fredag 10 augusti 2012

Text Book: Physics of Flight by Lande

The text book The Physics of Flight by Alfred Lande from 1946 is presented by
• When looking for a textbook on aerodynamics of the airplane for students after their first year of college physics and algebra, the author found a certain gap between elementary introductions and more advanced representations which require a full knowledge of calculus.
Lande warms up the reader by
• The airfoil, with its long span and curved ("cambered") crosssection, its blunt leading edge and sharp trailing edge, is an almost perfect instrument of pure sustentation.
• The physical explanation and evaluation of the amazing qualities of the transversal wing as means of sustentation consitute the chief topic of this book.
But Lande then only delivers the classical Kutta-Zhukovsky-Prandtl circulation theory:
• The aerodynamic process which leads to the production of lift and drag cannot be understood without studying the adhesion of the air to the surface of the wing, and the viscosity or internal friction (stickiness) of the air itself.
• Lift originates from the downward momentum imparted to the incoming horizontal air current.
• The downward deflection is the result of wind V and pure circulation w, as shown in the proof of K-J's law
• ....we found that vorticity superimposed on wind produces lift at right angles to the incident wind in two-dimensional flow, without giving rise to a force of drag parallel to the wind.
This is the standard presentation going back to Prandtl, which attributes lift to the "stickiness" of a viscous fluid satisfying a no-slip boundary condition. This is misleading physics since lift in subsonic flight in reality originates from incompressibility and a slip boundary condition modeling the small skin friction of slightly viscous flow, as shown on The Secret of Flight.

1 kommentar:

1. One problem for the correct understanding of lift may lie in the word "incompressible". How can there be increased pressure at the lower side and decreased pressure on the upper side of the wing if the air is incomresssible?
This contradiction troubles the students mind without him realising it and allows him to think in terms of conserved momentum etc instead of the more physically correct explanations.
D