Helicopter Flight Information

Autorotation Aerodynamics

Note that in this figure, which represents a normal 100% RPM autorotation, the inner red circle represents the area where the up-flow of air has created a "stalled region" in the rotor system. The middle green circle represents the area where the up-flow of air is actually driving the rotor system. The outer blue circle represents the "driven region" (driven by the inner driving region); this is also the region which is providing the lift.

Note that this figure depicts a vertical descent autorotation; the regions would be offset slightly in a depiction during forward flight. Refer to your 'Rotorcraft Flying Handbook'.

For an extended glide autorotation, the RPM can be pulled down to the lower limit of the allowable range specified in the relative RFM (POH) by the pilot increasing the collective pitch. At this lower RPM, you have accomplished two things; you have optimized the lift vs drag equations on the main rotor, and you have modified the aerodynamic regions of the rotor system in autorotation. This reduction of rotor rpm is commonly noted as increasing the efficiency of the rotor, but I strongly encourage you to remember the differences between powered and non-powered flight! Torque! You don't have any in non-powered flight therefore tail rotor thrust is not a significant factor, but you will lose tail rotor authority at a very fast rate when you let your rpm decrease in powered flight; at a very minimum, 3:1 ratio when compared to the main rotor.

When discussing techniques for modifying an autorotation, it is important to note that it is never acceptable to modify an autorotation until the autorotation is stabilized in the first place. This may seem to take forever to a student who is just beginning, but as time progresses you will have plenty of time. For this reason I like to begin teaching straight-in autorotations from a higher altitude to give students time to see and understand what is taking place.

This figure is also known as "L/D Max"; a graph which demonstrates where Lift verses Drag is at its maximum advantage point. This is simple when applied to an airplane, but a little more complicated with helicopters since we have two important factors to consider. Those two factors are airspeed, and rotor rpm. Rotor rpm can be decreased to optimize the lift vs drag ratio of the airfoil (rotor blades). An inexperienced pilot must be very careful with this.

Modification of the rotor rpm should never be attempted by a pilot with minimal experience since the risk of a stall is to great. Once a pilot has been taught more advanced autorotation procedures, then this technique can be practiced rather safely. It is most important to always remember that you must first stabilize your autorotation, then begin to modify those things which you can control to achieve a different result. When modifying the rotor rpm, a modified airspeed usually must be applied as well.

Another issue of extreme importance and which must be remembered is that it is better to accomplish a good autorotation to a bad area than it is to accomplish a bad autorotation to a good area.

Practice, practice, practice!! Fly with an experienced instructor and have a little fun while you learn!! 

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Last modified: 12/31/2011