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CG of a biwing

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CG of a biwing

Old 05-10-2005, 04:54 PM
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Rick M
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Default CG of a biwing

I have a old biwing that I am trying to ballance. and I do not know how to find the measuring point to ballance the plane at.
The top wing is 65" long and 12" wide and the bottom wing is 62" long and 10" wide with the back of the wings set equal in the fussaloge.
Any help would be great.
Thanks Rick
Old 05-10-2005, 10:36 PM
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tonystro
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Default RE: CG of a biwing

Rick M,

If memory serves me correctly a rough estimate of range for CofG is 25-33% of the wing chord on a monowing, and on a bipe would be half way between the calculated points on each wing. If that is valid, then I get:

25% of 12 inches (upper wing chord) is 3.0 ......... 25% of 10 inches (bottom wing) is 2.5
33% of 12 = 3.6 .............................................. 33% of 10 = 3.3

With the bottom wing leading edge 2 inches behind the upper wing's, then bottom wing points would be directly below points 4.5 and 5.3 inches from the leading edge of the upper wing.

So, theoretical CofG range would be on upper wing, 1/2 the difference between;

3.0 and 4.5 = 3.75 inches for 25%
3.6 and 5.3 = 4.4 inches for 33%

If I am wrong I hope someone more knowledgeable corrects me soon. I have two bipes nearing final assembly without plans or instructions for either, and I was going to use this method for intial CofG.[]

<edited for spelling>
Old 05-11-2005, 08:26 AM
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Rick M
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Default RE: CG of a biwing

Tony
Thanks for getting back to me. I am alittle lost, would I measure the back from the leading edge of the upper wing or the lower wing. I have measured back 4" from the leading of the top wing, and I would need to add about 2pounds of weight to ballance. that seems way to much weight the plane allready weights about 15 pounds.
thanks again
Rick
Old 05-11-2005, 09:21 AM
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LesUyeda
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Default RE: CG of a biwing

From Andy Lennon. This has always worked for me.

Les
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Old 05-12-2005, 10:31 PM
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tonystro
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Default RE: CG of a biwing

Les,

Deepest thanks for your posting of the diagram!! That's the one I had in mind, but couldn't remember where I put my copy. Since it shows 20% of chord, I obviously was in error in thinking it called for 25%.

Old 05-12-2005, 11:03 PM
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tonystro
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Default RE: CG of a biwing

I am alittle lost, would I measure the back from the leading edge of the upper wing or the lower wing. I have measured back 4" from the leading of the top wing, and I would need to add about 2pounds of weight to ballance. that seems way to much weight the plane allready weights about 15 pounds.
Rick,

I hope that the diagram Les posted helps your understanding. In my original rambling I wasn't clear in stating that I beleived a '25% of chord' would be a good starting point. That works out to 3.75 inches back of the leading edge, on the upper wing. The diagram Les posted recommends 20% of chord, which works out to approximately 3.2 to 3.25 inches back of upper wing leading edge. Based on your efforts so far, using either of these CG points would increase the amount of nose weight required.[]

The 4 inches aft of upper leading edge would work out to roughly 30% of chord. This might be OK, but I wouldn't know and I would hate to see you be tail heavy, with the resulting sensitive elevator response!

Two pounds does sound a little excessive, but perhaps not. As a comparison, my Midwest Super Stearman weighs 12.85 pounds, without fuel. It calls for "up to 1.20 4-stroke" and I have a Saito 1.80, as far forward as the mount allowed. I have a 1800 ma receiver battery (7.5 oz.) pushed up against the firewall, under the fuel tank, and I had to bolt 13.5 ounces of lead on to the motor mount, beside the engine, to get proper balance.

If you could identify the model you have, perhaps someone with experience with that model could better advise you.

Old 05-13-2005, 11:14 AM
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britbrat
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Default RE: CG of a biwing

Unless I'm missing something, the diagram seems to illustrate a typical location of the AC (aerodynamic center) for a bipe, not the C-of-G. Am I getting this wrong?
Old 05-13-2005, 04:34 PM
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Default RE: CG of a biwing

britbrat,

After taking a long, second look at the diagram I believe you are correct!

Old 05-13-2005, 04:38 PM
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kennyandannie
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Default RE: CG of a biwing

This is what I have used on my bipes, even my Midwest Super Stearmen, which if you used the CG on the plan, you had an unflyable airplane.

Set the plane close to level.
Plumb a line vertical from the leading edge of the upper wing, make a mark.
Plumb a line vertical from the trailing edge of the lower wing, make a mark.
Measure the distance between the two marks, and use 25 to 30% of that measurement, measured from the leading edge of the upper wing.

This only works with straight wing bipes.

Ken
Old 05-13-2005, 06:12 PM
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britbrat
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Default RE: CG of a biwing

That seems about right, & 25% is probably a safer bet than 30%
Old 05-14-2005, 09:21 AM
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LesUyeda
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Default RE: CG of a biwing

According to Lennon's article in Model Airplane News (from which the diagram was taken), CG=AC is the ideal location. CG foreward of that (nose heavy) makes a more docile craft, CG aft of that makes more aerobatic; just as in single wing aircraft.

Les
Old 05-16-2005, 11:21 AM
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britbrat
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Default RE: CG of a biwing


ORIGINAL: LesUyeda

According to Lennon's article in Model Airplane News (from which the diagram was taken), CG=AC is the ideal location. CG foreward of that (nose heavy) makes a more docile craft, CG aft of that makes more aerobatic; just as in single wing aircraft.

Les

I have some problems with that. The C-of-G should always be ahead of the center of pressure. Having the C-of-G & AC coincide would result in an unstable, & probably unflyable airplane (unless you have some sophisticated software flying the plane)
Old 05-17-2005, 09:11 AM
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LesUyeda
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Default RE: CG of a biwing

OK

Les

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