John Kallend's response to Ralph Glasser's letter to Parachutist about lift while tracking

[ryoder 1,590]

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Lift may be generated in the most amazing ways and we don't have to understand the process to know that it's lift.

Wonder what it's L/D is with the power off.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[billvon 2,998]

>whether a wing or object is flying pre-stall or post-stall.

I think everyone is placing too much importance on the term "stall." A Cessna that's on final approach has parts of its wing stalled; a Cessna doing an aggressive departure stall still has non-turbulent flow over much of its wing.

To most people, stall means the point at which drag starts going up very fast, while lift just goes up a little. In aircraft, the higher drag slows the plane, which reduces lift, which makes the plane drop, which inspires the pilot to pull back even more, which increases drag, which slows the plane etc etc until it falls dramatically out of the sky. But if infinite power was available, you'd get more lift at 30 degrees AOA than at 10, even if the flow was mostly turbulent. Heck, they've landed planes like that under computer control; with plenty of thrust and a vectored nozzle you can force a wing to 30 degrees and land it that way. Since you can get more lift out of it you can land at much lower speeds (and take off with a shorter runway.)

Skydivers have _lots_ of power available, since they have all that potential energy from their weight and altitude. So flying in a deep stall may be quite effective; flying in a partial stall may be even more effective.

[quade 4]

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Wonder what it's L/D is with the power off.

In all likelihood, pretty close to the same as with power on.

L/D is simply lift over drag. Doesn't really say anything about climb or decent. From straight and level flight and ALL other things being equal and without moving the control surfaces; add thrust and it climbs, lower the thrust and it will decend. Meanwhile L/D remains pretty much constant.

quade -
The World's Most Boring Skydiver

[pchapman 279]

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I think everyone is placing too much importance on the term "stall."

Ok, perhaps the problem is that once one explains to a jumper how their body in a track ISN'T acting like a wing in normal flight, then one should really explain how it IS acting. So the simple answer, you might suggest, is just to talk about lift and drag without worrying exactly how that is being achieved.

Understanding those differences does matter in some specific cases. For example, if a wingsuit design adds a pillow between the bottom of the rig and the jumper's butt, how much of an influence will it have on glide ratio? If one believes there's fully attached flow over the person's back, one might think the pillow would have a much greater effect than if one believes the flow is unattached (where a little bit of smoothing out of the jumper's profile won't really matter).

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But if infinite power was available, you'd get more lift at 30 degrees AOA than at 10, even if the flow was mostly turbulent.

Yes, more or less.
Few basic aerodynamic teachings say much about post-stall lift. They show a graph of an airfoil's lift increasing up to the stall point, then suddenly dropping, but nothing beyond that. So here's a graph to demonstrate what billvon is talking about, for a commonly studied airfoil (NACA 0015). The lift coefficient is plotted against the angle of attack. Well past the stall angle the lift goes up again, almost to what can be achieved pre-stall. Since the airfoil is pretty much acting like a plank (without the leading edge acting in the 'special' way that airfoils do when unstalled), the best lift ends up around 45 degrees angle of attack.

[Edited to add paragraph on drag:]
While one can achieve good lift well past the stall, drag keeps on going up rapidly past the stall. (Generally rising sort of like half a bell curve from zero angle of attack to a maximum at 90 degrees.) For that reason, someone in a wingsuit would still like to have as much attached flow on their wings as possible. As to what is actually happening over a wingsuit's wings, that's another topic.

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Skydivers have _lots_ of power available

180 lbs of skydiver at 120 mph = 58 hp :)

[pilatus_p 0]

already posted this elsewhere ...

Can't help it - made me laugh and seems appropriate to the discussion ...

http://www.amazon.com/exec/obidos/ASIN/1891830368/pageturners0c

Ross

http://www.teamtechnology.co.uk/troll.htm

[VectorBoy 0]

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John Kallend's response to Ralph Glasser's letter to Parachutist about lift while tracking

I loved the brilliant response by John. Atmonuati supporter or not Much respect to the professor