To: Anyone that will listen
The design requirements of the FAA dictate that the blades shall respond in the same sense as cyclic movement. It further states that even with the best design you are allowed to have several degrees of cross coupling. I would assume that this would be due to control geometry or aerodynamic influences (phase angle shift).
All helicopters have a design phase angle. This angle is 90-degrees. There is one exception and that according to Nick Lappos is the S-76 and it is not 90-degrees. You will have to get him to explain why. Also since I have no information on the BK series I can’t address the phase angle on those helicopters so we are stuck with Bell, Aerospatial and Sikorsky.
Now my definitions may be wrong but you will have to accept them as everything is predicated on these explanations.
Phase angle: The designed in control geometry that results in an angle of 90-degrees
Phase angle shift: The deviation from the 90-degree phase angle resulting from outside influences such as blade loading, air density, speed and any one of several other outside influences. This shift will result in the decrease of the 90-degrees to some figure less than 90-degrees.
Sikorsky helicopters have a 90-degree phase angle. The servos lead the direction of cyclic input by 45-degrees and the pitch horn leads the blade by 45-degrees. 45 + 45 = 90-degrees.
On two blade Bell helicopters the pitch horn leads or follows the blade by 90-degrees and the swashplate is displaced 90-degrees ahead of the blade.
On four blade Bells the pitch horn leads or trails the blade by 45-degrees and the swashplate displacement is 45-degrees. 45 + 45 = 90-degrees
On Aerospatial helicopters the pitch horn leads the blades by 60-degrees and the swashplate displacement is 30-degrees. 60 + 30 = 90-degrees. This may vary from model to model but the resultant is still 90-degrees
The Apache is much like the Sikorsky system.
The blades follow the swashplate movement in all cases and blade movement is as a result of Gyrodynamic Precession or Aeroscopic precession.
The Cheyenne was designed like the Sikorsky (45 + 45 =90-degrees) but the blades due to their aerodynamic shape and their stiffness never worked as designed. They got severe phase angle shift and the disc would not only move in a direction not commanded but they would diverge from the tip path plane and in certain cases would result in blade incursion. It took several years of design to get the blades to behave. This resulted in a sensing system being utilized to detect uncommanded movement and input was made to the main servo to make the blade move in the commanded position of the cyclic.
A similar system was incorporated on the Lynx. The Lynx is somewhat like the Robinson in that when rigging the helicopter the blade pitch is set with the blade 15-degrees ahead of the lateral axis. Without the protection of the black box the blades would dip down 15-degrees past the longitudinal axis when the cyclic was moved forward and because of the inherent stiffness in the rotor system this movement would be immediate.
Now, we get to the Robinson. Like the Lynx the blades have an offset but unlike the Lynx the Robinson is not effected like the Lynx. The Lynx has a 15-degree offset but a 90-degree phase angle. The black box adjusts the servo input so that when the cyclic is moved forward the servo input is adjusted to give 15-degree compensation to the right. The Robinson has an 18-degree offset but unlike the Lynx it has a 72-degree phase angle and according to Frank Robinson there is no need to make compensation for the offset. My god, that man is a genius.
So if the 15-degree offset is recognized by Westland and they compensate for it electronically then why doesn't the same apply to the R--22 and the 18-degree offset being compensated for by cyclic input.
It's Goose and Gander stuff the way I see it.
