Re: Misconceptions: Physics of Flight

[William Beaty <billb@ESKIMO.COM>]

On Wed, 11 Aug 1999, 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?

I for one would certainly agree.  The helicopter is an excellent model
because it focuses our attention on the fact that a wing behaves as a pump
or as a "reaction motor".


On the other hand, I've met people on Newsgroups who seem otherwise
rational, but who believe that helicopters do not deflect any air
downwards.  Instead they believe that helicopters are sucked upwards by
the low pressure at the upper surface of their blades!  (Evidentally we
are supposed to ignore all the air which streams downwards below the
craft, and also supposed to ignore all other similar systems: ignore that
boat propellors eject water backwards as they drive the craft forwards.)





Here's a subtle point about helicopters: if the rotor was to recirculate
the air in a torus-like motion, so that the air exiting from below the
blades was to flow up around the blades and re-enter the rotor from above,
then this might greatly reduce the lifting force.  Rather than generating
lift, the blades might simply cause the air within that torus-shaped
surface to spin around faster and faster.   Imagine placing a helicopter
in a large box and then throwing the box from a cliff.  If the blades
cannot eject mass-parcels downwards, then there can be no reaction effect
or lifting force.

When following the above reasoning I'm led to see that a helicopter MUST
pull air down from above in a radial pattern, and then eject the air
downwards in a coherent, non-radial stream.  The pattern of flowing air
above the rotor must NOT be identical to the pattern of flow below. A
component of motion of the inwards-moving radial pattern above the rotor
is a SIDEWAYS motion of the air, where air flows horizontally inwards
towards the rotor.  If air was bricks, then the rotor would be taking
bricks in from the sides and deflecting them vertically downwards, and
therefor experiencing an upwards momentum change which cancels out
gravity.

I suspect that the "tip vorticies" of the rotor blades have a lot to do
with the downwards-moving cylinder of air below the helicopter.  That
cylinder of air must have large vorticity at its "surface" (at the region
where the moving air meets the still air of the environment.)  Might this
vorticity by provided by the spiralling pattern of blade-tip-vorticies?
The idea makes sense, but I don't know if it is true.  If it is true, then
the air below the helicopter is not just a simple downwards-moving
cylinder.  The air is also flowing in a sort of tubular enclosure of
vortex-flow which constrains its shape and keeps it coherent.  The
"tubular enclosure" is being "knitted together" on the fly because of the
actions of the blade tips.


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