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Helicopter Lateral Flight

In this article, we examine how various values (flapping, pitch, torque, …) change with helicopter lateral speed. We consider a set of unaccelerated, trimmed flight conditions in which a helicopter is flying directly to the right (with 90 degrees of sideslip). The climb rate and forward speed are zero for all points, and we use the same aircraft weight and CG location throughout. Please note that this analysis is based on a particular helicopter model. Specific numbers (and even trends in rare cases) can be model dependent.

Drag and roll

When flying to the right, the fuselage and tail fin experience lateral forces pushing to the left, as shown in the plots below (normalized by gross weight, negative being left, positive right). These forces are overcome primarily by rolling the helicopter “right wing down” so that the main rotor thrust pushes right and counters these drag forces.

Plot of fuselage drag vs. lateral speed
Plot of helicopter roll vs. lateral speed

Torque, tail rotor thrust and pedal

Just as in forward flight, engine torque decreases with speed in this range. This is due to the main rotor moving out of its downwash, which reduces induced drag. The torque reduction necessitates lower tail rotor thrust to balance the net yaw moment on the aircraft. Normally, tail rotor thrust would be lowered by applying right pedal. However, in this case the rightward motion of the aircraft produces right-to-left airflow through the tail rotor, which reduces its thrust too much already. Hence, although less thrust is needed, left pedal is applied to balance yaw moments.

Plot of fuselage drag vs. lateral speed
Plot of pedal vs. lateral speed

Roll moments

At each speed, the net helicopter roll moment must be 0. The primary roll moments are from the rotors and fuselage aerodynamics. When moving to the right, the fuselage develops a substantial positive (“right wing down”) roll moment, as shown in the plot below. The tail rotor, being above the aircraft CG and thrusting to the right, produces a positive roll moment. As rightward speed increases and tail rotor thrust decreases, as explained in the prior subsection. Therefore, the roll moment due to the tail rotor decreases, as shown in the plot below. The decrease amounts to about half of what’s required to balance the fuselage roll moment. The remainder comes from the main rotor. Perhaps counter-intuitive, as the rightward speed increases, the main rotor produces more negative (“left wing down”) roll moment. The pilot is, however, required to provide right cyclic input to increase rightward speed. This is because of the tendency of the main rotor to flap back against the direction of motion. So, at higher rightward speeds, the main rotor wants to flap left (and decrease roll moment too much), so the cyclic must be moved right.

Plot of fuselage roll moment vs. lateral speed
Plot of tail rotor roll moment vs. lateral speed
Plot of cyclic vs. lateral speed
Plot of main rotor roll moment vs. lateral speed

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