To: NickLappos
On a typical Sikorsky, Bell, Aerospatiale and most other helicopters they have a rigged phase angle of 90-degrees. On most if not al of these helicopters the actual phase angle is less than 90-degrees (Phase angle shift). On a Robinson the rigged phase angle is 72-degrees with the assumption there is no if not a minimal shift in this phase angle.
No amount of modification of control input can compensate for a 90-degree pitch horn on a Robinson helicopter. The control input on the Robinson is identical to a two-blade system on a Bell. With forward cyclic the swashplate tips down over the nose and with left cyclic the swashplate will tip down over the left lateral axis. The opposite is true for opposite control inputs. On the Bell the pitch horn leads the blade by 90-degrees. When setting the forward pitch range on the Bell the blades are disposed over the lateral axis. In setting the lateral pitch ranges the blades are disposed over the longitudinal axis.
On the Robinson when setting the forward cyclic pitch settings the blade is disposed 18-degrees ahead of the lateral axis. In setting the lateral range the blades are disposed 18-degrees ahead of the longitudinal axis.
Now let’s discuss flapping on the Robinson. Flapping in this case is coning about the cone hinge.
On the R-22 with no pitch in the blades the pitch horn connect point with the pitch link is on or about coincident with the cone hinge so if a blade is raised from this static point there is no pitch flap coupling. However when collective pitch is increased the pitch horn will rise above the cone hinge so if the blade is moved about the cone hinge there will be pitch flap coupling with a decrease in pitch with up flap and an increase in pitch with down flap. This is normal.
However if you have a pitch horn that extends beyond the cone hinge you will have control problems. Using the above example with a 90-degree pitch horn if you raise the blade about the cone hinge with no pitch in the blades you will get an increase in pitch (on the R-22 there is no change in pitch. When collective is added and the blades cone there would be a significant increase in pitch over that which was input by the pilot. The kinematics of the blades in flight are beyond me but I do believe that with any flapping of the blades about the cone hinges the rotor would respond in a way that either increases the pilot input or acts in opposition to the pilot input.
The R-44 does not have a 90-degree pitch horn but it does extend beyond the cone hinge. Although the problems would not be as severe I believe that under certain conditions (autorotation) there could be a problem.
On most helicopters blade pitch settings are made and then the adjustable stops are set to limit any movement beyond that point. On the Robinson they have fixed stops and the blades are set to that stop. All pitch settings are set with the cyclic in the neutral position and the collective full down. Since any movement from the neutral position to the forward stop would be equal to the same movement to the aft stop. From that you might assume that the pitch settings for forward and aft cyclic would be the same but, they are not.
Both blades are set with the cyclic against the forward stop. Then the cyclic is moved the aft stop and the blade pitch is set to the aft settings (which are different). In setting the aft pitch point it is necessary to adjust the pitch links which destroys the setting for forward cyclic. The mechanic is then told to recheck the forward pitch setting which are now changed. At or from this point the mechanic is left to ponder what he must do as the maintenance manual is moot from this point onward. The lateral settings are the same.
In some cases it is necessary to make adjustments on the pitch-input rods to the swashplate (after having neutralized the swashplate). These adjustments can cause a binding on the swashplate monoball. The tail rotor settings are just as confusing.
Nick, I have a question. Put yourself in Frank Robinson’s place. You design a rotorhead and then construct a helicopter around it. The rotorhead is different in that it can flap about a teeter bearing and it an also flap about cone hinges. Good design says that the pitch horn can't extend beyond the cone hinge so that the helicopter has a rig angle of 72-degrees. Now design a blade that will respond in 72-degrees where all other helicopters have a rigged phase angle of 90-degrees and their blades respond accordingly.
