In a recent stack exchange question writers were comparing autorotation to an aircraft engine out rotation prop when something rather startling occured to me:

In an engine out the airflow will continue to rotate the prop on the same direction as with power. When helicopter loses power, WRT to rotor, it falls "backwards", creating force AGAINST the powered direction of rotation.

The thought is to go negative pitch and allow the rotor to build up angular momentum in the same direction as powered rotation, then switch to positive pitch to use that energy to flare and land.

Is this how it is done?


No, that is not how it is done: minimal collective blade pitch is not negative. The procedure is to lower collective immediately to the full down stop to minimise blade drag, upon which the helicopter will start to descend. The air upflow tilts the local lift vector forwards. Indeed, just like in a glider.

The bit that is not intuitively clear for fixed wingers is the reference frame of the airflow, which is the local blade velocity.

The inner bit of the rotor blade has a lower rotational speed than the tip. The up flow has a relatively larger influence on the lift vector tilt here, and upflow through the rotor drives the inner part of the rotor blade forward, while the outer part still experiences decelerating torques. From Leishman:

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It's similar to a fixed wing airplane really, except that it's happening in a circle. It's done by reducing the rotor blade pitch to the minimum when power is initially lost, by lowering collective, to ensure the blades are able to maintain airspeed (in the form of rotor RPM), then adjusting pitch with collective as necessary to keep RPM in the green arc.

Take away the motive force, the engine and prop in the airplane, or the engine torque acting on the blade root in the helicopter, and you have to lower AOA to maintain flying speed, then adjust AOA to maintain the desired speed (or in the helicopter, RPM). In autorotation, the flow switches from down through the rotor to up through the rotor because that's what it takes to maintain a flyable AOA.

The autorotating rotor is being driven around by the same forward thrust component of the lift vector that moves a gliding airplane forward. The autorotating rotor is just like two gliders passing each other and who's wingtips stick together because the tips were made of Velcro, forcing them to glide in a circle centered around the joined wing tips.

Because they are rotary wings going round, the AOA is high near the root and lower near the tip because the blade speed increases going outboard. Lift is perpendicular to AOA, so the thrust component of the lift vector is stronger near the middle and root than near the tip because the lift vector is canted forward, and the vertical lifting component of the lift vector is stronger near the tip because the lift vector is nearly straight up.

This is why they say most of the lift is coming from the outer part of the rotor disc and most of the driving force is coming from the inner and middle section of the disc (inner to mid span on each blade).

In an autorotation landing, inertia is used to convert forward energy of the blades to additional lift energy. As you come out of the autorotation after the landing flare, you go back into "powered mode" you might say, by pulling pitch, changing the flow through the rotor from upward to downward as when the engine was running. Except the "power" you are using isn't engine torque, it's just the inertia of the rotor blades. You have a couple of seconds to use this inertia before the RPM decays and down you come.

It's a bit like settling a gliding airplane on the ground by pitching the nose up and trading inertial energy of the plane into temporary increased lift.

  • $\begingroup$ Turns out this is a fairly complex subject. Apparently, if there is forward motion, the rotor can be "autogyroed" to generate lift. But if it is purely vertical, the blade pitch would have to be really cranked down. Seems that a forward motion is a big help with helicopters too. $\endgroup$ – Robert DiGiovanni May 19 at 21:45
  • $\begingroup$ Actually autogyros operate with a slightly positive blade pitch, one or two degrees, and they will autorotate straight down but the sink rate is too high for a good touchdown. Except for a few homebuilt gyros with special long stroke gear that CAN land vertically without damage youtube.com/watch?v=GlyR-aSEuig. $\endgroup$ – John K May 20 at 0:43
  • $\begingroup$ Plop! Good gear. There's a few things going on here. $\endgroup$ – Robert DiGiovanni May 20 at 14:14

Going to try an answer to bring a few power off lifting techniques to light.

  1. Parachute: uses drag force vector against gravity

  2. Glider: uses drag force vector against gravity, perpendicular to a forward facing oblique surface, to generate horizontal movement. Wing uses movement to create lift.

  3. Autogyro: Uses air flow in plane of rotor to spin it (much like an anemometer). Generates forward motion by pitching down. Spinning rotor creates lift.

  4. 100% Vertical descent with no forward component. In order to maintain same rotor direction and RPM, at least part of the rotor must go negative.

Notice the descending rotor has its leading edge pitched UP in some diagrams, whereas a gliders leading edge is always pointed down. Simple pool hall physics tells us which way the object will be deflected by the airstream.

There for, an autogyro, like a glider, will control its speed by pitching up or down, but a rotor in vertical descent with no sideways movement must alternate between generating RPM with negative pitch and lift with positive. In reality it can be set for a little bit of both, but the autogyro approach seems to be safer.


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