----[Please read http://ercoupers.com/disclaimer.htm before following any advice in this forum.]---- > if an Ercoupe prop is 3 feet from the datum, the extra 2 pounds > x 3 feet = a moment of 6 foot pounds. > > This means that the new prop is exerting 6 foot pounds of rotational > force on the center of gravity,
Just to keep you all honest, the CG and datum do not have to be, infact generally are not the same. The datum is some convenient point from which to measure. In fact, some "modern" aircraft have the datum in front of the forward most point of the plane, so all moment calculations are positive. Planes like ours typically used the leading edge of the wing, the firewall, or some other convenient point. (Ercoupes use the firewall bulkhead as the Datum, but the CG is around 28 inches behind the datum). Your analysis about adding weight for and aft of the CG is correct, though. So adding 2lbs to the prop may create a moment of -24 inch lbs, and adding 2 lbs in the tail cone may have a moment of +480 inch lbs. Going from a wood to a metal prop usually adds about 10-12 lbs. The (real) CG is usually somewhere near the center of the cord of the wing, so 2 lbs in the tail is probably about right. Your moments may vary John __________________________________________________ To unsubscribe from this list please send mailto:[EMAIL PROTECTED] ___________________________________________________________ T O P I C A The Email You Want. http://www.topica.com/t/16 Newsletters, Tips and Discussions on Your Favorite Topics
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