Stall

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Unhook679
Unhook679
WA
26 posts
WA, 26 posts
12 Mar 2008 9:25pm
What is the stall and why happens, i know when the kite fly backwards but aerodinamic, why??

Thanks

Good wind
lancekenny
lancekenny
SA
402 posts
SA, 402 posts
12 Mar 2008 11:09pm
Kites usually stall - choke - fall backwards because the power is being pulled on - bar to the chicken loop - when really you should be pushing away from the chicken loop to "refresh the air" in the kite. Although it may feel like the wrong thing to do - actually moving the bar to keep air moving over its surface rather than trapping it in the kite will generate more speed and lift.

Hope this helps... a good thing to do when you are learning is to go out on a 10kt day and just fly the kite (preferably somewhere safe) and maybe even body drag. The skills you learn flying in these conditions I find invaluable when out there going for it.
Neill
Neill
VIC
484 posts
VIC, 484 posts
13 Mar 2008 12:44am
en.wikipedia.org/wiki/Stall(flight)

have a look at the foil sections showing the flow separation. the theory is exactly the same for a kite, which is, in effect, just a wing. the main difference is that tube kites are terribly inefficient foil sections due to the solid LE and the large vortices induced directly behind the LE. hence a kite with a thinner LE will be more efficient.

makes me think that kites should be designed with a solid planform, imagine a flysurfer with 5 different inflatable (using a high pressure pump and bladders like a tube kite) bladders which were contained in a solid ripstop casing, with perfect cross-section foil shapes separating the bladders. getting it to turn and water relaunch could be interesting, but it would be rather efficient and you could tweak the foil sections to make it faster, slower, gruntier etc. it would certainly be more efficient in a straight line than a conventional tube kite if you could keep the flow laminar over the whole thing.
Kitehard
Kitehard
WA
2782 posts
WA, 2782 posts
13 Mar 2008 12:35am
Hi Guys,

Simply put, stalling is an aerofoil section with too high and angle of attack. The excessive angle of attack results in a seperation of the laminar flow of air away from the upper wing surface. The wing surface is covered with turbulent air instead of steady smooth flowing air. Stall starts at the trailing edge and advances more and more forward as angle of attack increases until the smooth airflow spreads away from the foil over the major part of the curve and kills off the high speed airflow and therefore the lower pressure zone.

This results in equal pressure on both sides of the wing and the wing loses lift and stops driving forwards and flies backwards. By pushing the bar away, you reduce the angle of attack and re-establish a laminar flow over the upper wing surface and again resuming a low pressure area and regaining lift and forward drive.

Some kites like Flysurfer and the new Ocean Rodeo have slots in the middle of the foil section which allows air to flow through the canopy to smooth out the turbulance on the upper wing surface. Basically it reduces stalling at very high angles of attack.

This probably wont make a lot of sense unless you first undertand the theory of flight and lift theory. Start with Bernouli's theorem of air losing pressure when traveling over a curved surface at higher speed. (Venturi).

Good winds,

au_rick
au_rick
WA
752 posts
WA, 752 posts
13 Mar 2008 12:27pm
good explanation, and since we're being techinical, the optimum angle of attack is 4 degrees, as determined by the Wright brothers, who incidentally tried to lodge a patent for the theory of flight.
Good job they didn't succeed otherwise we'd be paying royalties for the privilige of flying our kites
Neill
Neill
VIC
484 posts
VIC, 484 posts
13 Mar 2008 8:04pm
au_rick said...

good explanation, and since we're being techinical, the optimum angle of attack is 4 degrees


4 degrees for which foil section? is that for optimum speed (lowest drag) or for optimum lift? there is no "optimum" for every section, they are all different.
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