Torque & Translating Tendency

Torque effect

The engine drives the main rotor through the transmission. The rotor accelerates a large mass of air downward. By Newton's third law, the main rotor exerts an equal and opposite force on the engine, transmission, and ultimately the airframe. Without a corrective force, the fuselage would rotate in the direction opposite the main rotor's spin.

For the standard US-built helicopter (main rotor turning counter-clockwise as viewed from above), the fuselage tries to yaw right. Robinson, Bell, Sikorsky — most American designs work this way. Many European designs (Airbus/Eurocopter family, MBB, etc.) use a clockwise main rotor, and torque tends to yaw the nose left.

Torque is proportional to the power being applied. Pulling collective adds power, which adds torque, which requires more pedal to counter. This is why every collective movement in a helicopter is paired with a pedal input.

Anti-torque: the tail rotor

The tail rotor is essentially a small horizontal-thrust propeller mounted on the tail boom. It produces sideways thrust to counter the yawing torque of the main rotor. The pedals control the pitch of the tail rotor blades — pushing left pedal (US helicopters) increases tail rotor thrust to overcome torque and yaw the nose left; pushing right pedal reduces tail rotor thrust and lets the natural torque yaw the nose right.

The tail rotor is one of the highest-stress components on the aircraft and one of the most often-misunderstood. It eats roughly 5–15% of total engine power even in straight-and-level flight. It also faces a number of unique aerodynamic problems — chief among them Loss of Tail Rotor Effectiveness.

Anti-torque doesn't have to come from a tail rotor. NOTAR systems (No Tail Rotor) duct compressed air through slots in the tail boom, using the Coanda effect plus a directable jet. Fenestron (Eurocopter) is a shrouded tail fan. Tandem-rotor helicopters (Chinook) use counter-rotating main rotors and have no tail rotor at all. The aerodynamic problem is the same; the solution differs.

Translating tendency (tail rotor drift)

The tail rotor's thrust solves the yaw problem but creates a lateral one. The tail rotor pushes the helicopter to one side — for a CCW main rotor with the tail rotor pushing left to counter right yaw, the helicopter drifts right in a hover.

This is translating tendency, sometimes called tail rotor drift. Without compensation, a helicopter in a powered hover slides slowly sideways. The pilot counteracts it with a small amount of opposite cyclic input — for a US helicopter, slight left cyclic in a hover.

Many manufacturers compensate for translating tendency at the design level by mounting the rotor mast slightly tilted to one side, or by rigging the cyclic to introduce a small offset when centered. Either way, you'll often hover with the cyclic visibly offset from absolute center — that's not a malfunction, it's translating-tendency compensation.

Why this matters at low airspeed

Both torque and translating tendency are most pronounced when:

• Airspeed is low (no translational lift to reduce induced power demand)
• Power is high (high collective = high torque)
• Density altitude is high (rotor produces less lift per pitch, so collective is higher to maintain hover)

That combination — slow, high power, high DA — is exactly the setup for a Loss of Tail Rotor Effectiveness event. It's also where most pedal-control surprises happen: the helicopter that hovered fine at sea level on a cool morning may need full left pedal in a hover at 5,000 ft on a hot afternoon.