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Re: helicopter not equal rocket



At 03:21 PM 8/19/99 -0400, Michael Edmiston wrote:
John, you say:
<begin quote>
Yes, it expresses a dP/dt view. No, it does not come close to
explaining why a reaction engine should exhibit translational lift.
Consider the counterargument: Reaction engines, according to the
definitions everybody else is using, don't exhibit translational lift!
Certainly Bill B.'s machine gun (which started this thread) doesn't.
Bottom line: there is a fundamental difference between the
*observable* behaviors of helicopters and reaction engines.
You haven't come even 1% of the way to convincing me that the physics
is the same when the observed behavior is so different.
<end quote>

I stand by that.

This makes me reiterate what I said yesterday... we must have different
definitions of reaction engines. I was under the impression an engine
working by dp/dt was a reaction engine.

That very nearly says that anything adhering to Newton's laws is a reaction
engine. IMHO that definition is too broad to be usable. Under such a
definition, sure, helicopters are reaction engines, but the point I was
making about the physics remains: machine guns don't exhibit translational
lift. Using a machine-gun model to explain induced drag is bad physics.

That definition is fairly
broad and would include engines that do not behave identically.

Indeed.

For
example, even machine guns and rocket engines do not behave
identically. Since the rocket exhaust is (for all practical purposes)
continuous, and the machine gun "exhaust" is discrete, the thrusts from
these two reaction engines are very different. Yet they are both
reaction engines, aren't they?

That's a red herring. The discreteness is irrelevant. Surely Bill B. did
not intend to emphasize the discreteness of his machine-gun model. We are
all grownups; we can abstract away the irrelevant discreteness of the
machine-gun model and consider the average dP/dt.

Since you say I am using dp/dt, but not a reaction engine, I guess I
need your definition of a reaction engine.

If it makes you happier, we need not discuss reaction engines at all. When
I say machine-gun, I mean machine gun. That suffices to make my point that
a machine-gun model cannot provide a good model of induced drag.

While you're at it, could
you also tell me if you consider a jet engine as a reaction engine?

Well, since you ask so politely, I will answer (even though I say again
that the answers do not affect the physical points I have been making).

My "core" criterion is if it works in outer space, it's a pure reaction
engine. I generalize this a bit to things that "almost" would work in
outer space, and get the bulk of their momentum from processes that would
work in outer space.

By way of example:
*) I consider a rocket in outer space to be an excellent example of a
pure reaction engine.
*) I consider Bill's machine-gun model to be an example of a pure
reaction engine.
*) The term "jet engine" technically includes rockets (which emit a "jet"
of exhaust) but in common parlance refers only to turbojet and turbofan
engines.
*) Of these, I would say that a turbojet is a reaction engine to a fairly
good approximation. Yes, it needs air to breathe, but that is a fairly
minor contribution to the momentum budget.
*) A turbofan is, according to my chosen definitions, mostly *not* a
reaction engine. Most of its momentum comes from the action of the fan.
It's more akin to a propeller-type engine that it is to a rocket engine.
*) A typical "passenger jet airplane" these days is powered by turbofan
engines, not pure turbojets.

And how would a jet engine turned vertical compare to a helicopter?

*) A typical turbofan turned on its side would be more like a helicopter
than a rocket. A turbojet turned on its side would be more like a rocket
and not like a helicopter. This is perhaps more subtlety than you wanted,
but that's the way real-world physics sometimes happens.