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Old 02-25-2017, 07:59 PM
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BMatthews
 
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There's no error. As you make the fuselage longer the overall stability neutral point moves back. Make the fuselage excessively long enough and the NP ends up behind the wing. And the CG set for some degree of stability follows the neutral point.

Back in the 1950's there was a movement among free flight power flyers to make the fuselage longer and longer and the stabilizers larger and larger. This led to models such as the Civvy Boy where the CG was right on the trailing edge of the wing. And one particular extra long .049 competition power model had the CG indicated at 110% back from the leading edge. That's right, it was 10% behind the trailing edge.

What happens is that as the CG moves to 30% and further back is that the design starts to behave more like a tandem wing airplane. That is the stabilizer lifts upward same as the wing. The further back the more the stabilizer lifts.

What the CG calculator does not tell you is that for a smaller size stabilizer you can over load the lifting area and it will have too high a "wing loading" and not fly well at all. The stab ends up "stalling" when loaded too strongly unless it's a larger size stabilizer like those old free flight models had with their 25 and 30% size stabs. Bad and confusing things occur at about that time.