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Re: Airfoil



At 11:57 2/12/98 -0500, Rick Tarara wrote:

I've been convinced enough by all our previous discussions on this
to pull the airfoil from my instructional set (although it's still
in my Animated Chalkboard software--too lazy to replace that one),
but I still have questions.

Doesn't the air flow faster over the top of the wing than the
bottom for certain airfoils (for whatever reason) and doesn't that
produce a pressure difference--the essence of the Bernoulli >approach.
Yes other things are going on, but with such a shape is >NONE of the lift
attributable to a pressure difference? What about
low speed airfoils--I'm especially
thinking about para-foils and human powered craft such as the >Gossamer
Albatross--are those explainable totally in Newtonian versus Bernoulli(an)
terms?

rwt


You may rest easy with the conviction that the shape of a particular
airfoil is very important to a wing's performance.
People who build their own airplanes ( a particularly hardy and
iconoclastic breed) are warned of the necessity for stitching, gluing or
otherwise firmly attaching the upper surface of linen or polyester wing
covers to the structure.
They are warned that the consequence of not anchoring this potent source
of most of the wing's lift is to lift the upper third of the wing cover,
which then provides much increased tension on the relatively weak trailing
edge structure. When the trailing edge collapses inwards, the situation
then becomes dire.

There are other homebuilders who become disturbed by the disproportionate
effect of minor surface discrepancies of 0.2 mm on the upper surface of
their wings.
A case in point is the Dragonfly. This is a glass/carbon composite canard
of a type originally designed by Rutan ( of Voyager fame, etc.)
Rutan chose for the canard a particular ultra laminar wingfoil designed at
Glasgow University - and as it happens, he slightly modified it with a
french curve....
It transpires that when this particular wing section is even SLIGHTLY
affected by raindrops or fly-squash, its drag jumps considerably and its
lift declines appreciably too.
For pilots intent on flying a canard to evade a rainstorm - the effect can
be dramatic: the plane bobbles in the canard equivalent of a stall (
canards cannot be allowed to stall fully because of unfavorable recovery
consequences...) or sometimes cannot hold height at any power setting.

If we accept that most of the wing lift arises from upper surface
'suction' (or reduced air pressure, if this *s* word triggers your
antipathy), it STILL includes as a consequence that there is a resultant
downflow of airmass from the wing sufficient to balance the plane's weight.
That is to say, both Bernoulli and Newton can speak to lift ( and to drag).

The reason that no aircraft of any description uses flat plates is not the
lack of lift - but rather the presence of (too much) drag.

Sincerely

brian whatcott <inet@intellisys.net>
Altus OK