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Old 06-05-2007, 02:40 AM
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majortom-RCU
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Default RE: GSP Sukhoi very tail heavy. HELP!!!

I begin to think we may not be talking about the same plane here. The GSP/China Model Products Sukhoi I have is an ARF with molded fiberglass fuselage, two-piece wing and aluminum wingtube, such that the two wing halves are mounted to the fuselage for each outing, then removed for the trip back home. This wing is 80" span as mounted to the plane. It sounds like you are talking about a one-piece wing, probably smaller than 80" span, two halves glued together as part of initial assembly, then mounted into a big slot in the fuselage for flying, and removed in one piece for the trip home.

In any case, CG specifications are usually given as distance back from the leading edge as measured with the wing attached to the fuselage, and from the intersection of the wing LE with the side of the fuselage--not from the front of the notch that nestles between the sides of the fuselage.

The best way to assure yourself of having the right measurement is to plot your own Mean Aerodynamic Chord. There are several webpages devoted to this procedure, each with their own way of putting the basic concept. One good one is at [link]http://www.palosrc.com/instructors/cg.htm[/link]. Another is at [link]http://www.nsrca.org/technical/tip_tricks/mac_cg.htm[/link] . The general practice for aerobatic models is to start with CG at 33% of MAC. Some say 30%, some say 35%. There is no 'right' answer, since we are only looking for a close approximation to do our maiden and trim flights at, after which we will move things forward or aft or add ballast to get the plane trimmed to our personal taste and flying style.

After you've calculated or plotted the 33% point, you can verify that you are on target by noting that this point will be very close to either the wingtube if you have a two-piece wing, or to the main spar of a one-piece wing. If you can't see the main spar directly, it will be at the thickest part of the wing as measured vertically. It will also be just forward of the point where the balsa sheeting ends and the film-covered rib bays begin. After somewhere over a hundred kits & ARFs, I've yet to see any model that did not conform to this placement of the main wing spar.