[Phys-L] Re: thrust > drag

[John Denker <jsd@AV8N.COM>]

On 07/01/05 23:27, Chuck Britton wrote:

> I neglected to say that the question is accompanied by an
> illustration with four orthogonal directions clearly labeled weight,
> lift,(vertically directed), drag and thrust (horizontally directed).
> Also the velocity is clearly stated as being in the horizontal
> direction.

Oh.  Then I must retract most of the criticisms in my
previous note.  The question then becomes much more
focused as a probe for comprehension of Newton's laws.

Meanwhile my general remarks about the importance of
story problems, and the importance of having (or being
able to hunt up) the relevant domain knowledge remain
valid and presumably non-controversial.

> Our use of these terms in intro physics is indeed greatly simplified.

Yes.  In reality, drag is not necessarily horizontal,
and thrust is neither horizontal nor opposite to drag.

So one could criticize this question as embodying
misconceptions (or at least approximations) about the
definitions of lift, drag, thrust, and weight.  These
misconceptions/approximations, if consistently applied,
make the question easier to interpret.  The downside
comes much later, when the student has to unlearn the
wrong definitions.

> (but useful IMHO)

Agreed;  lift and thrust are _approximately_ horizontal
in normal flight.

Constructive suggestion:  If you want to make the question
more nitpick-resistant, just ask about the balance between
the _forward forces_ and the _rearward forces_.  This
falls under the heading of mean what you say, and say
what you mean.  Dressing up the question in technical
terminology (thrust and drag) muddies the water quite
a bit.

This sort of mud is harmless in the context of the
intro physics course, but it makes life harder for
people (e.g. me) who teach the down-line course where
the technical terms must be taken seriously.

> We'll get into the Navier-Stokes equations in a later course.

My answer has nothing to do with Navier-Stokes.

Start by imagining a Harrier jump-jet.  You can use
thrust to support part of the weight.  Now generalize
to aircraft in general.  In cruise conditions, the
amount of thrust you should optimally devote to this
is small (a few percent) but it's not zero.

This depends crucially on the precise definition of
lift, drag, thrust, and weight ... which are defined
with respect to three different coordinate systems.
  -- lift and drag are defined w.r.t the relative wind
  -- thrust is defined w.r.t the axis of the engine
  -- weight is defined w.r.t the "down" direction ...
   and even that is ambiguous during a turn, depending
   on your choice of reference frame (rotating or not)

You may find this distressingly complicated ... but those
are the official definitions, and they aren't liketly to
change anytime soon.

>         typo somewhere here?

Yes, I wrote (a) when I meant (b) ... thrust slightly
greater than drag.  Sorry.

> Newton had to state his number One Law because it is so definitely
> counter to the Aristotelian view of nature.

I think Galileo should get credit for the conceptual
breakthrough here.  Just because this is on Newton's
list of laws doesn't mean he invented it.

This is yet another instance of the general rule that
the pedagogy of science is markedly different from
the history of science.  Good introductory pedagogy
is simple and clean ... history is not.
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