Camber on launch

[Wave Glider]

This may sound like a retarded question haha, but I know with DLG's some people prefer to put camber on the wing to give additional lift on launch. Is ths the same case for a 2m plane on a hi-start or winch? Would you want to put camber in to acheive a higher launch or would it be better advised to just leave the wing flat on the bottom?

[rogerflies]

Dropping the flaps about 3-5 degrees makes a huge difference in winch-launching my Lovesong. I also put in just a little more up trim on the stab than is needed to offset the built-in down that's linked to the flaps.

It's hard to imagine any sailplane that wouldn't get a higher winch launch with some added camber. The amount you use with a high start might have to be less, depending on its strength.

Roger

[glidermang]

Wave Glider:

Roger is right. Using a winch or hi-start, flap usually helps.

DLGs usually employ a no-camber situation on launch. Once the DLG leaves the fingers, it's coasting and the goal is minimum drag. That comes when the wing is at zero lift. In fact, the best DGL fliers (that's not me, naturally) will launch vertically, so that the wing doesn't do anything at all on the way up.

Bigger gliders using winches or hi-starts benefit from camber just as a power airplane sometimes uses flaps for takeoff: more lift, given that the winch or hi-start is supplying energy all the way up.

I used to fly 2-meter, and no doubt about it: some flap improved things dramatically.

Now I fly DLG, and no doubt about it: minimize drag when launching! No flap deflection!

Yours, Greg

[aeajr]

That is correct, with DLGs you typically do not use camber on launch.

[Phil Barnes]

Greg is very close to the answer. A DLG is climbing verticaly or almost vertically for most of the launch and during that time, the wing should produce little or no lift, and drag will be minimized with minimal camber. The little bit that needs to beadded though, is that nobody launches a DLG vertically, or at least they shouldn't try to do that. The DLG is launched at an angle that is just a little above the horizon. The DLG pitches up to vertical in a short time frame after release. During that short time, the wing develops plenty of lift and it will likely benefit from some extra camber until the DLG has reached a vertical trajectory.

A larger glider that is launched with a strong winch or by hand tow (F3J), will most certainly benefit from extra camber. The camber should match the expected lift that the wing will be asked to produce on launch. Mark Drela's Supra plan shows a camber setting for launch which represents the camber that can produce the maximum possible lift for that wing. This position is used because Mark anticipates setting up the plane and flying a launch profile that will have the plane "pulling as hard " as possible (developing maximum lift) because that is what will maximize launch height. In fact, high performance competition planes are frequently set up such that on launch, an aileron command will only raise one aileron. The opposite aileron will not go down (that's how the programming is set up). This is done because the wing cannot develop any more lift with addidtional camber, so dropping an aileron below the launch camber position would only add extra drag and wouldn't increase lift any further.

For a weak winch or a high start, you want extra camber on launch, but not as much as you would use for a strong winch. You want the wing to be able to pull as hard as the high start or winch can pull. There is an optimal camber position to use to generate that amount of lift. You'll have to figure that out throigh experimentation unless you posses the ability to compute it.

[Phil Barnes]

Now I know that it took at least 13 minutes to type my last post ;)

[Wave Glider]

And a nicely spent 13 minutes! Glad to hear the responses, sounds like I'll have some fun tinkering until I get an optimal set up. Oh and this is only the flaps delfected down, not the ailerons coupled to them so that the entire TE is cambered correct?

[ACooper]

And a nicely spent 13 minutes! Glad to hear the responses, sounds like I'll have some fun tinkering until I get an optimal set up. Oh and this is only the flaps delfected down, not the ailerons coupled to them so that the entire TE is cambered correct?
You want the whole trailing edge cambered, Ailerons down too.

[Phil Barnes]

The entire TE should be cambered equally since you want the entire wing operating at the ideal camber. Keep the ailerons aligned with the flaps and use the correct camber position for launch which includes the flaps and ailerons.

I just did a search to see if I could find a Mark Drela quote about keeping the ailerons in line with the flaps for launch camber. I couldn't find but i did find the following:

http://www.rcgroups.com/forums/showthread.php?p=4363037&highlight=coriolis#post4363037

[aeajr]

For a weak winch or a high start, you want extra camber on launch, but not as much as you would use for a strong winch. You want the wing to be able to pull as hard as the high start or winch can pull. There is an optimal camber position to use to generate that amount of lift. You'll have to figure that out throigh experimentation unless you posses the ability to compute it.

Phil,

Looking at this from the plane's point of view, what about the case of a glider that does not have a super strong wing, what is the best approach there? Do you go for more camber or more speed? It sounds like you are saying optimize for more camber.


Some experimentation on a hi-start

I did some experiments with the Supra on a strong hi-start that can produce between 25 and 30 pounds of pull. That is stronger than the typical hi-start, but far less than the winch, so breaking the wing is not an issue on most 3M planes. It might be too much for a Gentle Lady. But I was thinking about this and was wondering what is the best approach when the pull is not as large as a winch can produce.

The Supra weighs about 4 pounds. With a 25 pound pull on the hi-start I can barely hold the plane. I can't really say I throw the plane. It is more of a release as the plane gets out of my hand so fast.

Through experimentation I have found that if I have no camber on as I release the plane, then add the launch mix a couple of seconds later I got better launches then if the mix was on from the start. It could be my technique or it could be that there was a better speed to pressure balance adding the camber later.

Phil,

Do you have any thoughts about this approach? Is it my launch technique rather that needs work, or does this delayed camber seem to have some merrit?

[rdeis]

Here's my limited noobie experience with launch camber on my Falcon:

My first several flights ignored camber altogether just to simplify things and get more comfortable with the idea of a fast, fancy airplane. I flew both from winch and highstart.

When I first added a little camber the launch didn't get much higher, but it got a lot more comfortable, that is, it felt much more like a smooth, solid launch than a mostly-controlled rocket-ride. It wasn't much steeper initially, either, but it was a lot more fun and a lot less less scary. More camber helped more-- I don't think I've pushed it to the point of using too much and inducing drag yet. It's going to take a lot more flying before I understand it well enough to experiment much.

At the moment I am drooping the ailerons slightly less than the flaps for added stall safety margin, but I'm fully aware that that's not aerodymanicly best.

I've actually been wondering if drooping the flaps a little MORE once the tow is established might be better on a swept wing in order to combat the loss of lift from the tips twisting that inspired the Supra's non-swept planform?

[ACooper]

This subject is also nicely covered in the RCA Masters Soaring DVD

[aeajr]

This subject is also nicely covered in the RCA Masters Soaring DVD

HUMM, I have that. Will have to go back and look again.

[nuevo]

For hi-start, I use max of 5-degrees camber. A hi-start just does not pull the plane hard enough to keep it from stalling, with say 15-20 degrees camber.



For winch launch, full-camber, say 15-20 degrees has been the norm for at least 10 years. Here are a few of my favorite articles on the topic, along with a few selected quotes from each.

Launch-Meister by Daryl Perkins
http://nesail.com/articledetail.php?articleID=2

"So, for best results, run full span camber, meaning, use the same amount of aileron deflection as flap deflection."

"I probably run between 15 and 20 degrees of full span camber."


Maximizing Your Launch Potential by Joe Wurts
http://nesail.com/articledetail.php?articleID=24

"I have full-span camber (ailerons and flaps down equally) of about 20-25 degrees"

[markdrela]

Assuming the launch energy input is fixed, the best launch is obtained by maximizing the L/D of the glider+line combination (i.e. by minimizing the drag losses).

A heavy winch line has lots of drag area, so here you want a large CL and hence large camber like 15 deg to slow down the glider and thus reduce the line loss.
In contrast, a hi-start line has relative less drag area, so a more modest launch CL and a medium camber like 10 deg is probably best.
In the limiting case of a very light hi-start line, the best camber is close to the thermal camber setting, 3-5 deg or whatever the airfoil likes.

The arguments above assume a fixed launch energy. But in reality the winch energy output increases with the glider CL, since a slower-winding winch draws more current. This favors even more camber than just what the total L/D dictates. Hence the 15-25 deg camber that Daryl and Joe advocate.
In contrast, a hi-start's energy output is the same for any glider CL, so the max total-L/D point is best.

[aeajr]

Thanks Mark.

CL = Coeficent of Lift?