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[Phys-L] Re: thrust > drag



Oops - thanks for catching that. Yes, I was being sloppy and
mixed lift and total aerodynamic force. Sorry that you had to
write a long correction.

However, after reading postings by others, I must make a comment
about the thrust vector. It is only purely horizontal for certain
speeds when flying at a constant speed and altitude. At slow
speeds the thrust vector can be quite a bit above horizontal. For
example, a Cessna 152 flying at 45 knots has the nose up at what
feels to be 15 degrees or more. One can fly at this speed and
maintain altitude by careful balance of pitch and power. In that
case, it is the sum of the lift and the vertical component of
thrust that equals the weight. As most have said, the drag in
this case is opposite to the velocity.

Bob at PC

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-
L@list1.ucc.nau.edu] On Behalf Of John Denker
Sent: Saturday, July 02, 2005 11:12 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: thrust > drag

On 07/02/05 08:58, Bob LaMontagne wrote:

I hate well meaning but incorrect questions. If a student has
a little
more
knowledge than the questioner, the student loses credit.

I agree with the general point ... but not with the
following example:

The lift vector is not in the opposite direction to the
weight during straight and level flight.

In straight and level flight in still air, lift
is directly upward, and weight is directly downward.

It is tilted somewhat to the rear
of the aircraft. If the wing does not have an effective angle
of attack,
there is no lift generated.

That statement appears to be based on an incorrect
definition of lift. A similar statement applied
to the _total aerodyanmic force_ of the wing would
be correct. The total aerodynamics force can be
resolved into two components, lift and drag.

Horizontally, you have thrust forward, with drag
and a horizontal component of lift acting backward.

Also incorrect, apparently for the same reason.

========

On 07/02/05 07:57, Aaron Titus wrote:

I've always thought of drag as being opposite the velocity
of the
object, the airplane in this case. This is probably true in
"still
air."

Yes, true in that special case, as stated. More generally,
drag is defined with respect to the relative wind.

However, I can see that if there's a crosswind, that would
change
the
direction of drag.

That's entirely true. Consider the limiting case of a parked
airplane. The object has no velocity at all, in the
terrestrial
reference frame, but it still has lift and drag.

===============================

For details on all this, see
http://www.av8n.com/how/htm/4forces.html
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