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I have been reading up on the controls of the CH-47 Chinook. I understand how yaw movement is created not by counter torque but by opposite cyclic on the front and back rotors. What I'm wondering about is adverse torque created when differential thrust via collective is used for pitch control. If one rotor is producing more thrust doesn't that mean that the increased drag on the rotor will produce more torque than the opposite rotor which is producing less lift? I understand that the rotors are locked in synchronization so that they can't produce different torque based on their RPM but what about torque created by the different aerodynamic effects on each rotor? How is would this unintended yaw be canceled out?

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  • $\begingroup$ Not an answer to your question, but rotors can and do produce different torque even at the same RPM. The fact that they're locked in unison does not imply anything beyond that they have the same speed. Ignoring the possibility of a stall, if one rotor blade has more angle attack the other, then it produces more lift and consumes more power. Consuming more power at the same rpm, means requiring more torque. $\endgroup$ Aug 17, 2023 at 12:47
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    $\begingroup$ Locking them in unison prevents the rotor blades from hitting one another - generally considered a Bad Thing™ $\endgroup$
    – FreeMan
    Aug 17, 2023 at 14:02

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What I'm wondering about is adverse torque created when differential thrust via collective is used for pitch control.

Differential thrust between the two rotors is indeed how the control around the pitch axis is achieved for a tandem helicopter.

If one rotor is producing more thrust doesn't that mean that the increased drag on the rotor will produce more torque than the opposite rotor which is producing less lift?

That's correct, thrust and torque are related to each other and in a quadratic way: doubling the thrust, the torque quadruples. The following plot show this quadratic relationship quite well (consider the dots):

 Rotor polar (Source - original plot from Principles of Helicopter Aerodynamics by J. Gordon Leishman)

So having two different thrusts between the two rotors generates as a side effect a net torque on the fuselage when seen from above, like in the following sketch where the thrust (and therefore the torque) on the rear rotor is for example lower than the one on the forward rotor, like in a pitch-up manoeuvre:

enter image description here (Source, modified by me)

How is this net torque counteracted? Simply tilting the two rotors slightly on the sides in order to generate an opposing torque. In our example, the forward rotor must be tilted to the left and the rear rotor to the right in order to generate a counterclockwise torque (in orange in the next sketch) which compensates for the previous net torque by the two rotors (in blue):

enter image description here

This tilting normally happens automatically, being it built inside the control chain of the helicopter, either mechanically or via the stability system.

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  • $\begingroup$ Wow, great answer. thank you for taking so much time in your answer. $\endgroup$
    – tavis
    Aug 20, 2023 at 8:45
  • $\begingroup$ @tavis you're welcome 🤗 $\endgroup$
    – sophit
    Aug 20, 2023 at 9:29

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