the streamlines only a short distance below the wing are completely undisturbed!
Well it is in a tunnel so where is it going to go?
A WING IS NOT A HALF VENTURI
You might find it amusing to note that ground school courses
often introduce the venturi as an example of how wings fly.
The presentation includes moving one of the walls of the
venturi so far away as to not influence the other wall. What is
left is a wall with a hump.The instructors
tell the students that because of Bernoulli’s principle this “half
venturi” has lift. But you now know that this is wrong.The
wall blocks the downwash, so there can be no lift. After
leaving the hump, the air is traveling at the same speed and in
the same direction as before the hump. As we know, if there is
no net change made to the airflow there cannot be lift. So,
what do you do when you see this on the FAA written exam?
Well, if you want to pass, you will have to give them the
answer they want to hear, even though it is wrong!
Forget Bernoulli’s theorem
WOLFGANG LANGEWIESCHE, STICK AND RUDDER, 1944
In brief, the air bends around the wing because of the Coanda effect.
When the air bends around the surface of the wing it tries to separate from the airflow above it. But since it has a strong reluctance to form voids, the attempt to separate lowers the pressure and bends the adjacent streamlines above. The lowering of the pressure propagates out at the speed of sound, causing a great deal of air to bend around the wing.This is the source of the lowered pressure above the wing and the production of the downwash behind the wing.
This reduction in pressure causes acceleration of the air via the Bernoulli effect
The acceleration of air over the top of a wing is the result of the lowered pressure and not the cause of the lowered pressure.
As is popularly quoted.
For me it goes -
Coanda -> Newton -> And some interesting little Bernoulli effects along the way.
The lift of a wing is proportional to the amount of air diverted per
time, times the vertical velocity of that air.
The Popular Description of Lift
Most of us have been taught what we will call the “popular description
of lift,” which fixates on the shape of a wing. The key point of
the popular description of lift is that the air accelerates over the top
of the wing. Because of the Bernoulli effect, which relates the speed
of the air to the static pressure, a reduced static pressure is produced
above the wing, creating lift. The missing piece in the description is
an understanding of the cause of the acceleration of the air over the
top of the wing. A clever person contributed this piece with the introduction
of the “principle of equal transit times,” which states that the
air that separates at the leading edge of the wing must rejoin at the trailing edge.
Since the wing has a hump on the top, the air
going over the top travels farther. Thus it must go faster to
rejoin at the trailing edge. The description is complete.
This is a tidy explanation and it is easy to understand. But
one way to judge an explanation is to see how general it is. Here one
starts to encounter some troubles. If this description gives us a true
understanding of lift, how do airplanes fly inverted? How do
symmetric wings (the same shape on the top and the bottom) fly?
How does a wing flying at a constant speed adjust for changes in load,
such as in a steep turn or as fuel is consumed? One is given more
questions than answers by the popular description of lift.One might also ask
if the numbers calculated by the popular description really work.
Let us look at an example. Take a Cessna 172,
which is a popular, high-winged, four-seat airplane. The wings must
lift 2300 lb (1045 kg) at its maximum flying weight. The path length
for the air over the top of the wing is only about 1.5 percent greater
than the length under the wing. Using the popular description of lift,
the wing would develop only about 2 percent of the needed lift at 65
mi/h (104 km/h), which is “slow flight” for this airplane. In fact, the
calculations say that the minimum speed for this wing to develop
sufficient lift is over 400 mi/h (640 km/h). If one works the problem
the other way and asks what the difference in path length would have
to be for the popular description to account for lift in slow flight, the
answer would be 50 percent. The thickness of the wing would be
almost the same as the chord length.
Understanding Flight, Second Edition
By David Anderson, Scott Eberhardt
David Anderson is a pilot and a lifelong flight enthusiast. He has degrees from the University of Washington, Seattle, and a Ph.D. in Physics from Columbia University. He has had a thirty-year career in High Energy Physics at Los Alamos National Laboratory, C.E.R.N in Geneva Switzerland and at the Fermi National Accelerator Laboratory.
Scott Eberhardt - Aerodynamics Engineer - Boeing (current) Professor University of Washington July 1986 – April 2006 (19 years 10 months)
Engineer NASA Ames Research Center - September 1994 – June 1996 (1 year 10 months)
Scott Eberhardt's Education - Stanford University Ph.D., Aeronautics and Astronautics 1982 – 1985 - Massachusetts Institute of Technology
M.S., Aeronautics and Astronautics - 1980 – 1981 - Massachusetts Institute of Technology - B.S., Aeronautics and Astronautics - 1976 – 1980
Available on
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in a range of digital or hard media.
Me? I'm sticking with the physicist and the Boeing dude!