The wrong "Bernoulli" explanation in Sci. American for April 2005
Jan-Olov Newborg
> Symmetrical airfoils will not generate lift if the angle of attack is
> zero. The F104 has a symmetrical airfoil and cruises at about 3 degrees
> positive AOA. Similarly, constant altitude inverted flight is possible
> only if the airfoil has a positive angle of attack relative to the
> earth. Lots of forward stick, in other words. In knife flight, the
> fuselage is yawed so it has a positive angle of attack and is
> generating lift and it is usually symmetrical, too. The slab-sided F4
> flew fine on its side at 500 KIAS. As for the 'true' explanation of
> lift, any time you have to juggle four simultaneous differential
> equations, pilot instruction becomes very unwieldy and the students
> doze off. This approach, to me, is picking nits.
>
> Walt BJ
It is very easy to see the importance of AOA (Angle of Attack) for
creating Lift of the Wings, when military aircrafts like Saab 37
Viggen.or Saab 39 Gripen, makes its approach for landing, at 11
degrees for normal landing and 15 degrees for short runway (800 m)!
The only way to physically create a local pressure gradient is to "turn
the airflow/ change direction" ( curved surface or wing at AOA)! Air
can tolerate maximum 8.5 deg, before separating from surface.
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!
Jan-Olov Newborg
Todd Pattist
>Bernoulli equation seems to very misunderstod by americans, even
>respected universities and Professors should know better about the
>physics!
As an American, I'm interested to hear what you think we
misunderstand about the Bernoulli equation.
>To change a veolocity a change in force is needed!
>There is no other way around!
I presume you meant to write "To change a velocity, a force
is needed." No "change in force" is needed - a constant
force will constantly change the velocity. A change in
force will change the acceleration. With this presumed
modification to your statement I would say that this is well
understood by physicists and aerodynamicists in all
countries of the world. Do you believe that the Bernoulli
equation is somehow inconsistent with this?
>I seems like Bernoulli equation does not explain the Windsock very
>good!
The Bernoulli equation applies to airflow through a windsock
as well as airflow past a wing. I'd be interested in any
evidence to the contrary.
>Some person has destroyed Otto Lilenthal Museum exibition in Anklam,
>Germany, by putting some Bernoulli rubbish in the halls!
>I asked if Otto Lilienthal himself wrote the false explanation, but that
>had been done by the museum chief!
Perhaps the museum chief is an American and knows what he is
doing :-)
--
The natural function of the wing is to soar upwards and carry that which is heavy up to the place where dwells the race of gods. More than any other thing that pertains to the body it partakes of the nature of the divine.
Plato, 'Phaedrus.'
Tom Sanderson
> I asked for years for an explanation of the airflow thru the Windsock,
> seen at every airport.
>
> I seems like Bernoulli equation does not explain the Windsock very
> good!
How do you figure?
Tom.
Aeroengineer
Gordon D. Pusch
> > 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...)
> 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