The wrong "Bernoulli" explanation in Sci. American for April 2005
Tom Sanderson
> All new airliners of Airbus or Boeing make and all top bizjets, have
> low drag "supercritical wings", with almost flat upper surface and
> curved lower surface (to host wing beam and fuel)!
>
> I have never seen Scientific American explain that wing and why it
> physically create Lift!
A supercritical wing is, by definition, designed to operate in a
compressible flow regime. You can throw Bernoulli out the window for such
wings.
They still generate lift the same way as all other wings; they induce a
circulation in the airflow. They have a "fat" shape to control adverse
transonic effects and prevent shock separation on the upper surface from
occuring too far forward.
As a side benefit, you can fit more fuel in there and use deeper chords, but
it's not strictly necessary...the 747 doesn't have a supercritical wing and
does just fine.
Tom.
Jan-Olov Newborg
> > > 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.washington.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 important 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...)
>
Late Professor Richard S. Shevell, Stanford Uni, chief aerodynamist for
making the DC-9, MD-80 and DC-10, once wrote in a private mail, "Dont
explain Lift using Bernoulli equation, but use it for the calculation
in connection with the Potential/ Circulation Theory model"! Shevell
described how the Potential theory model was use less for 150 years
(after Euler and others), but in 1904 the superpositioning of the
manually applied Circulation solved the problem of calculation Lift.
Today Newton based CFD (Navier-Stokes) calculates the best
aerodynamics.
Professor e.m John D. Anderson, Maryland Uni, writes in one of his
books about the venturi pipe:
"Strictly speaking Bernoulli equation can only be valid along the
centerline of the venturi, and then we have disregarded the
compressibility effects"!
A senior CFD aerodynamist once told me, that "the continuity equation"
only is a mathematical model, not a physical law.
Since the venturi pipe is a restriction in the pipe, it will increase
the drag, so to overcome that a increase in pressure infront of the
venturi is needed. The 1953 Piper Colt has as venturi pipe a
speedsensor without the convergent entrance part! Why? The front part
is not needed, only the divergent backpart makes the jobb by help of
the Coanda effect!
Jan-Olov Newborg
Merlin Dorfman
> Bernoulli equation seems to very misunderstod by americans, even
> respected universities and Professors should know better about the
> physics!
Everybody seems to want to be an aerodynamicist, and explain
Bernoulli's Theorem. See today's "Overboard" comic strip:
http://www.uclick.com/client/smc/ob/2006/08/13/