Re: helicopter not equal rocket

[John Denker <jsd@MONMOUTH.COM>]

At 01:32 AM 8/20/99 -0400, Stefan Jeglinski wrote:
> Regardless of the precise mechanism regarding translational lift, for
> example, it is not clear to me in the end whether you claim that the
> idea of "air being thrown downward" is not the mechanism for lift.

Well, yes and no.  Air being thrown downward is part of the story, but
unlike machine-gun bullets, you can't just throw the air down and forget
about it.  A lot of the air that you throw down in the downwash returns in
the upwash, and you'd better take that into account.

> Perhaps translational lift, for example, leads to more air being
> thrown down, and this adds to the overall lift.

As I have hinted more than once, "translational lift" is bit of a misnomer
-- it refers to power required, not lift _per se_.  It doesn't add to lift,
no matter what mechanism of lift we are talking about.

Real lift is a force which (by definition) is strictly perpendicular to
velocity, so it cannot contribute directly to the power budget.  Besides,
real lift must equal weight in or out of ground effect, in hover or forward
flight, so it obviously can't contain any contribution from what they call
translational lift.

> Or are you saying
> that there is a new mechanism at work that is wholly unrelated to air
> being thrown downward?

Wholly unrelated?  Of course not.

> To use an analogy from my field, solid state physics, I might take on
> the discussion of conductivity by electrons, or conductivity by
> holes, or conductivity by polarons. And I might say that the
> properties of each are quite different and that the conductivity of
> polarons absolutely cannot be explained by considering the electron
> alone. But I would never say that the conductivities in all three
> cases are not ultimately due to electrons.

This reinforces my suspicion that you are thinking of translational lift as
lift when it should be power required.

> Back to lift, I agree that the basic reaction engine (rocket) concept
> could not possibly -explain- translational lift (or induced drag, to
> cite another challenge)

OK!

> but if translational lift leads to more air
> being thrown down for example, one might say that translational lift
> leads to a greater "efficiency" of the reactive wing principle (that
> is, the idea that lift is a reaction to the wing throwing air down,
> regardless of the gory details).

I would have said that translational lift leads to more upwash rather than
more downwash.

The idea of induced drag can be summarized this way:  it is more efficient
to visit a large amount of air and yank it down gently, than to visit a
smaller amount of air and yank it down violently.  Newton's laws are upheld
in either case. (Duh!  Is anybody surprised?)  This depends, obviously and
crucially, on the existence of air that can be visited.

In contrast, a machine-gun model of lift production does not require an
atmosphere and produces no upwash.  Therefore it cannot reproduce the
observed dependence on wingspan, airspeed, and ground effect.