# B Helicopter torque

1. Apr 20, 2017

### wes

My question may be very simple for most of you but it's giving me great difficulty and your help would be greatly appreciated! So a helicopter with a counter clockwise rotation rotor will want to turn its nose in the opposite direction because of torque effect. This demonstrated Newton's third law. However when I take out the torque effect by removing engine power I will spin in the same direction as the rotor are turning. What force makes me spin this way since the helicopter is essentially a pendulum just hanging?

2. Apr 20, 2017

### A.T.

If the rotor spins the engine, then the torques are reversed. You have to decouple rotor and engine to avoid this.

3. Apr 20, 2017

### wes

Well technically it is by a sprag clutch, when throttle is rolled the rotor is automatically disengaged from the engine to let the inertia of the rotor blades drive the autorotation

4. Apr 20, 2017

### A.T.

Could be friction in the clutch. But if the rotor is freewheeling without friction then it exerts no torque on the helicopter via the shaft. If the helicopter still spins, then it is something else. Tail rotor still running? Downwash producing an aerodynamic torque at the hull? etc...

5. Apr 20, 2017

### wes

Your correct about the tail rotor still running and that's the only thing that enters my mind as well. I'm thinking that the anti torque pedals may be rigged in a way that there is still left pitch in the tail rotor even when the pedals are centered in the helicopter

6. Apr 20, 2017

### Staff: Mentor

Is it still running a generator to power the controls?

7. Apr 20, 2017

### wes

no the controls aren't hydraulic they are just push pull rods connected to pitch links ( all manual controls )

8. Apr 21, 2017

### A.T.

Could be. Is there no adjustment, that allows to set the neutral position? The other possibility would be some aerodynamic surface, that produces a counter torque from main rotor downwash.

9. Apr 21, 2017

### wes

Not for the pilot to adjust there isn't. The helicopter does have a vertical stabilizer but that only comes into affect with forward motion. I did find an article that said the left pedal when fully applied has about 19 degrees of positive pitch in the tail rotor as compared to the right pedal when fully applied has a negative ten degrees thus making me think there is still positive pitch in the tail rotor even with neutral pedals.

10. Apr 21, 2017

### 256bits

When When you are sitting on the ground in "idle" mode with the engine running does the helicopter spin around, or do you have to use the pedals to control rotation?
That should be the neutral position of the tail rotor would it not.

11. Apr 21, 2017

### Staff: Mentor

Perhaps it is just a surprising amount of friction in the drive train after the clutch?

12. Apr 21, 2017

### wes

13. Apr 21, 2017

### wes

That could be a possibility indeed!! It uses a sprag clutch to disengage the engine from the rotor system for auto rotational purposes. Once disengaged the main rotor is connected to the tail rotor via a drive shaft. Which would bring us round circle as to why would we spin with the direction of the rotor?

14. Apr 21, 2017

### jfmcghee

Which helicopter are you flying? I never noticed a tendency to rotate in the same direction as the rotor during autorotations.

15. Apr 22, 2017

### wes

It's a Robinson 22. Not necessarily in auto rotations because in an auto we have the vertical stabilizer to keep us from spinning but strictly referring to hover autos

16. Apr 22, 2017

### CWatters

I'm not a heli pilot but if you drop the collective pitch what happens to rotor rpm? If the rotor rpm goes up (and the rotor is driving the tail rotor) then the tail rotor RPM also goes up providing too much torque?

17. Apr 22, 2017

### A.T.

Yes, even at zero pitch the tail rotor and its drive shaft have some resistance. So the main rotor applies a torque to the rest of the helicopter in the direction it spins.

Last edited: Apr 22, 2017
18. Apr 22, 2017

### wes

In a normal autorotaton at say 500 ft we can build the rpm with the air moving through the rotor system however in a hovering autorotation at about 5 feet we drop the collective which in that case will maintain our rotor RPM and not build it. Then before touchdown at about 2 feet we pull collective which causes our rotor RPM to drop and tail rotor to lose authority.

19. Apr 22, 2017

### wes

Awesome that's what I was thinking but didn't know how to describe it. What would be the name for this? With power our opposite reaction would be called just a "torque reaction" with inertia the tendency for the helicopter to go with the rotation would be called what?