Counter-rotating props...

[Tinstaafl]

Not quite true to say they cause much less yaw, presuming you're comparing asymmetric conditions. It's not always easy to show much of a difference.

The 'problem' is that the downgoing blade produces more thrust than the upgoing blade at higher airframe AoA (to distinguish from the prop disk's AoA). The increased thrust of the downgoing blade offsets the thrust line of the whole prop disk a bit towards the downgoing blade, instead of the thrust line being in line with the crankshaft.

If both engines rotate the same way then the engine with the downgoing blade closest to the wingtip will have its thrust line a little bit further from the a/c longitudinal axis. If one engine is stopped then the rudder will be needed to counter the yaw caused by the remaining engine. The further offset the thrust line of the running engine then the more rudder will be needed to counter yaw if the other engine is stopped. Another way of saying it is that for the same amount of rudder input more speed will be needed to produce enough force from the rudder to counter the asymmetric yaw. Rudder input is limited (there's only so much it can move) so as speed is reduced there will come a point where you can't use more rudder to stop the yaw - Minimum Control Speed, Vmc - hence one engine is more critical than the other ie more speed will be needed at full rudder deflection if one engine fails than if the other engine fails. The failed engine that leaves you with the worst effect is the 'Critical Engine'. In other words, the engine that when it fails leaves the engine with the most offset thrust line still running.

Counter rotating installations are way of eliminating this effect. Instead of having the downgoing blade outboard on one of the engines, reverse the rotation direction so that the downgoing blade is nearer the fuselage. A mirror image of the (now formerly) critical engine.

Nothing's free. The extra cost of all of this is that many parts now can't be swapped between the two engines. You now have to keep individual spares for each engine/prop. Economies of scale of manufacturing are reduced a bit (producing two different engines instead of twice as many that are exactly the same) etc. Most times the benefit isn't worth the cost & hassle. Keep the speed above the worst case Vmc & you're covered in both cases.


Interestingly, I seem to recall at least one a/c type opted for the reverse case. The P38 Lighting had both engines with a downgoing blade outboard instead of inboard. This means you're in the 'worst case' scenario re Vmc if either engine fails. It was done to solve another problem (oil cooling? Tailplane vibration? Not sure) at the 'expense' of having a slightly higher Vmc than would be possible with 'conventional' counter rotating props.