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Most multi-engine aircraft have engines that rotate the same direction on both sides of the fuselage (e.g. the GEnx rotates clockwise, as viewed from the front, there is no 'mirrored' version).

What would the effect (drag, stability, turning performance) be, if an aircraft were to have oppositely-turning engines on each side of the aircraft (especially if the engines were rotating counter to the wingtip vortices)?

Note that propeller and jet engines may have significantly different answers!

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More of a comment than a complete answer, but the losses caused by swirl are not really on drag, stability, and turning performance. The swirl represents is a loss in total pressure in the axial direction, and so is a loss in shaft power extracted by the power turbine, or thrust provided by the engine if it’s a turbofan. This won’t reduce drag at the same aircraft velocity one iota, but does enable you to go faster if you reduce the swirl, due to the higher power (in a turboshaft) or thrust (in a turbofan). How much effect swirl has, I am not sure. I think it’s pretty small, because some engines have vanes at the turbine exit to remove, and others don’t bother, and instead choose to save the weight of these vanes.

Fairly obviously, extra power or thrust will help the vehicles turning performance, but I think it’s clearer to state this is due to extra power or thrust, rather than imply it’s a direct effect.

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