assemblies have two miniature permanent
magnets, 180 degrees apart. The motor runs at
a constant 120 RPM. The motor connects through
a shaft to the drum assembly.
The impeller assembly rotates over the motor-
drum shaft and mechanically couples to the drum
with the spiral spring. The pickup coils, one for
each assembly, are in line with the drum and
impeller assembly magnets. As the motor rotates
the drum, the impeller also rotates. When there
is no fuel flow, the magnets of the drum and
impeller assemblies align. As they pass their
respective coil, they generate simultaneous output
Figure 6-54.-(A) Fuel flow indicator; (B) fuel flow totalizer
indicator.
signals.
As fuel flow starts and increases through the
transmitter housing, it goes through straightening
receiver, a step-up gear train, a magnetic drag cup,
vanes. These vanes eliminate the swirling motion
and a calibrated spring. When fuel flows through
of fuel. The fuel then passes through straight
the fuel flow transmitter, an electrical signal goes
drilled passages of the rotating impeller. As
to the indicator receiver. This signal drives the
fuel flow increases through the impeller, a
synchro rotor to the proper position. Thus, the
proportional drag factor, or resistance to rotation,
is imposed on the impeller assembly. This
indicator pointer shows the rate of fuel flow.
Figure 6-54, view A, shows the face of the
resistance causes the spring to deflect, equalizing
single flow indicator. To determine the amount
the loading. The impeller magnets then deflect out
of fuel consummation per hour, multiply the scale
of alignment. This action produces a later signal
reading by 1,000. Figure 6-55 shows a schematic
than that of the drum magnets and coil. Thus,
diagram of the single fuel flow indicator.
an increased time span between signals of the
The following discussion uses the F-14 system
rotating assemblies becomes relative to increased
as an example of a fuel flow system. Other aircraft
fuel flow.
fuel flow systems operate in a similar way.
The fuel rate-of-flow power supply consists
The fuel flow transmitter consists of a
of a power transformer and power supply, two
synchronous motor, drum assembly, impeller
signal-conditioning channels, and a motor driver.
assembly, spiral spring, and pickup coils. The
The transformer receives 115 volts ac, 400 Hz,
transmitter housing has fuel inlet and outlet
and it feeds the power supply. The power
attachment flanges. The drum and impeller
supply provides low dc voltage to operate the
Figure 6-55.-Single fuel flow indicating system.
6-50
