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I know that in helicopters the N2 and Nr are coupled. I would appreciate a physical explanation leading to the cause of how the rotor blades could overspeed if collective (power) is lowered suddenly

I have heard that the rotor head needs to have a torque load in order not to overspeed. but I am not sure how that works.

The only thought I have in mind is if I am spinning something and suddenly I stop doing so, it is going to spin at the speed it had when I stopped the motion and/or less and decrease from there. But I dont see how can something could "overspeed" if the motion (energy) is stopped. The kinetic energy will be dissipated by lowering its speed but it cant increase, if so how and why? thank you

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On a turbine helicopter, the engine is being managed by a governor that maintains the rotor RPM (that you trim to a specific value in the green arc). When you move the collective, you are changing the torque demand on the engine and the governor senses the resulting incipient rotor RPM change up or down, and lowers or raises engine torque by increasing for decreasing the speed of the gas generator (the gas generator in a free-turbine turboprop or turboshaft is equivalent to the engine core on a turbofan).

There is some lag to this response loop. If you drop collective really fast while the engine is producing significant torque, there's a delay because the governor doesn't respond by reducing torque until the RPM starts to increase, the response itself has a bit of a lag as an internal control loop, and unloading the blades by lowering collective provides less resistance to the existing unreduced torque.

So basically if you slam the collective down fast enough, the engine gets unloaded faster than the governor can respond by reducing engine torque, and you might get a momentary overspeed until the governor catches up, especially if the RPM was near the top of the green arc when you did it.

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  • $\begingroup$ thank you, "less resistance to the existing unreduced torque" Are we talking about the resistance/drag caused by the high AOA on the rotor blades hitting the relative wind towards the head?? $\endgroup$ Feb 20 at 5:56
  • $\begingroup$ Yes. The blade is just an airplane's wing forced to go in a circle. Shove the nose down on an airplane with a high power setting and keep the power on until you actually see the airspeed start to rise before you cut it back. You'll gain a few knots to the extent the surplus thrust remained in place. The reactions are just way faster with a helicopter rotor. It's the opposite of cutting off power and delaying lowering collective for a second. RPM decays. Just like delaying lowering the nose in an airplane, after the power is cut, drops the speed. $\endgroup$
    – John K
    Feb 20 at 12:44
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The engine keeps pushing the rotors just as hard (at least at first), but the drag goes down so the rotors speed up.

You've got the metaphor backwards. Imagine instead that you are removing a bolt with a wrench. It's slow going and takes a lot of effort. If the wrench slips off the bolt but you're still pushing hard on the wrench, what's going to happen?

When you notice it start slipping, you'll stop pushing so hard. But not quickly enough that the wrench doesn't turn pretty fast at first. This is the same with the helicopter- the governor will work to reduce the torque and keep the rotor RPM the same. But if the collective is dropped too quickly the rotor will speed up at first.

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  • $\begingroup$ awesome thank you, "but the drag goes down so the rotors speed up" Are we talking about the drag caused by the high AOA on the rotor blades? and when the collective is lowered the AOA on the rotor blades reduces creating less drag? $\endgroup$ Feb 20 at 5:49
  • $\begingroup$ @YamchaAviator Yes. $\endgroup$
    – Chris
    Feb 20 at 6:21

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