THE BALANCE INDICATOR
CONSTRUCTION AND OPERATION
This part of the instrument uses a mechanical method to indicate that an aeroplane is correctly
banked for a given rate of turn. It uses the force of gravity, which acts upon a black ball in a liquid
filled glass tube, and maintains it in its true vertical position whilst the aeroplane is in straight and
level flight, as shown below.
The liquid acts as a damping medium for the ball, and two expansion chambers are concealed
behind the dial, to cater for temperature changes. The back of the tube is painted on the outside
with fluorescent paint to provide a contrasting background for the black ball, and the whole
assembly is firmly secured to the back of the dial by a bracket. The ball itself has weight and is
thus affected by aeroplane manoeuvres.
If the ball remains in the centre, the turn is balanced, and no slip or skid is present, as shown in
diagram (A) below.
Diagram (B) shows the aeroplane making a left turn at a certain angle of bank. During this
manoeuvre the indicator case and scale both move with the aeroplane. The ball is additionally
subject to a centrifugal reaction, since the aeroplane is in a turn, which forces the ball away from
the centre of the turn. If the turn is however carried out with the correct angle of bank, the two
forces are in balance, and the ball remains in the zero position. Any increase in airspeed during
the turn increases both the bank angle and centrifugal force. The ball continues to remain in line
with the resultant of the two forces, as long as the bank angle is correctly maintained.
If the angle of bank for a particular rate of turn is incorrect, for example, the aeroplane is under
banking, as shown in diagram (C), the aeroplane tends to skid out of the turn. This occurs
because the centrifugal force predominates, and the ball displaces away from zero toward the
outside of the turn. By comparison, if the aeroplane is alternatively over-banked (i.e. the angle of
bank is excessive for the rate of turn, as shown in diagram [D]), the aeroplane tends to slip into
the turn, since the force of gravity now predominates, and the ball moves away from its zero
position toward the inside of the turn. If the aeroplane skids or sideslips, the turn is unbalanced,
and if the ball remains in the centre, the turn is balanced.