Helicopter Flight Information
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Helicopter Flight Information |
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Rotor Efficiency - Where is your Rotor RPM? I recently read an article where the pilot stated that he "...increased the collective during the autorotation to improve rotor efficiency to extend the glide...". is this correct? Is the rotor more efficient in the lower limits of the green? Once while I was instructing in a Bell 47 with a manually controlled throttle, a student stated that he wanted to keep the rotor at the lower limits of the green because a former instructor had told him that the rotor was more efficient there and could be demonstrated due to the extended glide characteristics during the autorotation. Is this correct? The answer is NO, NO, and NO. The rotor is not more efficient with the rpm in the lower limits of the green. In fact, if you look at the lift equation, lift increases at the square of the airspeed. With regard to helicopters, this is rotor rpm. I want 100% (top of the green), 100% of the time in powered flight. During autorotation, rotor rpm can safely be decreased to the bottom of the green which will effectively increase the glide distance if airspeed is also increased to the optimal speed as well. This is not due to improved efficiency but rather due to the fact that the aerodynamics of the rotor has been changed.
The difference in forward descent as opposed to vertical descent is that the regions are offset. If the pilot increases the collective during the autorotational descent, the size of the driving and driven regions change i.e. the driving region decreases (lower rotor rpm) while the driven region increases (more lift) and consequently an extended glide results. Maximum extended glide will result if the airspeed is increased to the maximum stated in the POH for the relative extended glide configuration. Of significant importance is the fact that during autorotation, the most important factor is first achieving a stabilized descent before attempting to extend the glide. Do not reduce rpm to the point where the horn is continually activated since the pilot may then not notice further rotor rpm decay. It is equally important to understand that it is far better to accomplish a good autorotation to a bad area than to accomplish a bad autorotation to a good area. This can only be achieved by first focusing on a stabilized autorotation consistently; then work on altering the autorotation to achieve the desired results. Safety first always. Nothing will work when the autorotation goes bad.
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This
figure depicts three regions of the rotor while in autorotation: The outer
portion of the disk is the driven region and that which is providing the lift
during autorotational descent. The middle region is that where the up-flow of
air is driving the rotor, and finally, the inner portion is the stall region
which is doing nothing to benefit flight.