Re: Physics of Flight

[John Denker <jsd@MONMOUTH.COM>]

At 12:41 PM 8/18/99 -0400, Michael Edmiston wrote:
> We're getting several subject threads going here, so I'm not sure which
> one to respond to.  The ideas below are in response to Denker,
> Whatcott, Beaty and concern reaction engines, and pushing on the earth,
> etc.

I *thought* this thread grew out of my answer to Ludwig's question.  I had
said that a hovering helicopter was disrecommended as a starting point for
understanding an airplane in normal cruising flight, because of induced
drag and other complications that are of secondary importance to normal
cruising flight.

> I have been getting a lot of good thinking from all contributors to
> this discussion, so I hope we can sustain it a little longer without
> getting on each others nerves.

Right.  I'd like to stay focussed on elucidating the behavior of real
aircraft in a real atmosphere.

> I think we might be having a problem distinguishing between whether
> something happens, as opposed to whether it must happen.  Specifically:
> (1) Does a hovering vehicle that is employing a reaction engine also
> exert some force on the earth?  Answer: typically yes.  (2) Must it do
> so in order to hover?  Answer: absolutely not.

Agreed.  That will be of interest to the 0.001% of pilots who depend on
reaction engines to support their craft.

> In this regard I strongly disagree with Denker when he says:
>
> /jsd/ Gravity is a force between the earth and the aircraft;
> /jsd/ the only way to counteract it is a force (indirect or otherwise)
> /jsd/ between earth and aircraft.
> /jsd/ The only question is how indirect it is going to be.

I concede that my statement, if taken out of context, can be made to sound
like it is not always true.  However in the context of a closed system of
real aircraft in the real atmosphere, it makes a valid point.

Do we really need to have a discussion of whether the atmosphere is
relevant to a discussion of induced drag?

> As others have pointed out, reaction engines do not require the earth
> or any other object behind them upon which they push.  The only thing
> that needs to be pushed is the exhaust.  Reaction engines work in deep
> space regardless of whether the exhaust is aimed at earth, aimed at the
> moon, or aimed at nothing.

Which provides a hint as to why if you assume that an airplane's wing or a
helicopter's rotor is a reaction engine, then you will never understand
induced drag or translational lift.

> When a helicopter hovers, or a rocket takes off, or an airplane flies,
> the downward directed air (or exhaust) eventually exerts a force on the
> earth,

That's just what I was trying to say.  It is only a question of how
"eventually" it is.

> but that is not the force responsible for the flight/lift/etc.

OK, not directly responsible, but I never said it was directly responsible.
 This looks like nit-picky word games to me.

> The force responsible for the lift is the F = dp/dt in which the
> momentum change is that which the vehicle imparts to the air or
> exhaust.  Whether the earth is present or not is totally immaterial.

If you want to pick nits, you'd better watch out for other nits.  If the
earth were not present, there would be no atmosphere, and without the
atmosphere there would be no airplanes or helicopters.  So it's not
"totally" immaterial.

> In another post John says: "Rocket motors and machine guns work just
> fine in a vacuum.  Wings don't.  Doesn't that make you worry that your
> derivation isn't a reliable predictor of how wings work?"
>
> No.  That does not bother me at all.  A rocket motor won't work in a
> vacuum if you don't give it something to eject (i.e. fuel).  A machine
> gun won't provide thrust in a vacuum if you take away the bullets.  But
> that's exactly what you've done if you put a wing in a vacuum...you've
> taken away its ammunition, so to speak.

OK, so how to you use the reaction-motor model to explain translational lift?