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Hovering Autorotation

This maneuver is used to land from a hover without using the engine. This would normally occur because the engine or tail rotor failed. The name "hovering autorotation" is really a misnomer, because the helicopter actually never enters autorotation. Instead, the inertia of the spinning rotor system is used to produce thrust.

A JPEG and GIF sequence of photographs of a hovering autorotation are available.

Maneuver Description

The rotor system of a typical helicopter stores a fair amount of kinetic energy. This energy can be extracted as thrust by allowing the rotor RPM to decrease, giving up the stored energy. There is only a limited amount of energy available, and this determines the maximum height from which the helicopter can be landed without damage.

Collective and Throttle Controls

To practice a hovering autorotation, the pilot rolls the throttle to the off position while in a hover. The freewheeling unit allows the powerplant RPM to go to idle (or zero) while the rotor coasts. As the rotor RPM starts to decay, less thrust is developed and the helicopter will begin a descent toward the ground. As the ground is approached, the pilot can cushion the landing by increasing collective pitch. This bleeds the RPM down even more rapidly, but provides extra thrust to slow the descent rate of the helicopter. The helicopter will land gently if sufficient energy was available in the rotor system at the time the engine power output stopped.

Normally on entry to the maneuver, the collective pitch is not moved. This uses up considerable rotor energy as the helicopter sits in the hover, starting to descend very slowly. The pilot can achieve a softer landing if he initially lowers some collective. This starts the helicopter down toward the ground faster, and conserves rotor RPM. This is considered an advanced technique and is not normally taught to low time pilots.

Cyclic and Pedal Controls

When the throttle is rolled off, any torque being produced by the engine stops immediately. The pilot must reduce the anti-torque force being produced by the tail rotor, since there is no longer a torque force to counter. He does this by pushing right pedal (in most American helicopters) until the tail rotor is approximately at flat pitch. This will not be full right pedal, since the tail rotor is actually able to go past flat pitch and into "negative" pitch. The exact position of the pedals will depend on how the helicopter is rigged, but the pilot simply judges the amount of pedal by putting it in whatever position prevents any yaw from occuring.

While the helicopter was hovering, the tail rotor was producing thrust which not only prevents the helicopter from spinning due to torque, but also has the useless side effect of trying to push the helicopter sideways. This is called translating tendancy. This is opposed by tilting the main rotor to the left, providing equal and opposite thrust. When the tail rotor is placed in flat pitch, the translating tendency goes away, and therefore the lateral main rotor thrust must also go away. The pilot does this by simultaneously moving the cyclic to the right as he pushes the right pedal. The timing of the two must by synchronized. If the pedal is moved quickly, the cyclic must move quickly. If the pedal moves slowly, the cyclic must move slowly.

As the ground is approached and the collective is raised, student pilots will often want to push left pedal, because they are used to pushing left pedal as they increase collective. However in this case there is no torque being generated, so the pedals remain fairly motionless as the collective is raised to cushion the landing.

Common Mistakes

One of the difficult aspects of learning hovering autorotations is that they occur very quickly. The entire maneuver only lasts a 2-3 seconds.

Raising collective during the throttle rolloff

As the pilot rolls off throttle, it is possible to inadvertently raise the collective slightly. This has the unfortunate effect of using up some of the rotor RPM to gain altitude. This has the effect of placing the helicopter even further from the ground with less RPM available, and generally results in a firm or hard landing. The pilot has to be very careful that the collective does not move while the throttle is being rolled off.

Allowing the helicopter to yaw

The pedals must be moved fairly quickly to approximately the correct position if the pilot is going to avoid yawing the helicopter on entry to the maneuver. Errors in either the speed at which the pedals are applied, or the actual position the pedals are moved to will cause the helicopter to yaw. Minor mistakes can be corrected before the helicopter touches down, but a really good hovering autorotation will transition from powered to unpowered flight with no noticible change in aircraft attitude.

Allowing the helicopter to drift

The cyclic is another difficult control during this maneuver. The cyclic is normally moved in very small increments during a hover. In a steady hover, the cyclic will appear to be motionless even while corrections are being made, because the corrections are so small. At the entry to the hovering autorotation, the cyclic has to move a huge amount: an inch or more in some helicopters. The cyclic is so sensitive that it is difficult at first to move it so far, to exactly the correct position, and then revert to very small inputs to maintain level flight.

The result of this is that the helicopter is likely to drift either forward, sideways, or backward during the hovering autorotation. Forward drift is not normally a big problem because the landing gear is designed to allow touchdown with forward speed. Sideways and rearward drift is more of a problem. The landing gear is usually not designed to take very high side loads, and landing while drifting sideways can cause the helicopter roll over, or even if a roll over is avoided the side loads can damange the helicopter structure.

If the helicopter starts to drift, the pilot should make corrections with the cyclic just as in normal flight. Drastic sideways drift that cannot be corrected with cyclic may require that the pedals be used to align the landing gear with the drift. For instance, if the helicopter is drifting right, the right pedal could be pushed to yaw the helicopter nose to the right until it is now aligned with the direction the helicopter is moving.

Collective pull too soon or too fast

If the collective is raised too early or too rapidly, the helicopter may stop the descent for a moment, bleed of rotor RPM, and then descend again (but now with less RPM available to cushion the landing). This sort of mismanagement will often cause firm or hard landings because there may not be enough RPM left to maintain rotor thrust until the landing occurs. The helicopter may literally fall the last few feet, and this will result in a very hard landing.

Collective pull too late

Sometimes the pilot will fail to pull collective soon enough. This may simply mean that the landing is not cushioned as much as it could be. A possible problem is if the helicopter has flexible landing gear. If the helicopter lands hard enough to compress the landing gear, and the pilot continues to pull collective pitch, as the rotor thrust builds the landing gear can spring the helicopter back up into the air. The problem is that at this point no rotor RPM is left, and if the helicopter springs up to a 2 foot skid height, it will fall 2 feet back to the ground. 2 feet does not sound like a lot, but it's enough to damange many helicopters.

A word about hover autos in turbine aircraft

The first time anyone performs a hover auto in a turbine aircraft, they think wow because everything happens in slow motion compared to in a piston helicopter. However, there is something strange going on when the throttle is rolled off. The torque change is very gradual, as shown by the fact that the right pedal has to be moved very slowly. There are two possible reasons that I have come up with for this, but so far these are theories on my part. What I really think is happening here is that when you roll throttle off in a turbine aircraft, the fuel control is going to reduce fuel on a schedule designed to not flame out the engine. Even though the pilot may snap the throttle off, the fuel control is actually reducing fuel to the engine at a very slow rate. Therefore, substantial engine power is being developed during the hovering autorotation. If I am correct, a flameout will cause a rapid power loss, and the hover autorotation will happen at a pace similar to that of a piston powered helicopter. One other possibility is that the turbine itself may store significant rotational energy: it's turning at over 50,000 RPM. Why I don't like this theory as much is that in a free turbine engine, the power turbine is pretty small and light, and even at 50,000 RPM I doubt it can store really significant amounts of energy. If anyone has flamed out a turbine at a hover, or knows for sure why the loss of torque is so gradual, I'd be interested in hearing from you.

In any case, I've noticed a large discrepency in how much inertia I seem to have performing a hovering autorotation in a turbine aircraft, versus how much inertia I seem to have doing full down autorotations from altitude. I think that in this case, the turbine has plenty of time to wind down to idle while in the autorotational glide, and that the pitch pull and landing at the bottom of the auto accurately reflects how much inertia is actually available without the engine producing power.

The obvious danger with this situation is that if pilots practice hovering autorotations in turbine aircraft by rolling off throttle, they may get a false sense of the amount of inertia available to them. This may cause them to hover at altitudes which appear to be safe for a hovering autorotation (because the pilot has practiced hover autos from that altitude) but which may be too high to effect a safe landing if the engine actually flames out.

Paul Cantrell
paul at copters.com (replace " at " with "@" to email me - this avoids SPAMMERS I hope)

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