Does the CH-47 Chinook generate differential yaw as a side effect of generating differential lift?

Question

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?

Answer 1

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):

(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:

(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):

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