Torque
What is torque ?
Torque is a turning or twisting force. Anyone who deals with helicopters in anything other than just passing, becomes familiar with the term torque, but how many people actually know what it means ?
Here i'll attempt to get to the bottom of the torque mystery, and explain somethings along the way.
From a helicopter perspective torque is both of ally and our enemy, without it rotors would not turn and as a result lift would not be produced. Unfortunately because of torque a traditional single main rotor helicopter must have a means of overcoming the torque reaction in order to prevent it spinning around on breaking contact with the ground. The usual means of overcoming the torque reaction is the installation of a tail rotor on the end of a long arm (the tailboom) which itself produces a torque in opposition to the torque produced in driving the main rotor.
In a helicopter the ultimate source of torque is the engine.
The torque produced by the engine is transferred from the engine ouput shaft to the rotor system by means of a gearbox (or transmission if you are american).
Important concepts
A single torque acting on its own is known as a moment.
Moments produce both rotation and translation.
A pair of equal magnitude equally distanced diametrically opposed torques which are not inline with each other is known as a couple.
Couples produce only rotation.
Mathematically
τ = F * D * Sin * θ
- τ
- Torque [Nm]
- F
- Component of the Force perpendicular to the axis of rotation [N]
- D
- Perpendicular Distance from center of rotation that the force is applied [m]
- θ
- Angle of application of the force (greatest at 90°)
Example Content
τ = P / ω
- τ
- Torque [Nm]
- P
- Power [kg m2 s1] or [W]
- ω
- Angular velocity [radian s-1]
Horsepower = Power / 5252
Rearranging these gives us
(Power / RPM) * 5252 = Torque
So taking say the H300C as an example it produces 190hp at 3200RPM
(190/3200) * 5252 = 311.83lbft
Torque has dimensions of kg m
2 s
2 or lbf ft in non SI dimensions.
It has units of Nm or Newton Meters.
Example Content
What this means
An object will rotate at a constant rate if the torque applied to turn it is equal in magnitude to the torque applied to stop it rotating, this is of particular importance to helicopters as we must overcome the rotor torque due to drag with the correct amount of engine torque in order to maintain Rotor RPM.
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