How come there is no attempt at an "inter-meshing rotorprop" high-speed helicopter? There are some prototypes of high-speed helicopters, such as the Eurocopter X³ (X-Cubed), Sykorsky X2, and older ones such as the AH-56 Cheyenne.

At high speeds, forward and backward traveling rotor blades relative to high speed flight path induce asymmetric lift and drag: Forward helicopter flight speed minus backward rotating rotor blade speed may equal zero relative speed, or even a negative value.

So, how come there's no [known] attempt at making an inter-meshing rotorprop helicopter?


Inter-meshing like K-MAX
Rotorprop (pusher) like Sikorsky S-66


It seems to offer a far more symmetric configuration than the examples mentioned above, and as illustrated below:



Below an illustration of assymetry at some positive AoA (view is normal to relative wind):
If labelled 1 to 3 top to bottom, numbers 2 and 3 in the illustration are identical.

A pair of rotors that are counter-rotating. One set is co-axial, the other set is slightly separated. Aerodynamically (dissymmetry of lift problem), I don't see a difference.

In plan view it looks like this:

(Source)



My guess as to why it was not tried, is because the transmission for concentric shafts is simpler, less bulky, and lighter, compared to that of intermeshing rotors (shown below).

Also, in intermeshing rotors the lift is not completely vertical, which reduces the efficiency (source). High-speed aircraft need to be efficient.

(Source)