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Old 04-04-2017, 09:32 PM
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To amplify some of the previous posts. Typically the CG should be over the wheels when the nose is down 15 degrees from level. This can vary slightly depending on operation on grass or pavement. This does not meant change the balance of the airplane because that depends on flight dynamics, but rather sometimes the location of the landing gear is wrong.

Gear too far forward make the airplane more prone to ground loops, and bouncing on landing. Too far back, difficult to taxi and turn in high cross winds. Also a bit more prone to nose over.

A bit finer point, the so called CG point on plans indicates a balance point for the airframe with the fuselage level. The actual CG is usually above or below that mark, which is why the 15 degree down balance tests for possible problems. One might think gear location is always well worked out, but if the designer always flys from grass, that masks problems that pavement exposes.