OBCKGD03
The rule-of-thumb for gyroscopes is illustrated by Fig. 8-3. If aerodynamic forces are
applied to a pair of rapidly rotating rotor blades, the rotor blades, acting under the
laws of gyroscopes, will flap 90 degrees later in the direction of rotation. This flapping
will be seen by an observer as a tilt of the entire rotor disk (when a rotor rotates at high
speed, it is difficult for an observer to discern individual rotor blades, and the rotor
appears to be a transparent disk). The aerodynamic forces are created by changing
the pitch of the rotor blades, or by air turbulence.
On Lite Machines helicopters, to tilt the rotor disk backward the main rotor blades are
pitched to a high angle of attack as they pass around the right side of the fuselage and
to a low angle of attack around the left side of the fuselage. The aerodynamic forces
produced by this difference in angle of attack cause the blades to flap upward 90
degrees later over the nose and downward over the tail boom thereby tilting the rotor
disk backward.
Why 90 degrees, and not some other angle like 62 degrees or 127 degrees? The
answer is fairly technical, and involves the concept of mechanical resonance which is
a kind of back-and-forth motion such as the motion of a weight swinging on the end of
a piece of string. But, forgetting the technicalities, think of it like this: there is no good
reason why the point of maximum flapping should be any closer to one of the forces
than to the other. If two equal forces are located opposite each other (180 degrees
apart), then the maximum flap angle should occur right in the middle at 90 degrees.
Op er a tor's Guide
How Helicopters Work
LITE MA CHINES
8-5
Figure 8-3.
