shoves the air down with its bottom surface, and it pulls the air down with its top surface... In exerting a downward force upon the air, the wing receives an upward counterforce- by the same principle, known as Newton's law of action and reaction, which makes a gun recoil as it shoves a bullet out forward...
>Say that the wing is basically simply a plane, set at a slight inclination so as to wash the air down... But it was early found that the drag, lifting, and stalling characteristics of such an inclined plane can be improved by surrounding it with a curving, streamlined housing [emphasis mine]; hence our present wing "sections". The actual wing of an airplane is therefore not simply an inclined plane; it is a curved body containing an inclined plane.
[0]https://openlibrary.org/works/OL3483476W/Stick_and_Rudder?ed...
>Forget Bernoulli's Theorem
>When you studied theory of flight in ground school, you were probably taught a good deal of fancy stuff concerning an airplane's wing and just how it creates lift. As a practical pilot you may forget much of it. Perhaps you remember Bernoulli's Theorem: how the air, in shooting around the long way over the top of the wing, has to speed up, and how in speeding up it drops some of its pressure, and how it hence exerts a suction on the top surface of the wing. Forget it. In the first place, Bernoulli's Theorem does not really explain- the explanation is more puzzling than the puzzle! In the second place, Bernoulli's Theorem doesn't help you in the least bit in flying. While it is no doubt true, it usually merely serves to obscure to the pilot certain simpler, much more important, much more helpful facts.
>The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down.
>It shoves the air down with its bottom surface, and it pulls the air down with its top surface... In exerting a downward force upon the air, the wing receives an upward counterforce- by the same principle, known as Newton's law of action and reaction, which makes a gun recoil as it shoves a bullet out forward...
>Say that the wing is basically simply a plane, set at a slight inclination so as to wash the air down... But it was early found that the drag, lifting, and stalling characteristics of such an inclined plane can be improved by surrounding it with a curving, streamlined housing [emphasis mine]; hence our present wing "sections". The actual wing of an airplane is therefore not simply an inclined plane; it is a curved body containing an inclined plane.
[0]https://openlibrary.org/works/OL3483476W/Stick_and_Rudder?ed...
Don't compare the pressure of the air in the workshop to the fast moving air in the nozzle - compare the air in the system of the compressor.
In an air compressor, the lowest pressure air is the air moving through the hose and out the nozzle - the highest pressure air in the system is the 'still air' in the cannister. Think of an inflated balloon that you blow up and let go of, the highest pressure air is in the balloon, the lowest pressure air is immediately next to the mouth of the balloon, despite being the fastest moving.
It might feel surprising, but the air that moves faster across the top of the wing is lower pressure than the slower moving air below the wing. That both the air below and above the wing are higher pressure than 'all the rest of the air in the sky' is inconsequential to the the plane - we only need to consider the air directly interacting with the wing. (though this is not to deny the impacts of angle of attack etc etc.)
In an air compressor, the lowest pressure air is the air moving through the hose and out the nozzle - the highest pressure air in the system is the 'still air' in the cannister. Think of an inflated balloon that you blow up and let go of, the highest pressure air is in the balloon, the lowest pressure air is immediately next to the mouth of the balloon, despite being the fastest moving.
It might feel surprising, but the air that moves faster across the top of the wing is lower pressure than the slower moving air below the wing. That both the air below and above the wing are higher pressure than 'all the rest of the air in the sky' is inconsequential to the the plane - we only need to consider the air directly interacting with the wing. (though this is not to deny the impacts of angle of attack etc etc.)
The thing I never found satisfying was this notion that the air over the top moves faster because it has further to go - in what way does the length of a path that lies in the air's future have any effect on its speed now? As if the air over the top somehow has to match up with the air it was next to before the wing split it away below? What mysterious force would account for that?
The best I could arrive at was that the forward motion of the wing causes the back side of the curved wing top simply to pull away from the air in that region, reducing the pressure there, and incidentally (because Bernoulli) that air then moves faster as a result.
The best I could arrive at was that the forward motion of the wing causes the back side of the curved wing top simply to pull away from the air in that region, reducing the pressure there, and incidentally (because Bernoulli) that air then moves faster as a result.
>The thing I never found satisfying was this notion that the air over the top moves faster because it has further to go
On the one hand I agree that it is a stupid way to phrase it. On the other hand if the air doesn't "make it" then there is nothing where the wing just was aka a vacuum. The low pressure area that forms above the wing sucks the air along making it faster. Why doesn't all the air rush to fill the low pressure area? Well for air below the wing there is a wing in the way, air above the air flowing over the wing does rush down to fill the void providing lift, air behind the wing does as well creating some drag.
Same for angle of attack it deflects the air that would normally be above and behind the wing down (providing some lift),making a low pressure area form above the wing which the air speeds to fill.
On the one hand I agree that it is a stupid way to phrase it. On the other hand if the air doesn't "make it" then there is nothing where the wing just was aka a vacuum. The low pressure area that forms above the wing sucks the air along making it faster. Why doesn't all the air rush to fill the low pressure area? Well for air below the wing there is a wing in the way, air above the air flowing over the wing does rush down to fill the void providing lift, air behind the wing does as well creating some drag.
Same for angle of attack it deflects the air that would normally be above and behind the wing down (providing some lift),making a low pressure area form above the wing which the air speeds to fill.
> air above the air flowing over the wing does rush down to fill the void providing lift, air behind the wing does as well creating some drag
Just a nitpick, but these forces are never pulling, only pushing. The air rushing to fill the voids is not pulling the wing, is the air below or in front if the wing that pushes (and doesn't find an equal push on the other side).
Just a nitpick, but these forces are never pulling, only pushing. The air rushing to fill the voids is not pulling the wing, is the air below or in front if the wing that pushes (and doesn't find an equal push on the other side).
> As if the air over the top somehow has to match up with the air it was next to before the wing split it away below?
It's not a good explanation intuitively because it's not clear why that has to happen, and it's just wrong, because that doesn't happen.
It's not a good explanation intuitively because it's not clear why that has to happen, and it's just wrong, because that doesn't happen.
Imagine I fill a bathtub full of marbles - and I pull a solid semi circle through the marbles. The marbles that flat side moves past will barely have to move, the marbles that are displaced by the round side will have to 'move further'. They won't come out exactly at the same time, but they will have had to move further and move faster as the semi circle moves through the bath.
This is a great example, and the first time I've heard it phrased this way. Thank you, I'll file this away for later.
The speed of the wing is what causes the air to move around the two faces of the wing. The air has to move around the wing as it is being pulled through it.
Imagine pulling a fixed walled tube though the air, the air will move through the tube at roughly the speed that the tube is pulled through the air.
Now imagine pulling a funnel that starts off large and gets smaller. The same air will now have to move faster to get through the funnel (higher pressure at the mouth of the funnel, lower at the end).
Imagine pulling a fixed walled tube though the air, the air will move through the tube at roughly the speed that the tube is pulled through the air.
Now imagine pulling a funnel that starts off large and gets smaller. The same air will now have to move faster to get through the funnel (higher pressure at the mouth of the funnel, lower at the end).
> the air that moves faster across the top of a wing.
Except absolutely flat wings also work where the air is traveling the same distance. They aren’t nearly as efficient, but still produce lift.
Wings shape relates to skin effects, vortexes, turbulence, and drag. There’s a lot of complex interactions involved which don’t simplify to faster moving air creates lift.
Except absolutely flat wings also work where the air is traveling the same distance. They aren’t nearly as efficient, but still produce lift.
Wings shape relates to skin effects, vortexes, turbulence, and drag. There’s a lot of complex interactions involved which don’t simplify to faster moving air creates lift.
Does that flat wing work with a zero angle of attack (that is, parallel to the ground) or does it have to point upwards?
Race cars use downward pointing wings to generate the opposite of lift, to push the car into the ground. Of course even car wings have evolved into more efficient shapes, because there is a competition to win those races.
Race cars use downward pointing wings to generate the opposite of lift, to push the car into the ground. Of course even car wings have evolved into more efficient shapes, because there is a competition to win those races.
All wings need a positive or negative when upside down angle of attack to generate lift. People often draw the cord line incorrectly because the flat part of a wing isn’t zero and wings are mounted with a positive angel of attack so aircraft can be level in flight even with a ~15 degree angle of attack.
Car aerodynamics is complicated. People talk about spoiler downforce without really considering the details. If you push down on the rear spoiler of a toy F1 car the front end lifts up because it’s located behind the rear wheel. The goal is specifically downforce on the rear tires.
Similarly the rotational force on an axle wants to lift the front end. There’s another torque from the tires being located below the force of drag which again wants to lift the front of a car.
For strait line dragsters they accept the front wheels having reduced contact with the road for improved acceleration because they don’t need to turn. Where Indy and F1 uses front wings, but winged sprint cars pushed the classic spoiler forward on top of adding a wing for additional control. In racing it’s all about different trade offs for each sport.
Car aerodynamics is complicated. People talk about spoiler downforce without really considering the details. If you push down on the rear spoiler of a toy F1 car the front end lifts up because it’s located behind the rear wheel. The goal is specifically downforce on the rear tires.
Similarly the rotational force on an axle wants to lift the front end. There’s another torque from the tires being located below the force of drag which again wants to lift the front of a car.
For strait line dragsters they accept the front wheels having reduced contact with the road for improved acceleration because they don’t need to turn. Where Indy and F1 uses front wings, but winged sprint cars pushed the classic spoiler forward on top of adding a wing for additional control. In racing it’s all about different trade offs for each sport.
> "All wings need a positive or negative when upside down angle of attack to generate lift."
That would be true for symmetric wings, but is not the whole point of an (non-symmetric) airfoil or frisbee shape to generate lift while horizontal?
That would be true for symmetric wings, but is not the whole point of an (non-symmetric) airfoil or frisbee shape to generate lift while horizontal?
I should have said to generate lift in level flight. Drop anything with air resistance and it’s technically generating lift. However it’s important to separate the angle of attack relative to the airstream vs angle of attack relative to the ground for falling objects.
Anyway non-semmetric airfoils are about efficiency when the aircraft never flies upside down. Unfortunately you occasionally see mislabeled diagrams where the angel of attack seems to be zero when the wing is laying flat rather than the leading and trailing edge being level which creates a great deal of confusion.
PS: A frisbee shape is largely a question of grip as rings can fly further, but they both need positive angel of attack to achieve significant distances. https://web.mit.edu/womens-ult/www/smite/frisbee_physics.pdf
Anyway non-semmetric airfoils are about efficiency when the aircraft never flies upside down. Unfortunately you occasionally see mislabeled diagrams where the angel of attack seems to be zero when the wing is laying flat rather than the leading and trailing edge being level which creates a great deal of confusion.
PS: A frisbee shape is largely a question of grip as rings can fly further, but they both need positive angel of attack to achieve significant distances. https://web.mit.edu/womens-ult/www/smite/frisbee_physics.pdf
The Newton's 3rd law explanation and the Bernoulli explanation are both reasonable approaches and both work, very similar to the way that one can explain the path of a thrown ball both by Newton's laws and by the principle of least action.
NASA has a good explanation here [1]. Here's a brief summary.
The gas flow has to simultaneously conserve mass, momentum, and energy.
If you analyze lift by considering conservation of momentum you get that there are velocity differences in the flow at different parts of the wing. Integrate those around the whole wing and you find a net turn of the flow downward. Conservation of momentum (Newton's 3rd) requires an opposite upward force on the wing.
If you analyze lift by considering conservation of energy you also get velocity differences which lead to pressure differences. Integrate pressure over the whole wing and you get a net upward force on the wing.
[1] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/ber...
NASA has a good explanation here [1]. Here's a brief summary.
The gas flow has to simultaneously conserve mass, momentum, and energy.
If you analyze lift by considering conservation of momentum you get that there are velocity differences in the flow at different parts of the wing. Integrate those around the whole wing and you find a net turn of the flow downward. Conservation of momentum (Newton's 3rd) requires an opposite upward force on the wing.
If you analyze lift by considering conservation of energy you also get velocity differences which lead to pressure differences. Integrate pressure over the whole wing and you get a net upward force on the wing.
[1] https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/ber...
This doesn't really explain why those velocity variations occur in the first place, or am I missing something?
It sounds like "We observe velocity variations on the wing and these correspond to pressure variations that create lift due to conservation of energy." But it leaves the question on what is causing the velocity variations in the first place.
It sounds like "We observe velocity variations on the wing and these correspond to pressure variations that create lift due to conservation of energy." But it leaves the question on what is causing the velocity variations in the first place.
One other weird thing needed if you want your momentum numbers to balance: every plane in the sky is pulling the earth up towards it ever so slightly.
If you just mean there's a gravitational force on the Earth from the plane then, sure, but that's true for any object (including you and me). For an object on the ground, the normal force of that object on the ground balances their gravitational pull on the Earth, so the Earth experiences a net force of zero from the object and so isn't accelerated by it.
But I guess you mean that the plane has a net force pulling the earth towards it. But that would violate conservation of momentum – in fact the net force is zero, just as for an object on the ground. The plane's wings push the air down (the reaction to that is what keeps the plane up) and the resulting downdraft of that air exerts a force downwards on the earth.
That's all assuming the plane is in steady flight. If it's taking off, or just ascending, then overall it is pushing the Earth away from it. Conversely, when descending the Earth is pulled, overall, slightly towards it. The same thing happens when you jump: you push the Earth away from you (a much smaller distance than you travel away from it!), then on the way back down the Earth travels back towards you.
But I guess you mean that the plane has a net force pulling the earth towards it. But that would violate conservation of momentum – in fact the net force is zero, just as for an object on the ground. The plane's wings push the air down (the reaction to that is what keeps the plane up) and the resulting downdraft of that air exerts a force downwards on the earth.
That's all assuming the plane is in steady flight. If it's taking off, or just ascending, then overall it is pushing the Earth away from it. Conversely, when descending the Earth is pulled, overall, slightly towards it. The same thing happens when you jump: you push the Earth away from you (a much smaller distance than you travel away from it!), then on the way back down the Earth travels back towards you.
Crud I think you're right. I had it in my head that in steady flight, the plane had zero change in momentum, whereas, the air, collectively, gained net downward momentum. So to balance it all out, the earth must be gaining upward momentum (though of course spread over such an enormous mass as to make the velocity term imperceptible.)
Newton's third law doesn't explain stalls – or, at least, not their suddenness.
As angle of attack increases (or speed decreases) there comes a certain point where the lift suddenly drops in a dramatic way that wouldn't make sense from a naive application of Newton's laws. What's really happened is that the airflow has separated from the wings and Bernoulli's principle no longer applies. That's when you get a stall, and the plane starts falling rather than flying.
As angle of attack increases (or speed decreases) there comes a certain point where the lift suddenly drops in a dramatic way that wouldn't make sense from a naive application of Newton's laws. What's really happened is that the airflow has separated from the wings and Bernoulli's principle no longer applies. That's when you get a stall, and the plane starts falling rather than flying.
> It's pretty obvious that the wings push air down
The air being pushed down is actually a side-effect of the lift-creation process, not the cause of it.
A nice "counter example" is a wing in ground effect (flying very close to the ground), where there is less downwash, because of the ground, and yet the wing produces more lift. It's an effect that can make high aspect-ratio airplanes tricky to land.
The air being pushed down is actually a side-effect of the lift-creation process, not the cause of it.
A nice "counter example" is a wing in ground effect (flying very close to the ground), where there is less downwash, because of the ground, and yet the wing produces more lift. It's an effect that can make high aspect-ratio airplanes tricky to land.
> air being pushed down is actually a side-effect of the lift-creation process, not the cause of it
The turning of the gas is absolutely what causes lift. (Where the Newtonian explanation is misleading is in “neglect[ing] the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas” [1].
Put another way: if you know the mass and acceleration of the gas about the wing, you can calculate lift. (This is impractical for many reasons.)
> a wing in ground effect
VTOL aircraft also experience ground effect due to the fountain effect.
[1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
The turning of the gas is absolutely what causes lift. (Where the Newtonian explanation is misleading is in “neglect[ing] the physical reality that both the lower and upper surface of a wing contribute to the turning of a flow of gas” [1].
Put another way: if you know the mass and acceleration of the gas about the wing, you can calculate lift. (This is impractical for many reasons.)
> a wing in ground effect
VTOL aircraft also experience ground effect due to the fountain effect.
[1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
Curvature of streamlines is related to pressure gradient across said streamlines.
> Curvature of streamlines is related to pressure gradient across said streamlines
Sure. Ultimately just considering pressure or mass deflection doesn’t work without elaborate workarounds. Because neither describes the reality of an airfoil turning a moving viscous fluid.
Sure. Ultimately just considering pressure or mass deflection doesn’t work without elaborate workarounds. Because neither describes the reality of an airfoil turning a moving viscous fluid.
> The turning of the gas is absolutely what causes lift.
No. What causes lift is the differential in pressure between the top and the bottom surface of the wing. The rest is broadly speaking a side effect.
If the turning of the gas was the necessary mechanism for lift, planes in supersonic flight would fall out of the sky.
Instead of relying on an airfoil shape for lift, you could fly by sucking air from the top of your wing and dumping out the back of your plane.
No. What causes lift is the differential in pressure between the top and the bottom surface of the wing. The rest is broadly speaking a side effect.
If the turning of the gas was the necessary mechanism for lift, planes in supersonic flight would fall out of the sky.
Instead of relying on an airfoil shape for lift, you could fly by sucking air from the top of your wing and dumping out the back of your plane.
> If the turning of the gas was the necessary mechanism for lift, planes in supersonic flight would fall out of the sky
Why would pressure (Bernoulli out of Euler) propagate supersonically while momentum (Newton) does so subsonically?
> Instead of relying on an airfoil shape for lift, you could fly by sucking air from the top of your wing and dumping out the back of your plane
Wings (and the other bits that contribute to lift) are bigger than engines. That’s the leverage you get with a lifting body: you move more molecules than your thruster alone.
The correct answer here is unintuitive. But the very wrong answer is pressure alone. (As the article we’re commenting on clearly shows with its brilliant flat-cardboard example. You don’t need camber to have a lifting body, just angle of attack.)
Why would pressure (Bernoulli out of Euler) propagate supersonically while momentum (Newton) does so subsonically?
> Instead of relying on an airfoil shape for lift, you could fly by sucking air from the top of your wing and dumping out the back of your plane
Wings (and the other bits that contribute to lift) are bigger than engines. That’s the leverage you get with a lifting body: you move more molecules than your thruster alone.
The correct answer here is unintuitive. But the very wrong answer is pressure alone. (As the article we’re commenting on clearly shows with its brilliant flat-cardboard example. You don’t need camber to have a lifting body, just angle of attack.)
> Why would pressure (Bernoulli out of Euler) propagate supersonically while momentum (Newton) does so subsonically?
I'm sorry, I didn't understand the question.
But in supersonic flight, with a flat plate, you don't have any rotation in the game, as illustrated here [0]. And yet you will be producing a lot of lift.
[0] https://image.slideserve.com/251762/supersonic-flow-over-fla...
> But the very wrong answer is pressure alone.
No, it really is the pressure alone. And viscous drag, if you want to be pedantic. Those are the only forces at play, the rest is only a side effect of those forces.
I'm sorry, I didn't understand the question.
But in supersonic flight, with a flat plate, you don't have any rotation in the game, as illustrated here [0]. And yet you will be producing a lot of lift.
[0] https://image.slideserve.com/251762/supersonic-flow-over-fla...
> But the very wrong answer is pressure alone.
No, it really is the pressure alone. And viscous drag, if you want to be pedantic. Those are the only forces at play, the rest is only a side effect of those forces.
> you don't have any rotation in the game, as illustrated here
The arrows literally moved down!
> it really is the pressure alone
NASA, pilots and aerospace engineers would disagree with you. But yes, you can construct a working model of flight with just pressure. Same way you can make a Copernican model match our observations of how the stars and planets move.
The arrows literally moved down!
> it really is the pressure alone
NASA, pilots and aerospace engineers would disagree with you. But yes, you can construct a working model of flight with just pressure. Same way you can make a Copernican model match our observations of how the stars and planets move.
> The arrows literally moved down!
On the top part, you've got a supersonic free-stream deflected with an expansion fan to a supersonic parallel flow over a plate, deflected back and slowed-down to free-stream conditions through an oblique shock. The only thing the upper-surface "sees" is a parallel, supersonic flow.
On the lower part, you've got a supersonic free-stream deflected to a lower-speed supersonic flow through an oblique shock, creating a parallel supersonic flow over the lower surface of the plate, deflected back and re-accelerated to free-stream conditions through an expansion fan. The only thing the lower surface "sees" is a parallel, supersonic flow.
Now, unless you can come up with a force component emanating either from the oblique shocks or from the expansion fans and contributing to the lift vector, it fair to say that the flow deflection is not directly what is causing lift on the angled plate.
On the top part, you've got a supersonic free-stream deflected with an expansion fan to a supersonic parallel flow over a plate, deflected back and slowed-down to free-stream conditions through an oblique shock. The only thing the upper-surface "sees" is a parallel, supersonic flow.
On the lower part, you've got a supersonic free-stream deflected to a lower-speed supersonic flow through an oblique shock, creating a parallel supersonic flow over the lower surface of the plate, deflected back and re-accelerated to free-stream conditions through an expansion fan. The only thing the lower surface "sees" is a parallel, supersonic flow.
Now, unless you can come up with a force component emanating either from the oblique shocks or from the expansion fans and contributing to the lift vector, it fair to say that the flow deflection is not directly what is causing lift on the angled plate.
> No. What causes lift is the differential in pressure between the top and the bottom surface of the wing.
How do you explain airplanes that can fly with wing with symmetrical cross-section profile?
How do you explain airplanes flying upside-down?
How do you explain airplanes that can fly with wing with symmetrical cross-section profile?
How do you explain airplanes flying upside-down?
To create lift with a symmetrical airfoil, you are going to need a non-zero angle of attack. You can see the effect of a varying angle of attack on a symmetric NACA 0012 airfoil here [0].
The following plot shows the pressure distribution over a wing at 3 different angles of attack [1]. As you can see from the first plot, some lift is created at -8 degrees AOA, but clearly a lot less than the +10 AOA example, as that airfoil is optimized for positive angles of attack.
[0] https://www.youtube.com/watch?v=8uMlDL9HiaY
[1] http://avstop.com/AC/FlightTraingHandbook/imagese0.jpg
The following plot shows the pressure distribution over a wing at 3 different angles of attack [1]. As you can see from the first plot, some lift is created at -8 degrees AOA, but clearly a lot less than the +10 AOA example, as that airfoil is optimized for positive angles of attack.
[0] https://www.youtube.com/watch?v=8uMlDL9HiaY
[1] http://avstop.com/AC/FlightTraingHandbook/imagese0.jpg
Explanation based on Bernoulli effect requires longer path of air taking on top than on the bottom of the airfoil to create speed/pressure difference. With symmetrical airfoil both paths are the same regardless of the angle of attack. So when you mention AoA you implicitly lead to the explanation that lift, in majority, is not based on the Bernoulli effect.
I've read excellent article debunking the Bernoulli effect and lift many years ago, I'm not sure I can find it again...
I've read excellent article debunking the Bernoulli effect and lift many years ago, I'm not sure I can find it again...
Explanations based on the Bernoulli effect are trying to explain a speed differential by pretending that two particles that were separated on the leading-edge of an airfoil, to then travel one above the airfoil, one below, would then rejoin at the trailing edge of the airfoil. And so, if you were to change the upper-camber of the airfoil, the flow on the upper part would need to accelerate to be able to join the trailing edge at the same time. And that would create a lower pressure, therefore lift.
The nonsensical part of this model is that a particle on an upper streamline has anything to do with a particle on a lower streamline and that it is trying to keep up with it. Not so of course.
But the lift created by a pressure difference due to a locally faster flow still holds.
> So when you mention AoA you implicitly lead to the explanation that lift, in majority, is not based on the Bernoulli effect.
For a NACA 0012, you'll need an AoA, to have a faster flow on the upper part of your airfoil, as it it symmetric. Other airfoils are perfectly fine creating lift at 0 AoA.
The nonsensical part of this model is that a particle on an upper streamline has anything to do with a particle on a lower streamline and that it is trying to keep up with it. Not so of course.
But the lift created by a pressure difference due to a locally faster flow still holds.
> So when you mention AoA you implicitly lead to the explanation that lift, in majority, is not based on the Bernoulli effect.
For a NACA 0012, you'll need an AoA, to have a faster flow on the upper part of your airfoil, as it it symmetric. Other airfoils are perfectly fine creating lift at 0 AoA.
The Bernoulli effect only contributes to making wings more efficient. It isn’t fundamentally why lift occurs.
You can make almost anything fly if you have enough power and a tail. But how efficient will it be? Not as efficient as an airfoil that takes advantage of all the fluid motion properties.
You can make almost anything fly if you have enough power and a tail. But how efficient will it be? Not as efficient as an airfoil that takes advantage of all the fluid motion properties.
I think you wanted to respond to the parent comment. My questions have been a lead to debunk myth that the major contributor to the lift is the Bernoulli effect.
> How do you explain airplanes that can fly with wing with symmetrical cross-section profile?
> How do you explain airplanes flying upside-down?
Angle of attack is what causes lift. If you have a surface angled against the relative wind, it will produce lift.
> How do you explain airplanes flying upside-down?
Angle of attack is what causes lift. If you have a surface angled against the relative wind, it will produce lift.
I know all of that. I wanted to provoke parent commenter to let them see that the Bernoulli effect doesn't explain my two examples.
[deleted]
What causes the pressure differential then? It is my understanding that displacing\turning of the gas is what creates the pressure differential.
Good question!
The pressure differential, in essence, is created by a faster airflow over the airflow. As the total pressure in your flow stays constant, if you increase the local dynamic pressure (with a faster flow), the local static (measurable) pressure decreases.
So if you manage to shape your airfoil so that one surface experiences a faster flow (on average) than the other, you can create a pressure difference, and therefore lift.
And in effect it is true that the gas will most probably need to be turned and displaced, but that is really the airflow adapting locally to the obstacle (airfoil) it encounters. The nose of the airfoil, where the acceleration is high, can be a place where a lot of lift is created, but it is not necessarily so.
You can see example pressure distribution plots below:
http://avstop.com/AC/FlightTraingHandbook/imagese0.jpg
https://i.sstatic.net/UGurv.png
https://agodemar.github.io/FlightMechanics4Pilots/assets/img...
The pressure differential, in essence, is created by a faster airflow over the airflow. As the total pressure in your flow stays constant, if you increase the local dynamic pressure (with a faster flow), the local static (measurable) pressure decreases.
So if you manage to shape your airfoil so that one surface experiences a faster flow (on average) than the other, you can create a pressure difference, and therefore lift.
And in effect it is true that the gas will most probably need to be turned and displaced, but that is really the airflow adapting locally to the obstacle (airfoil) it encounters. The nose of the airfoil, where the acceleration is high, can be a place where a lot of lift is created, but it is not necessarily so.
You can see example pressure distribution plots below:
http://avstop.com/AC/FlightTraingHandbook/imagese0.jpg
https://i.sstatic.net/UGurv.png
https://agodemar.github.io/FlightMechanics4Pilots/assets/img...
It's not a cause and effect situation, because you can't have one without the other. A pressure differential can only exist if the flow is altered somehow, because a pressure differential means that the air molecules are subject to a net force which accelerates them. This is really just Bernoulli's theorem, which is Newton's second law. However, it doesn't tell you anything about why the flow around a wing arranges itself into such a configuration.
[deleted]
The Newtonian explaination of lift is partially but not completely correct. It only explains some of the lift which is empirically observed. Particularly the "push air down" model; the tops of wings also pull air down along themselves (assuming there isn't flow separation, e.g. a stall) and direct it down. To really explain that flow you need fluid dynamics.
> Do we still teach kids that planes fly because of Bernoulli's principle?
Not just for kids, but it is also in pilot training materials. I distinctly remember that it was how lift was explained in my PPL book.
Not just for kids, but it is also in pilot training materials. I distinctly remember that it was how lift was explained in my PPL book.
> distinctly remember that it was how lift was explained in my PPL book
TBF, the Handbook of Aeronautical Knowledge [1] does a better job.
[1] https://www.faa.gov/regulations_policies/handbooks_manuals/a...
TBF, the Handbook of Aeronautical Knowledge [1] does a better job.
[1] https://www.faa.gov/regulations_policies/handbooks_manuals/a...
In school when our physics teacher explained how the shape of an airplane wing creates lift and allows the plane to fly, I asked how it is that airplanes can fly upside down? I got the classic "that would be a great thing for you to research on your own time".
This is actually really cool, because an upside down airfoil will still create a high pressure ridge toward its leading edge. This causes air that would ostensibly flow along the bottom (high pressure) surface to sort of reverse and end up being pushed to the upper (low pressure) surface. The separation point is further down the leading edge than would be intuitively expected. This means the top stream of air still goes further, and faster, than the bottom stream of air.
So inverted wings still fly, just less efficiently.
So inverted wings still fly, just less efficiently.
If Newton’s third law sufficed, then the shape of the upper surface of the wing wouldn’t be important. In fact, it matters a lot.
There is no single explanation for why airfoils generate lift that works at a grade school level.
There is no single explanation for why airfoils generate lift that works at a grade school level.
Depends what we mean by grade school. For young kids (and honestly most adults) I don't think you need much more than this: "A wing, or anything that sends air moving past it down toward the ground will cause some lift (a push toward the sky), but also some drag (a push on your front toward your back). How much of each depends on the shape of the wing and how it's moving through the air. Really good wings cause a lot of lift without a lot of drag, which is good for not using a lot of fuel to get where you're going or for going really fast."
From my perspective, a much superior explanation would be something like: "A wing causes an aircraft to fly because its shape, and the angle at which it moves through the air, creates regions of higher air pressure under the wing, and lower air pressure above the wing. This causes an upwards force on the wing, and a corresponding reaction force downwards on the air itself."
The turbulence caused by a sharp leading edge of something like a flat board causes momentum transfer to the top of the wing. The problem isn't in explaining how a conventional airfoil works, Newton's law works well enough for that. The problem is in explaining what happens when things go wrong. Turbulence has been a problem for physicists for a long time...
The Newton's third-law explanation is "air bangs into a bottom of airfoil and pushes it up". Without having the concepts of boundaries layers, laminar and turbulent flow, flow separation and (more generally) the entire Navier-Stokes toolbox you don't have the tools for explaining why turbulent flow is a problem, for example.
The explanation based on Newton's laws of motion is more to the effect that the wing interacts with the air in such a way as to accelerate some of the air towards the ground. The reaction force is upwards.
The Navier-Stokes equations merely model fluid flows. Understanding them provides no understanding of the behaviour of such flows. That behaviour is emergent from the interaction of a great many particles.
The Navier-Stokes equations merely model fluid flows. Understanding them provides no understanding of the behaviour of such flows. That behaviour is emergent from the interaction of a great many particles.
>The explanation based on Newton's laws of motion is more to the effect that the wing interacts with the air in such a way as to accelerate some of the air towards the ground. The reaction force is upwards.
But that doesn't have any explanatory power at all. If we assume Newton's laws hold, then obviously if there's a force upward on the airfoil then there's a reaction force downward on the air.
It'd be like explaining the combustion engine by saying "the drive shaft from the engine rotates this way, and the reaction force - because the engine is more-or-less rigidly mounted to the frame - is resisted through the suspension by the wheels being in contact with the ground". OK, sure, but I still don't know how the engine actually works.
But that doesn't have any explanatory power at all. If we assume Newton's laws hold, then obviously if there's a force upward on the airfoil then there's a reaction force downward on the air.
It'd be like explaining the combustion engine by saying "the drive shaft from the engine rotates this way, and the reaction force - because the engine is more-or-less rigidly mounted to the frame - is resisted through the suspension by the wheels being in contact with the ground". OK, sure, but I still don't know how the engine actually works.
I dunno. If I look at even a very simple diagram of the flow of air around a wing I see air deflected downward on the bottom and air accelerated around a curve on the top. Both would be expected to produce a downward reaction force.
Added: Or more Newtonish (no action at a distance), there is more upward vertical force contributed by the particles in both cases than downward force.
Added: Or more Newtonish (no action at a distance), there is more upward vertical force contributed by the particles in both cases than downward force.
Yes, if someone tells you how the air flows around a wing you can immediately deduce that it's producing lift, since the air is deflected downwards. The real task is explaining why the air flows the way it does.
Newton's laws are absolutely not sufficient to explain how an airfoil works.
If I stick out a flat board from a moving car window, and hold it at an angle, it will "lift up". So indeed, airfoil shape does not matter. Angle of attack matters more, because that dictates the path of least resistance.
Planes fly by slicing through a lattice of air, with blades (wings) that only slice easily in directions that lie on a single plane. Orthogonal tail fins means that the vehicle doesn't go from side to side as easily, so it mostly keeps flying on a line. Take a `+`-shape and elongate it so you get a "dentastix" like shape, then hold that out the car window. It will go in whichever direction you point it.
Same idea of a boat rudder. And yet with boat rudders, we don't say "force of lift". The angle of attack changes, which means it now cuts through the water in a different direction (and the rudder piece wants to go straight in the direction it is pointing, since that way has comparatively little resistance in the water), which changes the way the rear of the ship moves which ultimately steers the ship.
Planes fly by slicing through a lattice of air, with blades (wings) that only slice easily in directions that lie on a single plane. Orthogonal tail fins means that the vehicle doesn't go from side to side as easily, so it mostly keeps flying on a line. Take a `+`-shape and elongate it so you get a "dentastix" like shape, then hold that out the car window. It will go in whichever direction you point it.
Same idea of a boat rudder. And yet with boat rudders, we don't say "force of lift". The angle of attack changes, which means it now cuts through the water in a different direction (and the rudder piece wants to go straight in the direction it is pointing, since that way has comparatively little resistance in the water), which changes the way the rear of the ship moves which ultimately steers the ship.
>So indeed, airfoil shape does not matter.
... what do you mean by that? If you mean "you can demonstrate the aerodynamic force using a flat plate", then yes you can do that. If you mean "a flat plate is a good tool to explain the aerodynamic force", then that's much less true. If you mean "in the real world, airfoil shape is irrelevant to aerodynamics" that's obviously false.
... what do you mean by that? If you mean "you can demonstrate the aerodynamic force using a flat plate", then yes you can do that. If you mean "a flat plate is a good tool to explain the aerodynamic force", then that's much less true. If you mean "in the real world, airfoil shape is irrelevant to aerodynamics" that's obviously false.
I was being a bit facetious, sorry. It matters, but I think what I was getting at is often simply overlooked in favor of airfoil shape and the pressure difference explanation. The situation of gravity no longer being a factor such as with vertical rudders seems often missed. Then, it's suddenly called "rudder force". Even though it's the same thing as "lift". It seems the field of physics has trouble with isolating this concept/phenomenon and coming up with an apt name for it.
Rudders are symmetric, i.e. don't have camber to create high/low pressure on one specific side all the time, and yet they work in redirecting (the relative) flow and thereby through Newtons 3rd redirecting the vessel!
Rudders are symmetric, i.e. don't have camber to create high/low pressure on one specific side all the time, and yet they work in redirecting (the relative) flow and thereby through Newtons 3rd redirecting the vessel!
The pressure difference explanation is the basic explanation, though. The name of the force is "the aerodynamic force", and there's not really any confusion on that point.
The difference of shape between hydrofoils and airfoils is determined by the properties of the masses in which they move, explained by the same theories of fluid dynamics, rather than any fundamental difference.
The difference of shape between hydrofoils and airfoils is determined by the properties of the masses in which they move, explained by the same theories of fluid dynamics, rather than any fundamental difference.
That's just because when you angle the sheet, more molecules of air hit one side imparting part of their kinetic energy, and fewer molecules on the other side to counteract this. I do realize I'm explaining pressure on a molecular level here, but to me it's still "slicing through" and "pushing against" a lattice of molecules.
It’s still Newton’s third law; Push air down and minimize pushing air sideways or in swirling vortices because pushing air the wrong way wastes energy as per newtons law.
That’s what the aerofoil does. It pushes air down but mimimizes wasting energy on drag. It’s still newtons law.
That’s what the aerofoil does. It pushes air down but mimimizes wasting energy on drag. It’s still newtons law.
For just a Newton's third law analysis, you have to have the air moving downward behind the wing. Doesn't the shape of the upper surface matter a lot in order to get the air moving downward?
>> The essence of the Bernoulli argument is that the top of the wing is longer -> air has to move further -> faster air has lower pressure "because Bernoulli" -> pressure imbalance means lift.
That argument doesn't hold up (no pun intended). Just because the distance is longer does not mean the air will go faster, it could just take longer to get there.
That argument doesn't hold up (no pun intended). Just because the distance is longer does not mean the air will go faster, it could just take longer to get there.
Not only kids but pilots. I got my license ten years ago and the answer to "How do planes?" was "Bernoulli."
Not only that, but depending on the particular FAA designated examiner you get, failing to tell him Bernoulli can result in a disapproval. I've heard of it happening.
Fortunately, none of this has ever mattered in the least for actually flying a plane, and there are plenty of sane examiners out there.
Fortunately, none of this has ever mattered in the least for actually flying a plane, and there are plenty of sane examiners out there.
You can experience faster air is lower pressure when you are trying to breath in strong wing (like sky diving or by an open window on a car on the motorway). It makes you usually gasp for air.
But yeah I was taught planes fly that way in the 90s.
But yeah I was taught planes fly that way in the 90s.
I don’t think I learned anything about flight as a kid. Frictionless cars, yes.
The Venturi effect does show that faster air is lower pressure though.
The qualifier is "along a streamline".
My mental model is that you can push hard on a wall while standing still, but the faster you run along the wall, the less you are able to push it.
>Additionally, a horizontal stabilizer in the back needs to be pitched down relative to the wings, creating downwards lift, pitching the plane up.
This is something I appreciate about the Lilium aircraft: they use canards to avoid this problem. Their latest design places the rear wing slightly above the canard[1], minimizing the downwash disadvantages[2] inherent in many canard configurations.
[1] https://www.youtube.com/watch?v=qZ73PftBfFg&t=273
[2] https://aviation.stackexchange.com/questions/83584/are-canar...
> > Additionally, a horizontal stabilizer in the back needs to be pitched down relative to the wings, creating downwards lift, pitching the plane up.
> Naturally, this is a fundamental source of inefficiency.
It's also not correct. The stabilizer in the back (or more generally: the wing in the back) needs to have a smaller angle of attack than the wing in front. In the common case (wing in back much smaller than wing in front) the wing in the back often is designed with negative angle of attack to create enough margin, but it's not strictly speaking necessary.
There are planes (like the Lilium you mention, or the planes designed by Burt Rutan) that have a small wing in the front (a canard) and a large wing in the back, providing most of the lift. In that case the wing in the back obviously needs to have a positive angle of attack to create the lift for the plane to stay in the air.
That may look like a special case, but it is not, aerodynamically speaking. The rule is the same for all types of planes: for stability, the back needs a lower angle of attack than the front, and that does not necessarily mean it needs to be negative. It also means the plane's center of gravity does not necessarily have to be in front of the front wing.
See https://www.av8n.com/how/htm/aoastab.html#sec-basic-stabilit..., and more specifically https://www.av8n.com/how/htm/aoastab.html#sec-canard-same.
I don't really know whether canard configurations solve the inefficiency problem of horizontal stabilizers with negative angle of attack: canards necessarily have a high angle of attack, which also creates drag.
(Note that all of the above is only relevant for planes with classic static stability (which is almost all of them); planes with relaxed static stability and fly-by-wire are kept stable by computer instead of aerodynamics.)
> Naturally, this is a fundamental source of inefficiency.
It's also not correct. The stabilizer in the back (or more generally: the wing in the back) needs to have a smaller angle of attack than the wing in front. In the common case (wing in back much smaller than wing in front) the wing in the back often is designed with negative angle of attack to create enough margin, but it's not strictly speaking necessary.
There are planes (like the Lilium you mention, or the planes designed by Burt Rutan) that have a small wing in the front (a canard) and a large wing in the back, providing most of the lift. In that case the wing in the back obviously needs to have a positive angle of attack to create the lift for the plane to stay in the air.
That may look like a special case, but it is not, aerodynamically speaking. The rule is the same for all types of planes: for stability, the back needs a lower angle of attack than the front, and that does not necessarily mean it needs to be negative. It also means the plane's center of gravity does not necessarily have to be in front of the front wing.
See https://www.av8n.com/how/htm/aoastab.html#sec-basic-stabilit..., and more specifically https://www.av8n.com/how/htm/aoastab.html#sec-canard-same.
I don't really know whether canard configurations solve the inefficiency problem of horizontal stabilizers with negative angle of attack: canards necessarily have a high angle of attack, which also creates drag.
(Note that all of the above is only relevant for planes with classic static stability (which is almost all of them); planes with relaxed static stability and fly-by-wire are kept stable by computer instead of aerodynamics.)
"See how it flies" that you linked is a wonderful resource for understanding the physics of aircraft and the skills needed to fly one.
Another, more math heavy treatment, of why airplanes fly that still aims to gain a conceptual understanding is "Understanding Aerodynamics" by Doug McLean: https://www.amazon.com/gp/product/B00B9QLBH0
Another, more math heavy treatment, of why airplanes fly that still aims to gain a conceptual understanding is "Understanding Aerodynamics" by Doug McLean: https://www.amazon.com/gp/product/B00B9QLBH0
I have that book! I bought it some time ago; I don't remember why I bought exactly that book, I guess it was recommended somewhere online much like you did just now.
Unfortunately I haven't found the time/energy to start reading it.
Unfortunately I haven't found the time/energy to start reading it.
I'm a huge fan of the Opener Blackfly, which has been physically designed for VTOL-like performance while providing a very simple flight mode during the transition from level to VTOL:
https://evtol.news/opener-blackfly/
The angle of attack of the lifting surfaces is intentionally offset so that, when the Opener reaches certain speed limits, aerodynamics take over and the plane switches modes automatically, without requiring much pilot input.
I can't wait to see these things buzzing around the skies - imho, this is the closest to 'the flying cars promise, fulfilled' so far ..
https://evtol.news/opener-blackfly/
The angle of attack of the lifting surfaces is intentionally offset so that, when the Opener reaches certain speed limits, aerodynamics take over and the plane switches modes automatically, without requiring much pilot input.
I can't wait to see these things buzzing around the skies - imho, this is the closest to 'the flying cars promise, fulfilled' so far ..
The Lilium Jet does not appear realistic to me, most VTOLs barely fly in the best of circumstances. It seems more like a scam to me. Not my field so I could be wrong but the added efficiency of a canard is small change compared to the other challenges involved.
Perhaps VTOL is entirely impractical and has no sizeable niche, but it appears Lilium is the least wrong out of all the existing VTOL physical layouts.
- No additional draggy/heavy structures for the VTOL components, they reuse the existing wings
- No large exposed props, which have noise and hazard concerns
- Propulsors are synergistically combined with the wing upper surface, enhancing lift during cruise
- VTOL mode actuators are synergistically combined with ailerons/elevators, reducing part count
- They have wings for efficient long-distance flight (surprisingly some don't!)
- Contingency ability for runway landing if the battery is too depleted
- The aforementioned canard advantage
>If any of the VTOL schemes are workable...
That reminds me of the "V/STOL Wheel of Misfortune", a diagram of 45 different V/STOL designs, of which four made it into service.
https://www.cmlx.co.uk/hawkerassociation/hamain/wheelmisfort...
That reminds me of the "V/STOL Wheel of Misfortune", a diagram of 45 different V/STOL designs, of which four made it into service.
https://www.cmlx.co.uk/hawkerassociation/hamain/wheelmisfort...
On a similar note, the Aviation week ranking for current Advanced Air Mobility projects is named a "reality index"!
https://aamrealityindex.com/aam-reality-index
(Lilium is in the upper middle of this, FWIW)
https://aamrealityindex.com/aam-reality-index
(Lilium is in the upper middle of this, FWIW)
On that case, past performance is absolutely not a guide for future results.
The weight/power ratio of electrical motors is so different from combustion engines that it's a qualitative difference already. Just because nobody has ever been able to solve that problem, it doesn't mean that a lot of people won't easily solve it now.
The weight/power ratio of electrical motors is so different from combustion engines that it's a qualitative difference already. Just because nobody has ever been able to solve that problem, it doesn't mean that a lot of people won't easily solve it now.
Joby aviation seems the furthest along.
Joby has completed many test flights that mimic its real-world route distances and energy-altitude flight regeimes.
Its flight avionics package (essential for VTOL fly-by-wire) has received FAA certification.
Related to Lilium and noise, Joby has a lower overall emitted dB energy and, specifically, a much lower dB signature in the human hearing range.
Joby has completed many test flights that mimic its real-world route distances and energy-altitude flight regeimes.
Its flight avionics package (essential for VTOL fly-by-wire) has received FAA certification.
Related to Lilium and noise, Joby has a lower overall emitted dB energy and, specifically, a much lower dB signature in the human hearing range.
It appears you’ve drunk the coolaid. I looked into it more and can now confidently say it’s a total scam. The stuff you’re talking about is window dressing.
Maybe that's true! If you can elaborate on your confident saying with facts, sources, or explanations, I would certainly give it my full consideration. It wouldn't be the first time I've been wrong!
What makes you think it's a "total scam"?
What makes you think it's a "total scam"?
My prior for eVTOL these days is scam, but I googled ‘Lilium Jet scam’ and hit this;
https://news.ycombinator.com/item?id=29651504
What I was looking for is someone to say it can’t work because of the math XYZ and for a person to say back that math XYZ is wrong because of ABC and that second part never happens.
The stuff they talk about is window dressing and doesn’t answer questions like, where are your 500wh/kg batteries?
I remember when Ballon Boy happened and I took one look at that Ballon and it was instantly obvious it wasn’t carrying a kid, but apparently others were still expecting to see a kid when the balloon landed.
https://news.ycombinator.com/item?id=29651504
What I was looking for is someone to say it can’t work because of the math XYZ and for a person to say back that math XYZ is wrong because of ABC and that second part never happens.
The stuff they talk about is window dressing and doesn’t answer questions like, where are your 500wh/kg batteries?
I remember when Ballon Boy happened and I took one look at that Ballon and it was instantly obvious it wasn’t carrying a kid, but apparently others were still expecting to see a kid when the balloon landed.
>My prior for VTOL these days is scam
_Why_ is your a priori expectation for all VTOL to be a scam? And does that generalized reasoning in fact apply to the specific case of Lilium?
>where are your 500wh/kg batteries
My prior is that batteries will continue on the same curve.
You can also check out their stock price. Down from peak 92%, so at least to investors it’s looking less likely they’ll deliver instead of more likely.
I gave a link in which others listed their rationale for why they don’t think it will work.
Their availability estimates for battery capacity is double the long term rate of improvement - which seems unrealistic. But yah know with enough wh/kg just about anything will fly.
I gave a link in which others listed their rationale for why they don’t think it will work.
Their availability estimates for battery capacity is double the long term rate of improvement - which seems unrealistic. But yah know with enough wh/kg just about anything will fly.
Imagine thinking stock price pegged to peak price indicates company value. :-/
Hopefully this can help clear up these (hastily-Googled) concerns.
I do expect in the nominal case that Lilium will take perhaps twice as long to come to market, so in the end the delays may roughly cancel out. Time will tell.
Note that battery density will effect all eVTOL startups equally, so it's not really a competitive disadvantage for Lilium per se, but rather an overall industry challenge. And I agree, there are many challenges facing the industry!
Good deep discussion, thank you. Cheers mate
[1] https://lilium.com/newsroom-detail/technology-behind-the-lil...
[2] https://lilium.com/newsroom-detail/youve-never-seen-anything...
> I gave a link
> my engineering mind recoils at the complexity of the design. The variable pitch blades, the adjustable exhaust nozzles, the tilt-wing vectored thrust system, etc. With complexity comes...
Hopefully this can help clear up these (hastily-Googled) concerns.
>Their availability estimates for battery capacity is double the long term rate of improvement
I do expect in the nominal case that Lilium will take perhaps twice as long to come to market, so in the end the delays may roughly cancel out. Time will tell.
Note that battery density will effect all eVTOL startups equally, so it's not really a competitive disadvantage for Lilium per se, but rather an overall industry challenge. And I agree, there are many challenges facing the industry!
Good deep discussion, thank you. Cheers mate
[1] https://lilium.com/newsroom-detail/technology-behind-the-lil...
[2] https://lilium.com/newsroom-detail/youve-never-seen-anything...
What you have shown me has not convinced me to change my opinion and I'm not trying to change yours. The people that I know in the industry focus on certification timeline being overly optimistic which while most defiantly true is less interesting to me than the battery tech, or even the sociology as to why people get so attached to this - like that Nuclear-Powered Sky Cruise concept that many people shared unironically.
No doubt a lot of amazing things can happen with an large increase of wh/kg which is why I think it's incredibly weird that so much effort was put into things that are not that, Lilium has special investments in battery tech with Ionblox, the only interesting question to me is 'is that paying off?'. Not how much % of efficiency can be saved with a canard design - that's window dressing. Also, if they have this amazing battery tech then isn't that the most valuable thing they have and is eVTOL really the best application for it, why not double the range of electric cars instead. Even if they think eVTOL is the best use case then why not run on a skeleton crew until the battery tech arrives, or at least properly test their designs with onboard generators.
Waiting for battery tech is like waiting for engine tech and since so many engines end up vaporware so do all the nice aircraft designs dependent on them.
But sometimes new engines do deliver. The RED Aircraft V12 engines are amazing and should enabled the Otto Celera 500L to work really well. The DeltaHawk engine but they might actually end up delivering a really nice reliable engine. Part of that is a combination of long term stagnant general aviation engine technology and reduced tooling costs for manufacturing with CNCs. So there was a lot of low hanging fruit waiting to be picked.
Obviously Lilium Jets initial claims were wildly unrealistic, I think their current claims remain unrealistic, perhaps by the time they 'deliver' they've scoped it down to small hops. If they plan on selling something that can carry 7 people (they've already sold 20?) then perhaps I would believe it more if they demonstrated something working with 1 person and a whole lot of performance to spare.
No doubt a lot of amazing things can happen with an large increase of wh/kg which is why I think it's incredibly weird that so much effort was put into things that are not that, Lilium has special investments in battery tech with Ionblox, the only interesting question to me is 'is that paying off?'. Not how much % of efficiency can be saved with a canard design - that's window dressing. Also, if they have this amazing battery tech then isn't that the most valuable thing they have and is eVTOL really the best application for it, why not double the range of electric cars instead. Even if they think eVTOL is the best use case then why not run on a skeleton crew until the battery tech arrives, or at least properly test their designs with onboard generators.
Waiting for battery tech is like waiting for engine tech and since so many engines end up vaporware so do all the nice aircraft designs dependent on them.
But sometimes new engines do deliver. The RED Aircraft V12 engines are amazing and should enabled the Otto Celera 500L to work really well. The DeltaHawk engine but they might actually end up delivering a really nice reliable engine. Part of that is a combination of long term stagnant general aviation engine technology and reduced tooling costs for manufacturing with CNCs. So there was a lot of low hanging fruit waiting to be picked.
Obviously Lilium Jets initial claims were wildly unrealistic, I think their current claims remain unrealistic, perhaps by the time they 'deliver' they've scoped it down to small hops. If they plan on selling something that can carry 7 people (they've already sold 20?) then perhaps I would believe it more if they demonstrated something working with 1 person and a whole lot of performance to spare.
Huh. What about Volocopter? Appears to be entering market.
https://www.volocopter.com
https://www.volocopter.com
All of these electric VTOL urban air mobility things are hopeless. Various tech companies including google have taken a stab at them over the past two decades and none are anywhere close to entering service. Google shut theirs down. I really want them to work, but nothing gets past the physics/economics of batteries having piss poor energy density relative to fossil fuels and the massive inefficiency of small rotors. I think Boom has a better chance of flying than Wisk. Meanwhile, Uber Copter is up and running in NYC with regular helicopters.
Well a scam that is flying and more or less working as advertised at this point. You can watch testflights on youtube. Same for the Archer, Joby, Beta Alia, and a few others. Several of those are now doing manned flights and shipping prototypes to early customers.
In short, they fly just fine. What makes you think things are a scam?
In short, they fly just fine. What makes you think things are a scam?
Well, what Lilium is flying is a demonstrator, not a prototype. The difference is that a prototype is close to the final product and test flights can be used in the early phases of the certification path, demonstrator flights cannot. Or to put harsher: all Lillium has is two model aircraft that have close to nothing in common with the 7 seater they are selling.
And one could call it a scam, when tuh product you sell has nothing to do with the product you show (and no, mock-ups at airshows don't count at all), and the product you sell has, so far, no clear timeline until certification. The aerospace version of vaporware. Whether or not it amounts to an actual scam woupd be for courts to decide. Right now it looks a lot like Nikola, without the option to use a hill to fake the product demo.
As a sidenote regarding test flights: last time I checked, those were unmanned, with a demonstrator and not a prototype and no longer than 6 minutes. Which is as far from what serious people in the field call a test flight of it could be. Good for PR and investors so, it looks cool.
Also, one can make everything fly, if you put enough thrust to it. Doesn't mean you have product that can sustain a business.
And one could call it a scam, when tuh product you sell has nothing to do with the product you show (and no, mock-ups at airshows don't count at all), and the product you sell has, so far, no clear timeline until certification. The aerospace version of vaporware. Whether or not it amounts to an actual scam woupd be for courts to decide. Right now it looks a lot like Nikola, without the option to use a hill to fake the product demo.
As a sidenote regarding test flights: last time I checked, those were unmanned, with a demonstrator and not a prototype and no longer than 6 minutes. Which is as far from what serious people in the field call a test flight of it could be. Good for PR and investors so, it looks cool.
Also, one can make everything fly, if you put enough thrust to it. Doesn't mean you have product that can sustain a business.
That prototype/demonstrator (let's not get silly about words here) looks like it's actually flying properly though. Transitions to horizontal and vertical flights and all. They are planning to have the first manned flight end of this year.
A scam would be intentionally misleading people about the ability of this thing to fly at all and then grabbing the money and run.
scam would also involve disgruntled investors trying to sue and getting their money back. There have been a few such cases about investors wanting their money back. But the headline is that Lilium is continuing to raise lots of money and making steady progress to getting their products launched. And those court cases seem to be going nowhere so far. The nature of VC funding is of course that things don't always go to plan.
Just because this company isn't satisfying your need for instant success and instead is following an entirely reasonable path to certification, which is slow for any airplane, doesn't mean it's a scam. By that logic anything is a scam until it emerges fully designed and manufactured on the market. That's not how things work in the real world.
This thing has investors, prospective customers with letters of intent, and flying prototypes.
Nikola is actually shipping trucks at this point too. Yes, they got caught with a non driving prototype running downhill and they got punished for that and the CEO might do some jail time for that. But the thing works now and they are selling lots of trucks that actually move cargo around. A scam would have been if the thing proved to be vapor ware. As it turns out, it wasn't. It was just running a bit late.
scam would also involve disgruntled investors trying to sue and getting their money back. There have been a few such cases about investors wanting their money back. But the headline is that Lilium is continuing to raise lots of money and making steady progress to getting their products launched. And those court cases seem to be going nowhere so far. The nature of VC funding is of course that things don't always go to plan.
Just because this company isn't satisfying your need for instant success and instead is following an entirely reasonable path to certification, which is slow for any airplane, doesn't mean it's a scam. By that logic anything is a scam until it emerges fully designed and manufactured on the market. That's not how things work in the real world.
This thing has investors, prospective customers with letters of intent, and flying prototypes.
Nikola is actually shipping trucks at this point too. Yes, they got caught with a non driving prototype running downhill and they got punished for that and the CEO might do some jail time for that. But the thing works now and they are selling lots of trucks that actually move cargo around. A scam would have been if the thing proved to be vapor ware. As it turns out, it wasn't. It was just running a bit late.
>A scam would be intentionally misleading people about the ability of this thing to fly at all and then grabbing the money and run.
There's a fine line between a scam and a business plan which bakes in assumptions that are unrealistic.
There's a fine line between a scam and a business plan which bakes in assumptions that are unrealistic.
I haven't been following those, but the GP's question wasn't about any of the things you enumerated.
The big question is: have they demonstrate a loaded plane flying through a useful distance while keeping enough reserve energy for satisfying the safety requirements?
Their videos are very well produced explanations about everything but this. There's some stuff about a few changes that reduce the reserve requirements, but still, I couldn't find anything about range.
The big question is: have they demonstrate a loaded plane flying through a useful distance while keeping enough reserve energy for satisfying the safety requirements?
Their videos are very well produced explanations about everything but this. There's some stuff about a few changes that reduce the reserve requirements, but still, I couldn't find anything about range.
While most of them fly just fine, the economics need still to be proven.
That usually comes after certification. Which some of these things are getting close to. I hear Delta is eager to start shuttling passengers from their terminals with Archer. At this point, you can start making some pretty informed statements about operational cost too. Battery life span and cost is a known factor. The cost of electricity is a known factor. There might be some nasty surprises with components, manufacturing, or scaling up production volume of course.
But mostly these things are starting to look like they are getting there.
But mostly these things are starting to look like they are getting there.
If this argument were sound, tailless designs (i.e. without a separate horizontal stabilizer either before or behind) would be optimal, but what matters to efficiency is the overall lift-to-drag ratio within all the other feasibility constraints. No-one obsesses over lift-to-drag ratios more than sailplane designers and their customers, and the fact that the medium- to high-performing sailplanes are neither tailless nor canards, despite there being airworthy examples of both canard and tailless gliders, is telling us something, at least in the range of Reynolds numbers relevant to glider flight.
Three issues with pusher-prop "tail-less" designs:
1) While they are more stable in nominal flight regimes, they are far harder to recover to stable flight from perturbations. It turns out, it's (overall) much safer to have a plane that will BOTH stall easier (more predictably) and recover easier than one that is less likely to stall in the first place, but difficult to recover from. One analogy I often use is the difference between a mid-engine car and a front-engine layout. While the mid-engine car has a greater overall theoretical "handling" performance ceiling, a front-engine car behaves more predictably (less twitchy) at the limits.
2) They are more susceptible to CG/balance issues so they have less practical cargo capacity because just a weee bit of pitch/yaw/roll trim results in a drastic drop-off in the aforementioned stellar lift efficiency.
3) They have much longer take-off and landing runway requirements due to less ground-effect and much less overall wing efficiency at near-stall speeds.
1) While they are more stable in nominal flight regimes, they are far harder to recover to stable flight from perturbations. It turns out, it's (overall) much safer to have a plane that will BOTH stall easier (more predictably) and recover easier than one that is less likely to stall in the first place, but difficult to recover from. One analogy I often use is the difference between a mid-engine car and a front-engine layout. While the mid-engine car has a greater overall theoretical "handling" performance ceiling, a front-engine car behaves more predictably (less twitchy) at the limits.
2) They are more susceptible to CG/balance issues so they have less practical cargo capacity because just a weee bit of pitch/yaw/roll trim results in a drastic drop-off in the aforementioned stellar lift efficiency.
3) They have much longer take-off and landing runway requirements due to less ground-effect and much less overall wing efficiency at near-stall speeds.
> If this argument were sound
Are you saying that having a horizontal stabilizer is not a source of inefficiency? This isn't an argument, it's a basic fact about airplane design. They necessarily contribute to overall drag.
In practical airplane design, there are considerations other than drag that make horizontal stabilizers a worthwhile compromise.
Are you saying that having a horizontal stabilizer is not a source of inefficiency? This isn't an argument, it's a basic fact about airplane design. They necessarily contribute to overall drag.
In practical airplane design, there are considerations other than drag that make horizontal stabilizers a worthwhile compromise.
Firstly, 'having a horizontal stabilizer is a source of inefficiency' is not an argument, it is a fact (one that might be a premise in an argument, but see below.) As you know this, how did you get from seeing that I said a certain argument is unsound to supposing that I am disputing this fact?
Maybe you think it is the only way that argument could be unsound, which brings us to the second point: the argument I am commenting on is not 'having a horizontal stabilizer is a source of inefficiency, therefore canards are more efficient', which would not even be valid. It is, instead, the argument that canards are more efficient because the conventional horizontal stabilizer usually produces a downwards force (incidentally, this is not always so [1].) While this may seem an obvious conclusion at first sight, it tacitly presumes a sharp separation of concerns which does not hold in practice.
The argument 'having a horizontal stabilizer is a source of inefficiency, therefore tailless designs have greater efficiency' (which was not made in the post I was replying to, but which is sometimes alluded to) does not hold up any better, on account of the compromises in making a stable and controllable tailless airplane (at least without active stability augmentation.)
[1] For some conventional airplanes, with the GofG near its aft limit, the horizontal stabilizer will produce an upwards force at low speeds (without being unstable as a consequence.) This happens to be the case for many gliders. A while back (and possibly now lost - at least, I have not been able to find it), there was an interesting article (by Wilhelm Dirks - co-founder of DG Aviation, I believe) explaining why, in practice, this cannot be exploited to get a little bit more performance out of a sailplane.
Maybe you think it is the only way that argument could be unsound, which brings us to the second point: the argument I am commenting on is not 'having a horizontal stabilizer is a source of inefficiency, therefore canards are more efficient', which would not even be valid. It is, instead, the argument that canards are more efficient because the conventional horizontal stabilizer usually produces a downwards force (incidentally, this is not always so [1].) While this may seem an obvious conclusion at first sight, it tacitly presumes a sharp separation of concerns which does not hold in practice.
The argument 'having a horizontal stabilizer is a source of inefficiency, therefore tailless designs have greater efficiency' (which was not made in the post I was replying to, but which is sometimes alluded to) does not hold up any better, on account of the compromises in making a stable and controllable tailless airplane (at least without active stability augmentation.)
[1] For some conventional airplanes, with the GofG near its aft limit, the horizontal stabilizer will produce an upwards force at low speeds (without being unstable as a consequence.) This happens to be the case for many gliders. A while back (and possibly now lost - at least, I have not been able to find it), there was an interesting article (by Wilhelm Dirks - co-founder of DG Aviation, I believe) explaining why, in practice, this cannot be exploited to get a little bit more performance out of a sailplane.
Could you explain the inefficiency in terms of energy transformation?
Potential energy change is zero (or negative)?
I get there may be sound or heat from air friction though would put that minimal. Also zero (negative) acceleration around axis due to rotation.
I'm curious how it's inefficient analysing through energy transformation.
Potential energy change is zero (or negative)?
I get there may be sound or heat from air friction though would put that minimal. Also zero (negative) acceleration around axis due to rotation.
I'm curious how it's inefficient analysing through energy transformation.
Because of the negative lift of the horisontal stabilizer, the main wing needs to provide more lift than the weight of the airplane. This increased lift requires flying the wing at higher angle of attack, which always comes with increased drag (https://en.wikipedia.org/wiki/Lift-induced_drag). Increased drag means lower efficiency.
Analyzing energy transformation is not so useful, because most drag ultimately ends up heating the air. (A little will heat the airplane skin.) There are several different mechanisms that makes that happen, and no easy way to figure out how large it is, but it's definitely not minimal. It's why (most) airplanes need an engine to stay up...
Analyzing energy transformation is not so useful, because most drag ultimately ends up heating the air. (A little will heat the airplane skin.) There are several different mechanisms that makes that happen, and no easy way to figure out how large it is, but it's definitely not minimal. It's why (most) airplanes need an engine to stay up...
Probably the best practical flight explanation website: https://www.av8n.com/how/
> With a simple rectangular wing, the center of pressure is 1/4 of the way along the wing from the leading edge
Is there a nice way to derive this? I find it interesting it's not the exact center though I guess it makes sense given the angle of the surface.
Is there a nice way to derive this? I find it interesting it's not the exact center though I guess it makes sense given the angle of the surface.
> Is there a nice way to derive this?
“It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the aerodynamic moment remains nearly constant with angle of attack. Engineers call the location where the aerodynamic moment remains constant the aerodynamic center (ac) of the airfoil” [1].
As you go faster, it goes from quarter chord to half. For a rectangle, the chord length is equal to the airfoil length.
Why quarter? It comes from thin-airfoil theory [2]. (Unintuitively, it holds across atmospheres.)
[1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
[2] http://aero-comlab.stanford.edu/aa200b/lect_notes/thinairfoi...
As you go faster, it goes from quarter chord to half. For a rectangle, the chord length is equal to the airfoil length.
Why quarter? It comes from thin-airfoil theory [2]. (Unintuitively, it holds across atmospheres.)
[1] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
[2] http://aero-comlab.stanford.edu/aa200b/lect_notes/thinairfoi...
I was asking myself the same question. I would love to see a derivation from e.g. the Navier Stokes equation for this. I think, intuitively, when you draw the streamlines under the rectangular wings, the applied force should be related to the curvature of the streamlines (which is larger at the beginning of the wing).
I made some simple 2D Navier-Stokes solver here where you can use the mouse to draw a section of a wing:
https://alexander-barth.github.io/FluidSimDemo-WebAssembly/
The color represents the pressure (I should add a proper color bar!)
I made some simple 2D Navier-Stokes solver here where you can use the mouse to draw a section of a wing:
https://alexander-barth.github.io/FluidSimDemo-WebAssembly/
The color represents the pressure (I should add a proper color bar!)
I remember being taught that Bernoulli's principle causes lift. I was skeptical—how does the air on top know to reach the other end at the same time as the air at the bottom? I think I did ask, and I was just told this is how it works, and that's the correct answer for the exam. This was before the internet, and I couldn't just look up the correct explanation.
I parked it in my brain as something I didn't really understand and forgot about it. This was until not so many years ago when I found a satisfactory answer on YouTube. It was criminal to have been raised in an era without the internet.
I parked it in my brain as something I didn't really understand and forgot about it. This was until not so many years ago when I found a satisfactory answer on YouTube. It was criminal to have been raised in an era without the internet.
Related, very deep, and amazing work: https://ciechanow.ski/airfoil/ https://news.ycombinator.com/item?id=39526057
For a more in-depth resource that is still very approachable at a high school level, I highly recommend John S. Denker’s book, See How It Flies, full text online
https://www.av8n.com/how/
Edit: added book title
Edit: added book title
>A common misconception about wings is that they need to have the classic airfoil shape to work. In reality, just about any surface can create lift and function as a wing
Left unsaid is why aircraft wings have airfoil-shaped cross sections with cambered (concave-down) shapes: they produce more lift for a given wing loading and angle of attack.
This is why aircraft have flaps, as well. They increase the camber of the wing so that the pilot can fly slower without pitching the nose up, which is important for example when maintaining sight of the runaway on landing approach.
Left unsaid is why aircraft wings have airfoil-shaped cross sections with cambered (concave-down) shapes: they produce more lift for a given wing loading and angle of attack.
This is why aircraft have flaps, as well. They increase the camber of the wing so that the pilot can fly slower without pitching the nose up, which is important for example when maintaining sight of the runaway on landing approach.
> You will notice that past a certain angle, the lift starts to decrease, and is replaced by a lot of drag, a force trying to slow down the wing. This is called a stall, and it limits how much lift a wing can create at any given speed. A proper airfoil geometry can generate more lift before stalling, and creates less drag for a given amount of lift, which is why most airplanes use them.
I'm a bit of a novice, so it sounds like this is the same explanation you're giving. What is the article leaving out?
I'm a bit of a novice, so it sounds like this is the same explanation you're giving. What is the article leaving out?
Speed. You'll always stall if you go too slow to get enough lift. Putting the flaps down increases lift and drag and lowers the stall speed.
As for why the article mentions decreased lift whereas flaps down increases lift, that's maybe a bit more complicated. Lift vs. Angle Of Attack is a curve that tops out at about 15°.[0] Flaps often go to steeper angles than that, but I'm not sure if lift actually starts to decrease again in that scenario. Certainly adding a curve to the back of the wing (flaps) isn't quite the same as changing the angle of the entire wing.
[0] https://commons.wikimedia.org/wiki/File:Lift_curve.svg
As for why the article mentions decreased lift whereas flaps down increases lift, that's maybe a bit more complicated. Lift vs. Angle Of Attack is a curve that tops out at about 15°.[0] Flaps often go to steeper angles than that, but I'm not sure if lift actually starts to decrease again in that scenario. Certainly adding a curve to the back of the wing (flaps) isn't quite the same as changing the angle of the entire wing.
[0] https://commons.wikimedia.org/wiki/File:Lift_curve.svg
> You'll always stall if you go too slow to get enough lift.
The amount of lift needed is determined by the load factor. In a turn, or when a glider launches via a winch, the load factor is higher than in level flight.
This means that the stall speed varies with load factor.
On the other side, you can still not be stalled below the straight and level stall speed, if the load factor is low enough. In parabolic flight you can see this, and with a load factor of 0 you cannot stall.
The amount of lift needed is determined by the load factor. In a turn, or when a glider launches via a winch, the load factor is higher than in level flight.
This means that the stall speed varies with load factor.
On the other side, you can still not be stalled below the straight and level stall speed, if the load factor is low enough. In parabolic flight you can see this, and with a load factor of 0 you cannot stall.
.
No, flaps increase CL. https://www.researchgate.net/figure/Figure-1-4-Effect-of-fla...
You might be thinking of total lift on a 3D wing, which is something you alluded to re: wing loading. But 3d wings vs 2d airfoils are really different concepts that you seem to be mixing up. Airfoils are advantageous regardless of wing loading (which is an inherently 3d concept) because of better L/D and stall characteristics.
You might be thinking of total lift on a 3D wing, which is something you alluded to re: wing loading. But 3d wings vs 2d airfoils are really different concepts that you seem to be mixing up. Airfoils are advantageous regardless of wing loading (which is an inherently 3d concept) because of better L/D and stall characteristics.
You are correct, I had a brain fart. Not enough sleep the last few days.
Except in dire circumstances, airplanes are not struggling to produce sufficient lift. What matters is to produce it with reasonably low drag, up to all the other constraints that go into making a practical airplane for its intended purpose.
There are some airplanes that can be in a situation of flying, but unable to climb or even maintain altitude, with flaps fully lowered. In such a situation, only carefully raising the flaps will allow it to climb away.
There are some airplanes that can be in a situation of flying, but unable to climb or even maintain altitude, with flaps fully lowered. In such a situation, only carefully raising the flaps will allow it to climb away.
Great explanation! It also decreases the stall speed too; which allows a plane to switch to ‘low speed mode’
It's a really fun set of tradeoffs to be honest. You get more lift at a given speed/AoA, which lets you go slower without stalling. But you get even more drag, which means the engines have to work harder to keep you flying at that slower speed.
Increasing drag is actually useful when landing. too. Trying to land an airplane with very little drag at a given spot is difficult because if you're just a little fast they'll float a long time.
Definitely! Flaps are one of the few “almost free lunch” things in aviation where almost everything else has harsh tradeoffs to consider.
Given how straightforward the physics are, what is the limiting factor that kept us from developing planes sooner? Insufficient propulsion power to overcome the drag of an inefficient wing?
Almost none of this is intuitive without hundreds of years of hindsight. The more subtle aspects of stability, such as avoiding oscillation mostly had to be determined experimentally. Then there is also the matter of actually constructing a plane, if you want it to be useful, it's going to need to be a lot more then some folded paper.
Thrust was definitely also a problem, a glider is not particularly useful unless it has a huge lift-to-drag ratio, which is only possible with modern materials and a solid understanding of airfoil design, which is a whole other can of worms.
Even things that seem so basic that we don't even think about them, like high were not at all obvious: just look at Sir George Cayley's gliders.
Thrust was definitely also a problem, a glider is not particularly useful unless it has a huge lift-to-drag ratio, which is only possible with modern materials and a solid understanding of airfoil design, which is a whole other can of worms.
Even things that seem so basic that we don't even think about them, like high were not at all obvious: just look at Sir George Cayley's gliders.
Paper planes have been a thing for a long while, but I guess no one thought that could scale?
It's like fire, super obvious in retrospective, and yet took thousands of years to become a technology.
It's like fire, super obvious in retrospective, and yet took thousands of years to become a technology.
It’s not easy to scale materials and retain the necessary rigidity. Like if you tried to scale up a primitive kite to support a human it would either be too heavy or too bendy. You need better materials and better construction techniques.
>Paper planes have been a thing for a long while, but I guess no one thought that could scale?
... and they were right, because aerodynamics is not scale-invariant.
... and they were right, because aerodynamics is not scale-invariant.
Well, airplanes do exist now, so that did definitely scale.
Airplanes were developed almost as soon as piston engines became available. Steam engines were too heavy for a plane. And even the early piston engine had a bad power/weight ratio. For french speakers, there is a video of a retired Dassault plane designer where he mentions (among other things) that developments in plane design basically followed developments in engines.
https://www.youtube.com/watch?v=lu4V9zn0wZ4
Also highly recommend his previous video about the design of the Rafale, it's basically a lecture in plane aerodynamics.
https://www.youtube.com/watch?v=XrpsptSLt90
https://www.youtube.com/watch?v=lu4V9zn0wZ4
Also highly recommend his previous video about the design of the Rafale, it's basically a lecture in plane aerodynamics.
https://www.youtube.com/watch?v=XrpsptSLt90
Leonardo Da Vinci lacked good propulsion. But otherwise people have been toying with replicas of his designs and getting them to fly. Compact, petrol engines did not emerge until late 19th century. And they weren't very light. So, that was indeed a factor. Putting a steam engine on a plane was not going to fly.
Though, they might have invented some kind of functional glider and used a steam powered winch. But, I'm guessing the aero-dynamics weren't that widely understood yet. And making stiff lightweight air-frames also took some time to happen. The contraption that the Wright brothers flew was a bit flimsy. And it barely flew. It was more of a proof of concept. Things like thermals and other updrafts that make gliders work weren't that well studied probably.
Though, they might have invented some kind of functional glider and used a steam powered winch. But, I'm guessing the aero-dynamics weren't that widely understood yet. And making stiff lightweight air-frames also took some time to happen. The contraption that the Wright brothers flew was a bit flimsy. And it barely flew. It was more of a proof of concept. Things like thermals and other updrafts that make gliders work weren't that well studied probably.
Control was an almost completely unaddressed issue before the Wrights took it up, even though it is also crucial to useful glider development.
This video has a nice delineation of the collection of breakthroughs needed:
This video has a nice delineation of the collection of breakthroughs needed:
https://youtu.be/EkpQAGQiv4Q?t=391Yes. AIUI, as bicycle riders the Wright brothers understood the need for banking in turns rather than thinking of maintaining a constant roll orientation throughout a turn
Basically, understanding airfoils. Back in the 1840s they put miniature steam engines in model planes and got them to fly. There were powered ground-effect planes in the 1800s that had more powerful engines than the Wright Flyer, but the Wrights were the first people to invent the wind tunnel, so they had a better understanding of lift-to-drag and control surface issues. They adapted the flow-pattern analysis tanks used by ship designers.
As mcarmichael said https://youtu.be/EkpQAGQiv4Q?t=391
As mcarmichael said https://youtu.be/EkpQAGQiv4Q?t=391
The basics are straightforward while having the hindsight of all the development we have enjoyed in the last hundred years. Stable flight was a hard nugget to crack. Better mechanical design, understanding of fluid dynamics, lighter materials, engines with higher power-to-weight ratios, etc all brought us to where we are today once the Wright Flyer was demonstrated.
My understanding is that it wasn’t power-to-drag but power -to-weight.
Although the Wright brothers deservedly get a lot of credit, I think they made progress once they enlisted the help of Curtis due to his knowledge of lightweight internal combustion engines.
The book Bird Men is a god read on this subject.
Although the Wright brothers deservedly get a lot of credit, I think they made progress once they enlisted the help of Curtis due to his knowledge of lightweight internal combustion engines.
The book Bird Men is a god read on this subject.
For anyone who has trouble finding this book like I did, the actual title is Birdmen (no space):
https://www.amazon.com/dp/B00GL3MGTI
https://www.amazon.com/dp/B00GL3MGTI
> The book Bird Men is a god read on this subject.
Thanks for that recommendation, I'll add that to my book reading list!
Thanks for that recommendation, I'll add that to my book reading list!
Weight. A typical steam train has enough propulsion power to overcome an enormous amount drag, but its power-to-weight ratio is low.
Yes, the wright flyer came fairly soon after internal combustion engines became light enough.
I've flown hundreds of hours using single-surface hang-gliders that effectively have little to no "flat plate" effect, and they make huge amounts of very draggy, slow speed lift. I've flown hundreds of hours using double-surface hang-gliders that make much less slow speed lift, but far less draggy lift at moderate to high speeds.
As in all things with aerodynamics - you optimize the design for the performance you want.
> Additionally, when plane speeds up, more drag is produced, slowing it down.
Wait, is it speeding up here or slowing down? Slowing down means deceleration, speeding up is acceleration, and it can't be doing both at the same time.
> When the plane slows down, it produces less drag, allowing to to pick up more speed.
Same deal?
I think what he's getting at is that drag increases with the square of speed, but it's a very confusing way of explaining it.
Wait, is it speeding up here or slowing down? Slowing down means deceleration, speeding up is acceleration, and it can't be doing both at the same time.
> When the plane slows down, it produces less drag, allowing to to pick up more speed.
Same deal?
I think what he's getting at is that drag increases with the square of speed, but it's a very confusing way of explaining it.
A common equation you will find in aerodynamics texts is:
Drag = 1/2 * fluid density * velocity^2 * C_d * Ref. Area
It approximates the drag experienced by objects as they move within a fluid (atmosphere). You can see that drag is proportional to the square of velocity, so going twice as fast induces 4 times the drag.
Ergo, when you speed up, you produce a lot more drag. This will slow you down until you reach an equilibrium between thrust and drag (unless you apply more thrust).
Drag = 1/2 * fluid density * velocity^2 * C_d * Ref. Area
It approximates the drag experienced by objects as they move within a fluid (atmosphere). You can see that drag is proportional to the square of velocity, so going twice as fast induces 4 times the drag.
Ergo, when you speed up, you produce a lot more drag. This will slow you down until you reach an equilibrium between thrust and drag (unless you apply more thrust).
So you're just agreeing with what I said:
> I think what he's getting at is that drag increases with the square of speed
> I think what he's getting at is that drag increases with the square of speed
https://m.youtube.com/watch?v=edLnZgF9mUg
I recommend this MIT opencourseware video to anyone interested in this topic
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Side note, but are there any international aircraft racing held ? For efficiency ?
There are efficiency races for general aviation aircraft, yes. Check out https://en.wikipedia.org/wiki/CAFE_Foundation
I suppose the Open category of the World Gliding Championships could be considered such a contest.
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Airplanes can do rolls and barrel rolls, i.e. they can fly upside down. Good luck with simple explanations ...
https://www.scientificamerican.com/video/no-one-can-explain-...
https://www.scientificamerican.com/video/no-one-can-explain-...
Easy, the natural angle of attack is determined by the elevator position. Flying upside down is totally possible with the right inputs.
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I remember learning about it and wondering why newton's 3rd law wouldn't suffice. It's pretty obvious that the wings push air down and it's not that difficult to understand (even as a kid) that newton's 3rd law works.
The essence of the Bernoulli argument is that the top of the wing is longer -> air has to move further -> faster air has lower pressure "because Bernoulli" -> pressure imbalance means lift.
Ok, cool, but the "Bernoulli principle" I got as a kid was "faster air is lower pressure", which is both empirically wrong (the air in a compressor hose is obviously moving faster than the air in the workshop) and logically inconsistent (speed is relative, after all). You add in a half dozen qualifiers and it becomes true, but I wonder if this is more complicated than "the wings push air down, the air pushes the wing up".