As I understand it, blades from a propeller have their own tip vortices, because the blades produce lift via the same aerodynamic principles as wings. So they must have tip vortexes too.

So with contra-rotating props, e.g. the NK-12, do the tip vortices interfere with each other in a good or in a bad way? Is their nature somehow better than those from single props?


1 Answer 1


Counter-rotating props are designed such that the second rotor should remove the swirl component of velocity that results from the parasite drag on the blades. This improves the efficiency.

Think of the counter-rotating blades as a single actuator disk (imagine the two rows being very close together) -- and then compare the counter-rotating actuator disk to an actuator disk made up of single-rotating propeller.

If the two disks have the same thrust and same diameter and same load distribution, then we would expect them to have the same induced power and the same strength tip vortices.

The viscous (parasite) power differences will depend on the number of blades, their chords, and several other details -- we'll assume it is the same.

So, if the counter-rotating disk can recover the swirl energy, it will be a more efficient system with the same tip vortex strengths.

That said, in a finite-blade system, the second row of blades certainly intercepts the wakes from the first row. This causes a great deal of noise and is one of the primary reasons you do not see counter-rotating props widely adopted. It is also the reason modern counter-rotating props typically have a different prime number of blades on each row -- 5,7 or 11,13, etc.


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