To: Dave Jackson
I read the article about the Cheyenne and the author got it almost perfect. However his reference in comparing the stabilizer bar on the Bell to the control gyro on the Cheyenne is incorrect. He properly states that the control gyro in its’ precession of 90-degrees is the motivating force in changing the pitch of the blades to cause a gyroscopic reaction 90-degrees later. He properly indicates that the pilots initial input to the swashplate and the compression springs is 180-degrees out of phase with the direction the pilot wants to fly. The input is made into the control gyro and it responds 90-degrees later and the blade disc responds 90-degrees after the control gyro change the pitch in the advancing and retreating blades.
One point he left out in his addressing blade response to control input is that the Cheyenne flies like a fixed wing aircraft. On a conventional helicopter the helicopter will continue to fly in a given direction as long as the cyclic is maintained in a given position that relates to the direction being flown. On the Cheyenne as soon as the pilot gets the desired attitude or direction he moves the cyclic stick back to neutral. The gyro having rigidity in space maintains the position the pilot had commanded it to and it continues the pitch change input necessary to maintain the direction or attitude of flight.
Here is a point to consider when looking at the crash of the first Cheyenne and the breakup in the wind tunnel. Once the helicopter attains sufficient speed so that the wings are lifting the helicopter the pilot lowers the collective and the helicopter flies like an autogyro. One problem with an autogyro is that it flies in low pitch and the blades in this condition are extremely sensitive to cyclic pitch input. Under the proper conditions the introduction of cyclic cause the blade to go unstable and maybe this was a contributing factor in the loss of the two helicopters and the death of the test pilot.
Regarding a comment referencing Ray Proutys’ explanation of the Cheyenne rotor system the Ring gyro was mounted on the proof of concept vehicle and only on that vehicle. The others used a gyro similar to that used on the Cheyenne.
Here is another point that he did not address. When sitting on the ground all cyclic input is locked out which immobilizes the stick. If the stick were displaced on the ground the helicopter would tip over.
If you will allow me to repeat myself, I asked that this thread be created so that individuals could expound on their theories relative to gyroscopic precession and when all inputs were made to compare notes and see if you could agree on a common and acceptable theory. Comparisons were made to spinning hockey pucks, bicycle wheel, bicycle wheels with only four spokes and several other strange concepts that were all equally difficult to follow.
Since most of you are pilots why can’t you reference the gyros that are spinning at great speed just a foot or two in front of your nose. Or, why can’t you reference a gimbaled gyroscope that most of us have seen in science class or flight or mechanics school.
[ 08 August 2001: Message edited by: Lu Zuckerman ]
[ 08 August 2001: Message edited by: Lu Zuckerman ]