Transcript Document

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
effect is for
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
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
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
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.