Need help on this Bipe
 

I've just finished with the electric conversion of this Bipe for a club member. The power is a Hacker A50-12L, ESC is a CC ICE 75 Amp, battery will be a 7S LiPo.

Question: Can anyone identify this model? The model is covered with silk and dope, and I'm guessing its about 20 years old or so. The top wingspan is 52 inches, bottom wingspan is 49 inches, chord on both wings is 8 5/8 inches. The fuselage is 48 inches from tail to prop.

What I need is the CG balance location for this Bipe.

Thanks everyone

DennyV

Looks like a skybolt, not quite like my (1970s-ish) SIG skybolt though.
Could be a slightly different size kit, or maybe the fellow whom partially built mine changed some things.
Top pic NOT mine...
http://pages.suddenlink.net/arlyn/P0002585.JPG

http://static.rcgroups.net/forums/at...8-DSC07779.JPG

http://static.rcgroups.net/forums/at...5-DSC07785.JPG

Denny, a safe start on bipes is 25% of the projected chord of both wings in plan view; in other words, if the distance from the LE of the top wing, to the TE of the bottom wing, is 12", start at 3" from the top wing LE. In this particular case, I'd ignore the top wing sweep, and measure from the foremost centre section, to allow a little extra safety margin.

Brad

Thanks Dave
Do your plans show where the CG is on your model?

I'll have to give it a look.

Generally it should be between the 1/3 of the leading edge of both wings... mark the 1/3rd line top and bottom, then ballance it in the center of your marks on the top wing.

Thanks everyone, I'll check the 1/3 top and bottom and see where it is.

(I'm glad I won't be doing the maiden on this one! Our club has a number of members very familiar with flying Bipes.)

DennyV

Denny .... the 30% back average of both wings is the safe recc'd on most bipes ... but with Bipes the incidence of wings to tailplane is a great influence on this. Just check out a Pitts against a Stampe or similar ...

One fact you must ensure - bipes have a lot of lift factor in the wings ... so downthrust is essential ... and contrary to even my own 'habit' are better not too overpowered.

Lovely bipe ... not too different to my Skymaster which is still undergoing rebuild !! It has distinct Pitts like lines .. short coupled fuselage, swept back top wing etc. Powered well, care with CoG - she looks like it could be a hot fun machine.

Nigel

The 25% of the projected chord of both wings as suggested a few posts back would (given the sweep of the top wing) be massivly conservative. So consevative that the CG may well end up at, or even in-front of, the main-gear and the thing will spend all it's time stood on it's nose!

There are a few CG calculators that do biplanes but if you want a rough method a safe CG is one thats halfway between the 25% MAC points of the top and bottom wings.
25% MAC of the bottom wing is straightforward as the wing is straight and un-tapered, but the top wing is swept so the 25% MAC has to be measured at a point just a hair inboard of half span.

This will make it a bit nose heavy but not so nose heavy that it will want to stand on it's nose.

PS... not all bipes need downthrust. I've got one (PA Ultimate AMR) that has none and it's perfect.

Steve

Most likely because designer has aligned wing and tail surfaces to gain better straight and level right way up and inverted ?

My comment was more for the general bipes .....

Nigel

The AMR is a funny set up actually. The motor, wing and tail is all lined up at zero degrees and wing and tail are symmetrical, Nothing so unusual there, but to get it trimmed level you have to add a fair bit of down elevator trim. The TE of the elevator ends up down by about 5mm for level flight:confused:

So it's trimmed with negative longitudinal dihedral, but it flies fantastically so one way or another it all works out. it's also got about 500W or power in a 38oz airframe, and it doesn't feel overpowered at all, so maybe the wacky set up helps.

That sure looks very similar to my pitts except for the cockpit area & size. My pitts measures 42" across & is 7" across the root. Just eyeballing the average of both my wings I got 12" total with the c.g. 4" back of the top wings l.e.
Just a rough average though