Is Bernoulli's principle the explanation of how an airplane wing generates lift?

In article <2s6f2p$hp@zip.eecs.umich.edu> positron@quip.eecs.umich.edu
(Jonathan Haas) writes:
> Isaac Kuo <isaackuo@lhasa.berkeley.edu> wrote:
> } In fact, because of Bernoulli's [Editor: "Bernouli's" in the
> } original] principle, I'd expect it would be very
> } easy to open the door at 60mph.  Just not all the way.
>
> Just wanted to point out here that David Karr does not, in fact,
> *believe* in Bernoulli's Principle.

Just to be consistent, why not point out that I do not, in fact,
believe in gravity, or that 2 + 2 = 4?

Of course I *do* believe in Bernoulli's Principle.  For example, it
allows Venturi tubes to be used as sensors for airspeed indicators.
What irks Mr. Haas is that I say that all his flight manuals are
snowing him when they give "Bernoulli's principle" as the explanation
for "how airplanes fly."

For a very nice, clear explanation of how airplanes fly (to be more
precise, how an airfoil generates lift), read "Bernoulli's Law and
Aerodynamic Lifting Force" by Klaus Weltner, in The Physics Teacher,
vol. 28 (Feb 1990), pp. 84-86.

(To summarize the debate, most popular explanations of the lifting
force say that the higher speed of the air flowing over the top of the
wing causes a drop in pressure by Bernoulli's principle.  Weltner
concludes that indeed Bernoulli applies, but "the other way around."
Low pressure on the top of the wing, created by reaction forces,
causes the air to flow faster there.)

A car door, of course, is in many ways unlike a typical airplane
wing, so I wouldn't presume to apply the same arguments blindly.

-- David A. Karr <karr@cs.cornell.edu>

[ Editor: two further postings by David Karr, April and June 1994; the
first from a thread "Plane flying upside down". ]

In article <1994Apr13.185628.6498@cs.cornell.edu> karr@cs.cornell.edu
(David Karr) writes:
>So altogether I find that I do not have a truly satisfactory
>explanation that would explain to a layman how airfoils generate lift.
>[...]  Is the "Bernoulli effect" explanation a
>pseudo-scientific myth, or a simple qualitative explanation that I
>just failed to find in any of the theoretical texts?

Thanks to Edgar Pearlstein who pointed out (in sci.physics) the very
nice article by Klaus Weltner in The Physics Teacher, Feb. 1990, p. 84
(and there's another relevant article by the same author in that
issue).

Weltner explains how lift is generated by the pressure gradient that is
generated when air is made to move along a curved path.  The Bernoulli
effect then explains that because the pressure on top of the wing is
lower, the air flows faster there.  Note that this is completely
reversed from the causal relation given by most popular writers
(including the writers of airplane manuals, apparently).

So when an airplane flies upside down, the air travels faster over the
flat "bottom" of the wing (which is now actually on top) due to low
pressure there, and slower over the curved "top" of the wing due to high
pressure there.

-- David A. Karr <karr@cs.cornell.edu>

Jonathan Haas responds to Isaac Kuo and myself without having read the
article by Weltner (and without coming up with a counterargument other
than the unsubstantiated assertions of some anonymous tech writers).
Jonathan, do your homework!

(An earlier version of Weltner's article appears in Amer J Phys, vol.
55 p. 50, but the later version seems better presented.)

Not only that, but many of the specific responses are misleading or
wrong:

In article <2si0r1$3ie@zip.eecs.umich.edu> positron@quip.eecs.umich.edu
(Jonathan Haas) writes:
>Isaac Kuo <isaackuo@lhasa.berkeley.edu> wrote:
[...]
>>a flat wing (barn door?) at a positive angle of attack.  Both the top
>>and bottom are the same length...
>
>Uh huh. Now, measure the airspeed you must move your barn door to make
>it take off. It's pretty darned high. Now, give that barn door a slight
>camber over the top and try it again. You'll find the airspeed necessary
>to generate the required lift for takeoff decreases dramatically...

Not all that dramatically.  Yes, the curved thin wing generates
somewhat more lift at a given angle of attack and velocity.  (Actually,
what the camber gives you is a negative angle of zero lift, or
equivalently, a positive lift even at zero angle of attack.)  The lift
of the curved wing also maxes out at a steeper angle of attack.  But
it's not as if you have to double your takeoff speed.  An increase of a
few percent would make up for the difference.

>because, while the deflection of air helps create some lift, the
>Bernoulli effect creates much more.

This is a typical non sequitur of the "Bernoulli causes lift" school.
We are still talking about very thin plate here.  We have given it
camber but not thickness.  The top and the bottom are still the same
length.  Where did the Bernoulli effect suddenly spring from?

>Your 'barn door', by the way, does not stall.

Define what you mean by "stall".  Do you mean "loses lift due to
separation of flow"?  If so, then the barn door most definitely *does*
stall, and at a much lower angle of attack than your cambered thin wing.

>It generates maximum lift
>at a 45-degree angle of attack,

Absolutely incorrect.  The maximum lift occurs at a relatively shallow
angle of attack, maybe 10 or 15 degrees.  (Less than the angle of
maximum lift for a curved plate, remember.)

>then generates less and less lift as
>angle of attack increases until it 'stalls' at vertical.

This part is more or less true, except that the stall (as noted above)
occurs much earlier.

>As a pilot,
>I can assure you that stalls occur at much, much lower angles.

Yes, at about 18 degrees for a Clark Y profile, for example.  The
coefficient of lift (CL) in this case is about 1.55.  As the angle
increases beyond 18 degrees, the CL falls off gradually to about 1.25 at
25 degrees, then about 1.05 at 30 degrees (from Fig. 115, p. 160 of Von
Mises, Theory of Flight).

-- David A. Karr <karr@cs.cornell.edu>

[ Editor: Jonathan Haas's posting of April 1994 that began the thread
"Plane flying upside down", in which Karr's April posting above
appeared. ]

Gabriel Duong <ar618@FreeNet.Carleton.CA> wrote:
>The wing of the airplane is curved on top so that when the air hits
>the front edge, a low pressure is created on top of the wing.  Thus
>the higher pressure below the wing pushes it upward and provides lift
>for the plane.  That is the general principle on how a plane flies.
>
>The question is :
>How come then can a plane fly horizontally UPSIDE DOWN ?

Because of a couple of things.  The first is, the Bernoulli effect is
only one of the two reasons why a wing generates lift.  The other is
that it is tilted upwards, causing wind to hit the lower surface and
deflect downward.  The reaction to this action pushes the wing upward.
This works even when upside down.

The other is that the Bernoulli effect is still working to generate
lift.  Think of the cross section of the wing as being roughly the
shape of an elongated teardrop.  During normal flight, the teardrop is
tilted upward so that the upper surface is "cambered" enough to cause
the air to accelerate when flowing past it.  When flying inverted, you
simply pitch the plane so that what was the lower surface is now
cambered, and the wing will generate lift in the *other* direction.

It's hard to describe without drawing a picture.

By the way, most planes can't fly inverted for any reasonable length of
time, because they are dependent on gravity to feed fuel from the tanks
to the powerplant.  Aerobatic planes designed to fly upside down have a
special inverted fuel tank.

-- Jonathan S. Haas <positron@eecs.umich.edu> (April 1994)