Dear Outside,
Dear Outside,
First of all the rigged position of the Robbo cyclic is right of the center of travel in line with the lateral axis and not in the center of allowed travel as dictated by the stop plate. In the rigged neutral position the stick is placed in the center of fore and aft travel and slightly to the right of the center axis of the aircraft. Because of the 18 degree offset if you moved the cyclic straight forward from the rigged neutral position the helicopter would fly to the left.
When a helicopter is in the certification process it is dictated that the helicopter fly in the same sense of cyclic stick movement. The FAA allows slight deviations in the amount of a degree or two, and they attribute this to pitch coupling. In order to demonstrate that the helicopter does in fact fly in the direction of cyclic movement they must employ a stick plotting board or some similar device that plots cyclic movement during flight. My report questions if this was done during certification.
The Robinson rotorhead was not designed with the offset in order to compensate for rotor aerodynamics as described in your post above. The offset results from the fact that the pitch horn can not cross the flapping hinge. Take a look at a Bell rotorhead. You will see that the pitch horn terminates at or about the teeter bearing. Look at any other helicopter and you will see that the pitch horn terminates at or about the flapping hinge and if your helicopter has elastomeric bearings like a Sikorsky S76 the pitch horn terminates at or about the apparent flapping point.
Because the pitch horn can’t always be in the exact ideal position of dead nuts on with the flapping hinge or teeter bearing it results in pitch coupling and that is what the pilot in a helicopter compensates for in adjusting his cyclic stick position. Of course, I am describing an ideal situation with no wind. Every thing you said about adjusting for a crosswind or a not so level landing spot is correct.
On the Robinson which has both a teeter bearing and flapping bearings the pitch horn terminates at or about the flapping bearing. If it were like a Bell but with flapping capability and the pitch horn did terminate at the teeter bearing the pitch coupling would be so severe that the helicopter would be uncontrollable. If you were familiar with a tail rotor that has a delta hinge you would notice that each time the blade flaps in or out the pitch that the pilot put into it would change. This is to equalize the lift across the tail rotor disc. The same thing would happen on the main rotor if the pitch horn were at or about the teeter bearing.
When the pilot of a Robinson pushes his cyclic forward from the rigged neutral position the swash plate tips down at the front and up at the rear. If the blades are laterally disposed (left & right) the pitch horn lags the lowest point of the swash plate by about 18 degrees. If you think back to helicopter aerodynamics 101 you will remember that the advancing blade will be at its’ lowest pitch at that point and with precession the effect will be 90 degrees later in the direction of rotation. With that point understood take a look where the Robinson pitch link is in relation to the lowest point on the swashplate. The pitch link must travel an additional 18 degrees before it reaches the lowest point on the swashplate. With precession, the blades will fully react at approximately 18 degrees left of the longitudinal centerline of the helicopter. The result is that the helicopter flies to the left. In order to counter this offset, the pilot must adjust his cyclic movement to compensate for the offset.
Regarding the rigging procedure,if the mechanic follows the instructions he could build in a jam of some of the flight controls, he could place the swash plate in a bind and he can rig in too much pitch. Even when the aircraft is tested for autorotation the mechanic can add in even more pitch and under certain conditions it could result in blade stall (not retreating blade stall).
Regarding side slip and out of trim flight, I believe Helidriver said it all in his response to Tilt Rotor.
I don't know if either you Outside Loop or, Rotor Nut have requested a copy of my report because it covers all of this. In answering these many posts I have retyped my report at least twenty times.
Please provide your email addresses and your fax numbers and I will send the report and two diagrams that explain everything.
My email address is
[email protected]
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The Cat
[This message has been edited by Lu Zuckerman (edited 11 October 2000).]
[This message has been edited by Lu Zuckerman (edited 11 October 2000).]