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Old 08-26-2005, 12:08 PM
Sky Sharkster
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Default C.G., Spar Location

Hi Mike, welcome to E-Flying! I'll try to help you out with the questions...the fact that the C.G. and spar locations are near each other (most planes) is more coincidence than anything else. Most model designers put the main spar at or near the thickest (deepest) part of the airfoil. Most room, easy to see and cut slots. On typical flat-bottom, semi-symmetrical and undercambered 'foils this is around 33% or further rearward which also happens to be a good "guesstimate" location for C.G. on a new design. Problem with using a Free Flight C.G. location for R/C is most FF designs have a lifting stabilizer so C.G. is more rearward. R/C designs usually move C.G. forward to (about) 28 % to remove download forces on stab, we have elevator control and (usually) a non-lifting stab airfoil so the self-stabilizing effect isn't needed, glide duration and stall recovery is more important on FF models. In fact it's a drawback for R/C trainer and sport models-although it IS used (rearward) on 3D and aerobatic models, the reason there is 3D's WANT to remove stability, non-lifting (fully symmetrical wing and stab) so they can hover and spin easily. So I'd put the spar at or near the deepest part of the chord and the C.G. at 28% to start. This C.G. will be very stable, maybe a little nose-heavy for optimum performance and after you trim the model you'll likely move it back for more control response. Remember to measure the ENTIRE wing chord (L.E., T.E., flaps or ailerons) when determining C.G. Also, this is a greatly simplified guide, factors like tail moment arm, stab size, nose length, tail volume coefficent, exact airfoil lift/drag coefficents etc. all play a part. This is just a starting point but should be a safe one. Good Luck! Sky Sharkster
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