User Guide
3: GROUND SCHOOL
3.9
HORIZONTAL MOTION
Unlike a fixed-wing aircraft, which is propelled forward by thrust from the engines,
a helicopter moves forward (or backward, or sideways) by tilting the lift force.
To move forward, a pilot adjusts
his cyclic control (see Cyclic
Stick, p. 4.3), which adjust the
angles of the individual blades
as they rotate so that the entire
rotor disc (the plane of rotation
of the rotor and blades) remains
tilted downward (see diagram).
When the aircraft hovers, the
rotor disc remains parallel to the
ground and all of the lift force it
generates pushes the aircraft
up. When the rotor disc is tilted,
the lift force is also tilted. When
the disc is tilted as in the
diagram above, more lift is being generated in front of the helicopter than behind
it, and the helicopter moves forward.
Because some of the lift force is being directed forward when a helicopter is
moving forward, the vertical component of the lift force is smaller. Thus more
total
lift must be generated to keep the aircraft moving forward at a constant altitude.
The same thing happens when you are turning (rolled to one side), as the lift force
is shifted to the side.
Translational Lift
Lift is initially produced as the rotor blades spin and their angled surfaces cut
through the air. When the helicopter switches from a hover into forward motion, air
passes over the helicopter from nose to tail. This air moves over the spinning
blades as well, creating additional lift. Lift created by the horizontal motion of a
helicopter is called
translational lift
.
Just as more “normal” lift is produced at higher
blade
speeds, more translational
lift is produced at higher
air
speeds — up to a certain point. Translational lift kicks
in at speeds near 20 knots, and, in the Longbow Apache, reaches a maximum
level at about 60-70 knots. Since this speed produces the best lift, it is the speed
at which the Longbow Apache achieves the best climb rate. Beyond this speed,
more power is required to overcome the effect of air resistance (or drag) against
the blades, and the amount of lift generated decreases.
Total lift
Total lift Vertical lift
Hover
Horizontal lift
Forward
Flight