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Re: pedagogical use of helicopters



At 05:23 PM 8/11/99 +0100, Ludwik Kowalski wrote:

Therefore, an explanation of a lifting force which begins with
a stationary helicopter is desirable, from the pedagogical point
of view. Then a helicopter moving horizontally and finally an
airplane with wings. Right or wrong?

Well, I wouldn't do it that way.

0) Think of the induced drag of an airplane. At high speeds, the airplane
visits a large amount of air and pulls it down gently -- creating very
little induced drag. At low speeds, the airplane visits a small amountof
air and pulls it down much harder -- creating lots of induced drag.

The same logic applied to helicopters predicts *infinite* induced drag in
the hover, and this prediction is closer to the truth than you might think.
Many helicopters simply *cannot* hover outside of ground effect -- they
don't have enough power.

The machine stirs up a huge torus of recirculating air; a typical air
parcel moves in a circle in some vertical plane, down through the rotor
disk and up in the far field, producing pathetically little vertical thrust
in comparison to all the commotion.

1) A hovering helicopter is a strange beast.
1a) From a distance, from a black-box point of view, a hovering
helicopter is rather similar to a rocket, hovering on pure thrust. It is
not analogous to an airplane.
1b) But if you zoom in on the region of air right next to the rotor
blade, the rotating wing is analogous to an airplane wing, except that in
the hover the wing sees a permanent downdraft that is quite unlike anything
an airplane has to deal with.

2) A translating helicopter is reasonably analogous to an airplane.


All in all, aerodynamics is a hard subject. Rotary-wing aerodynamics is an
even-more-difficult sub-specialty. Plain old airplane wings are much
easier to explain and to understand.