ikeisaacks

My first biplane. Where should the CG be measured at on a biplane setup? To make it even more confusing, this is a Beech c17e staggerwing. I measured the orginal setup using the conventional method using the lower wing, but it seems to be top heavy wanting to fall to nose or tail with the slighest breeze. one time it is nose heavy, the next with no weight added or removed tail heavy. Now what?

cody42001

I am building a Bipe, but am inexperienced at many things!
But, a longtime builder, Don Hoag [email protected] told me, that with wings
installed, measure from the top LE to the bottom TE, & consider that your chord!
Then, find 25 to 30% from the leading edge for your CG!
Guess it sounds simple enough--- Good luck!!!
Don Jeune
[email protected]

Matt Kirsch

My Feedback: (21)

A quick tip: Flip the airplane over and balance it upside down when balancing a low-wing airplane, or a biplane on the lower wing. It's too tricky and frustrating to balance a plane with the bulk of the mass above the balance point, like trying to stack a marble on the bottom of an overturned bowl.

cody42001 has got the method for finding the CG on a biplane, but let me clarify a bit: You need to measure from the LE of the forward wing to the TE of the rear wing when looking straight down from the top. Measuring diagonally to get that distance will give you a CG that's too far back if you then use your 25% figure and measure straight back on the forward wing.

Another option for finding the CG of a biplane is to find the CG for each wing separately, then split the difference in the fore-aft distance on the plane itself.

ikeisaacks

So, Matt being a staggerwing, I look straight down and measure from the LE of the bottom wing to the TE of the top wing, then 25-30% back from the bottom wing should be my approximate CG. Is this correct? Both wings are 4.75" wide but with this method it would become 6.75" because of the top wing being located 2" rearward.

Matt Kirsch

My Feedback: (21)

Nice bit of abstract thinking there. Two wings with 4.75" chord and a 2" stagger makes 6.75". That's the stuff.

Now, I don't remember offhand, but are the wings on the Beech swept back or straight? On straight wings, it's a simple 25-30% measurement, but it's a little more involved with swept back wings. 25% will be way nose heavy on a swept-back wing, though you might be able to get away with it for initial flights if the sweep is not very dramatic. Just be prepared to move the CG back.

http://www.airfieldmodels.com/inform...amic_chord.htm

Matt Kirsch

My Feedback: (21)

More:

http://www.modelaircraft.org/templates/ama/0903_15.asp

Campy

My Feedback: (1)

Go to http://homepages.ihug.co.nz/~atong/ and scroll down to MAC. This will tell you how to figure out the CG for both single and multiple wing planes.