> > A recent issue of Scientific American carries a mind virus! :)
> >
> > In the "Working Knowledge" column, the author presents an incorrect
and
> > long-debunked explanation of how wings work:
> >
> > "because the wing top is curved, air streaming over it must travel
> > farther and thus faster than the air passing underneath the flat
bottom.
> > According to Bernoulli's principle, the slower air below exerts more
force
> > on the wing than the faster air above, thereby lifting the plane."
> >
http://www.scientificamerican.com/article.cfm?chanID=sa006&colID=9&articleID=00009993-1F9A-1417-9F9A83414B7F012B
> >
> >
> > The above explanation is wrong, and is known as the "Equal
Transit-Time
> > Fallacy." This incorrect explanation is widespread among children's
> > books, but I'm suprised to see it in a reputable magazine like SciAm.
> >
> > It's debunked here:
> > NASA GRC: Incorrect lifting-force theory #1
> > http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html
> > http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html
> >
> >
> > Also see other debunkings:
> > Anderson/Eberhardt: a physical description of flight
> > http://www.aa.wa****ngton.edu/faculty/eberhardt/lift.htm
> >
> > Denker: How Airplanes Fly
> > http://www.av8n.com//how/htm/airfoils.html#sec-inverted-camber
> >
> > Irrotational inviscid flow: Streamline animations
> > http://www.idra.unige.it/~irro/profilo1a_e.html
> >
> > Or just ask yourself this: if airfoils *must* be more curved on
> > top, doesn't this prove that symmetrical airfoils cannot work,
> > and that upside-down flight is impossible?
>
> Yes, but the NASA page *does* say the air flow over the top of the an
> airfoil that is producing lift *is* faster, and furthermore that this
> *does* create lift from the Bernoulli principle!
> The only fact that is incorrect is that the streams over the top and
> bottom have to meet at the trailing edge. In fact the flow over the top
> is faster even than this. The question is *why* does it have to be
> faster than the flow over the bottom?
There is a vortex flow circulating around the wing cross-section,
superposed onto the chordwise flow. The vortex circulation is necessitated
by the _Kutta condition_ at the trailing edge; both these points are
nicely
explained at the "How It Flies" website:
<http://www.av8n.com//how/htm/airfoils.html#sec-circulation>
Note that the Kutta condition implies that _trailing-edge angle_
is far more im****tant than "leading edge angle" or "angle of attack" ---
which is why it is more effective to put control surfaces at the trailing
edge of an airfoil than at the leading edge. (The "lift control" surfaces
at the leading edge are really _separation control_ surfaces, not "lift"
surfaces...)
> Also, notice in the animation with a symmetrical airfoil there is *no*
> lift at zero angle of attack. Then this might suggest in the
> symmetrical case it is just Newton's force laws at work.
No, it is explained by the fact that for a symmetrical airfoil at
zero angle of attack, there is nothing to break the symmetry between
the upper and lower airflows, so the circulation around the cross-section
is necessarily zero.
By "blowing" the wing, one breaks the symmetry between the upper and lower
airflows and artificially generates a vortex flow that is independent of
the
Kutta condition; hence, in the "blown" case, there could be lift even at
"zero angle of attack," as predicted by the Kutta-Joukowski lift theorem,
<http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/kutta.html>.
(Note that such "forced" circulation explains why a spinning baseball can
"curve," in spite of the fact that its spherical rotational symmetry makes
it
impossible to even _define_ a "chord" to compute an "angle of attack"
from...)
-- Gordon D. Pusch
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