Thermal wing:CO8....

[Redster]

I've always wanted to build and fly a flying wing glider.I found some plans at Andy McDonald's flying wing page,the CO8.I plan to put a Kontonik drive 501 to launch it.My question is that I know there is disadvantages on thermaling on flying wings,but is it such a disadvantage that it's not worth while of building one.Is is really that bad to thermal?Anyone with building and flying experience with wings please give me advice,it sure looks like an efficient flying machine.

Redster

[Ollie]

I have been interested in flying wing sailplanes for about 50 years.

Here are some things to consider:

1. A pure flying wing without any fuselage or vertical tail(s) is stealthy in that it is very hard to see, edge on. Vertical tails are added to make it easier to judge pitch attitude but they are a source of the parasitic drag that the tailless design is trying to eliminate.

2. To have a wide speed range, low camber, low maximum coefficient of lift airfoils must be used. This raises the stall speed compared to a tailed configuration.

3. In the swept back wing configuration, the tip ends of the wings are producing little or even negative lift. This lift distribution decreases the effecive aspect ratio, increasing the induced drag. At thermalling speeds, induced drag is more than half the total drag making this a big disadvantage.

4. In the swept wing configuration. there is a "middle effect" which increases drag over the center portion of the wing. This can be overcome by gradually curving the 25% chord line of the wing rather than having it meet at an angle on the center line. This complicates the planform.

5. The swept wing configuration puts great demands on the rigidity of the structure in order to avoid aeroelastic effects that change the pitch trim.

6. Winch and highstart launching of wings is not as high because the wings can't reach as high coefficients of lift.

I could list a few advantages of flying wings but they wouldn't apply to flying in thermals.

[Daemon]

On the other hand, a straight leading edge plank like the Moth from NCFM (http://www.northcountyflyingmachines) is so strong and so fast and slippery that it can be bungeed instantly to an altitude where you can then catch thermals. Certainly a different approach to the problem. You might want to talk to Harris Nelson (have to give him a call as he has no email) about his flying wing airfoils. He's been making some foils with zero pitching moment, and very low drag. The CG is pretty sensitive, but they're pretty efficient.
We're talking foamies that fly a lot more like crunchies. It'd be interesting to see what would happen with one of these foils on a bagged or moulded ship.

BTW, the real big disadvantage I see with wings and thermalling is no camber control. No camber, no flaps, no crow. Real spoilers might be interesting. And if it does get flipped over by turbulence at altitude, reestablishing orientation can be tricky. That said, I've specked out various combat wings many times.

ian