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Old Jun 27, 2012, 04:16 AM
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Joined Mar 2011
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Help!
launch camber on highstart

Hi
I would like to know why most guys use launch camber (where available). Is there any advantage of using it on a highstart? It seems reasonable to use it on a winch to keep the tension on the line while 'kiting' the plane up. But on a highstart, doesn't the flaps just bleed energy from the bungee without adding much height?
I talked to a guy in my club the other day about it. He had 15 years of experience on full-scale gliders and in his opinion flaps should never be used in a glider during any kind of launch. He also have loads of experience on winch-starting full-scale gliders and in his opinion launch camber shouldn't be used in that type of launch either. Just so you know why I am questioning what seems to be common practice in the R/C soaring community.
We don't need to go into a discussion about the difference between full-scale and model gliders. What I want to know is if there really is any benefit of using launch camber, especially on highstart. What, if any advantages are there? Or is the guy right about not using it on the highstart? Is it more dependent upon what size glider on any given size highstart i.e a large glider on a small highstart with lots and lots of stretch in one end of the spectrum vs a small glider on a relatively stiff highstart on short stretch?
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Old Jun 27, 2012, 04:47 AM
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Thermaler's Avatar
Buchanan Mi
Joined Apr 2005
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I have noticed a greater launch height either in on winch or hi-start with launch camber from my 2M Osprey to my F3J Victor.

Joe
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Old Jun 27, 2012, 05:09 AM
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R.M. Gellart's Avatar
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On a high start I just use thermal camber for launch, not full blown launch camber. It allows for more pull, but allows the speed to stay up.

Marc
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Old Jun 27, 2012, 06:28 AM
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Dallas, TX
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I use thermal camber unless the wind is above above 10 mph then I will use launch camber and can get a small zoom off a 3m Hosemonster high start with a Supra Pro.

Alan
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Old Jun 27, 2012, 06:33 AM
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On a highstart or onewinch I use 1/5 or so the camber that I use on a winch.
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Old Jun 27, 2012, 07:49 AM
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If your model is set-up for winch launching or J launching it will require similar tension to get a reasonable launch. Most sport high starts do not generate this kind of tension. So generally speaking if the wind is light to calm, use good tension and minmum camber for the best launch. If there is enough wind to increase the stretch of the high start rubber once the model has been launched, then launch camber can be very useful and that stretch can be converted in to a good zoom . LJ
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Old Jun 27, 2012, 11:56 AM
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I didn't notice any difference in launch height with and without camber when launching my Alex XL (Spider 2M) but did notice a big improvement when I was able to stretch the bungee more even though the line was 40% shorter.
Does anyone know if the Xplorer series of gliders need any special thermal camber? I would guess it only need launch and speed camber as it is a thermal duration plane, any thoughts on that?
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Old Jun 27, 2012, 09:06 PM
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Evan D's Avatar
Charlotte, NC
Joined Jan 2004
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Same here... It seems if I put more than a few mm it doesn't get as high.


Quote:
Originally Posted by OVSS Boss View Post
On a high start I just use thermal camber for launch, not full blown launch camber. It allows for more pull, but allows the speed to stay up.

Marc
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Old Jun 27, 2012, 11:43 PM
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Seems to be a relationship between available power and appropriate camber. My winch motor is unusual in that it is wound for 12 volts. Most winches are wound for 6 volts. On 12 volts, in still air, flaps were no help. When I changed to 24 volts, using a lot of camber helped. Similarly, my 3M sized hi start, when used with a 3 meter plane, didn't seem to launch as well with lots of camber. When there's a fair breeze, lots of camber enables the glider to get high while maintaining good tension, so it can zoom, and also release with the rubber stretched out a bit for more height even at release.

If the line didn't have any drag, then thermal camber or slightly less would be more appropriate as you'd want best L/D for the system. However, when the drag of the line is large relative to the drag of the airframe, increasing lift is the only way to improve L/D very much.

I think we may be able to dismiss the full scale guys ideas. A 3 meter glider may use something like 1 horsepower per lb. on a hot winch launch. Unless those launch winches for full scale are, say, 800 horsepower or something, and pulling several times the weight of the full scale glider, they're much less power relative to the weight. Also, since flying speeds are higher, a good zoom would take even MORE power to be comparable. And we already know that in the underpowered case, camber won't help all that much. Plus, line size in relation to aircraft size is likely to be less, so the line has less drag. And the chute is probably smaller in relation to the aircraft. So he's right about full scale but not about a relatively powerful winch launch with an RC model.

BTW, since line drag can be significant, it helps to use a thinner line and smaller chute. 50 lb test braided line is enough. Or, at least, 50 lb test squidding line. If you're willing to replace more than once a decade or two, you can probably go with even less. For the last couple of years, I've extended my hi start with some 50 lb braided kite line which is even thinner than the squidding line. Very inexpensive, but I've forgotten the brand.
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