A Technical Disclaimer of the Vision in Aeronautics Lessons
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"
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
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
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".
- Beginner's Guide Home Page