User Guide

3: GROUND SCHOOL
Angle of Attack
When a helicopter powers up, its blades are parallel to the ground. Once they’re
spinning fast enough, the pilot angles the blades. When the blades cut through
the air at an angle, they generate lift.
The angle at which the blade hits the air is called the
angle of attack. This angle
changes as the pilot changes the angle of the blades with his flight controls and
as the direction of the air moving over the blades changes (as, for example, in
gusty wind conditions).
To understand how blade
angle of attack increases lift,
imagine holding your hand
outside a car window while the
car is moving. If you hold your
hand so that your palm faces
the ground, the edge of your
hand cuts through the air with
relatively little resistance. If
you hold your hand perpendic-
ular to the ground, the force of
air rushing against your palm
pushes it back. But if you
angle your hand so that its
front edge is tilted slightly upward, the force of air will push your hand slightly up
as well as back.
Similarly, when air
moves over an
angled blade, the
force of the air
pushes it slightly
upward, contribut-
ing to the total lift
force on the air-
craft. The lift force
generated
increases with the
angle of the blade
— up to a point.
As you saw with
your hand, there
is a point at which angle of attack becomes too steep, when the flow of air over the
blade is disrupted and the force pushing the blade backward is greater than the
force pushing it up. The blade is no longer generating lift. This is known as a
stall .
3.3
Angle of attack is dependent
on the direction of airflow
over the blades
Direction of
Airflow
Angle of attack = 0°
Dynamics of a Stall
Airflow
Disrupted
Airflow