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Biplane Setup Question

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Biplane Setup Question

When building a biplane, if you change the top wing from straight to swept, would that move your CG forward or back? The MAC on a swept wing is back farther than on a straight wing, so wouldn't that mean that the tip of the swept wing would be farther forward than the center of the straight wing, but thte actual CG location would be the same?

Thermals---I need thermals!
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Biplane Setup Question

I used to know this stuff, I believe it has to do with tip stall characteristics. I'll try to find my aerodynamics book and see if I can sort it out. This is an interesting question.
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Biplane Setup Question

"One thing that makes a bipe fly good is 2 separate incidence angles on the wings… the top wing should be 1-2 deg more angle of attack, and set further forward on the plane….This allows it to stall clean, but the lower wing to continue providing lift". "When you figure CG, take the CG location of the top wing, and the CG location of the bottom wing and then balance the plane at 25-30% between those two spots". There's more to it, I'll try to summarize it.
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Biplane Setup Question

What brought this up was a question from Mike Mclain who has an Aeromaster kit that can be built with either a straight or swept top wing. Their suggestion for the CG on the plans sounded a little sketchy to me. I have a bunch of stuff for figuring out the CG on a biplane, but I'd expect that the fore and aft location of the upper wing is probably still constrained by some aerodynamic rules. Maybe it is decided by the fact that the tip of the upper wing would be directly above the lower wing rather than forward of it. That could put the MAC potentially at about the same location as if the upper wing was straight but positioned ahead of the lower wing. It seems that the forward most point on the upper wing would then be forward of where the leading edge of the straight wing would have been. Making any sense at all?

Thermals---I need thermals!
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Biplane Setup Question

Here's Pete Bowers explanation for the swept wings on his Flybaby bipe: Another reason for using a separate set of biplane wings instead of merely adding one is the problem of pilot access. Since, in the monopla ne, the pilot sits right on the center of gravity, adding another wing would mean that the new wing would have to be directly over the existing straight wing to maintain the sam e center of gravity and center of lift. However, a wing right over the cockpit would make it nearly impossible for the pilot to get in unless a door were cut in the side of the fuselage. Since this is undesirable, it was necessary to move the center se ction of the upper wing ahead of the cockpit. This puts a lot of lifting area ahead of t he existing center of gravity and makes the airplane tail heavy. The solution to thi s problem was to sweep the wing back to move the center of lift aft. Since the necessar y sweepback for the upper wing alone would be too great, BOTH wings were swept.
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Biplane Setup Question

Ah, but both are not always swept. How far aft the center of lift ends up would also depend on the degrees of sweep. Pete has a really good point about cockpit access. I dunno why I never really thought about that. I just found an article by Gerry Yarish and another by Andy Lennon talking about finding the CG on biplanes with only the top wing swept. I'm too tired to digest it right now tho.

Thermals---I need thermals!
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Biplane Setup Question

Don't you lay the outline of one wing over the other and use the combined area to find the MAC? Then you'd use the percentage of that to find the CG location. Right? So wouldn't the actual CG relative to the fuselage remain the same?
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Biplane Setup Question

i could put mine together as it flys fine u can ck the cg on it

400 ' is high enough to get were you are going
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Biplane Setup Question

Your what? Swept upper wing or straight? This thread has raised an interesting question that as modelers someone else has usually figured out for us.
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Biplane Setup Question

my aeromaster///swept

400 ' is high enough to get were you are going
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Biplane Setup Question

All we need is another one with straight wings to compare with it.

Thermals---I need thermals!
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