Doug Riley
Platinum Member
- Joined
- Jan 11, 2004
- Messages
- 6,968
Ferran:
Fortunately, the answer is yes. Cyclic pitch control works normally in zero G and negative G, in the sense that the rotor still tilts in response to stick movements. Without rotor thrust, the change in the angle of the disk is not felt in the frame, but it still occurs.
When the frame rotates nose up, the spindle tips aft relative to the rotor disk, as long as the spindle maintains a fixed angle to the frame (i.e. the stick is fixed). The rotor will experience the usual cyclic pitch change and react accordingly. That is, the advancing blades' pitch will increase, and the retreating blades' pitch will decrease. The disk will respond by flying to a new, positive angle of attack.
Note that the cyclic pitch change itself does not increase the rotor's thrust. A cyclic pitch change does not alter total thrust; it merely alters the distribution of thrust across the disk. The rotor's thrust becomes positive in the advancing sector and negative in the retreating sector, but the net thrust is still zero.
However, as the disk tips aft in response to the momentary dissymmetry of forces, the disk AOA becomes positive. This, in turn, increases the AOA (not the pitch, just the actual AOA) of each blade in each sector of the disk. Therefore, rotor thrust increases, restoring positive G.
If you view the video of the PPO in Japan that Scandtours posted in another thread, you may doubt this analysis. You can see the pilot level the rotor during a zoom climb. The gyro begins to pitch over. You then can see the rotor tip aft, presumably because the pilot pulls the stick aft. The pushover continues, though, even with the rotor disk tipped well aft.
I believe the reason for this is that the gyro was still climbing from its own momentum when the nose dropped in response to forward stick. The nose rapidly rotated so low that the gyro was still climbing by the time the pilot reacted by pulling aft. The climb angle was sufficient that even a full-aft rotor disk only met the air at a zero angle of attack.
If my analysis is correct, the video demonstrates the importance of the H-stab. The stab would keep the nose from dropping while the aircraft was climbing. If the gyro in the video remained pointed into the relative wind, prompt aft stick would probably have saved the pilot's life. If the airframe had had a nose-up bias as I suggested earlier, then the aft stick would not even be necessary; the pilot could just hold the stick still and the frame would supply the aft-cyclic effect.
A Dominator, with its low thrustline, is an example of a gyro that provides automatic cyclic inputs that tend to maintain positive G -- as long as you hold the stick still. I was able to duplicate this behavior in my Gyrobee (despite a slightly high thrustline) by using a large immersed H-stab with -3 degrees of incidence.
I don't know if any of the Magni clones uses either of these design tactics. I understand that the Magni aircraft themselves do not.
Fortunately, the answer is yes. Cyclic pitch control works normally in zero G and negative G, in the sense that the rotor still tilts in response to stick movements. Without rotor thrust, the change in the angle of the disk is not felt in the frame, but it still occurs.
When the frame rotates nose up, the spindle tips aft relative to the rotor disk, as long as the spindle maintains a fixed angle to the frame (i.e. the stick is fixed). The rotor will experience the usual cyclic pitch change and react accordingly. That is, the advancing blades' pitch will increase, and the retreating blades' pitch will decrease. The disk will respond by flying to a new, positive angle of attack.
Note that the cyclic pitch change itself does not increase the rotor's thrust. A cyclic pitch change does not alter total thrust; it merely alters the distribution of thrust across the disk. The rotor's thrust becomes positive in the advancing sector and negative in the retreating sector, but the net thrust is still zero.
However, as the disk tips aft in response to the momentary dissymmetry of forces, the disk AOA becomes positive. This, in turn, increases the AOA (not the pitch, just the actual AOA) of each blade in each sector of the disk. Therefore, rotor thrust increases, restoring positive G.
If you view the video of the PPO in Japan that Scandtours posted in another thread, you may doubt this analysis. You can see the pilot level the rotor during a zoom climb. The gyro begins to pitch over. You then can see the rotor tip aft, presumably because the pilot pulls the stick aft. The pushover continues, though, even with the rotor disk tipped well aft.
I believe the reason for this is that the gyro was still climbing from its own momentum when the nose dropped in response to forward stick. The nose rapidly rotated so low that the gyro was still climbing by the time the pilot reacted by pulling aft. The climb angle was sufficient that even a full-aft rotor disk only met the air at a zero angle of attack.
If my analysis is correct, the video demonstrates the importance of the H-stab. The stab would keep the nose from dropping while the aircraft was climbing. If the gyro in the video remained pointed into the relative wind, prompt aft stick would probably have saved the pilot's life. If the airframe had had a nose-up bias as I suggested earlier, then the aft stick would not even be necessary; the pilot could just hold the stick still and the frame would supply the aft-cyclic effect.
A Dominator, with its low thrustline, is an example of a gyro that provides automatic cyclic inputs that tend to maintain positive G -- as long as you hold the stick still. I was able to duplicate this behavior in my Gyrobee (despite a slightly high thrustline) by using a large immersed H-stab with -3 degrees of incidence.
I don't know if any of the Magni clones uses either of these design tactics. I understand that the Magni aircraft themselves do not.