To: Grainger and Flight Safety
The solution Grainger proposed in his modification of the drawing looks as if it would work. Whether it would change the mechanical advantage offered by a vertical pitch link I can’t say. Also, I do not know if it would alter the dynamics of the rotor system or not.
However, no matter what is proposed to alleviate the problems caused by the 18-degree offset nothing will change. Frank Robinson designed this rotor system by incorporating cone hinges and in doing so, he established the physical characteristics of the pitch horn in relation to its’ attachment to the rotor blades. The pitch horn cannot cross the cone hinge. The cone hinges were incorporated to minimize the bending loads on the blades and to allow the blades to flap in relation to the teetering head. This minimized the transmittal of flapping loads into the head. There were restrictions made to the flight envelope to further minimize flapping loads.
Graingers solution to rotate the attachment of the pitch link in relation to its’ attachment to the blade was attempted on the Cheyenne AH-46 Attack Helicopter. On the Cheyenne the Phase Angle (note, in the UK phase angle is the lead of the pitch horn in relation to the blade. In the USA phase angle is the precession angle normally indicated as 90-degrees) would change due to blade loading, maneuvering loads and speed. This problem caused the blades to diverge and hit the fuselage. In one case it killed the pilot and in the other it destroyed a very large wind tunnel in California. Lockheed and Parker Bertea worked for over two years and they finally solved the problem by doing what Grainger suggested in his post (rotate the swash plate in relation to the head). It worked but the control system became so complex and it had so many single point failures that the program was cancelled.
In any case the Robinson head will not be changed and the pilots whether they will admit it or not must adjust the cyclic to the right to compensate for the offset.
Whether the offset contributes to the loss of the rotor or rotor incursion is open for review.
Another point to consider is that with the capability to flap the blades will also lead and lag. The rotor head however has no lead/lag hinges so any tendency to lead and lag will be reacted by the cone hinges and the teeter hinge and eventually grounded through the shaft. The blades on the Robinson are not very robust and as such will suffer spanwise bending which will tend to make them very unstable, which could result in divergence.
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The Cat