Robinson: Tech Q's
Once again, I must admit to a major error. The error was in my statements about the 18-degree offset on the rotating swashplate, in relation to the stationary part of the swash plate. The fact that when the blades are disposed over the lateral axis creates an 18-degree difference between the two elements of the swashplate means absolutely nothing and, I am sorry that I took up so much of your time in trying to justify the offset. I tried to tie this into gyroscopic precession and the left tilt of the rotor disc. I also referenced the fact that this difference could cause an increase in pitch possibly resulting in blade stall. On this particular point I was really wrong. In Frank Robinsons’ explanation of this problem he talked about delta effect and that pretty much explained his side of the story.
How I came to this conclusion was by reading and re-reading the rigging procedures. Some of you may have read my posts on this thread where I compared a Bell Rotor system to that of a Robinson rotor system. You may recall I suggested a test on both helicopters where the blades were positioned in a specific way and the cyclic was to be moved in a specific direction. In the case of the Bell the blades would not move but on the Robinson they would move. Then I suggested that the Robinson blades be rotated until the pitch links were above a specific axis and then the test was to be repeated. When the cyclic was moved the blades did not move. I used this point to show that there was an 18-degree offset on the swashplate and if not compensated for the helicopter would move to the left when the cyclic was moved forward. This conclusion was correct but the basis for the conclusion was incorrect.
Prior to going any further I have to assume that most of the participants on this thread all believe in phase angle and 90 degrees of gyroscopic precession. If you don’t agree with this concept you can stop reading this post right now.
Here is the real story. When the Robinson R22 and R44 are rigged for cyclic control ranges the blades are rotated so that the pitch links are directly over the lateral or longitudinal axes of the helicopter. Unlike the Bell where the blades are disposed over either the longitudinal or lateral axes of the helicopter the Robinsons’ blades are offset 18-degrees ahead of the respective axes.
This means that when the cyclic is pushed forward from the neutral center the blades will have reached their maximum pitch input and with gyroscopic precession they will have their maximum response to that input 90-degrees later. That means, that the blades will tip down left of the longitudinal centerline. The ultimate conclusion was correct but the means of reaching it was flawed. Frank Robinson knew that my assumption was correct but, but my reasoning was not, so he weasel-worded his way out of it by stating that the engineering explanation about delta hinge effect was too complex to put into words in the forum.
Let’s hear you comments
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The Cat