Public Informative Post - Blog Posts

This turned into a longer post than I anticipated but whatever.

Something I've been seeing quite often in the comments under helicopter posts that make it to the broader internet spaces is discussions on autorotation. These discussions are mostly incomplete information at best and outright wrong at worst. A lot of people seem to be able to recall it as a fact about how helicopters can glide to a safe landing, but aren't aware of the actual process. So here's a guide on what an autorotation is, how its performed, and some of the nuances to it.

For the uninitiated, an autorotation is a maneuver that every helicopter is capable of performing which allows it to land safely in the event of a power failure. Even more simply put - its how a helicopter glides.

I've already made previous posts about helicopter controls and some principles of flight which I recommend checking out first if you're unfamiliar with those.

Under normal flight the engine(s) drive the rotors at a constant flight rpm and all control is made by pitching (changing the angle) The blades to make more or less lift. Essentially the same process as sticking your hand out the window of a moving car and making rise or fall in the wind. However the rotors are experiencing a lot of drag (wind resistance) which requires the engine to produce a lot of power to overcome and maintain rpm.

When an engine failure occurs there is no more power driving the rotors and the high drag will cause the rotor rpm to start to decay rapidly. If nothing is done about that then the rpm will fall so low that the rotors will stall or worse and the helicopter will fall out of the air like a rock. Thankfully we have the option to autorotate instead of that outcome.

The first thing that happens to initiate an autorotation is to fully lower the collective. This will flatten out the blade pitch and minimize the drag on the main rotor, slowing the rpm decay. As the collective is lowered the cyclic will need to come aft slightly to prevent the nose from dropping. Also the right pedal will have been pushed in as the power failure initially occured to prevent yawing.

Now the helicopter is in a steep descent and the autorotation has begun. The airflow through the main rotor has reversed from normal flight. Instead of being drawn from above and expelled downward there is a diagonally upward flow of air through the main rotor.

Now the rotor rpm will begin to rise again thanks to the special design of the rotor blades. A rotor blade has an airfoil shape which is sort of like an elongated teardrop with the wider end on the leading edge. This shape minimizes drag and maximizes lift. But the blade is also slightly twisted. It has a positive pitch at the root where the blade attaches to the rotor hub which gradually transitions to a negative pitch at the tip.

Because of this twist and the difference in relative speed along the blade length (tip travels relatively faster than the root) the blades will develop three distinct regions. These are the driven, driving, and stall regions

The driven and stall regions at the blade tip and root are still producing drag but the middle driving region is actually producing lift, in an upward and slightly forward direction. This forward lift is the thrust that causes the rotor rpm to increase during an autorotation.

So now you are in a descent and recovering rpm back to the normal flight range. If you leave the collective fully lowered the rpm will now start to increase past the normal range and begin to overspeed. If the overspeed becomes too great the blades will be damaged and one could eject. Not ideal.

You have to manage the rpm manually to prevent it from becoming too low or too high. You also do this with the collective. Remember, to start the auto you should lower the collective fully to minimize rpm loss initially and then to start recovering it. As the rpm reaches the normal range the collective should be raised again just a bit to "catch" the rpm. Now you can manually adjust rpm with a tiny amount of collective movement. Rpms a little too fast? Raise it a bit. A little too slow? Lower it a bit. What this is doing is changing the size of the driven and driving regions of the blade, thanks to the twist. Lowering the collective grows the driving region and shrinks the driven region, and vice versa for raising it.

Now the helicopter is safely gliding and can be steered to a landing spot. There's not much to do until you're approaching the ground. The next maneuver will be the level and flare. The height at which you initiate the level and flare depends on the helicopter. Generally a larger helicopter will have more momentum and need to start the maneuver sooner.

Starting with the level off. You will be gliding with a high rate of descent and forward speed in an autorotation. The purpose of the level off is to drastically reduce the rate of descent. By using some aft cyclic input you will pull the nose up and put the helicopter in a level flight attitude. This causes the upwards lift of the rotor disc to act as a sort of parachute and arrest the descent.

Now with the descent rate minimal you apply more aft cyclic to pitch the nose up further and neutralize the forward speed. This is the flare and its the last opportunity to build rotor rpm in an autorotation.

Now you are just over the ground with little to no forward speed and the helicopter will start to settle and sink. Apply forward cyclic to level the helicopter parallel with the ground and use the pedals to keep the nose pointed straight ahead. Then you have whatever rpm is built up to cushion the landing. Smoothly raise the collective fully as the helicopter sinks to touchdown and the landing can be shockingly smooth.

What an autorotation really comes down to is energy. You often start at a high-ish altitude with some forward speed and this becomes the potential and kinetic energy you trade to power the rotors instead of the engine. The energy is an absolute requirement though. If you dont have enough of a combination of speed and/or altitude then an autorotation can be impossible. There are phases of flight and certain missions where you have to accept the risk of a power failure and rely on the crash-worthiness of the airframe.

Despite that, I've done a lot of engine failure procedures in small planes and helicopters and 9 times out of 10 I would rather experience a real one in a helicopter.