To: Imlanphere
Ray Prouty said: "Some helicopter designers arrange the cyclic control system such that a stick movement directly to the right produces maximum pitch over the nose and rely on the pilot to take care of the rate cross-coupling. Others, such as Robinson, rotate the controls to minimize this crosscoupling while accepting some acceleration crosscoupling". My question is, how do the pilots counter this cross coupling. Which way do they move the cyclic?
The control movement and the resulting swashplate movement is exactly the same on a Robinson as it is on a Bell. Using the illustration above, when the pilot moves the cyclic to the right the Bell blade will have reached its' maximum pitch over the nose and its' minimum pitch over the tail and as a result the P R E C E S S I O N of 90 degrees will cause the blade to rise over the left side and drop over the right side and the helicopter is flying to the right.
The control movement for the Robinson is exactly the same. However, when the blades are over the nose or the tail they must rotate an additional 18 degrees to achieve maximum and minimum pitch change. When the have rotated this additional 18 degrees they will be at the maxim/minimum pitch change points and, the P R E C E S S I O N OF 90 degrees will cause the blade to rise 18-degrees past the lateral axis, and the helicopter will fly to the right, with a slight forward component. So, Mr. Prouty is wrong in this case. However, I do agree with his remarks about the laws that apply to a gyroscope. Grey Area are you reading this?
With regards to what Mr. Prouty said about articulated rotorheads or rotor heads with real or theoretical offset hinges he was talking about the positioning of the swashplate relative to the direction of flight. Most helicopters of this type follow what Sikorsky has been doing for years. The movement of the controls are the same as described above, however the movement of the swashplate is different. If the pilot moves his cyclic to the right, the swash plate will tilt up somewhere between the 10 and 11 o clock position and the swashplate will tip down somewhere between the4 and 5 o clock position. This offset is 45-degrees. The pitch horn lead is 45-degrees making a total phase angle of 90-degrees. With the right cyclic input as described above the disc will tilt up over the 9 o clock position and will tip down over the 3 o clock position. This movement is the result of
PR E C E S S I O N which was caused by a force imbalance across the disc which was induced by the movement of the cyclic stick and the resultant pitch change.
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 December 2000).]
[This message has been edited by Lu Zuckerman (edited 15 December 2000).]