of freedom, as used with gyros, shows the number
of directions in which the rotor is free to move.
(Some authorities consider the spin of the rotor
as one degree of freedom, but most do not.)
A gyro enclosed in one gimbal, such as the one
shown in figure 6-34, has only one degree of
freedom. This is a freedom of movement back and
forth at a right angle to the axis of spin. When
this gyro is mounted in an aircraft, with its spin
axis parallel to the direction of travel and capable
of swinging from left to right, it has one degree
of freedom. The gyro has no other freedom of
movement. Therefore, if the aircraft should nose
up or down, the geometric plane containing the
gyro spin axis would move exactly as the aircraft
does in these directions. If the aircraft turns right
or left, the gyro would not change position, since
it has a degree of freedom in these directions.
Figure 6-35.-Action of a freely mounted gyroscope.
A gyro mounted in two gimbals normally has
two degrees of freedom. Such a gyro can assume
gyroscope holds its position relative to space, even
and maintain any attitude in space. For illustrative
though the earth turns around once every 24
purposes, consider a rubber ball in a bucket of
water. Even though the water is supporting the
hours.
ball, it does not restrict the ball's attitude. The
The factors that determine how much rigidity
ball can lie with its spin axis pointed in any
a spinning wheel has are in Newton's second law
direction. Such is the case with a two-degree-of-
of motion. This law states "The deflection of a
freedom gyro (often called a free gyro).
moving body is directly proportional to the
d e f l e c t i v e force applied and is inversely
In a two-degree-of-freedom gyro, the base
proportional to its mass and speed." To obtain
surface turns around the outer gimbal axis or
as much rigidity as possible in the rotor, it has
around the inner gimbal axis, while the gyro spin
great weight for size and rotates at high speeds.
axis remains fixed. The gimbal system isolates the
rotor from the base rotation. The universally
To keep the deflective force at a minimum, the
mounted gyro is an example of this type.
Restricted or semirigid mounted gyros are those
basic flight instruments that use the gyroscopic
property of rigidity are the gyro horizon, the
mounted so one plane of freedom is fixed in
relation to the base.
directional gyro, and any gyrostabilized compass
system. Therefore, their rotors must be freely or
Practical applications of the gyro are based
universally mounted.
upon two basic properties of gyroscopic action:
Precession (fig. 6-36) is the resultant action or
deflection of a spinning wheel when a deflective
1. Rigidity in space
force is applied to its rim. When a deflective force
is applied to the rim of a rotating wheel, the
2. Precession
resultant force is 90 degrees ahead of the direction
of rotation and in the direction of the applied
Newton's first law of motion states "A body
force. The rate at which the wheel precesses is
at rest will remain at rest, or if in motion will
inversely proportional to rotor speed and directly
continue in motion in a straight line, unless acted
proportional to the deflective force. The force
upon by an outside force." An example of this
with which a wheel precesses is the same as the
law is the rotor in a universally mounted gyro.
deflective force applied (minus the friction in the
When the wheel is spinning, it stays in its original
gimbal ring, pivots, and bearings). If too great
plane of rotation regardless of how the base
moves. Figure 6-35 shows this principle. The
a deflective force is applied for the amount of
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