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Re: helicopter not equal rocket



Here are some comments followed by some questions.

(1)
John Denker was bothered by my statement that a machine gun and rocket
engine have different properties because one provides continuous thrust
and the other provides discrete thrust. This was not intended as a
"red herring" or nitpicking. It also was not the result of
misunderstanding which aspects of reaction engines may or may not be
germane to our discussion. Rather, this was intended as a counter
argument to John's strong implication that things having different
observable properties cannot be examples of the same thing. Earlier
he said, "there is a fundamental difference between the *observable*
behaviors of helicopters and reaction engines." My point is that
"observable differences" is not always sufficient to place two things
into different categories. John and I no doubt look different, and we
probably exhibit significantly different observable behavior. Yet we
are both humans. We are both scientists. I presume we are both male.
And so forth. By pointing out that machine guns and rocket engines
(a) exhibit different observable behavior; yet (b) are both reaction
engines; I was trying to make the case that not all reaction engines
have identical physical behavior. That a helicopter shows some
behavior that some reaction engines do not show is insufficient to
disqualify the helicopter as a reaction engine.

(2)
In the same vein, it is interesting that John distinguishes between a
turbojet and a turbofan... two engines that do not yield very different
physical behavior. He appears inclined to call the turbojet a reaction
engine, but not the turbofan. I understand these are borderline for
John because it appears he would prefer that reaction engine only
pertain to things that work in space (more on this later).

It should not be a surprise that I think both the turbojet and turbofan
are clear examples of reaction engines. They are both ejecting mass
from a nozzle/cowling/etc. and provided thrust as a result of that mass
ejection. The primary difference between the two is that all of the
exhaust from the turbojet has been heated to fairly warm temperature
whereas some of the exhaust of the turbofan has bypassed the bulk of
the heating. In some designs the bypass air is not heated at all, and
in others it is heated by mixing with the "combusted air" prior to
ejection from the engine.

An aside, but a comment I will make, John is incorrect when he says "
yes, [a turbojet] needs air to breathe, but that is a fairly minor
contribution to the momentum budget." If you look at the momentum of
the exhaust, i.e. look at the mass ejected, the air "breathed in" is
essentially all that is ejected... the fuel is inconsequential.

(3)
I strongly disagree with John's assertion that an important part of the
"reaction engine" definition is that it must be something that works in
space. John implies that this definition is widely understood, but I
disagree. My experience (and also the result of a small survey) is
much more in line with a recent posting by William Beaty under the
subject heading: re-define "reaction motor???

(4)
John has said several times that he would rather drop the reaction
engine discussion, and the helicopter discussion, etc. and get back to
the discussion of lift of a wing in regular airplane flight. The
reason we Newton-ists can't do that is because it is at the heart of
our view of a wing. We (at least me) acknowledge the Bernoulli-ists
view and we agree those equations work and have value. But we are at a
loss as to why we are not allowed to view the wing as a reaction
engine. The jet engine gets thrust by causing air to be thrown out its
back. The wing gets lift by causing air to be thrown downward.

(the questions)
(q1) Would a vertical jet engine (turbojet or turbofan) exhibit
translational lift if tipped slightly so that it translates? This
might be somewhat related to asking if it is more difficult for a
harrier jet to hover than for it to translate (at slow velocities
before the wings develop very much lift).

(q2) Conversely, would translational lift for a helicopter disappear
if we put a cowling around the blades so that they were more like a
turbofan? I realize this is a complicated question because the rotor
blade pitch needs to change during rotation so that translational
motion may begin. And questions arise about how long the cowling is
and whether it gets tipped, etc.

These questions arise because ideas like ground affect and
"experiencing more air as it translates" would still be true if the
cowling is in place (and ought to be true for the jet engine air
patterns). But the cowling would change the idea of adding the
translational air velocity to the rotational velocity as we examine air
speeds across the blades.

Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817