ORIGINAL: drac1
That's exactly what i was going to say Matt. You are spot on.
While there would be some spiraling of airflow as it comes directly off the prop, by the time it travells down the nose and over the wing (which would act like vanes and straighten the airflow), by the time it reaches the tail i can't see how the airflow would be still spiralling. Just as with contra rotating, how can the airflow be straight? The rear prop would still spiral the airflow albeit in the opposite direction. I think that torque is the biggest cause which is being confused with spiralling airflow. That's why contras don't need right thrust, props turning in the opposite direction cancel out the torque effect not spiralling airflow.
Of course this is just my opinion as i see it.
Actually, there are more than one torque generators in a propeller driven model. One, the actual crankshaft motion. Two, the propeller turning. Props have mass and are turning in the direction of the crankshaft in most cases. These two tork effects are additive. Three, the simple action of pulling in and throwing back air;; air is massive and it requires considerable force to pull in and throw back in a spira manner. That also has tork components that add to the other two plus it has the precession factor that adds to the overall reactive force of rotation.
A contra cancels out a large portion of the tork and P forces attributed to the props. Motor or engine tork is still there.
Relax, crankshaft tork is relatively minor in models. Although the larger IMAC models can take advantage enough of crankshaft/prop tork to do real tork rolls (meaning aileron unassisted) at idle or a hair above. At idle, there is very little reactive tork from pulling in and throwingback air (or precession for that matter). Nowhere near as easy to do one of them Real Tork Rolls with a 2 meter Pattern model or smaller.
BTW- I do believe that the spiralling air coming from the prop is massive enough (meaning, carries force) far enough down the fuses of most models to cause significant trimming issues. I'm not sure if I mentioned it here (too lazy to go back and read my scribbles), but one experiment I have done is to add cannards immediately behind the prop. With the engine typical offset (right thrust), the model went right, following the thrust vector.
I have not reset the offset to zero and try the cannardexperiment again, but it is an experiment begging to be done. If it works as expected, cannards immediately behind the prop of a 2 meter pattern model can be a poor man's contra prop. I will do the experiment but will report in the Pattern column.....