The Wronger Bernoulli etc

[Tom Sanderson]

> And someone tell us how to explain how circulation induces lift
> to a budding pilot.

Track the path of a single air particle as the airfoil approaches and
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
pressure from the leading edge, then get pulled back and down along the top
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
lift. Airfoil design is all about getting the maximum lift for the drag and
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.

[stau...@usfamily.net]

> Well. I can't add to the Bernoulli discussion because it's over 30 days
> old or has been closed by the moderator. (Remove it from the list,
> then). And someone tell us how to explain how circulation induces lift
> to a budding pilot. Bernoulli is easier to understand and demonstrate.

> 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. And if in ground effect, lift is still developed but
> because circulation is limited by the presence of the earth's surface
> below the wing, less drag is experienced, hence 'float' is experienced.

A point to consider. Only the flow in the boundary layer is
constrained to follow the exact profile of the airfoil, and that only
when there is no seperation. Smoke tunnel photographs show that flow
at reasonable distances from the airfoil surface for a symmetrical
airfoil do not look unlike those for a non-symmetrical airfoil.

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.

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.

[StratMatt]

> Someone tell us how to explain how circulation induces lift

> to a budding pilot. Bernoulli is easier to understand and demonstrate.
> 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. And if in ground effect, lift is still developed but
> because circulation is limited by the presence of the earth's surface
> below the wing, less drag is experienced, hence 'float' is experienced.

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), 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.

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).

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. The pressure differential between the
upper and lower surfaces is not enough to lift the aircraft if you calculate
it. 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.
After you read the site I linked to you will see that I basically agree
with everything the author described. I've read every site I can find about
how wings lift and I think this is the one.
What do you all engineers think of the site?

When I teach my students about lift I show pictures of airliners flying
in conditions conducive to contrails where the downwash, wingtip vortices and
reduced pressure above the wing is clearly visible. You can go to
www.airliners.net and search for aircraft type "747" and keyword
"condensation".
A picture puts the mystery to rest in my opinion. I was also quite surprised
to learn how far above the aircraft the area of low pressure extends- it
really is like a big scoop.
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.

So my take on it is that Bernoulli explains that the postive angle of
attack causes a pressure reduction and that pressure reduction causes the
airflow to accelerate. That accelerated airflow departs the trailing edge in
a downward motion as downwash which is an action which lifts the aircraft in
reaction, as explained by Newton.

As I said, I'm not an engineer and have no formal education on this
subject.
What do you think of my thoughts on the subject?
Corrections are requested.

thanks,

Matt L
Seattle, WA