I intend to get a collective-pitch RC helicopter in the future; in the meanwhile I purchased an imitation radio with USB connection and AccuRC to practice in my PC to gain some experience.

I've gone through the training scenarios in AccuRC and I'm getting fairly good at hovers (even inverted) with a variety of 450 to 700 class models. Next, I'm trying to practice autorotations.

A book on flying actual full-size helicopters ("Learning to Fly Helicopters, Second Edition" by R. Randall Padfield) indicates that an autorotation starts with bottom collective -- so far so good -- and aft cyclic. The reasoning for aft cyclic goes that you need to maintain rotor RPM (too low, no lift; too high, you risk structural damage), so you want air flowing through the rotor plane, not in parallel with it. Almost always you'll have forward airspeed so the angle of the wind is such that aft cyclic is required to get airflow through the rotor plane.

I tried to follow the same procedure within AccuRC, but I get the feeling that aft cyclic just causes loss of forward speed and appears to be actually counterproductive.

I understand the physics of full-size helicopters will not be identical to those of RC helicopters due to different scaling factors for weight/area/volume, so perhaps the autorotation procedure in an RC helicopter may be different.

Can someone describe in detail the exact steps for autorotation of an electric, 450 to 700 class, RC helicopter, and what to watch out for? E.g. the book mentioned above emphasizes quite strongly the need to maintain rotor RPM -- is this as much of an issue with RC helicopters as well?


1 Answer 1


I'm not really an expert, but you're right about aft cyclic. A full-size heli is probably cruising along faster than they want to autorotate, and can trade some speed for rotor energy. A model could be doing anything, and once you are autorotating you want to maintain some forward speed so the airflow over the tail helps to keep you straight.

Also, most models are aerobatic and set up with more negative pitch than you need for autorotation. Too much negative pitch will result in a very rapid decent, and could over-speed the rotor. The correct amount is usually about the same as for an inverted hover. It still comes down pretty fast!

Model RPM drops much faster than full-size. There's less momentum in the blades, and much less forward momentum. I think the big difference here is that a full size pilot is probably surprised by an emergency and not used to using negative pitch. They're looking for places to land, trying to restart the engine, and making a mayday call, and need to remember that maintaining RPM is the most important task.

With a model, maintaining RPM is not hard, but it is still the primary task. Listen to the rotor speed and adjust the pitch to keep it about the same as normal. Staying level and steering are secondary.

The hard bit is knowing when and how fast to add positive collective. That's different for each model. You can get used to it by hitting the kill switch from a low hover and trying to stay a few inches above the ground, and by trying to autorotate to a head-height hover and then switching back on.


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