Musings on Lift

[PT20J]

The March 2025 AOPA Pilot magazine got me thinking: I wonder why it seems so hard to understand how our wings produce lift?

Bernoulli? Newton? Both? Neither?

It's not as hard as we make it, but it is not intuitive and trying to make it match our intuition seems to me to be what trips up those that try to explain it in simple terms.

Consider Bernoulli. Yes, the air flowing over the top of the wing speeds up and produces a lower pressure. But why does it speed up? It is NOT because the air above the wing is funneled through some sort of half venturi. This is easily shown by the fact that a flat sheet will generate lift at a positive angle of attack, but, since there is no camber, there is no half venturi.

Consider Newton. Lift is not created by downwash. Downwash creates drag. Downwash is due to the tip vortices of finite span wings. 2D airfoils generate lift but have no downwash. Ground effect reduces downwash with the effect that drag decreases and lift increases. If downwash was responsible for lift, lift would decrease as we neared the ground and our Mooneys wouldn't float so much.

There are two problems that impede our understanding of lift. The first is that many of the drawings of streamlines flowing past an airfoil are incorrectly drawn. An airplane wing actually affects the airflow at a considerable distance ahead, above, below and behind the wing. The second problem is that we are used to simple systems that have discernable cause and effect. But fluid dynamics isn't like that. If the air speeds up, is it because the pressure changed? Or, does the pressure change because the air speeds up?

What is really happening is that the wing presents an obstacle that the air must go around. And, there are laws of nature that the air must obey in so doing. The air must change direction to get out of the way. Air has mass so changing its direction requires a force. Force on a fluid such as air is a pressure. So, as the air flows around the wing there will be pressure gradients. And pressure gradients cause the air to speed up or slow down. It's not correct to say that the pressure causes the direction and speed changes or that the direction and speed changes cause the pressure changes because all three parameters are part of one system. 

[Hank]

8 hours ago, PT20J said:

The March 2025 AOPA Pilot magazine got me thinking: I wonder why it seems so hard to understand how our wings produce lift?

Bernoulli? Newton? Both? Neither?

. . .

Consider Newton. Lift is not created by downwash. Downwash creates drag. Downwash is due to the tip vortices of finite span wings. 2D airfoils generate lift but have no downwash. Ground effect reduces downwash with the effect that drag decreases and lift increases. If downwash was responsible for lift, lift would decrease as we neared the ground and our Mooneys wouldn't float so much.

Yes, that is certainly "a" downwash, but as you pointed out, it also certainly does not create lift.

The Newtonian downwash theorized to produce lift is spanwise along the wing, not way out just at the tip. As the airfoil moves through air, it displaces air in a downward direction along its length, and by the 3rd Law, the downward moving air creates an upward force on the wing. This is why introductory aerodynamics discusses infinite length airfoils, to eliminate the special causes of drag created at the tips.

Then there is the Bernoulli Effect.

Our airplanes fly because of both the Newtonian upward force caused by pushing air down, and the Bernoulli effect cause by pressure drop atop the wing. Newton pushes up, Bernoulli pulls up, and away we fly.

Newtonian lift explains why a flat sheet (or your tilted hand stuck out the car window) generates lift. The lift can be increased by changing the plate / hand into an airfoil shape, and the airfoil can be optimized for a particular flight speed and air density; NACA generated A LOT of data covering this decades ago that designers still use.

[PT20J]

The spanwise vortex sheet is a consequence of the tip effects. Lift can be calculated by either accounting for the pressure differences above and below the wing or the momentum changes in the air as it moves around the wing. But the latter is not the same as ‘forcing air down” as the example of ground effect demonstrates. Aerodynamics text books first describe 2D airfoils in order to derive equations of lift and drag and then go on the describe 3D wings in order to account for the effects of planform and span. The NACA catalog of airfoils is 2D wind tunnel data because the wing models completely filled the test section from wall to wall so there were no tips.

[0TreeLemur]

One of my favorite views of lift generation in action.  Streaklines in a wind tunnel at an angle of attack near stall.

Newton's second law F=ma written perpendicular to the streamlines does  a better job of describing the physics that produces lift better than any other equation.  It clearly shows that streamline curvature produces a pressure gradient in the direction opposite of the radius of curvature.  The minus sign indicates that the pressure decreases in the direction pointing towards the center of curvature, the n direction on the figure.   The wind tunnel observation also shows why our stall detectors are placed where they are.

 

[varlajo]

7 hours ago, Hank said:

Newton pushes up, Bernoulli pulls up, and away we fly.

Well said Sir, well said!! 

[Joshua Blackh4t]

I remember a comic from a flying magazine. It had graphical illustrations of a quiz.

What makes a wing fly? Is it:

A: Bernoulli

B: Newton

C: Coanda effect

D: Bucket loads of cash

[AH-1 Cobra Pilot]

I have not yet read that issue of AOPA Pilot, but the conversation above makes me think of three things:

As nobody above has mentioned it, does the article talk about compressible vs. incompressible flow?

Pressure is simply Newton's Third Law in gas.

Most of these things are really obvious if you take some time to play with a wind tunnel that has smoke generators and a rotatable airfoil.

[N201MKTurbo]

19 hours ago, PT20J said:

 

What is really happening is that the wing presents an obstacle that the air must go around.

So, not really. The air is stationary (sort of) and the wing must pass through it. But your analogy does not make the analysis incorrect. 

[N201MKTurbo]

So, the disturbances ahead of the wing travel forward from the wing at the speed of sound. When the wing is moving faster than these disturbances you hit the sound barrier. This drastically changes the airflow over the wing. But we don’t have worry about these things….

[PT20J]

A lot of YouTube videos are just wrong. Here’s a good one by a retired Boeing engineer (his book goes into more detail)

This one is good also.

 

[Utah20Gflyer]

Does a propeller create thrust by creating lower pressure in front of it or by moving the air from in front of the propeller to behind it!  A propeller is a miniature wing.  
 

Lift is created by forcing air downward.  The significance of the lower air pressure above the wing is that the air is being sucked downward and therefore is creating extra lift through the direction change downward.   The process of changing the direction of the air creates drag.  
 

This explains ground effect perfectly since the downward moving air is now hitting a solid surface and the air can’t move as freely as it does at altitude.  
 

Without forcing air downward you can’t push a plane up.  It really is just that simple.  

[MikeOH]

Not sure what the point of this thread really is....starts out with "it's not as hard as we make it" and the next thing you know we've got the triple-integral of Biot-Savart!

@Utah20Gflyer get's it, IMHO 

[EricJ]

47 minutes ago, Utah20Gflyer said:

Does a propeller create thrust by creating lower pressure in front of it or by moving the air from in front of the propeller to behind it!  A propeller is a miniature wing.  

They do both.   Standing behind a running prop demonstrates the thrust from the high pressure pretty readily.   Sucking up rocks during a static stationary run demonstrates the low pressure created in front of the prop.   Wings are the same.   When the sun is low it's pretty easy to see the skin on top of the wing puffing up in between the ribs and spars, like it's being inflated, but it's just the low pressure on top of the wing pulling the skin up.  It's kind of cool to realize that that force pulling up the skin is a lot of what's holding the airplane up.