I thought Brenner would jump in and answer this one, but since he didn't here goes. Yes, the motor does produce a torque that reacts against the airframe, and it produces rpm, in order to transmit power to the props. Power is a function of torque x rpm. The props absorb this power and if we assume 100% efficiency for a second, the product of torque and rpm would be the same at the prop as it is at the motor. Then given a 10 to 1 gear ratio, the props are turning at 1/10 the rpm of the motor and have 10 times the torque. Conclusion, the airframe sees 1/10 the torque it would if it were a direct drive setup like an outrunner with the same prop load at the same rpm . So it isn't eliminated, but is greatly reduced and as Tony said, it is probably not as significant a factor as the spiral slipstream .

Jim O