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My understanding is that during power-off autorotation the main rotor is still coupled to the tail rotor, and the flight controls execute the same functions as during power-on flight. Two questions:

  1. Do any (heavy) helicopters have power-assisted controls? If so are those difficult to handle in the event of total power failure?

  2. How is the input during autorotation different from the input for the same attitude under power? E.g., during a given glide and flare to land are the rotor blade angles of attack and swashplate orientation identical to when the a rotors are powered?

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Yes and yes.

A certification requirement of helicopters with powered controls is that they have a manual reversion so that control can be maintained in the event of a power failure. However, the controls can be really heavy with no power and accidents have occurred when hydraulics have failed at critical moments or pilots have lifted off with hydraulics off and have been caught by suprise with the effort needed to maintain control.

The control inputs during autorotation are the same. For any given glide, or flare, the lift required is the same powered or unpowered. Lift is proportional to the speed of the blades, which is independent of powered or unpowered, and the angle of attack.

During autorotation, the energy (power) going into the rotors is derived from the airflow coming up through the disc. In this sense, an autorotation is still powered but the power comes from the airflow rather than the engine.

During autorotation, since the airflow is from below, rather than above, the pitch required is lower for the same angle of attack to generate the same amount of lift. So, the swashplate is lower in autorotation but lowering collective still decreases lift and raising it increases lift. The primary difference is that rather than controlling lift and power, the collective controls rotor speed since increasing pitch increases the angle of attack which increases lift and therefore increases drag, slowing down the rotor. Lowering the collective has the opposite affect.

The cyclic control is the same in all respects (you tilt the rotor thrust in the direction you want to go) since it increases, or decreases the pitch, and therefore the angle of attack, cyclically. The movement of the swashplate about the X and Y axis is independent of the position of the swashplate in the Z axis.

The pedals work in the same way too.

In summary, the only difference in the controls is the vertical position of the swashplate.

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    $\begingroup$ Do you want to have translational motion during autorotation? $\endgroup$ – TomMcW Oct 29 '15 at 21:37
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    $\begingroup$ @TomMcW Absolutely, although it's possible to autorotate most of the way with zero airspeed. You need it at the bottom though to be able to flare, to increase rotor RPM so that you can use the collective to generate lift and cushion the landing. You change from converting height into rotor RPM to converting speed to RRPM. Most autorotations are flown at max range speed, or minimum descent speed or some other nominal figure, typically at around 60 kts since this is where most helicopters require the least power. $\endgroup$ – Simon Oct 29 '15 at 21:48
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    $\begingroup$ '"A certification requirement of helicopters with powered controls is that they have a manual reversion so that control can be maintained in the event of a power failure."' This is incorrect. There are many helicopters with hydraulic only, fully irreversible flight controls: if all hydraulic power fails, the helicopter cannot be controlled. $\endgroup$ – Koyovis Feb 4 '18 at 11:45
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Heavy helicopters tend to be hydraulicly controlled to such an extent that there no longer is a physical link between the controls and the blades (like big jets). You're just opening and closing hydraulic valves one way or another. So... no hydraulics, no controls. The hydraulic pumps are generally driven by the main gearbox so that you will always have hydraulic pressure (main and backup) as long as the rotors are turning faster than a minimum speed. Usually you will fall out of the sky before that rotor speed falls below that threshold.

There is a rotor speed that if it falls below that rpm, you will have lost the ability to get it back up and will have lost the ability to auto rotate.

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Image source

In heavy helicopters such as the CH-53E Super Stallion, the swash plate is actuated hydraulically only, there is no manual reversion and if all hydraulic power is off, the helicopter cannot be controlled. That is why there are redundant hydraulic systems, powered by pumps mechanically connected to the rotor transmission: as long as the rotor turns, the pumps provide hydraulic power. Loss of all hydraulic power is considered highly unlikely.

The main hydraulic actuation cylinders are located at the swash plate and at the tail rotor, and the cable run to the flight controls is of considerable length, creating some friction and lack of precision. Also, there is a mechanical mixer installed which provides coupling between collective-pedals-cyclic: when collective is increased, the mixer provides extra tail rotor input without the pilot having to move the pedals. Without any additional measures, the mechanical mixer also creates cross coupling between flight controls which would further complicate precision control: it is a big ship!

In order to make the stick forces simple and light, there is a second hydraulic actuator at each flight control, just behind the cockpit: they are irreversible, and the AFCS system can independently move these and add autopilot inputs. The feel characteristics are as follows:

  • With AFCS ON, only artificial feel forces (including trim) are felt plus the friction of the short run to the AFCS system. If the pilot moves a control, the mixer does its work without any other flight control being back driven.
  • With AFCS OFF, the higher friction of the run to the swash plate primary actuators is felt, plus back driving forces from the mechanical mixer.

During autorotation, the coupling between the swash plate and the flight controls is exactly the same as when in powered flight. However, there is considerable difference in the feel and in aircraft response with AFCS ON or OFF. Both conditions are modelled in flight simulators and the pilots are experienced in controlling the ship in either condition.

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