I always do the math on the CG location listed in the instruction booklet. The instructions for this biplane indicate the initial CG location be set at 5.5" aft of the leading edge of the top wing. I did the math to set the CG at 27% and the balance point is 4.0" aft of the leading edge.
If you balance this plane at the suggested location it would exceed 33% and most likely snap roll on liftoff. Am I wrong? Please offer assistance.

im not too familiar on how to find CG for a bipe but maybe it has something to do with the offset wings?


matt

Hey you guys that have experience with this plane please help out here. My GP Stearman is now ready to test fly and if the CG is off I would sure like to know it now.
Flycatch, what formula did you use? Did you take in to account the lower wing?

Yes, indeed, it does have to do with the stagger between the wings

With the wings level, measure from the front of the top wing to the back of the bottom wing.

Not diagonally, but parallel to the ground. That dimension will be the used as the wing chord.

Let's say, FOR EXAMPLY ONLY, it's 20". 27% of that is 5.4". Measure back from the leading edge of the top wing 5.4" and that's the 27% point of the the total chord.

Get out your ruler, do some measuring and you'll find the spot that GP suggests is pretty good to start off with.

Mr Scale
Flew my superstearman last weekend with the cg at 5.5 back from the top leading edge just like the book says and it flew very well. Have a 1.80 Saito installed and it balanced without any added weight.

Husker, you are lucky, it took 18 ounces of lead up front to get mine to balance at 5.5" with an O.S. 1.08... that is why I was worried when I saw that 4" was the requirement.

Mr Scale
Let us know hoe she flys with the 108.

I'm the orginal poster and I thank everyone for there replies. I stand by by opening remarks. I test flew the plane at 5.5" and it was extremely tail heavy. After the initial flight I added 12 ounces, which moved the CG forward to 4.1" or about 27% MAC. After this correction it flew great. I also reduced the airleron throw for both upper and lower wings to +/- 1 1/8 inches. The plane is powered by an OS 1.08 engine which is more than needed.

Geez, now I am worried again. One thing is for sure. This plane is not going up until I am convinced where the CG is supposed to be. I hate to add more weight to the already 18 ounces of dead weight. []
flycatch, are you saying you added an additional 12 ounces or that you had no extra weight initially?

I have found the following formula for calculating CG on a biplane. Based on this Great Planes is within 1/16" of an inch of being correct at 5 1/2". Heck, I could off that much just in doing the measurements.

CG needs to be about 25% of the chord back from the leading edge. If you have a biplane then draw a line connecting the leading edge of the wings and make a mark at the half way point, then from that point move aft 25%.

Mr. Scale,

I test flew with 6 ounces of weight installed in the ballast box. The plane acted tail heavy. I increased the ballast weight to twelve ounces and it flew fine. The poster who says I'm wrong with my calculations is using the wrong information. His calculations are based on a biplane with no stagger. The Sterman has positve stagger and you must take this into account. Think of both wings as one assembly. Measure from the leading edge of the top wing to the trailing edge of the bottom wing. This measurement is the chord of both wings combined. Since the chord is constant the math for finding the MAC is simple.
I have a total of twentyseven flights on this airframe since Thursday of this week and it is a pleasure to fly.

Read my full "User Review".

I have a simple method of determining this. I raise the tail wheel until the top wing is level, then hang a string with end weights ( I tie nuts onto the string ends) over both wings. The distance between the strings is the whole chord, and you apply your percentage to that. The typical percentage on these older bipes is anywhere from 25 to 28% back from the front string, and I think I used the 25% on my Waco YMF, which flew beautifully.

I haven't set up my GP Super-Stearman yet, in fact I came in to see what the general consensus is, and there clearly isn't one here in this thread. On to the next thread!

Jim [8D]