Teaching Lift to Seventh Graders

> >> I don't think there's any real debate, unless it involves lift demons.
> >> The physics-based way lift works is that the wing shoves the air down.
> >
> > Does it?  How about in something like:
> >
> >
http://www.aether.demon.co.uk/coolkit/graphics/ekranoplan/antishipping.jpeg
> >
> > ?
>
> It's still the same priciple: Basic Newtonian action and reaction.

The principle is indeed the same, but I think it rasies some interesting
issues.  It all depends on what you mean by "shoves the air down".

If you mean that the wing "applies a force" to the air, then I agree, but
I'd argue that the explanation is facile.  "How does a bridge create the
support for its load?  The load shoves the bridge down and the Newtonian
action and reaction principle (third law) means that the bridge pushes up
with an equal and opposite force on the load."  It's all true, but, for me
at least, it doesn't help much with "how it works".

If you mean that the wing "changes the momentum" of the air, then I also
agree, but you need to take great care in the quantitative aspects.  The
formal, quantitative description of that in fluid mechanics is the momentum
theorem.  That says that if you draw an arbitrary control surface through
the fluid, the momentum flux into the control volume *plus* the pressure
forces on the control surface add up to the force on that volume of fluid.

For an aerofoil in flight at altitude, you can concoct a control surface
where the pressure forces are insignificant and the entire force on the
volume is accounted for by momentum flux.  [You can also concoct control
surfaces where the opposite is true -- the key is that however you slice and
dice it, the sum is always the same, equal to the lift on the aerofoil.] In
the case of an aerofoil in ground effect, there is no control surface that
gives that result.  The higher pressure on the ground beneath the wing has
to add into the equation.  The momentum change of the air is never enough to
account for the size of the lift produced.  You can't get away from the fact
that the aerofoil is, in effect, riding on a cushion of high pressure air,
which presses on both the aerofoil and the ground.  It's not shoving
*enough* air down to generate the lift.

So what's that got to do with explaining all this to seventh graders.  Only
this I think...

Newton's second law can be couched in the terms that every change of
momentum has a force responsible for it.  If there is a change in momentum,
there *must* be a force.  That is *not* the same as saying "if there is a
force, there *must* be a change in momentum".  I've often heard it said that
lift *must* come from the downward deflection of air because of Newton's
second law.  It's not true -- that's a bad application of the second law.
The wing does not *have to* deflect the air to create lift, and in the case
of the ekranoplan, it doesn't, or at least it doesn't deflect enough.

> The difference here is that the air that has been "shoved down"
> by the wing has another surface to interact with ("ground effect"),
> which results in a modified flow-field around the wing that significantly
> increases its efficiency at "shoving air down" --- and more importantly,
> an =ENORMOUS= reduction in the induced drag, because the interaction
> of the two counterrotating wingtip vortices with the ground causes
> them to move toward each other and cancel each other out a relatively
> short distance behind the plane.

Of course, that's all true, and I've taken some liberties by talking about
the ekranoplan as above.  The true talent of the ekranoplan is that
reduction in induced drag, which comes from the elimination of wing tip
vortices because it flies within about a span of the surface.  The
aerofoil-in-ground-effect stuff I discussed above is really
two-dimensional -- it comes from the fact that the aerofoil is flying within
a chord of the surface too.

Julian Scarfe