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Center of Gravity

Old 03-29-2007, 04:44 PM
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Default Center of Gravity

As I look at my plans of my different kits I see where the C of G should be. I take it this is checked after the model is finished, but with the engine or without? The reason I ask is this.

When I was working at my local hobby shop back in 1981 this guy came in and gave me all his control line stuff as he was getting out of the hobby. The Banshee was part of his stuff. I never flew it. Always had it hanging around. Well yesterday I was looking it over real good and noticed that he had glued weights on the fuselage right under the stabilizer. one on each side. Hmmm. Checked C of G with engine, balanced perfect. Took engine off then checked C of G again and the tail slams to the floor.

The Banshee does look to have a very long nose.

Any thoughts on this subject?

Thanks
Old 03-29-2007, 05:38 PM
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Default RE: Center of Gravity

Always balance with the engine, fuel tank empty!

Think about it ... are you gonna have the engine on it when you fly it? The rule of thumb is, nose heavy planes fly poorly, tail heavy planes fly once. That's why you balance with the tank empty, so as the fuel is used up the plane doesn't become unflyable.

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Old 03-30-2007, 06:45 PM
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Default RE: Center of Gravity

Ukie,

Depends a bit on who built and trimmed the model...

A highly polished stunt flier might have it balanced a bit aft for sharper response. That can be a bit too touchy for starters...

Also, is all the equipment the same as when the model was trimmed out? A lighter or heavier engine can make a noticeable change.

Do you know where the plans show the intended CG? For starters, it should be safe to balance it up to a half inch forward of there to restrain the responsiveness until you're ready to use it. Banshee is a good flying design, and you should enjoy it.



Old 03-31-2007, 05:42 PM
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Default RE: Center of Gravity

As a general rule for older designs, 15% mean aerodynamic chord is a good starting point. On your Banshee measure the total chord, flaps and all, halfway out the wing, and figure 15% back from the leading edge. I suspect that is forward of where the plans show. A Twister balanced thusly flies very nicely. I like to say, "A nose-heavy airplane can be retrimmed; but you may not be able to repair a tail-heavy airplane."
Old 04-01-2007, 07:03 AM
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Default RE: Center of Gravity

Almost all my plans show CofG right along or just ahead of the wing spar, with no engine or tank. I never used to pay attention to this stuff. Just built them and flew them. Maybe that explains why some of my builds didn't fly that great compared to others. I will check balance now after installing engine and with dry tank. Thanks.

IMO if the weight distribution correction could be made during building, would be better than having to add weight after. Just doesn't make sense to try and build a model light, then add heavy weights after.
Old 04-01-2007, 07:35 AM
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Default RE: Center of Gravity

You are correct, to a certain degree. The thing is, think of the airplane as a teeter-totter. The heavy engine is on a short arm, balanced by a longer, lighter tail. If you build heavy, it's like putting heavy weights up and down both fulcrum arms. The plane gets heavy, but it doesn't necessarily balance any better. You might still need some weight to balance the engine. If you build light, but need a little weight at the back to balance the weight of the muffler, or a larger engine, the plane will still be lighter, and fly better, than if built heavy.

What we always try to avoid, if possible, is a plane that's heavy in the tail needing nose weight to balance. Because, with a shorter moment arm, it takes more weight at the nose to balance a heavy tail. I'd much rather have to put an ounce of lead in the tail of a plane than eight ounces on the nose!
Old 04-01-2007, 05:15 PM
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Default RE: Center of Gravity

The Banshee as a design was built around an Unmuffled, light Fox 35 and built with light balsa in the right areas. Any modern engine or engine with a muffler screws that all up. The plans to a new kit Banshee tell you to cut 1.5 inches off the nose, I'd go a little further but an OS 25 would probably fly the plane, if it was built out of light wood. SIG generally doesn't put that in the kits I get so an OS40 is usually what I end up with taking 2 inches off the nose and adding 3 inches behind the wing. You could also just glue the wing cutout back in, ifyou have it, and cut it out again 2 inches farther forward. But you'll probably still have to add weight somewhere.

Looks good with that long nose, hard to achieve from the kit form. Oh, and you'll need a good running Fox 35 too.

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