On May 23, 5:20*pm, Mxsmanic wrote:
Dudley Henriques writes:
Actually, if I'm reading you right, I would rephrase this just a bit,
as it feeds into the problems we as instructors have in "re-
explaining" lift to students.
STRESSING either Newton or Bernoulli in the lift explanation causes
more than a modicum of confusion UNLESS it's done by including BOTH
theories in the explanation. You've done that actually. I would just
enhance things a bit more :-)

Agreed.

The problem is that all these effects interact, and explaining lift is often a
matter of preferred viewpoint, as you imply.

But it is true that lift always involves the acceleration of an air mass,
which is a matter of Newton's third law. How this acceleration is accomplished
is irrelevant, provided that it occurs. Bernoulli's effect and many other
effects help to explain why air flowing over an airfoil with a positive angle
of attack is accelerated at right angles to the direction of flow, but these
effects don't produce the lift directly, it's the acceleration that produces
the lift.

If you build something that accelerates an air mass in the same way without
any connection to Bernoulli et al., it will still fly. On the other hand, if
you build something that demonstrates Bernoulli's effect but does not
accelerate air perpendicular to its flow, no lift results.

When I dealt with the lift issue with instructors in seminar, my
personal approach was to favor the Newtonian explanation as in my
opinion student pilots can grasp Newton a lot easier than Bernoulli,
but I've ALWAYS made it habit NEVER to leave Bernoulli out in the
cold.

Lift is produced by diverting the air flow, thanks to Newton. The diversion in
an airfoil is in part produced thanks to Bernoulli.

Again correct but with a slightly different approach from me. It's
fine to quote the need for an accelerated air mass (relative wind
actually) as a necessity for lift creation. The statement is
absolutely correct, but again we have to be careful when dealing with
someone wishing to dissect Bernoulli and Newton.
The plane simple truth of it is that YES, we need relative wind to
create lift, and YES, we also need a positive angle of attack to
create lift. An airfoil no matter how efficient, at rest with no
relative wind in play, creates no lift. Same for the plank of wood.
Produce a relative wind on either and introduce a positive angle of
attack and INSTANTLY you have lift that can be explained completely
EITHER by Bernoulli or by Newton.
All we do when we stipulate that a relative wind must be present for
lift to be created is to stipulate the CONDITION under which Bernoulli
and Newton require for either to produce and explain lift.
It's a round robin that always ends up with both of these guys staring
us right in the puss with neither of them winning OVER the other .
Bernoulli 100% Newton 100% Newton the easier of the two to use as an
explanation, but NOT at the expense of Bernoulli! :-))
Dudley Henriques
Dudley Henriques