Hmm. REALLY not understanding circulation

Thanks, Roger.

I've seen your page before, but the thing that has confused me about it and
especially that circulation diagram is what would happen to a tab-style
stall warning device installed at the front of the wing in that diagram.
Wouldn't it be going off all the time?

"Roger Long" wrote in message
...
Discussion and animated graphic of circulation he

http://www.avweb.com/news/airman/183261-1.html

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Roger Long



"xerj" wrote in message
...
Every time I think I have it sort of worked out, I fall in a heap. I've
read through "See How It Flies", but it's still a bit beyond me.

I can understand why a spinning ball would create a circular flow around
itself, but I can't really grasp why a wing would.

Does anyone have a lay explanation as to why?

I've seen your page before, but the thing that has confused me about
it and especially that circulation diagram is what would happen to a
tab-style stall warning device installed at the front of the wing in
that diagram. Wouldn't it be going off all the time?

The diagram doesn't show the flow at fine enough scale to infer that
kind of thing. Even though there is a general upward movement of air
ahead of the wing it isn't a clockwise flow relative to the wing.
There is a line at the leading edge called the stagnation line. The
air that hits the wing above this line goes over and the air below the
line goes under. As angle of attack increases, the stagnation line
moves back and down. At high angles of attack, it can get far enough
back that air does flow forward across the wing. This lifts the stall
warning tab and sets of the horn. This is related to but is not the
same thing as circulation.

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Roger Long

Not quite. The stall warning simply is reporting that the stagnation
line has moved below the stall tab installation point. It's at some
greater angle of attack that most of the wing stalls. Otherwise, you
wouldn't need the warning, the stall would be warning enough.

"Tony" wrote in message
ups.com...
Not quite. The stall warning simply is reporting that the stagnation
line has moved below the stall tab installation point. It's at some
greater angle of attack that most of the wing stalls. Otherwise, you
wouldn't need the warning, the stall would be warning enough.

How is this different from what Roger posted?

Roger said
At high angles of attack, it can get far enough
back that air does flow forward across the wing.

indicating that is what lifts the stall warning tab.

That's pretty deep into a stall. In airplanes I've flown the tab lifts
at high angles of attack, but I doubt very much the wing is very much
stalled. The horn sounds well ahead, and at lower angles of attack, a
stalled wing. For sure, though, the stagnation line had moved below the
tab, lifting it.

"Tony" wrote in message
oups.com...
Roger said
At high angles of attack, it can get far enough
back that air does flow forward across the wing.

indicating that is what lifts the stall warning tab.

I must be missing something. I read Roger's post simply to mean that as the
stagnation line moves back and down, the flow above the stagnation line
heads away from the stagnation line, toward the stall warning tab, causing
it to move and turn on the stall warning device (buzzer, light...whatever).

I also read your post to say the same exact thing. I'm not getting where
you and Roger disagree.

Inasmuch as the tab is mounted slightly below the leading edge of the wing,
the air has to be moving from a position behind the stall warning tab,
toward the leading edge of the wing. It seems to me that this is what Roger
wrote, and is also what you wrote (essentially).

I suppose Roger could jump in and clarify the disagreement.

Pete

I said that air moves forward across the wing which was clumsy wording
since it could easily be misinterpreted. Air near the wing surface
above the stagnation line moves from front to back so, if the
stagnation line has moved back under the leading edge, there can
actually be a small bit of flow with a forward motion relative to the
wing. This is very localized and only occurs very near the wing and
only at high angles of attack.

A stall warning tab could sense the stagnation line in one of two
ways. It can be blown up and forward by the reversed flow or it can
simply be spring loaded so that the switch is engaged when air flow
drops below a certain point as the stagnation line approaches. I'll
confess that I haven't looked at one closely enough to know which way
they are set up. They may even be different on different aircraft.

When I sail, (which seems to be more than I fly now) I have lengths of
yarn taped near the leading edge of the jib. If I get to too high an
AOA, the one on the "bottom" of the sail will start to point straight
up and even forward. Even though the sail is still pulling hard, the
stagnation line has moved well around to the windward side. There is
a slight drop in efficiency but no dramatic stall.

Pop quiz class:

Sails don't stall and suddenly lose their lift causing the sailboat's
heel to suddenly decrease. Can anybody tell us why? (Hint: Assuming
you had long enough landing gear to get to stall AOA while rolling
along the ground, you couldn't create the same kind of sudden loss of
lift that you experience in the air.)

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Roger Long