As shown in Fig. 8-2, in addition to a pitch axis, rotor blades are generally hinged near
the rotor hub to allow the blade to flap up and down and swing forward and backward.
This allows the rotor blades to react to the constantly changing aerodynamic and
gyroscopic forces encountered in flight. Without these hinges the rotor blades would
likely be unstable, and would need to be built stronger and heavier to withstand
in-flight forces
OBCKGD02.
Helicopter dynamics are substantially different from airplane dynamics. The rotating
main rotor on top of a helicopter acts like an immense gyroscope. As such, the rotor
obeys the physical laws of gyroscopes. The basic operating principles of gyroscopes
are not obvious, so read the next sentence over a few times because this
rule-of-thumb is the key to understanding gyroscopes and helicopters.
Forces applied to tilt a rotating gyroscope produce motion 90 degrees later in
the direction of rotation
.
Why? Because the gyroscope is rotating. If the gyroscope was not rotating, the
forces would cause it to tilt where the forces were applied. Since the gyroscope is
rotating, it starts to tilt where the forces are applied, but the rotational motion
effectively carries the tilting motion along with it. The maximum tilt actually occurs 90
degrees later in the direction of rotation.
Op er a tor's Guide
How Helicopters Work
8-4
LITE MA CHINES
Figure 8-2.
