Hi guys, I'm doing a scratch build Bi-plane, now I know why they call it "scratch" build.

One problem with Bipes that I'm having is their wing arrangement (difference in incidence between both wings, positioning on the fuselage, struts, flying wires etc.) . I've seen some plans, and all of them are different in their own ways. Currently I am planning to place the top wing further towards the nose than the bottom wing.

I currently plan to put the top wing in front of the bottom wing. What do you think?

csisfun, yes that is the normal arrangement, called positive stagger. If the upper wing were aft of the lower wing, it is called negative stagger. Depending a lot on other factors in the design, I have found that most model bipes fly best with the upper wing (with positive stagger) incidence at about 1.5 degrees less angle of incidence than the lower. This is not always true as there are other factors involved but; on the Phaetons, Lazy Aces, Aeromasters that I have flown, all flew best with that arrangement. Now, if you were wanting highest efficiency and least drag at cruise, you might find quite a different arrangement gives better results.

Thank you sir, for that tip.

Now I've asked around, and it seems that the CG for a bi-plane is NOT the highest point of the top wing contrary to most monoplane designs. Is there anyway to calculate the CG for flying? Or else where would a starting point be. My scratch built has a positive stagger and same amount of incidence.

This has always worked for me.

Hmm... don't quite get it.

What is this picture trying to say? Sorry, can't make anything out of it. Is "AC" supposed to be CG?

"AC" is "Aerodynamic Center".

An easy way to pick a starting point (if the wings are of the same size) is to look at the projection of the surfaces looking straight down from above and set the CG at 25 % of the projected area. With positive stager this would be 25 % of horizontal component of the distance from the leading edge of the upper wing to the trailing edge of the lower wing measured from the leading edge of the upper wing.

AC does equal CG.

Les