Negative TR trust
Jim,
1. Conform the quote from the handbook, a fully symmetric helicopter would require a small amount of negative trust in autorotation because in autorotation a small amount of power (5-10 %) is drawn from the rotor to drive a number of systems: this requires an opposite force to the usual torque canceling force in powered flight.
2. For efficiency reasons, many helicopters are not symmetric, and an aerodynamic torque canceling force will be provided by (for instance) the tail fin in order to reduce the amount of power that the tail rotor has to provide in forward flight. In autorotation this aerodynamic configuration needs to be neutralised increasing the need for negative trust.
3. As stated by Steekvlieg, Fast lateral flight (or compensation of strong lateral wind) creates a situation where negative trust is needed in powered flight.
4. Dynamic conditions can arise where negative trust is needed : for example in a hover power failure, delay in control inputs will most likely make the helicopter rotate. To arrest that motion negative trust is needed.
5. Possibly (fast) backward flight or hover with strong tail wind, could create a condition where negative trust is required to control the helicopter.
d3
edited point 5 added