[Phys-L] Re: thrust > drag

[John Denker <jsd@AV8N.COM>]

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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