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Torque Reaction - PowerPoint PPT Presentation

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Torque Reaction. The fuselage’s reaction to the turning of the main rotor system is Torque Reaction. Newton's third law of motion states that for every action, there is an equal and opposite reaction. The engine power on most American built single engine helicopters causes the rotor system

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Torque Reaction

The fuselage’s reaction to the turning of the main rotor system

is Torque Reaction

Newton's third law of motion states that for every action, there

is an equal and opposite reaction. The engine power on most

American built single engine helicopters causes the rotor system

to rotate in a counterclockwise direction. The reaction is that

the fuselage will rotate clockwise. The degree of right yaw is

directly proportional to the amount of power applied.

 As viewed from the top


Translating Tendency is the movement of a helicopter in the direction of Tail Rotor thrust. This is caused by trying to cancel a turning moment about the mast with a thrust force and moment from the tail rotor.

The potential for movement is proportional to the amount of power applied and the amount of tail rotor thrust needed to overcome torque reaction.



effect is for


to drift to

the right

Torque Effect

Tail Rotor Thrust


Corrections for translating tendency:

  • Rigging of the cyclic system
  • Application of some left cyclic, the method used in most American-built helicopters
  • Tilting the rotor mast to the left
  • Programmed mechanical inputs from Automatic Flight Control System (AFCS), Stabilization Augmentation System (SAS), Mechanical Mixing Unit (MMU), or any combination of the three

When left cyclic is applied to prevent the right translating tendency, the force of the main rotor is applied to the left. If the left force created by the main rotor is a greater distance from the center of gravity then the right force of the tail rotor, the a left rolling moment will occur.

This will cause the helicopter to hover left skid low and will be more pronounced in a tail low hover (aft CG)


Translational Lift

The additional lift obtained through the increased efficiency

of the rotor system with airspeed obtained either by horizontal

flight or by hovering into wind


6-10 knots

As airspeed increases, the helicopter starts out running

major downwash, causing the relative wind to become more



At 16-24 knots the rotor system has outrun the effects of

downwash. The airflow is nearly horizontal through the rotor with little recirculation back into the rotor. This horizontal flow

significantly reduces induced

flow which increases angle of attack.

Just as the main rotor gains efficiency with horizontal airflow,

the tail rotor too becomes more efficient during this transition

to forward flight. As the tail rotor gains efficiency, it produces

more thrust and causes the nose of the helicopter to yaw left.

During a takeoff where power is constant, the aviator must

apply right pedal as speed increases to correct for the left yaw.


At a given angle of incidence,

a more vertical airflow increases induced flow and aerodynamically reduces the angle of attack, creating the need for more pitch in the blade to maintain a constant lift vector

Reduced inflow velocity causes

angle of attack to increase with

no increase in blade pitch. This

results in an increase in lift with

a decrease in induced drag. The

reduction in induced drag results

in a more vertical lift vector for

each rotor blade


Transverse Flow Effect

Simply stated, Transverse Flow Effect is the difference in lift

between the forward and aft portions of the rotor disk.

Because of coning and forward tilt of the rotor system, air

moving over the forward half of the rotor disk is more

horizontal then air over the aft portion of the rotor disk

The result is an increase in induced drag in the aft portion

of the rotor system caused by the air having a greater

downwash angle in the aft portion of the rotor disk.


Airflow over the aft half of

the rotor with a greater

induced flow and a reduced

angle of attack

Airflow over the forward portion

of the rotor with more horizontal

airflow, reduced induced flow

and a greater angle of attack


The increased angle of attack in the front half of the rotor increases lift of the blade at that location. This in turn causes the blade to flap up. Due to phase lag, the

maximum upflapping displacement occurs over the left

side of the helicopter.

The decreases angle of attack in the rear half of the rotor

causes the blade to flap downward. Phase lag causes

the maximum downflapping to occur over the right side.

The combined effects result in the rotor disk tilting to the

right and changing the direction of the lift vector.












Oversimplified illustrations of Transverse Flow Effect before and after Gyroscopic Effect

The pilot can recognize Transverse Flow Effect because of increased vibrations in the helicopter as airspeeds increase towards ETL on take off and decelerating through ETL during landing. The greatest lift differential occurs at those speeds.


At higher airspeeds, lift

differential between the

fore and aft portions of

the disk begins to

decrease. The cyclic

must be moved back to

the right at higher cruise


As the pilot senses the

right tilt of the rotor, he

must apply left cyclic

to prevent a change in

the attitude of the disk.