Originally Posted by
shumway76
Helicopters require tail rotor to counter the torque caused by the rotor blades.
Fixed wing single engine propeller aircraft also suffers from torque effect, but doesn't need something as dramatic as a "counter torque" mechanism.
Is the torque effect caused mainly by the length of the blade?
- helicopter has long blades, so more counter torque is required.
- fixed wing single engine propeller blades are short, so not such a great counter torque is required
Is my guess above correct?
Thank you!
Fixed wing aircraft do have anti-torque devices. They're called wings. If the aircraft starts to rotate in roll the different changes in angle of attack of the two wings (one will increase, the other decrease) applies an anti-roll force opposite to the rotation. Airplanes are inherently roll "stiff", in normal flight - they have very heavy roll damping. Ailerons are used to adjust this mechanism, and provide a small net angular momentum in either direction, at the pilot's discretion. The vertical stabilizer provides an effective mechanism for preventing the accumulation of angular momentum around the aircraft's normal axis. The rudder is used to trim this anti-rotational force.
The main difference between an airplane's anti-torque devices and those of a helicopter comes about because an aircraft is in forward motion: passive flight surfaces have a predictable airflow over them at all times and can be used. A helicopter's anti-torque mechanisms have to work while the aircraft is in hovering flight, moving only very slowly, or moving in a non-forward direction.
The torque effect from a helicopter comes from two contributions:
1. Spinning the rotor up to speed. As long as you do this while the helicopter is on the ground, the angular momentum imparted to the blades is balanced by twisting the planet the other way, through the wheels or skids. This effect stops when the rotor is at speed, as its angular momentum remains constant.
2. Spinning the air, as a by product of pushing it downward to generate lift while under power. This is a continous effect. To avoid building up angular momentum in the helicopter (which would cause the body to rotate around the rotor axis) the helicopter needs a source of angular momentum equal and opposite to what it's feeding into the air - which it gets from a tail rotor.
3. A rotor under autorotation doesn't apply any net angular momentum to the air. Gyrocopters don't have tail rotors.
4. What most people call "torque" effects in airplanes are precessional effects.
Other notes: torque is not a force. It has different physical units.