Lu is on one side of the 'argument' and Frank Robinson is on the other. As I understand it, Lu has a background in aircraft reliability and Frank Robinson is the principal of the Robinson Helicopter Company. For them to have divergent positions on the subject of the Robinson rotor does not seem to be totally illogical.
Lu appears to have an intuitive feeling that the R-22 rotor may not be perfect, and he has expressed it on numerous occasions. Frank Robinson has placed one
post on PPRuNe where he presents an explaination about
"a highly technical subject".
Between these two extremes are the participants of this forum. I would humbly suggest that none of the regular participants are fully conversant with the difficult subject delta3 & phase-lag. Even Nick has said that it took two or three revisions, during the development of the S-76, to get the phase-lag correct. This means that with all the 'scientific' mathematics on delta3 & phase-lag it still took additional work in the form of 'art' to obtain the optimal phase-angle. It should also be noted that the S-76' s tail rotor only has collective control, whereas the R-22 main rotor has collective and cyclic control to consider.
How can any PPRuNe participant take the side of Lu or of Frank Robinson?
Either, neither or both may be partially or totally, correct or incorrect.
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The coyote has given a very clear description. He concludes with;
" You are flying the rotor with the sticks and the fuselage with the pedals (and air), and the hazard lies when one asks the other to do too much too quickly."
In support of Frank Robinson;
It is said that large helicopters have greater stability than small one. Since the R-22 is at the light end of certified helicopters, the use of a basic teetering rotor would have resulted in minimal stability. Frank Robinson probably implemented delta3 as a type of stability augmentation system. The objective being similar to that of the early Bell gyrobar and the Hiller paddle.
In support of Lu; (for those who say Lu does not technically back his arguments)
Robinson's patent 4,131,391 has a interesting segment.
The patent says;
"Referring to FIG. 6, a condition is illustrated which the present invention avoids. If it is assumed that line 70 represents the true axis of rotation, destabilizing moments tend to create a condition in which the center of the rotor 71 is displaced, off the true axis of rotation. One destabilizing moment is due to the offset of the rotor thrust vector from the teeter hinge 61 and is equal to T .times. Z. The other destabilizing moment is due to the centrifugal force created by moving the center of gravity of the rotor off the center of rotation by a distance Z.
The condition illustrated in FIG. 6 cannot occur, however, since by the present invention, the stabilizing centrifugal moment is always greater than the destabilizing moments for all conditions. Structurally this is achieved by separate coning hinges 63, 64, for each blade, the coning hinges being offset a sufficient distance such that the stabilizing centrifugal moment (2 .times. CF .times. y) is always greater than the destabilizing moments where y is the displacement between the pitch change axes of the opposed rotor blades."
Will not his stabilizing centrifugal moment be half of what is stated I.e (2 .times. CF .times. half of y)? Might a sudden out-of-plane excitation, which had the same harmonic as the flexible blade,
cause the condition illustrated in FIG. 6 to occur? Would not his second destabilizing concern then attempt to cause lead-lag between the blades, since the coning hinges will be acting as flapping hinges and the blades will want to independently adjust their speeds to compensate for their changes in radius? Could delta3 then exasperate the problem?
I am certainly not suggesting that there IS a problem here. However, if the coning hinges are to do all that he wants them to and definitely not do what is shown in FIG 6, IMHO, the two coning hinges should have been linked together. Linked together so that they can cone in unison and cannot flap independently. This concern has been brought up previously. The last time was the posting and picture in Lu's thread [R-44 Rotorhead. Sorry guys I just couldn't help myself.]
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No one, including Lu, has given cause to believe that the R-22 rotor is flawed, but conversely, can anyone say that this light rotor is without fault?
Dave J.
Edited for clarification.