Here is the response from John D. Anderson, Curator of Aerodynamics, Smithsonian National Air and Space Musuem, to my question how lift of a wing is explained by state-of-the-art expertise of today:
- There are several explanations that have been offered for how aerodynamic lift is produced on an airfoil.
- Some of these explanations are given with almost religious fervor, which sometimes generates heated intellectual debate over which is correct.
- Such discussions are a matter of philosophy as to which explanation is the most fundamental.
- The following is my philosophical conviction as to the most fundamental mechanisms used by nature to generate lift:
- The lift is mainly due to the pressure distribution on the surface; shear stress has only a small effect on lift. In general, for an airfoil that is generating lift, the average pressure exerted on the top surface (pressing down on the airfoil) is smaller than the average pressure exerted on the bottom surface (pressing up on the airfoil). With lower pressure on the top and higher pressure on the bottom, presto -- lift on the airfoil.
- Therefore, let us concentrate on how nature creates this situation, i.e., how nature produces the pressure distribution over the surface of the airfoil.
- First, the flow velocity is faster over the top surface and slower over the bottom surface. Why? Because mass must be conserved.
- Now let us invoke the famous “Bernoulli principle”:
- Because the flow speeds up as it flows over the top surface of an airfoil, as explained above, then the pressure over the top surface must go down.
- The flow velocity over the bottom surface is lower than over the top, hence the pressure over the bottom surface is higher than on the top. This imbalance of pressure (higher on the bottom and lower on the top) creates an upward force on the airfoil – the lift.
- Please note: In nature, the velocity increase does not come first, and the pressure decrease does not come second, or vice versa.
- Nature does all this simultaneously, applying the fundamental laws all at the same time. Nature sets up a velocity field and a pressure field simultaneously, fields that obey the fundamental principles.
- So there you have it. The absolute fundamental principles of mass conservation and Newton’s 2nd law are the reasons why lift is produced, and we have just explained how nature invokes these two principles to create lift on an airfoil.
- This is what nature does.
- I can not advance a philosophy on the generation of lift more fundamental than this, although it takes a full page to explain it.
But this is science reduced to empty triviality. Anderson sends me the following excuse:
- My explanation is based on the fundamentals of physics that apply to any flow, incompressible to hypersonic. This explanation is different from yours, but neither is necessarily “wrong.”
But a Museum of Science which is empty has no role to play, nor does a scientific theory which is a triviality.
PS1 Note that Anderson does not mention circulation theory, the only non-trivial theory of lift presented in text books, presumably because Anderson share our insight that it is an unphysical and thus incorrect theory.
PS2 The aerodynamics experts at KTH (Art Rizzi and Ulf Ringertz) are smarter than Anderson; they simply refuse to answer any question posed by me about state-of-the-art. But to say nothing is less than repeating a triviality.
PS3 Anderson does not say that our New Theory of Flight is "necessarily "wrong"", which could mean that Anderson believes that it could well be correct. But Anderson does not say that he has studied the New Theory to find out if is correct or not. It may be that Anderson would rather see our theory published by AIAA as new input to a discussion without conclusion since 100 years, than a rejection aimed at stifling discussion. I have asked him about his opinion of the New Theory and will report the answer.
PS4 The generation of lift of a wing is described by Anderson as follows on the museum's web site How Things Fly educating the next generation of US scientists:
- A wing is shaped and tilted so the air moving over it moves faster than the air moving under it. As air speeds up, its pressure goes down. So the faster-moving air above exerts less pressure on the wing than the slower-moving air below. The result is an upward push on the wing—lift!