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Helicopter Overtorque

- question from Tieo Jing Jin

When discussing helicopters, the term "overtorque" simply refers to the ability of a helicopter pilot to apply torque (or throttle) to the engine above the specified 100% limit. It is not uncommon to see torque levels as high as 130%.

There are, of course, more details to the concept of overtorque. First of all, it is best to understand how a helicopter works from a pilot's point of view. (See previous questions on the torque effect and helicopter controls.) Most helicopters have what is known as a constant speed rotor. This is similar to a constant speed prop in fixed wing aircraft. No matter what phase of flight the helicopter is in, the rotor is always turning at the same speed. The variation in thrust or lift is achieved by changing the pitch of all the blades simultaneously (pulling or pushing on the collective). As one might imagine, as the pitch of the blade increases, the torque required to keep the rotor turning at a given speed increases. For comparison, imagine yourself with a paddle on a canoe. If you turn the paddle edgewise to the water flow, it is very easy to move it through the water. If you turn it so it is perpendicular to the water flow, the paddle becomes a lot harder to move through the water. Fortunately, most modern helicopters have the ability to automatically link the throttle and collective controls. With this system, the helicopter flight control system automatically increases or decreases the torque to keep the rotor turning at a constant speed. In some situations, the pilot might require extra lift or thrust, and will "pull in" a lot of collective. As the engine tries to maintain the constant rotor speed, the torque can exceed the 100% limit.

The confusing part of overtorque is that the 100% limit is somewhat arbitrary. The limit is usually set according to fatigue limits of the engine, so exceeding the 100% limit for short periods of time won't cause the engine to catastrophically disassemble. In fact, there are many failsafe systems (including simple physics) that prevent the engine from reaching a stage of failure due to overtorque. The requirements dictating engine overtorque vary depending on the helicopter and its use. Usually there are rules specifying the amount of time that can be spent above 100% torque. In addition, other rules specify that when a helicopter exceeds 100% torque at any point during its mission, certain parts must be inspected visually, or, depending on the amount of overtorque, some parts must be completely replaced. Of course, from a pilot's point of view, the fact that the helicopter needs to be inspected by a mechanic after a flight is of little concern when the bad guys are running towards you with big guns!
- answer by Doug Jackson, 25 March 2001

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