How does Aerodynamic Lift really work?

[DA299]

I've done a lot of reading on this, but still don't quite fully understand the mechanics of lift.  There are several contradictory sources on the internet saying that Lift is due to Bernoulli's principle, or due to Pressure differentials, or due to Newton's Third Law. The vast majority of airfoil cross-sections also further complicate matters, as well as the myriads of wing geometries available. 

I'm not an aerospace engineer, but my understanding is that none of these explanations are the whole picture, because fluid dynamics is complex, and lift is itself a phenomenon which is intuitively difficult to understand.

The first thing I understood, is that airfoils exist not only to generate lift, but to maximize the Lift-to Drag ratio (L/D). Even a brick will generate lift at a high enough AOA, but the resulting drag penalty makes it all but useless as an airfoil.  So you can say that anything can make aerodynamic lift, but airfoils are much more efficient at doing so.

Next, you have to understand that Lift is generated through entirely difficult mechanisms at different flight regimes:

1) At low-subsonic regimes(=<0.5 Mach), the airplane is moving slowly enough, that simply deflecting the air downwards wouldn't produce much lift. If you used a flat plate at slow speed flight, you could theoretically generate enough lift to fly, but you'd either need huge wings, or very high angle of attack to generate any appreciable amount. So, while they would generate lift, flat-plates would also generate a lot of drag, which would require a lot of engine to compensate. Thus the 'air deflection model', while being reasonably accurate, isn't actually a good model while designing an aircraft for these flight regimes. The pressure differential model is much more accurate, and it dictates the airfoil cross-sections of low-speed flight, which are usually thick, strongly curved teardrops or under cambered. Incidentally, these cross-sections are also observed in hydrodynamic wings, as both water and air(at low airspeeds) are incompressible. Thus, it can be said that while a flat-plate does generate lift, it does not have  a great L/D, and properly designed airfoils are vastly superior at low airspeeds. 

2)At Tran-sonic regimes(0.5-1 Mach), a flat plate becomes more and more efficient at generating lift, due to the increased airspeed, it need not have a high angle of attack, and so is actually useable at transonic regimes. However, it is still not the absolute best method, as using thinner, flat-bottomed or symmetrical airfoils will still outperform it in terms of L/D. A properly designed airfoil('supercritical airfoil') will generate a lot more lift than a flat plate, as it actually keeps the airflow subsonic at these airspeeds. Thus, it can be said, that both 'air-deflection' and 'pressure differentials' contribute to generating lift at these airspeeds.

3) At supersonic regimes, and especially at high-supersonic regimes, a flat plate(with sharp edges) becomes the best at generating lift with the highest L/D. The standard airfoil shape isn't as efficient, because at these speed, air is no longer incompressible, and is better approximated as a series of tiny balls hitting the wing(Newtonian theory). At these speeds, the air no longer wants to smoothly follow the contours of an airfoil, and it is best to disturb it as little as possible. Thus, the best way of generating lift at supersonic regimes, is in fact by deflecting air down. A pure supersonic airplane design will have very thin wings, and will fly with a slight positive AoA for the absolute maximum L/D ratio.  Case in point are most jet fighters, and the Concorde. However, in principle, even these aircraft have wing cross-sections that resemble very thin airfoils, because they are not pure supersonic aircraft(at the very least, they have to land and take-off at low subsonic speeds!) 

4) At very high supersonic and hypersonic regimes(>5 Mach), the effect of compression lift is also present. This basically works by trapping the bow-shock beneath the wing, and using it o compress the air between it and the wing. The high pressure air effectively pushes up on the aircraft, resulting in lift. This method is very efficient at high hypersonic speeds, but to date, very few aircraft have actually utilized it(The only manned one was the the XB-70 Valkyrie, and it didn't even fly fast enough for the compression lift to actually become the primary cause of total lift.). This method of lift/ flight is also called waveriding.

 

One of the prime examples of different lift mechanisms at different flight regimes, is the Concorde. It used a delta-wing planform with high sweep, and had very thin wings. At subsonic speeds it L/D ratio was actually less than at Mach 2, because it needed higher AoA at lower speeds. I should also note, that the airfoil cross-sections are by far the best way of maximizing L/D, generally speaking. The most L/D at supersonic airplane can hope to achieve is less than 20, while a subsonic airplane can easily achieve an L/D ratio of 30 or more(i.e. Gliders, and Sailplanes).

So, this is about complete an understanding of lift as I have, if there's anything wrong with it, or anything needed to be added, please do so. I'd love to hear any differing opinions as well.

[RCgothic]

Ultimately it's about conservation of momentum whilst minimising turbulent flow.

When an aerofoil has a positive angle of attack, air molecules collisions with the underside are increased (as is pressure), accelerating the flow downwards.

On the upperside the surface falls away from the air molecules, and collisions (and pressure) are reduced. The random motion of gas molecules causes the gas to move into the vacated area, and again the flow is accelerated downwards.

Whether you view the lift as arising from the difference in pressure between the two surfaces or equal to the impulse imparted to the gas, it's just conservation of momentum. The gas is forced down (and a little bit forward - drag), the plane is pushed up (and back).

The different shapes of aerofoils are all about keeping the flow attached to the aerofoil predictably and avoiding turbulence, which moves the air molecules more than necessary  and so robs additional forward momentum from the aerofoil (drag), but also disorganises the flow such that the average deflection of air is less (lift reduces).

How much the flow is diverted by an aerofoil is a function of angle of attack and speed, so if you vary your speed but want the same lift you also have to vary the angle of attack. But if the angle of attack is too large, the flow will detach and the wing will stall.

Subsonic aerofoils like smooth transitions, because the gas can follow the curve of the surface.

Supersonic aerofoils can't keep the flow attached to the upper surface, so they're always going to get drag and turbulence because of that. Instead they design around minimising transitions. Supersonic aerofoils don't like transitions, because the upstream flow can't see them coming and so each transition generates a shockwave, and shockwaves sap energy. A curve is effectively a series of infitinte transitions, so that's bad. Supersonic aerofoils are therefore sharp and flat with a minimum of transitions.

Also, in an atmosphere everything is an aerofoil. A brick is an aerofoil. This is where body lift comes from. It's not as efficient as a well designed aerofoil though, and tends to come with a lot of drag.

The only objects that don't produce at least some lift are balls and cylinders perpendicular to the flow because they can't vary their angle of attack. But even these can induce lift if spinning and moving in an airflow. Friction accelerates the boundary flow in one direction and decelerates it in another, which changes the exit angle of the airflow just like an aerofoil. This is called the magnus effect, and it's how sports balls are made to curve through the air among other effects.

I think you've basically got all of this down already, sounds like your understanding is pretty good.

Edited September 5, 2022 by RCgothic

[mikegarrison]

 

There is nothing contradictory about describing lift as "pushing the air down" or as a result of the pressure distribution on the wing. These are just two different ways of looking at the same thing.

If you draw a boundary around the skin of your airplane, no air is crossing that boundary. But the air is pushing on it (pressure), and the net of that pressure equals the forces being applied to the airplane.

On the other hand, if you draw a boundary like a box in the air that your airplane is inside, then you don't care what is happening inside in terms of pressure. What you care about is the net change in momentum of the air as it flows through the boundaries of your box.

These two ways of looking at this are physically linked and always equal, so you can't get pressure lift without momentum lift or vice versa.

[Ultimate Steve]

On 9/4/2022 at 1:29 PM, RCgothic said:

Ultimately it's about conservation of momentum whilst minimising turbulent flow.

When an aerofoil has a positive angle of attack, air molecules collisions with the underside are increased (as is pressure), accelerating the flow downwards.

On the upperside the surface falls away from the air molecules, and collisions (and pressure) are reduced. The random motion of gas molecules causes the gas to move into the vacated area, and again the flow is accelerated downwards.

Whether you view the lift as arising from the difference in pressure between the two surfaces or equal to the impulse imparted to the gas, it's just conservation of momentum. The gas is forced down (and a little bit forward - drag), the plane is pushed up (and back).

The different shapes of aerofoils are all about keeping the flow attached to the aerofoil predictably and avoiding turbulence, which moves the air molecules more than necessary  and so robs additional forward momentum from the aerofoil (drag), but also disorganises the flow such that the average deflection of air is less (lift reduces).

How much the flow is diverted by an aerofoil is a function of angle of attack and speed, so if you vary your speed but want the same lift you also have to vary the angle of attack. But if the angle of attack is too large, the flow will detach and the wing will stall.

Subsonic aerofoils like smooth transitions, because the gas can follow the curve of the surface.

Supersonic aerofoils can't keep the flow attached to the upper surface, so they're always going to get drag and turbulence because of that. Instead they design around minimising transitions. Supersonic aerofoils don't like transitions, because the upstream flow can't see them coming and so each transition generates a shockwave, and shockwaves sap energy. A curve is effectively a series of infitinte transitions, so that's bad. Supersonic aerofoils are therefore sharp and flat with a minimum of transitions.

Also, in an atmosphere everything is an aerofoil. A brick is an aerofoil. This is where body lift comes from. It's not as efficient as a well designed aerofoil though, and tends to come with a lot of drag.

The only objects that don't produce at least some lift are balls and cylinders perpendicular to the flow because they can't vary their angle of attack. But even these can induce lift if spinning and moving in an airflow. Friction accelerates the boundary flow in one direction and decelerates it in another, which changes the exit angle of the airflow just like an aerofoil. This is called the magnus effect, and it's how sports balls are made to curve through the air among other effects.

I think you've basically got all of this down already, sounds like your understanding is pretty good.

I'm in the third year of an aerospace engineering degree and this is the best explanation of this by far I have ever come across. Thank you.

[mikegarrison]

XKCD giving away all our secrets:

[Dinlink]

I would like to point out some details on the origin of subsonic lift phenomena:

- When explaining lift everyone centers around describing how airfoils are able to deflect the flow field and hence, cause a pressure difference... But very little try to explain why... Why an obstacle on the flow field is capable of deflecting it at subsonic speeds...

- Turbulence... airfoils works in turbulent régime Indeed, turbulence is not the same as boundary layer detachment, which is the culprit of lift loss. Turbulence is a flow regime where inertial forces are higher than viscous ones (High Reynolds number) and hence the flow becomes unstable and create a cascade of swirls, big to small, to the smallest at molecular level where energy is dissipated (Kolmogorov scale)... This disordered regime appears almost from the leading edge (front tip) of the airfoils on normal conditions... Only at very low speed the laminar-turbulent transition can be delayed... Or even you can have a full laminar flow regime in very small flying animals or in very viscous media...

- The magic is all due to the viscous boundary layer, the fundamental phenomenon ignored by most science educators. If your remove viscosity completely, lift becomes impossible, no shape would be able to create any lift at any subsonic speed... It's called potential flow, and has exact mathematical solution. When you add a little bit of viscosity, even a very small amount, viscous effets become very important very close to the walls, this viscous effects create "detachments" of the flow, that means, the flow instead of following the walls of the profile and go around the edges as they would in potential flow, they detach. When the flow is below the airfoil (at the intrados) and tries to go around the trailing edge (rear tip) as it would in potential flow, it detaches, causing a depressions that guides the flow on the upper side of the airfoil to the trailing edge, causing the deflection and hence the lift... No viscosity, no lift... But it can be detrimental too, if the angle of attack is too high, the flow can detach trying to go around the leading edge, creating a big recirculation zone on the upper side (extrados), and causing the upper side of the flow to ignore the curvature of the airfoil and pass straight, destroying the deflection and hence the lift. This can also be observed on laminar regime and is called "laminar recirculation Bubble".

 

I hope these explanations helps to clarify some confusions that can arise from the information found on the internet about the origin of lift.

 

PS. I would like to add a Direct Numerical simulation of an airfoil in a slow flying glider, showing the laminar - turbulent transition and the corresponding turbulent flow on the extrados, intrados and wake. The trailing edge detachment zone is shown too (incipient separation). Hope this serve as complement to the explanations above.

 

Edited September 30, 2022 by Dinlink