If the 10x6 is just clearing the grass, an 11x5 maybe slightly more efficient, but I doubt that it will solve the problem.
I'd say that either the prop or the engine is not the right size for the model. An engine has a "preferred" RPM range, and generally develops the maximum
power within the range.
The basic rule of thumb is to first weigh the model.
Then determine the required power.
There are a lot of different ways of doing this.
I prefer the watts/LB method
75-100 watts / Lb for typical trainers, and so forth. "Gentle" flyers a bit less, maybe 50-75W/Lb
100-150 (I prefer this range) for mild to moderate aerobatics.
150+ aggressive aerobatics up to 3D
746 W = 1HP
A "good" .60 2cy will develop ~1400-1700W or around 2 HP.
An example of what a favorable power/weight ratio can do.
P-51 "40" Hanger 9 Mustang ARF, electric power, retracts, all up weight 10-11 LB (Don't believe the 7.5Lb spec)
Motor Eflite Power 52, 1650W maximum continuous. 12x8x3 prop (grass clearance, etc.)
~HP 2.2 at full throttle
Max speed (GPS 88mph)
Ground run is determined by thrust side effects and tail wheel configuration.
Slowly increase power with elevator up. Once the rudder, elevator, and ailerons are effective, reduce up elevator and somewhat gently,
Add more power and up elevator as appropriate. Too much power too soon will result in a ground loop or nose over. (Much like the full scale A/C)
After it's cleared the ground, rotate and apply full throttle (if not already applied) for vertical climb out.
Electric motors do have a major difference in comparison to engines. An electric motor develops maximum torque
at a much lower RPM than the typical engine. A three blade prop can take advantage of this at the expense of efficiency at higher speed.
Last edited by chuckk2; 12-20-2013 at 10:08 PM.