RV7 Flyer
lvl.1
United States
Offline
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If you're asking why the current, fixed-pitch type of quadcopter can't autorotate, the answer has been given many times. As stated, a *real* helicopter (or an RC helicopter) has a swashplate which is used to control the pitch of the rotor blades at all positions throughout its rotation. The "collective" portion of a helicopter's controls moves the swashplate uniformly up or down to control the pitch of the main rotor identically at all points. Watch a helicopter lift off sometime. The pilot will increase the throttle, the blades will spin faster and faster, and then as he "raises the collective", the pitch angle of the blades increases and the aircraft will lift up vertically. Decrease the collective, and the blade angle decreases, resulting in less lift, and the aircraft will descend (basically). BTW, this has *nothing* to do with forward airspeed, which is controlled by the cyclic control.
In a loss of power, or retarded throttle for practice, the pilot "dumps the collective" and as the aircraft essentially falls, airflow over the rotor will cause it to continue spinning (or increase its rotational speed). Just prior to landing/crashing, the pilot now increases the collective, using the stored energy of the rotating mass to keep the rotor turning, and increases the blade pitch angle, which results in lift, preventing the crash. BTW, on some helicopters, you have to dump collective *very* quickly when power is lost or this won't work.
Forward flight uses cyclic control of the swashplate, tilting it cause the blade angle to be more at some points in the rotation and less at others (and there's the matter of gyroscopic precession, etc., which determines where in the rotation the blade inputs have to be effected by the plate).
Now, if you're asking "why can't someone make a quadcopter that has variable pitch blades and a collective so that it can autorotate", well...have at it! |
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