To: heedm
Granted, the blades do fly to their assigned disc position due to the introduction of cyclic pitch, which substantiates the theories proposed by many individuals on this forum. Many of these individuals discount the theory of gyroscopic precession being involved in any displacement of the blades / disc with the introduction of cyclic input. If gyroscopic precession plays no part in the tilting of the disc either as a single entity or as individual blades then please explain this.
Lets assume that the optimum phase angle is 90-degrees although with what Nick indicated this is not always true. However for the purposes of this explanation we will accept 90-degrees as the phase angle for the helicopter systems being discussed. When Sikorsky helicopters were designed it was assumed that the phase angle was 90-degrees and to accommodate pitch input and proper pitch response they offset the servos by 45-degrees and the pitch horn led the blades by 45-degrees. When the pilot pushed forward cyclic the fore and aft servo moved downward. Assuming that the advancing blade was disposed over the right side of the helicopter it would be at the minimum pitch position. The opposite blade would be at the highest pitch position. 90-degrees later the blade on the right side would be at the low flap position and the blade that was over the left side would be at the high flap position. However as the advancing blade approached the longitudinal axis it would already be increasing in pitch. The opposite would be true for the blade passing over the longitudinal axis over the tail.
A similar situation exists on the Bell rotor but in the Bell the pitch horn leads by 90-degrees and the cyclic pitch activity would be the same as that on the Sikorsky. All other helicopters are similar in that the pitch change is maximum 90-degrees prior to where the maximum result will take place. The only two helicopters that I know of that do not have a 90-degree offset in the controls are the Lynx and the Robinson. Now, maybe Nick might be aware of others that don’t have a 90-degree phase angle at least in the way the controls are set up. One that Nick mentioned was the S-76 and the reason the controls were not set up in the normal Sikorsky design concept was depending on who tells the story due to an engineering screw-up. Sikorsky had to do a lot of test flying to design compensation into the mixing unit in order to counter the lack of a 90-degree offset in the control system.
In the light of the above I ask why the 90-degrees if gyroscopic precession is not involved. Why couldn’t they just put the control inputs anywhere and just let the blades seek their own position.