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expression of the heat or energy content per pound
or gallon that is released during combustion. This
value, which is usually expressed in British thermal
units, influences the range of an aircraft. Where the
limiting factor is the capacity of the aircraft tanks, the
calorific value per unit volume should be as high as
possible, thus enabling more energy, and hence
more aircraft range, to be obtained from a given
volume of fuel. When the useful payload is the
limiting factor, the calorific value per unit of weight
should be as high as possible, because more energy
can then be obtained from a minimum weight of fuel.
Other factors which affect the choice of heat per unit
of volume or weight, must also be taken into consideration;
these include the type of aircraft, the
duration of flight, and the required balance between
fuel weight and payload.
Fuel system
118
111. Turbine fuels tend to corrode the components
of the fuel and combustion systems mainly as a result
of the sulphur and water content of the fuel. Sulphur,
when burnt in air, forms sulphur dioxide; when mixed
with water this forms sulphurous acid and is very
corrosive, particularly on copper and lead. Because it
is impracticable to completely remove the sulphur
content, it is essential that the sulphur be kept to a
controlled minimum. Although free water is removed
prior to use, dissolved water, i.e. water in solution,
cannot be effectively removed, as the fuel would reabsorb
moisture from the atmosphere when stored in
a vented aircraft or storage tank (para. 118).
112. All gas turbine fuels are potentially dangerous
and therefore handling and storage precautions
should be strictly observed.
Vapour locking and boiling
113. The main physical difference between kerosine
and wide-cut fuels is their degree of volatility, the latter
type of fuel having a higher volatility, thus increasing
the problem of vapour locking and boiling. With
kerosine-type fuels, the volatility is controlled by distillation
and flash point, but with the wide-cut fuels it is
controlled by distillation and the Reid Vapour
Pressure (R.V.P.) test. In this test, the absolute
pressure of the fuel is recorded by special apparatus
with the fuel temperature at 37.8 deg. C. (100 deg. F.).
114. Kerosine has a low vapour pressure and will
boil only at extremely high altitudes or high tempera-
Fig. 10-21 Relationship between calorific
value and specific gravity.
tures, whereas a wide-cut fuel wilt boil at a much
lower altitude.
115. The fuel temperature during flight depends
upon altitude, rate of climb, duration at altitude and
kinetic heating due to forward speed. When boiling
does occur, the vapour loss can be very high,
especially with wide-cut fuels, and this may cause
vapour locking with consequent malfunctions of the
engine fuel system and fuel metering equipment.
116. To obviate or reduce the risk of boiling, it is
usual to pressurize the fuel tanks. This involves
maintaining an absolute pressure above the fuel in
excess of its vapour pressure at any specific
temperature. This may be accomplished by using an
inert gas or by using the fuel vapour pressure with a
controlled venting system.
117. For sustained supersonic flight, some measure
of tank insulation is necessary to reduce kinetic
heating effects, even when lower volatility fuels are
used.
Fuel contamination control
118. Fuel can be maintained in good condition by
well planned storage and by making routine aircraft
tank drain checks. The use of suitable filters,
fuel/water separators and selected additives will
restrict the contamination level, e.g. free water and
solid matter, to a practical minimum. Keeping the fuel
free of undissolved water will prevent serious icing
problems, reduce the microbiological growth and
minimize corrosion. Reducing the solid matter will
prevent undue wear in the fuel pumps, reduce
corrosion and lessen the possibility of blockage
occurring within the fuel system.
Fuel system
119
Rolls-Royce RB211-535C
Metrovick G2
Following the successful operation at sea of
the Metrovick F2-based 2500 hp Gatric marine
gas turbine, the Royal Navy ordered four
larger sets with a maximum operational rating
of 4500 shp. Developed from the Metrovick
F2/4 Beryl axial-flow aircraft engine; the G2s
were installed in the Motor Gunboats ’Bold
Pioneer1 and ’Bold Pathfinder; the former
going to sea in 1951.
INTRODUCTION
1. Two separate systems are required to ensure
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