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.

Direction of Rotation Torque Effect

Torque Effect Tail Rotor Thrust Overall effect is for helicopter to drift to the right 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 Hover 6-10 knots As airspeed increases, the helicopter starts out running major downwash, causing the relative wind to become more horizontal

Just as the main rotor gains efficiency with horizontal airflow, 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.

+ - + - Before After 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 speeds 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.

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