User Guide

LONGBOW 2 CHAPTER 3
Torque
A bizarre function of early
helicopter models frustrated
designers for centuries.
Whenever a model took
off, it would keel over in
one direction or spin and
hop uncontrollably. This
behavior was later under-
stood to be the result of a
torque force: the rotor
shaft was spinning in one
direction, while the rest of
the craft was twisting in
the opposite direction.
Why does torque occur?
Newton’s Third Law of physics states that for every action, there’s an equal and
opposite reaction. In this case, the reaction is torque. A helicopter body spins
opposite the direction its rotor is spinning.
Tail Rotor
The Longbow Apache has a tail rotor to counteract torque. The pilot uses direc-
tional control pedals to apply force in the opposite direction of the torque created
by the spinning blades. The tail rotor generates this force in the same way the
main rotor generates a lift force – the directional control pedals increase/decrease
the blade angle of attack, which in turn increases/decreases the amount of force
the tail rotor creates. The tail rotor can also be used to yaw the aircraft in a hover
and sharpen turns (see Directional Control Pedals, p. 4.4).
Hovering and Translating Tendency
During hovers, helicopters with a tail rotor tend to drift to one side because the tail
rotor “pushes” the craft sideways. This is called
translating tendency . Helicopters
with blades that rotate counterclockwise (most helicopters built in the U.S.) drift to
the right. To compensate for this drift, the pilot can tilt the entire rotor disc to the
left. This causes the main rotor to apply more force against the tail rotor.
Some modern helicopter designs have built-in features that take translating
tendency into account. The main rotor may tilt slightly to the left when the cyclic
control is centered. Or, the control system may tilt the rotor to the left when
collective is increased during a hover. (See Collective Stick and Cyclic Stick,
pp. 4.2 - 4.3, for more information.)
3.8
Tail tends to turn in
opposite direction
Tail rotor
counteracts
drift of tail
Direction
blades spin