Negative Torque System
Under certain flight conditions with the engine throttled back to idle and the prop in fine/flat pitch, the prop may produce zero thrust, or drag. The drag in this condition is known as the ‘flat plate drag’. Imagine the prop disc as being a large solid plate. Turboprops with their large diameter props are particularly susceptible to drag under this condition, the fixed-shaft turboprop being affected more than the free-shaft turbine engine. The CSU would naturally select a fine/flat pitch setting, but this produces the highest drag and negative torque with the engine idling. To alleviate the high drag and negative torque at low power, turboprop engines employ a negative torque system incorporated within the CSU/reduction gear assembly. The negative torque system senses when negative torque is being produced by the prop and commands the CSU to turn the blades to a more coarse pitch setting. At this setting the blade angle will be correct to absorb a predetermined amount of horsepower. As soon as the negative torque is removed with increased power selection, the CSU resumes normal operation
Autofeathering is a feature more common on large transport aircraft, particularly turboprop transports. A turboprop with a reduction gear of around 20 to 1 will have its engines driven up to a speed of twenty times the prop speed. This could place enormous stress on the transmission between the engine and propeller and cause excessive prop drag. With the engine being driven by the prop, the negative torque sensor will activate the autofeathering system and feather the blades automatically.
The autofeathering system operates on a different basis to the negative torque system, which senses when the engine is still running under power but producing negative thrust, as opposed to an autofeathering system that senses a failed engine condition and turns the prop blades into the feathered position. The negative torque system operates on a continuous basis ‘as required’ at low power settings. Autofeathering acts only once – when the engine has failed.