Aircraft Systems and Flight: Negative Torque System in Constant Speed Propellers

A constant speed propeller is a propeller that rotates at a constant rotational speed (RPM) regardless of the flight condition. Whether the aircraft is flying at slow or fast speeds, the propeller spins at the same RPM. Speed adjustments are achieved by changing the propeller’s blade angle or pitch. Propeller blades are just like airfoils and produce lift as the blades rotate through the air. This, in consequence, produces the aircraft to move forward or thrust. Engines with constant speed propellers offer a variety of advantages over fixed pitch propeller engines. For example: they offer better performance at different flight conditions by constantly adjusting the propeller’s blade angle, the ability to change the blade angle to produce thrust reversing to stop an aircraft at landing, and the ability to feather the propeller in flight. The term feathering the propeller is known as the ability to maintain the propeller’s blade angle in a position where it does not induce excessive drag on the aircraft when an engine needs to be shutdown in flight. There are aircraft malfunctions that require an engine to be shutdown in flight, for example, loss of oil quantity. When not rotating, an unfeathered propeller presents a flat surface to the air stream and produces excessive drag. Another condition that arises when an engine is shutdown is the propeller free spinning due to the airstream passing through it. Since the propeller is not being controlled anymore, the airstream can continue to spin the propeller causing it to drive the shutdown engine. This is called negative torque and its dangerous because it causes excessive drag and can further damage a shutdown engine by driving the engine. This is where a Negative Torque System (NTS) comes in to play. When an engine is shutdown in flight, a Negative Torque System feathers the propeller by maintaining blade angle and prevents the propeller from windmilling in flight. If the Negative Torque System was to malfunction in flight during an engine shutdown, the propeller won’t be able to feather and the propeller will spin freely. These conditions can ultimately cause loss of aircraft control due to the increased drag from an unfeathered propeller and a free spinning propeller can cause a fire on an engine that was shutdown due to a low oil quantity by driving the engine again.

References:

Federal Aviation Administration (FAA). (2016). Pilot’s Handbook of Aeronautical Knowledge (PHAK). Chapters 7 and 14.