To: Try_Cyclic,
Dear Try,
I don’t know where to begin. To start off with we must make a comparison between the Robinson and Bell Helicopter rotor systems as both have two blades and both have a teeter hinge. The Robinson rotor system also incorporates flapping hinges. The only point I will make about the flapping hinges is that they reduce the flapping moments of the blades. Anything beyond that point and I will have to admit my ignorance. The teeter head is underslung to minimize lead lag action and spanwise bending, which is the same for the Bell. The fact that the Robinson blades can flap, they can also lead and lag. Since they are restrained from leading and lagging by the flapping hinge the lead lag action is transmitted to the shaft by the teeter bolt. This results in a cyclical load +/- applied to the mast at a rate of four times the rotor speed. This can set up cyclical fatigue in the mast and cause unusual wear on the teeter bolt and hinge bearings.
On the Bell rotor system when the pilot pushes the cyclic forward the swash plate tips down forward (on some Bell models the swash plate tips up forward and dips down at the rear in order to fly forward. On this type of rotor system the pitch horns are on the rear of the blades as opposed to being on the leading edge of the blade)
In either case, the advancing blade is at its’ lowest pitch when it is over the right side of the aircraft. The retreating blade is over the left side of the helicopter and is at its’ highest pitch angle and with a precession or phase angle of 90 degrees the blades will be down forward and up aft and the helicopters will fly forward.
On the Robinson Helicopter the swashplate movement is the same as on the Bell. When the cyclic is pushed forward the swashplate will tip down forward and up aft. The pitch horns on the Robinson lead the blade by approximately 72 degrees. So, when the advancing blade is over the right side of the helicopter and the retreating blade is over the left side of the helicopter neither blade has reached the maximum pitch change (+/-). The pitch horns must travel another 18 degrees for the blades to reach their maximum pitch change. I don’t care how much you think of the Robinson engineering designers, they are unable to counter the laws of physics that deal with rotating masses. When the stick is pushed forward the disc will tip to the left. The only way to counter this phenomenon is to displace the stick to the right.
Many years ago the Cheyenne helicopter had a related problem. On that helicopter the blades were designed in such a way that the phase angle would change and the engineers worked for about four years to figure out how to compensate for this condition. They finally solved the problem but it so complicated the control system that a single point failure (of which there were many) could cause loss of control.
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The Cat