The Wronger Bernoulli etc
T o d d P a t t i s t
>> And someone tell us how to explain how circulation induces lift
>> to a budding pilot.
....
>A particle
>going over the wing will initially move forward and up due to positive
>pressure from the leading edge, then get pulled back and down
....
>Any shape which imparts downward momentum to the airstream will generate
>lift.
Any net downward motion of the air does result in an upward
force (lift), but "circulation" theory (at least in its 2-D
simple model case) is all about the fact that lift but does
not require that net resulting downward air motion. The
simplest case is a rotating cylinder, which raises air at
the front, pushes it back down at the back, and produces a
lift force proportional to the spin without resulting in any
net downward air motion. Just as much air goes up as goes
down.
"Circulation theory" is all about superimposing this type of
flow with circulation onto a theoretical inviscid flow
without any circulation (which generates no lift), so that
the resulting flow closely approximates the real flow of air
where the air flows smoothly off the trailing edge of the
wing. All of the lift is due to the "circulation" of the
flow, and that's where the name comes from.
--
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.'
T o d d P a t t i s t
>Unfortunately, there IS no simple explanation of the effect of airfoil
>camber without getting into advanced fluid mechanics and differential
>equations. I liked the explanation of a former sci.aero moderator,
>Mary Shafer- action of the "lift daemons". It is the only thing
>understandable without getting really into the nitty-gritty of fluid
>dynamics.
I'm not convinced. I like the humor in "lift-demon" theory,
but in the end it's a cop-out - just throwing up our hands
and saying it's too complex to explain. Simpler
explanations are possible, but they are even harder than
hard mathematical explanations.
>
>This is true for any situation where viscous flow dominates. At low
>enough air density where the mean free path becomes a sizeable portion
>of the airfoil chord, things change, momentum flow can give a good
>approximation of lift. But no airplanes except the shuttle fly in this
>regime.
Tom Sanderson
> I choose not to teach the circulation theory to my students because I
> don't think it exists in the manner the term suggests it does. I
> understand
> what people are trying to explain by using the term "circulation"
> (basically
> upwash and downwash),
Circulation theory of lift and upwash/downwash are different things. A 2D
airfoil doesn't have any upwash but still has circulation.
> but because the airflow does not circulate completely
> around the wing in a uninterupted circular fashion round and round (it
> stops
> at the stagnation points on the leading and trailing edges) I personally
> think the term "circulation" is misleading and even confusing.
You only need circulation theory when you assume inviscid flow. Circulation
theory can't be explained with physical intuition because it's not a
physical thing...it's a math trick put in to correct for the fact that
you're ignoring viscosity, compressibility, time-variance, and a bunch of
other stuff to try to simplify the Navier-Stokes equations.
> I am no engineer, but my current opinion is that Bernoulli's theory
> only
> explains how the airflow accelerates over the top of the wing AS A RESULT
> OF
> the reduced pressure. Not vice versa.
Correct. In order to get a stable flow field around the airfoil, you have
to have a low pressure area near the back of the wing. Bernoulli correctly
describes what's happening to the streamline, but isn't the cause.
> The reduced pressure over the upper surface, caused by a positive angle
> of attack, causes the airflow to accelerate and the airflow's
> viscosity/coanda effect causes it to follow the upper surface beyond the
> trailing edge where the high velocity airflow continues off the wing in a
> downward motion in the form of the downwash which lifts the airplane.
Coanda effect doesn't cause anything...it's a description (like Bernoulli),
not a cause. Viscocity doesn't do it either (the air is not "stuck" to the
wing). It's the pressure field that causes the air to follow the airfoil
surface.
> What do you all engineers think of the site?
It's pretty good. Better than most I've seen. It does make some physical
whoopsies (you can't pull on air, for example): "Air is pulled from above".
It's explanation of the Coanada effect is completely bogus. They use the
illustration of water flowing over a glass...this looks like the Coanada
effect, but the physical are totally different. Air does not stick to the
airfoil surface.
Tom.
Don Stauffer
> I choose not to teach the circulation theory to my students because I
> don't think it exists in the manner the term suggests it does. I understand
> what people are trying to explain by using the term "circulation" (basically
> around the wing in a uninterupted circular fashion round and round (it stops
> at the stagnation points on the leading and trailing edges) I personally
> think the term "circulation" is misleading and even confusing.
>
> stagnation points where there is upwash at the leading edge and downwash at
> the trailing edge BECAUSE of Bernoulli's theory, however Bernoulli's theory
> alone does not explain lift. For a Cessna 172 to be lifted soley as a result
> of Bernoulli's theory it would have to be travelling at 400 knots!!! (from
> http://www.aa.washington.edu/faculty/eberhardt/lift.htm an _excellent_
> site).
>
What we need to consider with circulation theory is that the circulation
function is superimposed on the LINEAR flow due to the speed of the
foil through the air, or the air over the foil. With no relative
airflow, there IS NO circulation, so it cannot be considered as the ONLY
flow. It is like studying force vectors on a force at an angle to a
chosen coordinate system. We seperate a single force into two or more
components to ease the mathematics in studying our problem.
Yes, it is a mathematical construct. But airflow around an arbitrary
shape is VERY complex. We could never design good airplanes if we did
not use such mathematical constructs (and even more arcane ones).
WaltBJ
If you are able to get very close to the approach end of a runway
serving heavy jets (727 and up) you can usually see the downwash effect
on the smoke trail as the airplanes "cross the fence". AMAF if you can
get very close to the centerline of the runway you can usually feel the
downwash. On a humid day you can usually see the condensate streamers
graphically outlining the downwash. 27 at Stapleton was a wizard place
to observe this effect but alas it is now lost and gone forever.
Walt BJ
Merlin Dorfman
> My feeling is that there is no full circulation because it stops at the
> stagnation points where there is upwash at the leading edge and downwash at
> the trailing edge BECAUSE of Bernoulli's theory, however Bernoulli's theory
> alone does not explain lift. For a Cessna 172 to be lifted soley as a result
> of Bernoulli's theory it would have to be travelling at 400 knots!!! (from
> http://www.aa.washington.edu/faculty/eberhardt/lift.htm an _excellent_
> site).
Moved to http://home.comcast.net/%7Eclipper-108/lift.htm
...
> I'm sure everyone has seen the photos of the bizjet flying over a fog
> bank over Salt Lake leaving behind it vortices and a large depression,
> showing exactly what is going on.
I don't recall this picture--can you provide a URL?
Jan-OlovNewborg
> > And someone tell us how to explain how circulation induces lift
> > to a budding pilot.
>
> passes. It will move, roughly, in a circle (aka circulation). A particle
> going over the wing will initially move forward and up due to positive
> surface. A particle going under will go the opposite direction. However,
> the particles won't end up where they started...they'll both be lower than
> they were initially (assuming positive angle of attack). Net result is
> downward momentum transfer to the air, which results in an upward force on
> the wing (lift).
>
> Any shape which imparts downward momentum to the airstream will generate
> avoiding problematic flows (separation, shocks, etc.)
>
> > Oh, yeah, FWIW, a supercritical wing produces lift because it's at an
> > angle of attack, just like a symmetrical or plane surface airfoil is to
> > develop lift.
>
> True, because a supercritical wing gives imparts momentum to the airstream
> just like a flat plate or normal airfoil. The major point with a
> supercritical thing is that it will continue to do that efficiently (with
> low drag) at a higher speed than a conventional airfoil.
>
> Tom.
Here is the best downwash/waketurbulence picture I have seen at this
site :
http://www.airliners.net/open.file/239080/L/
It,s taken by a pilot in a meeting plane at FL350
Jan-Olov Newborg