2.3.
The principles and operation of fuel controls used on current
engines are discussed in this paragraph. Depending upon the type of
engine and the performance expected of it, fuel controls may range
from simple valves to automatic computing controls containing
hundreds of intricate parts.
Strictly speaking, a pilot of a gasturbinepowered aircraft
does not directly control his engine. His command over the engine
corresponds to that of the captain of a ship who obtains engine
response by relaying orders to an engineer below deck who, in turn,
moves the throttle of the engine. But before he moves the throttle,
he monitors certain operating pressures, temperatures, and rpm that
are not apparent to the captain. The engineering officer then refers
to a chart and computes a fuel flow or throttle change which will not
allow the engine to exceed its operating limitations. If you think
of the pilot as the captain of the ship, then think of the automatic
controls as the engineer. They, too, monitor operating pressures,
temperatures, and rpm, and make the necessary fuel and throttle
adjustments.
Fuel controls can be divided into two basic groups:
hydromechanical, and electronic. There are as many variations in
controls as there are engines. Although each type of fuel control
has its particular advantage, most controls in use today are
hydromechanical. Some fuel controls are extremely complex devices
composed of speed governors, servo systems, valves, metering systems,
and sensing pickups.
This section limits discussion mainly to fuel control theory
of the hydromechanical type. A schematic of one is shown in figure
2.2. A fuel control in the simplest form consists of a plain
metering valve to regulate fuel flow to the engine. A
hydromechanical fuel control consists of the following main
components, but it is not limited to only these.
1. Pump to pressurize fuel.
2. Governors to control rpm.
3. Relief valves to protect the control.
4. Manual control systems (emergency control system).
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