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1.5 - Aircraft Hydraulic
System Reservoir Design
Functions of the Reservoir
- Provides air space for expansion of the oil due to
- Holds a reserve supply of oil to account for
- thermal contraction of oil.
- normal leakage - oil is used to lubricate piston
rods and cylinder seals. When the piston rod
moves, it is scraped to remove impurities that
might collect on the rod when returning into
actuating cylinders. If many actuating cylinders
are operating at the same time, then the amount
of oil lost is greater.
- emergency supply of oil - this case occurs
only when the hand pump is used.
- volume changes due to operational requirements -
oil needed on side 2 of piston head is less than
that needed on side 1 of cylinder piston (which
occurs during actuation).
- Provides a place to remove air or foam from liquid.
- Provide a pressure head on the pump, that is, a pressure
head due to gravity and depends upon the distance of the
reservoir above the power pump.
Construction of Reservoir
In the construction of a reservoir, one must know:
- Material: for the reservoir itself 5052 aluminum has been
used. It is weldable and ductile, it can work in the
needed temperature range and it must work when it is in
any position and orientation to the earth (example: 1. In
space, it is on its side; gravity is pulling on the
reservoir's "sides"; 2. during blast-off,
gravity is forcing the liquid to the tank's bottom.)
- Size: To obtain the size of the reservoir needed, one
must calculate the volume of oil needed for one emergency
actuation. This means finding the amount of oil needed
for all emergency equipment to work. Then, it is
necessary to calculate the volume for thermal contraction
by taking all oil volumes of the hydraulic system,
finding the coefficient of contraction and multiplying it
by the number of degrees in the temperature range
expected during operation. You must do the same for all
oil volumes in operational requirements, thermal
expansion, leakage, etc.
- Shape: You must look at the space available to fit the
tank. A sphere is the best shape to use because
uniform stresses are generated by the interior pressure.
Its one major disadvantage is that it is difficult to
mount. The next best shape is a domed cylindrical shape.
Not only can it be mounted easily, but it can be made to
A stand pipe to
the power pump is needed and is always in the middle of the
tank. Regardless of variation in its orientation (upright
or on its side), it will be submerged. The return pipe from the
rest of the hydraulic system is put near the top of liquid in the
tank, at a tangent to the tank surface, so that the fluid
entering releases all its energy through swirling at the top and
dissipates it through release of bubbles of
Baffles within the tank are used for two reasons:
- they strengthen the tank against pressure from within and
outside of the tank, and, more importantly,
- they are used to stop the swirling effect of the return
oil from producing a whirlpool. This effect would only
make the stand pipe in the center of the tank suck in the
column of air.
Filler pipe. Such a pipe eases the replenishing of the
reservoir liquid. Since liquid seeks its own level, we put
the filler pipe so that its mouth has the same level as the
design level in the reservoir.
atmosphere- Initially, vents were introduced because a vent will
not allow a void to form within the tank. However, as ceiling
altitudes increased, pressure within the tank and the hydraulic
system was being lost and cavitation occurred. To
stop this phenomenon from happening, pressurized reservoirs were
created (see section 1.7).
Dipstick-Sometimes filler pipes could not be used to add oil
and tanks would have to be filled from the top. This made it
difficult to measure the oil. The dipstick was therefore
introduced. A long stick with marks on it, its job was to measure
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