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## A Technical Disclaimer of the Vision in Aeronautics Lessons

by

Tom Benson

Aerospace Engineer, NASA Glenn Research Center

November 15, 2001

Between 1993 and 1996 the NASA Lewis (now Glenn) Research Center Learning Technologies Project supported a program for high school students to design, build and use a subsonic wind tunnel. The program was called a "Vision in Aeronautics" and the reports on the project are available at this web site. In general, this was an excellent piece of work by all involved. The "Lessons" on aerodynamics, however, does include some technical errors which I feel must be addressed in my role as technical advisor for Glenn LTP.

The error occurs in Section F on aerodynamic Lift. Lift is a simple topic to observe but very difficult to explain correctly. At the Glenn LTP Beginner's Guide to Aerodynamics (BGA) we have included a lengthy and detailed explanation of the origins of lift and the major incorrect theories which can be found in popular literature. I suggest the reader begin at the "What is Lift?" slide and follow the "Theories of Lift" guided tour to get the complete picture. In Section F, the author is presenting a variation of the "Equal Transit", or "Longer Path" incorrect theory. A claim is made that because the surface area of the upper surface of the airfoil is greater than the bottom that the velocity will be higher and that the pressure will be lower. For a lifting airfoil, the upper surface does have higher average velocity and lower average pressure, but it is not a function of the surface area. The claim is further made that it is the "high air pressure under the airfoil which creates the Lift" which is totally incorrect. The aerodynamic force is the integrated normalized pressure times area around the entire surface as explained at the BGA. Some variations of Bernoulli's equation are presented in Section F which are used to "quantify" the lift. This is a mis-application of Bernoulli's equation and will not give the proper value of lift for a given airfoil shape.

Section G on angle of attack effects has similar errors. The claim is made here that changing the angle of attack changes the stagnation point on the airfoil leading edge (which does occur), and that the change produces more area on the "top" of the airfoil which again produces more lift (which is totally incorrect). Lift does increase with angle of attack, but the increase is because of greater induced flow turning not because of increased "surface area". For a more correct explanation of angle of attack effects please see the page at the BGA on inclination effects.

I have debated about completely deleting the "Lessons" for the "Vision in Aeronautics" because of these errors, but have decided instead to leave them as written with this disclaimer. Most of the material from this program is excellent, and I don't wish to "throw the baby out with the bath water".