TURN POINTER. --The turn pointer
for its indications. The power for the turn
operates on a gyro. The gimbal ring encircles the
indicator gyro is either electrical or vacuum.
gyro in a horizontal plane and pivots fore and aft
in the instrument case. The major parts of the turn
BALL. --The ball portion of a turn-and-bank
portion of a turn-and-bank indicator are as
indicator (fig. 6-40) consists of a sealed, curved,
glass tube. The tube contains water-white kerosene
follows.
and a black or white agate or common steel ball
bearing. The ball bearing is free to move inside
A frame assembly used for assembling the
the tube. The fluid provides a damping action and
instrument.
ensures smooth and easy movement of the ball.
The curved tube allows the ball to seek the lowest
A motor assembly consisting basically of
point when in level flight. This is the tube center.
the stator, rotor, and motor bearings.
A small projection on the left end of the tube
The electrical motor serves as the gyro for
contains a bubble of air. The bubble lets the fluid
the turn indicator.
expand during changes in temperature. There are
two markings or wires around the center of the
A plate assembly for mounting the
glass tube. They serve as reference markers to
electrical receptacle, pivot assembly, and
show the correct position of the ball in the tube.
choke coil and capacitors for eliminat-
The plate that holds the tube and the references
ing radio interference. It also mounts the
are painted with luminous paint.
power supply of transistorized indicators.
The only force acting on the ball during
straight flight (no turning) with the wings level
A damping unit that absorbs vibrations
is gravity. The ball seeks its lowest point and stays
and prevents excessive oscillations of the
within the reference marks. In a turn, centrifugal
needle. The unit consists of a piston and
force also acts on the ball in a horizontal plane
cylinder mechanism. The adjustment screw
opposite to the direction of the turn.
controls the amount of damping.
The ball assumes a position between the
reference markers when the resultant of centri-
An indicating assembly composed of a dial
fugal force and gravity acts directly opposite to
and pointer.
a point midway between the reference markers.
When the force acting on the ball become
The cover assembly.
unbalanced, the ball moves away from the center
of the tube.
In a skid, the rate of turn is too great for the
The carefully balanced gyro rotates about the
lateral axis of the aircraft in a frame that pivots
angle of bank. The excessive centrifugal force
moves the ball to the outside of the turn. The
about the longitudinal axis. When mounted in this
way, the gyro responds only to motion around
resultant of centrifugal force and gravity is not
a vertical axis. It is unaffected by rolling or
opposite the midpoint between the reference
markers. The ball moves in the direction of the
pitching.
force, toward the outside of the turn. Returning
the ball to center (coordinated turn) calls for
The turn indicator takes advantage of one of
increasing bank or decreasing rate of turn, or a
the basic principles of gyroscopes--precession.
combination of both.
Precession, as already explained, is a gyroscopes's
natural reaction 90 degrees in the direction of
In a slip, the rate of turn is too slow for the
angle of bank. The resultant of centrifugal force
rotation from an applied force. It is visible as
and gravity moves the ball to the inside of the
resistance of the spinning gyro to a change in
turn. Returning the ball to the center (coordinated
direction when a force is applied. As a result,
turn) requires decreasing the bank or increasing
when the aircraft makes a turn, the gyro position
the rate of turn, or a combination of both.
remains constant. However, the frame in which
The ball instrument is actually a balance
the gyro hangs dips to the side opposite the
indicator because it shows the relationship
direction of turn. Because of the design of the
between angle of bank and rate of turn. It lets
linkage between the gyro frame and the pointer,
the pilot know when the aircraft has the correct
the pointer shows the correct direction of turn.
rate of turn for its angle of bank.
The pointer displacement is .proportional to the
.
aircraft rate of turn. If the pointer remains on
6-34
