From my brief Wikipedying, if counter-rotating props' tops go towards each other, each has a reduced P-factor so the loss of either engine won't yaw the aircraft as hard as if the bottoms come together.

As I don't really care about safety of my aircraft's occupants (usually a couple ants after it sits on the grass) or reliability, I'd prefer to increase the P-factor for fun and flat spin potential. Out of curiosity, does the direction of counter-rotating props (rather, counter- or similar- in a twin) affect the efficiency?

  • $\begingroup$ If your aim is to increase the P-factor, why not try a twin whose props both spin the same way, such as a Dragon Rapide. But please don't try to spin a historic aircraft if you don't know what you're doing. $\endgroup$ – Dan Hulme Sep 6 '15 at 22:08
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    $\begingroup$ This is a model aircraft (I suppose my "ants" comment was a bit thick) $\endgroup$ – Nick T Sep 6 '15 at 23:29
  • $\begingroup$ You might find reading about the P-38 Lightning or F-82 Twin Mustang interesting. If I'm not mistaken both of these airplanes had counter-rotating propellors such that the p-factor was least favorable on both sides. I believe this was done to make the unfavorable characteristics from slipstream less and to reduce stall speed with both engines operative. In any case, this setup is definitely less stable and would probably give you some great spins in a model. Then again, with a model the p-factor difference is probably immeasurable. $\endgroup$ – ryan1618 Sep 7 '15 at 1:00

It might. Depending on how close to the wingtips your props are.

If you put the props right at the wingtips, then the conventional rotation to minimize precession and torque actually decreases efficiency by promoting tip vortex (the direction of the prop is the same as tip vortex thereby adding to tip vortex). This reduces lift for the same amount of airspeed.

On the other hand, rotating it the other way round - away from each other - reduces tip vortex by twisting the slipstream the opposite direction as the tip vortex. This increases lift (technically, it reduces the reduction of lift due to tip vortex) for the same amount of airspeed.

One plane that exploited this was the Vought XF5U - the Flying Pancake. The direction of prop rotation is fairly obvious in this picture:

Vought XF5U

The interesting result of this was that even though the XF5U had very short wingspan it could do short take-offs.

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  • $\begingroup$ Since you're flying model aircraft I thought you'd get a kick of this video of prototypes of the XF5U including an RC version doing a hover long before 3D bacame a thing: youtube.com/watch?v=LfpTDOAfj7Y $\endgroup$ – slebetman Sep 7 '15 at 15:29

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