A proper CG location in acrobatic models is the first steep for excel. In the first part of this blog we discuss the CG location in sport planes, here is the method for acrobatic models or for final adjustment. We assume that the fuel tank is over the CG, and then the influence of the fuel is null, if not, you should try this with the tank almost empty. Always test all this at a safe altitude in a calm day, and repeat the test until you get consistent results.
Let’s go, at full throttle passing in front of you start to climb at 45° and then roll 180° to make it climb 45° but inverted. Then, release the elevator stick and look, four things can happen:
1) If the model climbs straight in the same line, the CG is ok for 3D but a little fore for F3A
2) If the model climbs drawing a soft curve reducing the climb rate, the CG is ok for pattern but may be a little aft for 3D
3) If the model start to climb at more than the initial 45° angle, it means that the CG is too aft
4) the model start point down quickly, it means that the CG is too fore
Cases 1 and 2 are ok most of the cases and adjustments will be small; cases 3 and 4 are not acceptable for most of the acrobatic pilots. Anyway, for final adjustments, the CG should be located in the place that the pilot feels it more comfortable for his style.
Every change in the CG must be small and documented. Be aware that, at after some point, a little further change will have a huge effect in the model’s stability, so be prudent.