Now, if you stop the gyro top and turn its axis horizontal, and then start it
spinning again, balancing one end on a pivot, (figure. 1.3), it won't fall. The top's axis will
stay horizontal, resisting the tendency of gravity to change its direction Although the
gyro will RESIST the force that gravity applies to it, the gyro will still RESPOND to that
force. The gyro responds by moving its axis at a RIGHT ANGLE to the APPLIED
FORCE. The axis will tilt in a direction 90º away from the applied force. This is called
PRECESSION.
1.7 Figure 1.4 is another view of the same gyroscope. Its far end is still balanced on
the pivot. Gravity is pulling down on the gyro. If the gyro rotor is turning in the direction
shown by the arrow, the near end of the frame (axis) will move to the left. If the rotor
were turning in the opposite direction, the frame would move to the right. Note that in
each of these examples the direction of movement was displaced from the applied force
(gravity) by 90º. The axis stays horizontal, but the gyroscope responds to the force of
Fig. 1.4 Gyro precession
Gyro action may be summarized as follows: A spinning gyro tends to keep its
axis pointing in the same direction. This is called RIGIDITY. If you apply a force that
tends to change the direction of the spin axis, the axis will move at a right angle to the
direction of the applied force. The direction of precession will be 90º clockwise from the
applied force if the rotor is spinning clockwise (when viewed from the "free" end of the
rotor's axis); if the rotor is spinning counterclockwise, the precession will be 90º
counterclockwise. If the axis is horizontal, and you try to tilt it, the axis will turn. If the
axis is horizontal, and you try to turn it, the axis will tilt. This second characteristic of a
gyro is called PRECESSION.
1.9 Because of precession, we can control the direction that the spin axis points.
This enables us to aim the spin axis where we want it to point. Without precession, the
rigidity of the gyro would be useless.
Basic Gyro Elements
1.10 The gyro shown in figure 1.5 is a basic, universally mounted gyro, sometimes
called a free gyro. Its components are rotor, inner gimbal, outer gimbal, and base or
support. Gimbals are devices that permit the rotor to assume any position and retain
that position when the support is tipped or repositioned. Note that in figure 1.5, the
support may be moved about all axes without the rotor position being disturbed.
1.11 As you know, gravity is a force that acts along parallel lines upon each particle of
matter. A plot of the resultant gravitational force on a body such as a gyro would be
equivalent to the sum of these separate forces. The point at which the resultant force is
applied is called the CENTER OF GRAVITY. To have a balanced gyro, the center of
gravity must be located at the intersection of the three axes of the gyro.
Rigidity
1.12 A gyroscope is a spinning body that tends to keep its spin axis rigidly pointed in
a fixed direction in space. What do we mean by "fixed direction in space"? A fixed
direction on Earth is by no means fixed in space, because the Earth turns once on its
axis every 24 hours, and makes a complete revolution around the sun every year. The
sun itself is moving through space, taking the Earth and the other planets with it.
Because of these motions, the expression "fixed direction in space" as used in this
explanation is theoretical. For all practical purposes, we can say a line from the Earth to
a distant star is a fixed direction in space. If the spin axis of a spinning gyro is pointed at
a distant star, it will remain pointed at the star, as the Earth turns. Gyro rigidity is the
strength with which a gyro resists any external force that would tilt its rotor spin axis.
There are three factors that determine gyro rigidity: weight of the rotor, distribution of this weight and rotor speed.
the system is equal to the input energy. Hence the energy necessary to spin the gyro
rotor is contained in the rotor as angular momentum, which is a function of rotor weight
and the speed of rotor rotation. The heavier the gyro rotor the larger the torque
necessary to spin it, and the greater the angular momentum of the rotor. If we have two
rotors with identical shapes but different weights spinning at the same velocity, the
heavier of the two will be more rigid in its spin axis since it has the greater angular
momentum.
1.14 Now let's look at the effect of weight distribution in the rotor of a gyro.
Consider three rotors of the same weight, as shown in figure 1.6, view (A), view
(B), and view (C), with the diameter of one rotor half the diameter of the other
two. Now, when we spin these rotors at the same speed, we find that the rotors
with the greater diameter are much more rigid than the one with the smaller
diameter. Next, we find that we can make the rotors equally rigid by causing the
rotor with the smaller diameter to spin faster than the larger rotors. Thus rigidity
depends both on speed and distribution of weight. The weight of the larger rotor
being farther away from the axis of spin causes it to be more rigid. This effect is
even more pronounced if we shape the rotor as shown in view C of figure 1.6.
Shifting as much weight as possible to the outer rim of the rotor increases rigidity even futher.
