Contra Rotating Propellers
on full-size aircraft the usual reason for contra-rotating props is not so much to cancel out torque, its to avoid having a long undercarriage necessary for a single large prop on a high powered engine.They were frequently used on early turbo-prop aircraft, for example the Westland "Wyvern" and Fairey "Gannet" naval aircraft, and the russian "Bear" reconnaisance aircraft. They are less efficient than a single prop, not least because of the frictional losses and extra weight of the necessary gearing.
Another way of avoiding torque problems is "push-pull" power from two engines, example the Dornier 335 heavy fighter.Of course the snag there is to keep the rear prop from striking the ground.
I have flown a few "push-pull" surveillance RPVs and can confirm that they are very nice to fly.
By the way, dont confuse the effects of torque, (usually manifested as a rolling tendency, especially at low forward speeds, or a strong yaw, example during take-off) with the effects of assymetric disc loading, which can result in a strong yaw/roll moment.
To clarify ; when you pull the nose up,(assuming a counter clockwise rotation of the prop) and especially during climb-out from take-off, the right(starboard)) side blade, going downwards, is taking a bigger bite at the air than the port, up going side, so the aircraft tries to turn/roll to the left (port).
If the airspeed is insufficient for the rudder/ailerons to hold the plane straight, the result is sometimes a strong left turn culminating in a roll and dive into the ground. Remedy, back off on the throttle damn quick!
Saw just this happen to a .90 powered Mustang a couple of weeks ago! A black bag job!
Another point is that the spiral airflow coming off the prop will "blow harder" on the left side of the fin than on the right,, also causing a left turn tendency. Which is why we often use some side thrust on the engine mount to counteract the effect.
Here endeth the lecture. Heh heh heh.