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Combined acceleration and speed control
51. The combined acceleration and speed control
system (fig. 10-9) is a mechanical system without
small restrictors or spill valves. It is also an all-speed
governor system and therefore needs no separate
governor unit for controlling the maximum r.p.m. The
controlling mechanism is contained in one unit,
usually referred to as the fuel flow regulator (F.F.R.).
An H.P. fuel pump (para. 85) is used and the fuel
pump servo piston is operated by H.P. fuel on one
side and main spray nozzle (servo) pressure on the
spring side.
52. The F.F.R. is driven by the engine through a
gear train and has two centrifugal governors, known
as the speed control governor and the pressure drop
control governor. Two sliding valves are also rotated
by the gear train. One valve, known as the variable
metering sleeve, has a triangular orifice, known as
the variable metering orifice (V.M.O.), and this sleeve
is given axial movement by a capsule assembly. The
V.M.O. sleeve moves inside a non-rotating governor
sleeve that is moved axially by the speed control
governor. The other valve, known as the pressure
drop control valve, is provided with axial movement
by the pressure drop control governor and has a
triangular orifice, known as the pressure drop control
orifice, and a fixed-area rectangular orifice. The
speed control governor is set by the throttle lever
through a cam, a spring and a stirrup arm inside the
regulator.
Fuel system
105
Fuel system
106
Fig. 10-9 A combined acceleration and speed control system.
53. At any steady running condition, the engine
speed is governed by the regulator controlling the
fuel flow. The fuel pump delivery is fixed at a constant
value by applying the system pressure difference to
the fuel pump servo piston. This is arranged to
balance the servo piston spring forces.
54. When the air intake pressure is at a constant
value, the rotating V.M.O. sleeve is held in a fixed
axial position by the capsule loading. The fixed
throttle setting maintains a set load on the speed
control governor and, as the r.p.m. is constant, the
governor sleeve is held in a fixed position.
55. The fuel pump delivery is passed to the annulus
surrounding the V.M.O.; the annulus area is
controlled by the governor sleeve, and the exposed
area of the orifice is set by the axial position of the
V.M.O. sleeve. Consequently, fuel passes to the
inside of the sleeve at a constant flow and therefore
at a constant pressure difference.
56. The pressure drop control valve, which also
forms a piston, senses the pressure difference
across the V.M.O. and maintains the fuel flow at a
fixed value in relation to a function of engine speed,
by controlling the exposed area of the pressure drop
control orifice.
57. When the throttle is slowly opened, the load on
the speed control governor is increased, so moving
the governor sleeve to increase the V.M.O. annulus
area. The effect of opening the V.M.O. is to reduce
the pressure difference and this is sensed by the
pressure drop control governor, which opens the
pressure drop valve. The reduced system pressure
difference is immediately sensed by the fuel pump
servo piston, which increases the pump stroke and
consequently the fuel output. The increased
compressor delivery pressure acts on the capsule
assembly, which gradually opens the V.M.O. so that
the fuel flow and engine speed continue to increase.
At the speed selected, centrifugal forces acting on
the speed control governor move the governor
sleeve to reduce the V.M.O. annulus area. The
resultant increased pressure difference is sensed by
the pressure drop control governor, which adjusts the
pressure drop valve to a point at which the pump
servo system gives an output to match the engine
requirements. The function of the governors and the
control of the fuel flow is shown diagrammatically in
fig. 10-10.
58. During a rapid acceleration, the initial degree of
overselling is mechanically controlled by a stop that
limits the opening movement of the speed control
governor sleeve. A similar stop also prevents the fuel
supply from being completely cut off by the governor
sleeve during a rapid deceleration.
59. Changes in altitude or forward speed of the
aircraft vary the fuel flow required to maintain a
constant engine speed. To provide this control, the
capsule assembly senses changes in H.P.
compressor inlet and delivery pressures and adjusts
the V.M.O. accordingly. For instance, as the aircraft
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