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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?

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

enter image description here

Below an illustration of assymetry at some positive AoA (view is normal to relative wind):

positive AoA Kamov / Kmax

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  • $\begingroup$ Beyond the obvious (Kaman is likely not as rich in R&D funding as the others) what about the intermeshing rotor configuration in Kmax makes you belief that it's a better fit for high speed flight? Your diagram only tells part of the story. $\endgroup$
    – KorvinStarmast
    Commented May 2, 2017 at 17:43
  • $\begingroup$ It really doesn't matter the two rotors are coax or inter-meshing if rotor is not rigid: they are both as not suitable for high speed as conventional. $\endgroup$
    – user3528438
    Commented May 2, 2017 at 23:22
  • $\begingroup$ @user3528438 I was thinking about rigid rotors, with no dihedral/anhedral angle at all. $\endgroup$
    – user21228
    Commented May 4, 2017 at 15:02

1 Answer 1

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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:

enter image description here
(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.

enter image description here
(Source)

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