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.
The climb rate and forward speed are zero for all points.
The same helicopter type, weight, and CG location are used throughout.
Drag and roll
When flying to the right, the
fuselage and
vertical fin
experience lateral forces pushing to the left, as shown in the plots below (normalized by gross weight).
These forces are overcome primarily by rolling the helicopter “right wing down”
so that main rotor thrust pushes right and counters these drag forces.
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.
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 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 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 remaining roll moment 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.
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