## måndag 6 augusti 2012

### Theory of Flight on Wikipedia

Unphysical incorrect explanation of lift by National Air and Space Museum, Smithsonian Institution.

Wikipedia (Lift Force) gives a good account of state-of-the-art as concerns the scientific explanation of how the wings of an airplane generates a lift force balancing gravity, which makes it possible to fly at affordable power balancing drag. Let us scrutinize Wikipedia's message in the light of the New Theory of Flight. Wikipedia starts out with:
• An airfoil is a streamlined shape that is capable of generating significantly more lift than drag.
• There are several ways to explain how an airfoil generates lift.
• Some are more complicated or more mathematically rigorous than others; some have been shown to be incorrect.
• For example, there are explanations based directly on Newton’s laws of motion and explanations based on Bernoulli’s principle. Either can be used to explain lift, but each appeals to a different audience.
OK, so there are several ways to explain lift, some of which are incorrect.

After presenting empty explanations based on Newton's 3d law (lift by redirecting air) and Bernoulli's law (lift from pressure difference form velocity difference above and below the wing), Wikipedia continues with:
• Explaining lift while considering all of the principles involved is a complex task and is not easily simplified.
• Lift is generated in accordance with the fundamental principles of physics
• The most relevant physics reduce to three principles:
• Conservation of Momentum, Conservation of Mass, and Conservation of Energy,...
• which results in the Navier–Stokes equations which are notoriously difficult to solve, but in many instances approximations suffice for a good description of lifting airfoils.
OK, so the Navier-Stokes equations describe the physics, but the equations cannot be solved and thus need to be simplified (a "complex task"):
• In large parts of the flow viscosity may be neglected.
• One further simplifying approximation is to assume that the flow is irrotational, i.e. that the flow does not rotate around itself, ... with two nice properties: 1) an irrotational flow can be expressed as the gradient of a scalar function cis alled a potential (the flow is then called potential flow) 2) solutions to complex flows can be expressed as the sum of simpler flow fields. These two properties make the calculations tractable.
• In particular, solutions can be expressed as the sum of a uniform flow (i.e. a steady flow equal to the free stream velocity) plus a free vortex flow, i.e. a circular flow around the airfoil with the speed inversely proportional to the radius.
•  In order to arrive at a unique (physical) solution, one can apply the Kutta condition, which says that for steady flow the rear stagnation point is coincident with the trailing edge of the airfoil.
• The magnitude of the vortex flow is adjusted so that the Kutta condition is met. Another way to say this is that the airflow around the airfoil develops enough vorticity to satisfy the Kutta condition. Vorticity is measured by a number called the circulation, so an airfoil in a steady flow will develop sufficient circulation to satisfy the Kutta condition.
This is the Kutta-Zhukovsky circulation theory developed 100 years ago, which however is unphysical and thus is one of the many incorrect theories.

Wikiepedia then concludes with:

It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental.
— John D. AndersonCurator of Aerodynamics at the National Air and Space Museum[11]
• Many other alternative explanations for the generation of lift by an airfoil have been put forward, of which a few are presented here. Most of them are intended to explain the phenomenon of lift to a general audience. Although the explanations may share features in common with the explanations above, additional assumptions and simplifications may be introduced. This can reduce the validity of an alternative explanation to a limited sub-class of lift generating conditions, or might not allow a quantitative analysis. Several theories introduce assumptions which proved to be wrong, like the equal transit-time theory.
The article thus leaves the reader effectively without explanation, which is not surprising since the New Theory of Flight is the first correct physical explanation and has not yet passed the refeere's at AIAA, which have to defend the state-of-the-art described by John D.Anderson (who may be one of the referees) and Wikipedia.

The New Theory of Flight was declined on August 3 2012 by AIAA in the first round, since AIAA apparently clings to the Old Theory described by John d. Anderson and Wikipedia,  and now the second round starts with an examination of the referee's reports. The referee's have a double task of defending the existing theory (which is a "complex task") and showing that the evidence of the New Theory is not enough (also a "complex task"). Stay tuned....