I don't think either 2 strokes or 4 strokes are necessarily better for this application. I think people forget that torque is just a function of power and rotational velocity (rpm). Since a heli rotor is typically a constant rpm system, we can find the power needed to drive the rotor at this rpm at max pitch and then take this power as a constant in our drive system design. We can then calculate a gear reduction ratio that will give us the right torque from any engine that can output the peak power we need.


Torque, power and rpm are described by:

Power = Torque x rpm


So as an example, say it takes 1.6 hp to turn your heli rotor at 1600 rpm. We can take these values as constants and then work out the other values.


We know from published data that a certain .46 2 stroke makes 1.6hp. This is the right amount of power so we will use this engine. We also know that it makes this power at 16000 rpm. So we can calculate that we need a 10:1 gear reduction. Obviously the torque at the rotor head is 10 times that at the motor output shaft, but the power is constant so everything works fine.

Now, we find a 4 stroke to do the job. A .91 will fit the bill, but it makes 1.6hp at, say, 9000 rpm. Now the math says we need 5.625:1 gear reduction.

Note that the peak torque values for these 2 engines never entered into the equation.

Now how to decide which motor to use? That depends on what your goal is - highest power/weight, greatest fuel efficiency, lowest noise, best reliability, mechanical simplicity etc...