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[Phys-L] Re: the plane truth



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

anthony_lapinski@PDS.ORG 2/3/2006 2:54:44 PM >>>
In teaching about fluids, I was discussing that airplanes fly 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?
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