Helicopter Torque: Newton's 3rd Law & Pendulum Effect

[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?

 

[A.T.]

However when I take out the torque effect by removing engine power I will spin in the same direction as the rotor are turning.

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

 

[wes]

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

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

 

[A.T.]

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

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...

 

[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

 

[russ_watters]

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

Is it still running a generator to power the controls?

 

[wes]

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

 

[A.T.]

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

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.

 

[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.

 

[256bits]

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

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.

 

[russ_watters]

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

 

[wes]

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.[/QUO
Well that's kind of a difficult question because when we are on the ground at %100 power, there isn't any torque on the blades because our collective isn't up. (the collective basically tells the engine when to add more power) however when the collective comes up engine power is being supplied to the rotors thus meaning we need more anti torque at that given power setting. so the more power we pull with our collective the greater the anti torque pedal needed because of the helicopter wanting to yaw because the torque factor.

 

[wes]

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

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?

 

[jfmcghee]

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

 

[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

 

[CWatters]

Once disengaged the main rotor is connected to the tail rotor via a drive shaft.

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?

 

[A.T.]

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?

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.

 

[wes]

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?

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.

 

[wes]

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.

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?