john flynn

Gidday, my dad has just rebuilt an old model bi plane that was smashed years ago. It looks like a single seat stearman (is there such a thing?) but we don't know for sure. Anyway the question is, how do you determine the cg of a bi plane? Is it measured on the bottom wing or the top and where on the wing is a good starting point. Any help would be appreciated.

MinnFlyer

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The rule of thumb that I use for a bipe is to pick it up from behind the top wing at the center. If it sits slightly nose down, it should fly well enough to let the flight characteristics determine any changes needed.

Rodney

A good starting point is to look straight down on the bipe. Go back one third or slightly less from the leading edge of the forward wing to the trailing edge of the aft wing. Initial trials might be best at 1/4 the distance between these two points although that might be slightly nose heavy. Also, be sure that the upper wing has slightly less angle of attack than the lower wing. On all of mine (at least seven different models) the best setup seemed to be about 1.5 degrees less angle of attack on the upper wing relative to the lower wing. This is with planes where the upper wing is forward of the lower wing. Thrust line was also somewhat critical, all require some down and right thrust, typically 3 degrees; adjust the thrust line for best flying at all throttle settings after getting the angles of attack firmed up.

Morgothius

I have a Kyosho Super Stearman (one cockpit), a Waco biplane (Sportsman Aviation/Hobby People), and a Great Planes Fokker Triplane. I just finished measuring the CG on all three, which I presently fly. Suprisingly, all three measured the CG at 62.5% of the total distance from front of top wing to rear of bottom wing! Thats 62.5% from trailing edge. IE: Total distance on Stearman was 12" and CG was 7.5" from trailing edge of bottom wing. Same with the Fokker. The Waco total distance was 10" and the CG was 6.25 inches from the trailing edge of the bottom wing. I should add that I tend to fly my planes very slightly tail heavy, with about 1 or 2 degrees of down elevator, which I would guess works out to about 1/4" aft CG at the very most. I would like to add as an aside, that I fly all three without right thrust in the engine, and use a bit of right trim in the rudder instead, if needed. Hope this helps.

john flynn

Thanks fellas for the input. We now have a good starting point. I will put a photo of the model online as soon as I can and perhaps one of you could id it for sure. Thanks again men.
Chow for now, John.

john flynn

Gidday again fellas, I have finally got some pics of the plane. When dad rebuilt the wings he had no idea where the centre struts should go so the top wing may be in the wrong position. Any info on that would be appreciated as well as the identification of the plane.

john flynn

Another view

john flynn

and another

MinnFlyer

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Wow, that wing has quite a stagger to it!

Without dragging out the algebra book, I would try balancing it at the LE of the lower wing. But I would do some high speed taxis and maybe let it lift and settle back down before I actually let it get airborne.

Morgothius

My Waco has a pretty significant stagger too. Probably at least as much as these pictures show. Mine balanced at 62.5% as I stated above. I would be concerned about the apparent positive incidence on the top wing though, and this will increase the lift created by the top wing, thereby throwing your required CG forward. The leading edge of the bottom wing may be a good idea for starting your tests.

john flynn

Thanks alot men, but what is it. I though maybe a stearman, what do you think?

hauckf

Posted this same question on another website and got this answer. Haven't tried it, but it sounds logical. "Find the MAC of the top wing and the bottom wing. Draw both MAC's in the side view. Draw a line from 25% of the top wing's MAC to 25% of the bottom wing's MAC. Divide the last line into two segments. The top segment length is proportional to the area of the bottom wing, and the bottom segment length is proportional to the area of the top wing. The point that is between the two segments is the aerodynamic center of the top and bottom wing together. Put your CG there to start with, and move it back in small increments between test flights until the desired reduction in stability is obtained." Good luck.

ps I put together an Excel workbook to do this and a couple other related calculations, like finding the MAC of a wing, etc. Anyone who would like a copy of the file can E-mail me.