Re: helicopter not equal rocket

[Michael Edmiston <edmiston@BLUFFTON.EDU>]

Okay, John, I was not aware of this power difference between hovering
and translating.  But now that you have told me about it, I think it
does make sense in the reaction-engine view... and this partly comes
from the information you told us about how a slow airplane makes worse
vortex wake than a faster plane.

A translating helicopter is visiting/affecting much more air than a
hovering helicopter.  It doesn't need to "jerk the air downward" as
violently in translation as it does in hovering because when it
translates it affects a much larger quantity of air.  I believe this is
essentially the same type of reasoning you used to explain the
tremendous wake behind a slow plane.  This type of argument is a
reaction-engine (F = dp/dt) type of argument.

Worse, a hovering helicopter partially experiences the same air over
and over.  To the extent that the air just goes down, out, up, in, and
back down in a continuously circulating "smoke ring," we aren't
throwing significant net amount of air downward.  Away from ground
effect, this mechanism could be significant and I can imagine the
difficulty in hovering.  Interaction between the vortex ring and the
ground would interfere with this recirculation... it would spread out
the downward moving air along the ground and result in a greater
quantity of "new air" being brought into the blades.  I think this type
of reasoning is also consistent with a reaction engine view.

As you have stated, the Bernoulli, circulation, etc. views/analyses
have to be consistent with the Newtonian (F = dp/dt) views.  You
understand a way to view "translational lift" using the traditional
aeronautical engineering views of Bernoulli, et.al.  But there also
must exist an F = dp/dt way of viewing this.  I think what I said above
does that.


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



-----Original Message-----
From:   John Denker [SMTP:jsd@MONMOUTH.COM]
Sent:   Thursday, August 19, 1999 11:43 AM
To:     PHYS-L@lists.nau.edu
Subject:        Re: helicopter not equal rocket

At 11:12 AM 8/19/99 -0400, Michael Edmiston wrote:
> Denker says: "I keep daring people to explain translational lift using
> the helicopter-as-rocket model.  Still no takers."
>
> John, please define "translational lift" so that I am sure I know what
> you're asking.  Then maybe I'll give it a try.

Fact: A helicopter requires significantly less power when it is moving
(translating) than it does in the hover.  This is called translational
lift.  (This is the standard, if not exactly brilliant, terminology).

The effect is so significant that many helicopters cannot hover outside
of
ground effect.

Hint:  The physics is similar (albeit not quite identical) to what we
call
induced drag in a fixed-wing aircraft.

As the saying goes, learning proceeds from the known to the unknown.
 It is
droll to see people who have never heard of translational lift using
helicopters to "teach" other folks about induced drag.