Starting CG for a unusual shape wing

[Ampmister]

I have designed and
built a plane unusual to my regular thinking , it is best described as a sort of smaller electric powered Sig Wonder . I started with constant cord U-control wing and added swept out and to the rear tips ( WW1 German ) to offset tip stall . The horz. stab. starts at the trailing edge center with elevator in center between two vertical fins . The fuselage pod is also a fully symetrical airfoil in shape , motor in front .
Should I follow the thinking of a constant cord flying wing ,or add some width to the calculation to figure in the horz. being in the center ? Iam currently thinking to follow the rule of starting safely at 15% and working rearward to possibly 23% or more on the wing airfoil section only . :o :)
Thanks everyone for what suggestions you have to offer .
Ampmister ,

[Butch777]

Any pics??

[Roger Long]

Here are some first try pics.

[CloudyIFR]

Go to www.TailwindGliders.com and download "Flying Wing Calc", you'll need MS Excel, Open Office or some other spreadsheet program.

I'd start with a static margin of 5-10%.

Curtis
Montana

[BMatthews]

By all means run the calc program. Also if you can find info on the Wonder's CG location your version is so close to it that the same location would be fine for you as well.

It's not a pure plank either. By adding the sheet spacer to move the elevator back you've added a rather short but significant tail moment length as well as an attached stabilizer area. So with that in mind 15% is unneccesarily safe. You'd be fine starting at 20% and perhaps run dual rates on the elevator until you determine how sensitive it is.

[MudMarine]

Your CG, (Just by looking at it) is going to be just behind the main spar within 1/2 inch or so. You are gonna want to put some reflex on your horizontal control surface to control forward pitch moment.

[vintage1]

Aha! R/C control line combat plane!

Fairly forward..about 18=20% of chord without plugging it all in to a calc.

Looks a complete hoot. I hope your reflexes are up to it...

[MCarlton]

I'd concur with MudMarine on this one, its a pretty similar layout to the old Obelix slope soarer, which flew best with a fairly forward CG and with a bit of reflex on the elevator and on the ailerons (but that was a fairly lifty section, yours appears to be symmetrical? In which case, you may not need the reflex on the ailerons.

I would make a start with the CG on the rear edge of the spar and go back from there.

Looking at the layout again, if one took the "centre section" to be part of the wing, and drew on the 25% position, and did so on the tips, then it looks to me like the 25% MAC line actually gives an aerodynamically swept forward wing, in much the same way as a conventional wing with a straight LE and tapered TE.

In that case, you may need a more forward CG than it appears?

Someone with more brain than me will no doubt respond...

[peterangus]

For the umpteenth time.......

Experiment with a simple chuck glider with a scaled planform.