Helicopter Flight Information

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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 green arc (indicated on the rotor tachometer) by the pilot increasing the collective pitch. The rotor is not more "efficient" at this lower RPM, you have changed the aerodynamics of the rotor system in autorotation. By increasing the collective pitch, you have decreased the middle driving region and increased the outer lifting region. The rotor RPM decreased because you made the driving region smaller.

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. The speed where this occurs will be the minimum rate of descent in autorotation, the best rate of climb in powered flight, and minimum rate of descent in a powered descent. The speed where this is achieved will vary from one aircraft to another. E (the yellow line) represents the best airspeed.

This graph can be found where? In the RFM.

The airspeed depicted in this graph is often slightly slower than what is taught as the optimum climb airspeed, optimum autorotation airspeed, and best initial approach airspeed. This can be confusing to students. Relative to autorotations, (the topic here), the optimum airspeed will give us the best flare and best handling qualities.

The airspeed depicted here might be the best choice if an air-start were to be attempted. An air-start is never an option at low altitudes and likely not an option in average conditions of helicopter flying since you would likely be flaring just about the time you get stable. Remember that if you are flying at 500-ft. AGL, you are just 20-seconds from touch down in the event that you experience an engine failure, so you best be doing everything just right and according to your learned procedures.

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Last modified: 01/19/08