If you study the literature you will find that there is no correct theoretical explanation of flight. The explanations offered can be classified as
- trivial and correct (for uneducated)
- trivial and incorrect (for uneducated)
- nontrivial but incorrect (for educated).
What is needed is a nontrivial correct theory for both educated and uneducated and this is what we offer on this site. The trivial or incorrect theories often overlap by combinations of trivial or incorrect elements. The understanding of the beauty and usefulness of the nontrivial correct theory is facilitated by understanding what trivial or incorrect theories say. To properly appreciate what is nontrivial and correct requires knowing what is incorrect or trivial. A Google image search on “airfoil lift” gives a large collection of images all incorrect none depicting anything near the truth.
NASA STATE-OF-THE-ART TRUTH
NASA presents several incorrect theories for lift listed below, but no theory claimed to be correct. NASA argues as if by discarding all incorrect theories, what remains would be the correct theory, but science does not work this way because there are infinitely many incorrect theories, and the correct theory does not come out from a negation.
NASA ends with an cryptic message connecting back to Bernouili and Newton:
- To truly understand the details of the generation of lift, one has to have a good working knowledge of the Euler Equations.
This is a cover-up of the fact that NASA does not know what keeps airplanes in the air, which of course cannot be admitted.
In pilots handbooks it is common to simply avoid the question of how lift is generated, by suggesting that it is so natural that no explanation at all is needed, as in
telling a lot about generation of drag but nothing about lift.
1. Trivial and Correct: Newton’s Third Law
Air is deflected downwards (downwash) and as a reaction there is an upward force on the airplane which is lift. This is correct but trivial, since it does not explain why air is deflected downwards. It is often used in presentations directed to people without formal training in aerodynamics:
2. Trivial and Incorrect
2.1. Longer Path Theory
Watch the incorrect longer path theory being carefully explained in
and contradicted by
2.2. Downwash Theory
2.3. Bernoulli’s Principle
A popular explanation of lift is to refer to Bernouilli’s Principle which connects high velocity to low pressure and say that because the velocity of air on top of the wing is bigger than the below, the pressure on top will be smaller than below and thus create lift, which is the message in
- Bernoulli’s Principle
- The Aerodynamics of Flight
- Physics in a minute: How wings really create lift
- How Airplanes Fly
- Forces of Flight
- Aerodynamics for Pilots 1
- Why Do Airplanes Stall?
- Theory of Flight- MIT.
But it is not explained why the velocity is higher on top, and so the explanation of lift presented is no real explanation, or more precisely a trivial truism: There is lift because the pressure on top is lower than below.
2.4. Coanda Effect
The Coanda effect is the tendency of a fluid jet to be attracted to a nearby surface as in the kitchen sink experiment with the cylinder being pulled to the left by the deviation of fluid flow which appears “to be sticking” to the cylinder surface:
A connection is made to the lift of a a wing created by the air flow above the wing being similarly deviated by “sticking to” the upper surface of the wing. What remains to be explained is why the flow “sticks to the surface” and does not separate before the trailing edge. A common misconception is expressed in How Airplanes Fly: A Physical Description of Lift, by flight experts Anderson and Eberhardt:
- Why should a fluid follow a curved surface? The answer is viscosity: the resistance to flow which also gives the air a kind of “stickiness.”
The new theory shows that this incorrect: the flow “sticks” because it is incompressible and the viscosity is so small that a slip boundary condition applies, not because the flow is viscous with a viscous boundary layer.
3. Non-Trivial but Incorrect
3.1. Kutta-Zhukosky Lift Theory
The only nontrivial theory of lift is that by Kutta-Zhukovsky presented at the time of the first powered flight in the early 20th century. Kutta and Zhukovsky found that by modifying zero lift/drag potential flow by a large scale flow circulating around the wing section, a flow with lift (but no drag) was created as illustrated in the following generic picture of the theory:
The logic was that since circulation gives lift and there is lift, there must be circulation. But this is not correct logic, because it is possible that lift is created by something else than circulation, and this turns out to be the case as shown in the new theory.
Nevertheless, Kutta-Zhukovsky circulation theory is the standard “educated” explanation of lift by e.g.
- US Centennial Flight Commission
- Physical Principles of Winged Flight
- NASA Introduction to the Aerodynamics of Flight
- The Origins of Lift
- See How It Flies
- Two-Dimensional Airfoil Theory.
Kutta-Zhukovsky theory invents lift from a mechanism which does not occur in reality: The new theory explained on this site shows that lift and drag are both generated from a basic instability mechanism of potential flow which is not Kutta-Zhukovsky’s circulation. Here you find more about Kutta-Zhukovsky:
- The Spell of Kutta-Zhukovsky’s Circulation Theory
- Wikipedia Coverup
- Kelvin’s Theorem Unphysical.
3.2. Prandtl Drag Theory
Kutta-Zhukovsky did not offer any explanation of drag and it was the young German physicists Ludwig Prandtl who took on this burden in Motion of Fluids with Very Little Viscosity, which made him the father of modern fluid mechanics. In this 8 page note Planck suggested that drag was an effect of a very thin boundary layer attaching the flow to a solid boundary, which when separating causes a wake of turbulent flow creating drag.
The new theory shows that Prandtl speculation has nothing to do with real physics: Drag originates from the same instability of potential flow which gives lift to a wing, and does not come from any boundary layer.