Mart,
i would be interested to know how frequent rotor strikes are with two blade intermeshing designs.
Objection:
The risk of blade to blade contact.
Information:
re: Flettner
50 pilots were being trained to fly the Flettner FL-282. This included blind and bad weather flying, to which some accidents were attributed. One fatal accident on Nov 10, 1944 occurred when blind flying in a cloud and it is assumed that the pilot dived his machine then pulled back so violently on the controls that the rotor blades were forced into each other or the tail. Subsequently the diving speed was limited to 109 m.p.h.
"All attempts to get blade interference in flight have been unsuccessful." from US evaluation of Flettner FL-282.
[Source ~ ETF p.25]
re:
Kellett
The initial flights of the XR-8 showed that there was insufficient directional control. To solve this, the rotors were redesigned to give opposed longitudinal cyclic. The two rotors have three blades each and this additional flapping caused the blades of opposing rotors to occasional make contact with each other. They then produced and tested 2-blade rotors, like the Flettner, but this resulted in severe vibration. A rigid rotor system was then proposed but proved untenable because of the significant reengineering required and the unavailability of composite construction in those days.
The XR-10 was then build incorporating the lessons learned from the XR-8 but there was blade contact during an autorotation. On a subsequent test, the control system failed, killing the test pilot.