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05-09-2010, 10:39 AM | #1 |
RCC Master Contributor
I am: Jeff.S
Join Date: Nov 2007
Location: Beaumont Alberta
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CG of a Biplane
My brother is trying to balance his Seagull 40 size Stearman using the manual suggestion of 3.5 cm on the top wing. With the engine moved on the mounts an additional 3/8" and the 4cell battery outside the firewall the plane is still tail heavy. We think the instructions are wrong because it uses metric and standard units throughout the book. Does anyone know the proper CG for this plane or a method to calculate it? Any info would be good.
Thanks Jeff |
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05-09-2010, 11:04 AM | #2 |
RCC Master Contributor
I am: Chris K.
Join Date: Aug 2006
Location: Guelph, Ontario
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Re: CG of a Biplane
A good rule of thumb or starting point for the CG is 25-30% of the mean cord of the wing. For a biplane the mean cord is the distance from the leading edge of the top wing to the trailing edge of the bottom wing. The best way to measure the mean cord is to set the plane up on a table in its normal flying attitude and then use a square to measure from the leading edge down to the table and then from the trailing edge down to the table, the distance between the two marks on the table is the mean cord. Your CG would then be 25-30% of this distance back from the leading edge of the top wing.
If the wings are tapered the measurement process becomes more difficult which hopefully someone else can comment on. Good luck. |
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05-09-2010, 12:18 PM | #3 |
RCC Contributor
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Re: CG of a Biplane
The balance point must be measured along the "Mean Aerodynamic Chord". The "MAC" must be established first.
See the picture. It illustrates what Cricklewood was saying. The diagram is for a monoplane. For a biplane use the same diagram. For the tip and root chord dimensions use what you see from directly above the assembled wings. Typically for a biplane, at the root, the leading edge of the top wing to the trailing edge of the lower wing. For the tip chord it can vary, but whatever you see. Ed s
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05-09-2010, 09:17 PM | #4 |
RCC Apprentice
I am: Larry M
Join Date: May 2003
Location: Strathmore,AB
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Re: CG of a Biplane
My guess is that your instructions are right... This not a new plane design and was originally built for a very heavy motor. With our new lighter RC motors you may have to put the battery inside the fuse and add a few oz. to the fire wall. We just got done building a Fokker DVII bi Plane it originally had a rotary engine. We put a 68cc gas engine in it and added 5 POUNDS yes pounds to the fire wall to balance it. believe the instruction book & add weight as need.
Good Luck |
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05-09-2010, 10:04 PM | #5 |
RCC Master Contributor
I am: Jeff.S
Join Date: Nov 2007
Location: Beaumont Alberta
Feedback: 6 / 100%
Posts: 1,007
Total Props: 10
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Re: CG of a Biplane
Thanks Gents,
For the info. I think we can figure out the balance now. I already piled this plane in on my brother because it was nose heavy. I need to save face and prove it was not balanced right, because there is no other reason for this plane to reacted the way that it did on take off. (Maiden) Jeff |
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