Aircraft Hydraulic Systems
Aircraft Hydraulics Definition
It is a system where liquid under pressure is used to transmit this energy. Hydraulic systems take engine power and convert it to hydraulic power by means of a hydraulic pump.
Hydraulic power may be reconverted to mechanical power by means of an actuating cylinder
(1) - A hydraulic pump converts mechanical power to hydraulic power
(2) - An actuating cylinder converts hydraulic power to mechanical power
(3) - Landing Gear
(4) - Engine power
Some Hydraulic Systems in Aircrafts
1. Primary control boosters
2. Retraction and extension of landing gear
3. Sweep back and forth of wings
4. Opening and closing doors and hatchways
5. Automatic pilot
6. Shock absorption systems and valve lifter systems
7. Dive, landing, speed and flap brakes
8. Pitch changing mechanism, spoilers on flaps
2.Principles of Operation
Part of the hydraulic system is the actuating cylinder whose main function is to change hydraulic (fluid) power to mechanical (shaft) power. Inside the actuating cylinder is a piston whose motion is regulated by oil under pressure. The oil is in contact with both sides of the piston head but at different pressures. High pressure oil may be pumped into either side of the piston head.
The selector valve determines to which side of the actuating cylinder the high pressure oil is sent. The piston rod of the actuating cylinder is connected to the control surface.
As the piston moves out, the elevator moves down. As the piston moves in, the elevator moves up. The selector valve directs the high pressure oil to the appropriate side of the piston head causing movement of the piston in the actuating cylinder. As the piston moves, the oil on the low pressure side returns to the reservoir since return lines have no pressure!
The differential in oil pressure causes movement of the piston. The force generated by this pressure difference can be sufficient to move the necessary loads. Each cylinder in
the plane, boat, etc., is designed for what it must do. It can deliver the potential it was made for; no more, no less. Air loads generally determine the force needed in aircraft applications.
Hydraulic System
A hydraulic system transmits power by means of fluid flow under pressure. The rate of flow of the oil through the system into the actuating cylinder will determine the speed with which the piston rod in the actuating cylinder extends or retracts. When the cylinder is installed on the aircraft, it is already filled with oil. This insures that no air bubbles are introduced into the hydraulic system, which can adversely affect the operation of the system.
Pascalâ„¢s Theory
In a confined stationary liquid, neglecting the effect of gravity, pressure is distributed equally and undiminished in all directions; it acts perpendicular to the surface it touches. Because the actuating cylinder is not vented, the force delivered through the piston to the surface of the fluid is translated into a pressure on the surface of the fluid.
The pressure (p) acting on the incompressible oil does work [(pressure) x (Area of piston) x (piston's stroke) = Work].
3.Hydraulic Pressure Regulated Power System
The system in drawing below represents a pressure regulated power system comprised of two parts:
1) the power system, and
2) the actuating system part of the overall hydraulic system.
Parts of the Power System
1. Reservoir -- holds an extra supply of fluid for system from which oil was drawn when needed, or oil was returned to it when not needed.
2. Accumulator -- absorbs pulsation within the hydraulic system and helps reduce "linehammer effects" (pulses that feel and sound like a hammer has hit the hydraulic tubes). It is an emergency source of power and it acts as another reservoir.
3. Filter -- removes impurities in the hydraulic system and in the reservoir. The reservoir has one big filter inside the tank.
4. Power Pump -- it changes mechanical horsepower (HP) to hydraulic HP.
5. System Relief Valve -- relieves pressure on system as a safety.measure and takes over as a pressure regulator when pressure regulator fails.
6. Pressure Regulator -- as the name implies, regulates the pressure in the hydraulic system. When it senses a built-up in pressure in the lines to the selector valves, it acts so that the system automatically goes to bypass.
4.Aircraft Hydraulic System Reservoir
Functions of the Reservoir
1. Provides air space for expansion of the oil due to temperature changes
2. Holds a reserve supply of oil to account for
a. thermal contraction of oil.
b. 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.
c. emergency supply of oil - this case occurs only when the hand pump is used.
d. 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).
3. Provides a place to remove air or foam from liquid.
4. 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.
The best shape is a domed cylindrical shape. Not only can it be mounted easily, but it can be made to order.
5.Aircraft Hydraulic System Power Pumps
Function:
1. The function of the hydraulic system power pump is to change mechanical horsepower to hydraulic horsepower.
Types of Power Pumps
There are two types of power pumps, a gear pump and a piston pump.
1. Gear pumps have efficiencies that average about 70-80% overall efficiency, where overall efficiency is defined as:
overall efficiency = (mechanical efficiency)*(volumetric efficiency)
Gear pumps move fluid based upon the number of gear teeth and the volume spacing between gear teeth.
2. Piston pumps move fluid by pushing it through the motion of the pistons within the pump. They can generate overall efficiencies in the 90-95% range.
Principles of Operation:
Gear type pumps are ideal when working with pressures up to 1500 lb./sq.in. As mentioned previously, the volumetric efficiency of gear pumps depends upon the number of teeth, the engine speed and the tooth area.
As the liquid comes from the reservoir, it is pushed between the gear teeth. The oil is moved around to the other side by the action of the drive gear itself and sent through the pressure line. What makes the oil squeeze in between the gear teeth? gravity and the pressure head. To prevent leakage of oil from the high to the low pressure side from occurring, you can make the gears fit better.
You might want to increase the pressure used to move the fluid along. However, the higher the pressure, the higher the friction loading on the teeth. Friction will develop heat which will expand the gears and cause the pump to seize (parts will weld together and gears will stop rotating). In order to stop this, you can have the pump case, the gears, and the bearings made out of different materials, (e.g., steel gears [1-1/2 inch thick], bronze bearings, aluminum casing). Normally, the gear speed is higher than the engine speed (normally 1.4 times the engine speed).
Oil can leak over and under the gears. To prevent leakage, you can press the bearings up against the gears. This decreases seepage but this decreases the mechanical efficiency when friction increases. Even though oil acts as lubricant, seizing can occur when oil is drained from the hydraulic system.
As mentioned previously, we can push the bearings up against the gears to decrease leakage. As F increases, M decreases, thus, the gears and bushing increase in friction and mechanical efficiency decreases. When you increase the pressure on the inlet side of the pump, leakage will increase around the gears. To reduce the leakage, you must push the bearings and gears closer, causing an increase in friction. That is why inlet pressures over 1500 lb/sq in, are not used.
Principle of the Shear Shaft
Gear pumps are built using a shear shaft principle. That is, if the pump fails, the shear shaft breaks and this allows each of the gears to rotate in its own part of the system
(pump side or engine side) and nothing else will happen to the system. This phenomenon is similar to a fuse in an electrical system. When the electrical system overloads, the fuse breaks, causing the circuit to break without damaging the rest of the electrical circuit.
Principle of the Reciprocating Piston Pump
These kind of pumps attain volumetric efficiencies of up to 98% and they can maintain pressures from 1500 to 6000 psi. They can achieve overall efficiencies of up to 92% and can move fluid volumes up to 35 gallons per minute.
As the cylinder block rotates, space between the block and the pistons increase, letting in more oil. As the block rotates from bottom dead center, the reverse occurs and the pistons push oil out through the outlet. When the pistons move down, the suction caused by the vacuum from the space, created by the movement of the piston, pulls in oil. Changing the angle between the swash plate and the cylinder block gives a longer pumping action and causes more fluid to be pulled in. As the cylinder block rotates, the piston cylinder openings over the inlet and the outlet vary. When cylinders 4-6 take in hydraulic fluid and act as the inlet to the pump, then cylinders 1-3 push the hydraulic fluid out and act as outlet to the pump.
Aircraft Hydraulic Architectures
Learjet 40/45
The aircraft hydraulic system supplies hydraulic pressure for operation
of the aircraft landing gear, brake, flap, spoiler/spoileron and thrust reverser
systems. Hydraulic fluid flows from the main hydraulic reservoir
through two firewall shutoff valves to the main engine-driven
hydraulic pumps for distribution to the required systems upon demand.
The hydraulic system has both a main and auxiliary source of hydraulic
power. These sources are totally separate up to the source selector
valve. An auxiliary dc motor-driven hydraulic pump is installed to provide
auxiliary hydraulic pressure to the brake system through the
brake source shuttle valve and to the landing gear and flap system
through the source selector valve in the event of a malfunction. The
auxiliary hydraulic system only provides pressure for the brake system
while the aircraft is on the ground.
A 260 cu. in. (4261 cc) reservoir supplies hydraulic fluid to the main and
auxiliary hydraulic systems. The reservoir is designed with a separation
wall (partition) to contain fluid for either the main or auxiliary system.
Reservoir pressure is maintained at approximately 20 psi (138 kPa)
by bleed air supplied through a pressure regulator. A bleed air pressure
relief valve releases pressure in excess of 20 psi (138 kPa), and a vacuum
relief valve prevents negative pressure in the reservoir. A thermal shutoff
valve prevents high energy bleed air (>390° F [199° C]) from entering
the reservoir in the event of a hydraulic line failure.
The main and auxiliary hydraulic pumps will each maintain a nominal
pressure of 3000 psi (20,685 kPa) for their applicable systems. A precharged
(1500 psi [10,343 kPa]) hydraulic accumulator is installed to
dampen and absorb pressure surges within the main hydraulic system.
A separate brake accumulator, fed by the auxiliary system, maintains
pressure for the emergency/parking brakes. Two high-pressure filters
and two return filters prevent hydraulic fluid contamination within the
main and auxiliary systems. These filters incorporate bypass valves
which will open in the event they become clogged. A hydraulic pressure
relief valve, installed between the high-pressure and return lines
in both the main and auxiliary system filters, will open to relieve pressure
in excess of 3700 psi (25,511 kPa
Flight Controls: Manual
•One main system fed by 2 EDP’s
•Emergency system fed by DC electric pump
•Common partitioned reservoir (air/oil)
•Selector valve allows flaps, landing gear, nosewheel steering to operate from main or emergency system
•All primary flight controls are manual
Safety / Redundancy
•Engine-out take-off: One EDP has sufficient power to retract gear
•All Power-out: Manual flight controls; LG extends by gravity with electric pump assist; emergency flap extends by electric pump; Emergency brake energy stored in accumulator for safe stopping
The primary flight controls (ailerons, elevator, and rudder) are mechanically
operated through the control columns, control wheels, and rudderpedals. The flaps and spoilers are hydraulically actuated and
electronically controlled. Airplane trim systems (pitch, roll, and yaw)
are electronically controlled.
AILERON
in the fuselage area and one in each of the left and right wing area. In
addition, a disconnect mechanism is incorporated into the pilot’s control
wheel which allows the disconnection of the aileron control system
(in the event of a jam) and switching to spoileron system for roll control.
The fuselage control circuit connects both pilot’s and copilot’s control
wheels together, and each wing control circuit is connected to the aileron
drive mechanism. The three control circuits are connected together
via a common sector assembly. In normal operation, whether by an input
from the autopilot or by manual input to one of the two control
wheels, the two control circuits will move in unison to drive the two aileron
panels. The aileron control system is considered the primary system
for roll control and is interfaced with the spoileron system for roll
augmentation.
ROLL DISCONNECT
If ailerons become jammed, the aileron control system can be disconnected
and the spoileron system can be used for roll control. The pilot’s
control wheel is disconnected from the aileron cables and copilot’s control
wheel by the red lever labeled ROLL DISC located on the hub of the
pilot’s control wheel. This will also disconnect and prevent engagement
of the autopilot. Safe flight can continue on spoilerons alone. For
more information on roll disconnect, see Spoileron (ROLL DISCONNECT)
system.
GULFSTREAM G500/550 HYDRAULIC Systems
