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Balancing a bipe

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Old 11-15-2012, 10:28 AM
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acdii
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Default Balancing a bipe

We have a bipe that the upper wing is swept back, the lower straight, with the upper being far foward of the lower, and is very short coupled. Where would you balance this kind of plane? Over the landing gear, forward of it? It flew tail heavy. and we found the cowl, which was removed, has some weight in it.
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Old 11-15-2012, 02:56 PM
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Default RE: Balancing a bipe

Droop a string over both wing's as close to the fuse as you can. The string has to reach below the bottom wing both front and rear.  Measure the distance of the string, this will give the total of the the top leading to the bottom trailing, edges. Balance 30% of that measurement from the front of the top wing close to the fuse. That's how I do it anyway.
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Old 11-16-2012, 07:42 AM
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Default RE: Balancing a bipe

Use this calculator to get the recommended CG for both wings, then split the difference.

http://www.geistware.com/rcmodeling/cg_calc.htm
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Old 11-16-2012, 08:01 AM
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Default RE: Balancing a bipe

If the plane has already flown and seems tail heavy, but not uncontrollable it's pretty close, so just add or move weight forward until it flies right. The calculations probably won't get you any closer to "right" than you already are, then you'll just have to start moving weight around anyway. The swept back wings make things more difficult than a simple method will deal with.
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Old 11-16-2012, 08:23 AM
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Default RE: Balancing a bipe

Yep, the upper wing is swept, the lower one straight so the tips almost meet.  He hasnt flown it since putting the cowl back on, so hopefully it is better balanced now with the extra nose weight.
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Old 11-16-2012, 08:28 AM
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Default RE: Balancing a bipe

I have always heard....30% back from the leading edge of the upper wing then draw an imaginary line to 30% back from the leading edge of the lower wing. Where the imaginary line is 1/2 way down the fuselage is your CG point
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Old 11-16-2012, 12:10 PM
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Default RE: Balancing a bipe

Treat the plan view of both wings as if it was one wing, do the normal balance calc or use 25% MAC as you starting point.
Evan, WB #12.
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Old 11-17-2012, 07:10 AM
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Default RE: Balancing a bipe

"I have always heard....30% back from the leading edge of the upper wing then draw an imaginary line to 30% back from the leading edge of the lower wing. Where the imaginary line is 1/2 way down the fuselage is your CG point "

But the upper wing is swept back; where along the sweep would you draw your line?

Les
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Old 11-17-2012, 08:46 PM
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Default RE: Balancing a bipe

That method of considering the wings as one can work if both wings are the same and there's no sweep, but with this plane it could make him really nose heavy. It only takes about 5 minutes to take the measurements and plug them into the calculator above. That will give him the correct CG for each wing, and then he locates it in between the two on the plane. That will be the technically correct CG, which he can then adjust to suit his tastes afterward.
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Old 11-18-2012, 05:41 PM
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Default RE: Balancing a bipe

I fully agree with Jester, except that the location of the total CG also depends on the area of each wing and the type of stagger.

The CG is located at the middle point only if both areas are equal and if the stagger is negative.

If not, the CG will be closer to the CG of the higher and bigger wing.

Note that the point which is determined is the Aerodynamic Center (AC) on the Mean Aerodynamic Chord (MAC) estimated for the two wings working as one (disregarding the effect of the AC of the horizontal tail).

Note that the AC is expected to be located more toward the leading edge of the upper wing, because the lower wing lift is expected to be reduced (about 65%, depending on gap height and stagger) by the washout produced by the upper wing.

http://www.geistware.com/rcmodeling/cg_super_calc.htm
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Old 11-19-2012, 06:03 AM
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Default RE: Balancing a bipe

Your diagram points to right about where the main spar is on the upper wing, which also places it center on the main gear. We had taken a guess as to this being the sweet spot and balanced it at that point, so we shall see if that is correct when it gets flown again. He wants to wait for a calm day when both of us are there in case he needs trim assist like the first time.
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