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Hi alll-_______________________________________________
As an aero engineering undergrad I learned that a wind
tunnel is a
horrible place to learn about free flight. In order to transform
wind-tunnel results into their free flight equivalents, one has to
correct
for wall effects and the effect of the strut that holds the model
and I'm
sure I've forgotten other things. The good part is that some of the
corrections involve the same virtual image techniques that we
practice in
electrostatics. But the wind-tunnel labs were messy and tedious -they
helped persuade me to become a theorist.
Regards,
Jack
On Fri, 3 Feb 2006, Rick Swanson wrote:
This opens the door for an often had discussion about flight and
Bernoulli's prinicple. It is actually, like most fundamental
principles when you do more than scratch the surface, a very
subtle and sophisticated subject. It is handled very well on a
"Physics of Flight" videodisc -- lots of good pictures,
explanations, and demonstrations. It is probably not in many
collections.
Bottom line is, machines can fly because the are able to direct
air downwards -- it is an action-reaction, conservation-of-
momentum thing. Helicopters are the easiest example. For
airplanes, streamlines coming off of an airfoil travel downward.
A calculation of the change of air momentum downward, which is not
easy, yields the lift.
With the right angle of attack, a flat wing will produce lift --
airplanes flying upside down have a big angle of attack. Ait hits
the bottom of the wing, is deflected down, and the plane goes up.
We physicists get ourselves in trouble sometimes by poo pooing the
Bernoulli explanation and we can rightfully be taken to task by
aeronautical engineers who know their stuff. Most of the
introductory texts that I have seen recently do a good job of
giving a balanced conceptual explanation. Airfoil designers, the
Wright brothers being two of the first, do measure pressure
differences top to bottom of the wing and use the results to
design airplanes and you can relate the pressure differences to
the velocity of the air. The important concept for we physics
teachers to be aware of is that the medium that the planes fly
through is a fluid and the fluid near the wing knows that the
fluid away from the wing is there, and it makes a difference --
that is why ducks like to glide close to the surface of a lake.
Boundary layers, separation, turbulence, vortexes ... great stuff!
Cheers,
Rick Swanson
P.S. Regarding the question, "Why does it go faster?" "It has to
meet up with the air that went below" just does not cut it, in my
opinion. I like to explain it in terms of the continuity
equation. The air going over the top of an airfoil has a smaller
area to flow through. This makes sense if you look at an airfoil
in a wind tunnel but I am not sure it translates easily as an
explanation to real flight conditions. By this time, however, I
am usually on to oscillations and waves and don't think about it
again until the next semester.
Richard E. Swanson, Ph.D.
Dean of Instruction
Physics Professor
Sandhills Community College, Pinehurst, NC 28374
swansonr@sandhills.edu (910) 695-3715
In teaching about fluids, I was discussing that airplanes flyanthony_lapinski@PDS.ORG 2/3/2006 2:54:44 PM >>>
because of
Bernoulli's equation (principle). The shape of the wing allows air
going
over the wing to travel faster than that going under the wing.
Then the
pressure under the wing is higher than that above, providing
"lift." Here
are my two issues with this:
1) Exactly how does the air travel faster over the wing? If the
plane is
going at a constant speed, then shouldn't the speed of air
relative to the
wings be the same? I've heard that the air at the front of the
wing must
meet again at the back of the wing. But what would happen if the
plane
were to somehow move through a more dense fluid, say, water? Would
the
same thing ("lift") happen?
2) I know that some planes can fly upside down. How can this be
understood
in terms of lift and Bernoulli's equation?
Can anybody offer insights into these issues?
--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn
Valley