Modern aircrafts include many different types of subsystems. These subsystems are very closely interlinked to each other. One of these subsystems is hydraulic subsystem, which is usually used for actuating most of the mechanical subsystems, such as landing gear, flight control surfaces, weapons system etc. Thus the hydraulic system is a very essential part of the aircraft and its reliability and functionality are very essential to the flight worthiness of the whole aircraft.
Aircraft Hydraulics Definition
(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 (mechanical HP)
If an electrical system were used instead of a hydraulic system, a generator would take the place of the pump and a motor would take the place of the actuating cylinder
Some Hydraulic Systems in Aircrafts
2. Retraction and extension of landing gear
3. Sweep back and forth of wings
4. Opening and closing doors and hatchways
5. Automatic pilot and gun turrets
6. Shock absorption systems and valve lifter systems
7. Dive, landing, speed and flap brakes
8. Pitch changing mechanism, spoilers on flaps
9. Bomb bay doors and bomb displacement gears
Principles of Operation
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.
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].
Hydraulic Pressure Regulated Power System
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.
Aircraft Hydraulic System 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).
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
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:
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
(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.
As the shaft and swash plate rotate, the piston will suck oil into the cylinder block and as the shaft and swash plate keep on rotating, the piston pushes oil out through the outlet. Pumps can be made to move more or less oil volume.
6.Hydraulic System Check Valves
Function of Check Valves
Check Valves are hydraulic devices which permit flow of fluid in one direction only.
Check Valve Used In Aircrafts
Check valves are designed so as not to tolerate leakage. The purpose of the light spring is only to keep the poppet on the seat.
Most manufacturers use sharp-edged, very hard seats and soft, maybe plastic, poppets. Parallel seats are very good except that they are too prone to trapping contaminants between the seat and the poppet.
To limit the pressure of some section of the hydraulic system when the pressure has reached a predetermined level. That pressure level may be considered dangerous and, therefore, must be limited.
Principle of Operation
The adjustment screw at the top of the pressure relief valve is set for a certain pressure value, let us call it P2. In general, even with a pressure of P1, the poppet would lift up, except that the spring is strong and has downward force forcing the poppet closed. Poppet will not move until a pressure greater than that required is felt by the system (i.e., P1>P2). When the pressure increases, the poppet will move up, forcing the excess
liquid to move through opening at high velocity. On other side of seat, pressure is zero because the back side of the relief valve is connected to the return line. When the
pressure in the system decreases below maximum, poppet will return to its seated position, sealing the orifice and allowing the fluid to follow its normal path. These type of pressure relief valves are only made to be used intermittently.
Circuits Using Pressuring Limiting Devices (PLDs)
1. The power system where the system relief valve is used to back up the regulator is an example of a use of the PLD. In such a system, the pressure setting, P2, is set 125% above the system pressure. Rate of flow is dependent upon engine speed.
2. Thermal relief valves are set at 150% of system pressure. When the temperature (T) changes, the liquid expands more than the expansion of the hydraulic tubing. Since T increases, the pressure (P) increases. Thus, the tubing will burst unless there are thermal relief valves in the system. Set at one pressure, the thermal relief valves are connected to the return lines because the pressure there is close to nil. This only works when the selector valve is set in the neutral position.
3. Force Limiting Device (FLD). Suppose that we want 1000 pounds of force to move a certain control surface. But our system can deliver 3000 pounds per square inch. If that pressure can be delivered on a 2 square inch piston head that moves the control surface, we would be= 6000 lb, a much higher force than is needed. We can put a force limiting relief valve (FLD) which would limit the force to 1000 lb by adjusting the FLD to act when the pressure reaches 500 psi (1000 lb/ 2 square inches).
5. Blow up devices. When a plane is coming in for landing on a carrier deck, the brakes are set and the selector valve is put at neutral. If the plane is waved off on its landing attempt, the brakes must retract quickly so that the plane does not stall. Therefore, when the pilot is waved off, he will push the throttle to get more speed to get away from carrier. In doing so, the air pressure force acting on the brakes, F, is so great that it moves the brake. In doing so, the piston moves to right, causing fluid to flow and to push on the relief valve. This action allows more oil into the other line which in turn pushes on the piston and repeats the process. After the pilot reacts to this situation, he will change the selector valve position (if he has to change it), to move the b
Hydraulic System Accumulator
Principle of Operation
is greater than the pressure of the accumulator, the diaphragm will deploy (inflate). Using Boyleâ„¢s Law, the compressed gas will increase in pressure as its volume decreases. The diaphragm will move up or down, depending on system pressure.
When the diaphragm is at half way, the gas volume will be Ã‚Â½ as much as it was initially, while the accumulator pressure will be twice as much as its pre-load pressure (i.e., 1/2 system pressure). Therefore when the accumulator is at half volume of gas, it will be charged at full system pressure.
Uses of an Accumulator
1. Absorbs the shocks due to rapid pressure variations in a hydraulic system
2. Helps maintain a constant pressure within the hydraulic system
3. Helps the hydraulic pump under peak pressure loads
4. It is an emergency source of power (the braking system has its own accumulator)
The preload is checked every day. Nitrogen and helium are preferred to compressed air. Oxygen leaks into the oil will cause spontaneous combustion and that is why it is not used in the accumulator. Carbon dioxide (CO2 ) is not used because it
liquefies at 800 or 900 psi (which is considered low pressure compared to the pressure requirements of the system).
The accumulator has to withstand about 450,000 lb of force. The spherical shape is used because a monocoque (single shell) body is the strongest and can withstand high pressures before failing.
Pressure Regulation in
Douglass Pressure Regulator
When an actuating cylinder finishes its motion and stops, a high pressure will be felt through the system. If so, this high pressure oil coming from the power pump (right side of diagram) will keep check valve C open and also act on piston A. In its
movement, piston A pushes Ball B off seat D. The oil, taking the passage of least resistance, goes through passage D into the center chamber back to the reservoir. The pressure on the right side of check valve C will drop and will be less than the pressure on the left side of C, therefore, causing the ball to seat itself in check valve C. When the
hydraulic system pressure drops, the pressure on piston A decreases, causing a decrease in pressure on B as well. The path of least resistance through D will close and the oil will move in the direction towards check valve C. Now, because the pressure on the right side of C is greater than on the left of C, the check valve will be forced open and the oil will move toward the selector valve side of the system (left side of diagram).
The range of operation of the pressure regulator is defined by the difference in force required for bypass and the force required at actuation.
Electrol Pressure Regulator
When the hydraulic pump is charging the system, balls 1 and 2 are seated on their seats but ball 3 is letting oil pass through to the actuating cylinders. When actuation stops, the pressure in the system builds to maximum. The spring holding ball 1 onto the lower seat is designed to withstand the force produced by the maximum pressure of the hydraulic system. As maximum pressure is reached, ball 1 is made to move to the upper seat, thereby letting high pressure oil reach the A side of piston. But, ball 3 has already moved to its seat due to the higher pressure felt on side C than on side D. Therefore, oil coming from the hydraulic pump is at a charging pressure less than the maximum pressure of side A of the piston, causing the piston to move down, in turn pushing ball 2 down. Oil, wanting to take the path of least resistance, goes by ball 2 to the return line.
If actuation restarts, then pressures at C side of ball 3, at A side of piston and at F side of ball 1 decreases. Ball 1 falls onto the lower seat, since the spring force is greater than
the force generated by the hydraulic system's oil pressure. Piston A will then move up due to the pressure drop on side A, causing ball 2 to close the orifice to the return line. The oil being pumped from the hydraulic pump now has greater pressure on side D of ball 3, causing the ball to move to the left and the hydraulic system oil goes in the direction of the actuating cylinder.
The range of operation depends on ball 1. As you can see, if the force of the spring holding ball 1 is to be overcome, a certain pressure (equal to the spring force divided by the opening's cross-sectional area) at the lower seat would be needed. If the ball were to move upward to the upper seat, the exposed surface area of the ball on which the pressure acts would increase (while the spring force would essentially remain the same) thereby causing the pressure in the system to decrease. Ball 1 would then move from the lower seat to the upper seat very quickly as more surface area is exposed, causing the pressure required to move the ball to drop. When the pressure delivered is much higher than the pressure required to move the ball, then the ball will move quickly and bang shut against the upper seat. The opposite reaction occurs when the pressure in the system is lowered to within operational limits.
Hydraulic System Hand Pumps
2. acts as an Emergency system of power
Principles of Operation
The hand pump converts the power of a human being to hydraulic horsepower. For this reason, such pumps are used in older model aircraft.
1. Single action - single impulse (S.A.S.I.)
2. Double action - double impulse (D.A.D.I.)
Analysis of a S.A.S.I. Pump
The pump works on the principle of mechanical advantage.
When pilot moves the handle away to L (see diagram below), a low pressure is caused to form in the chamber of the pump, Z. Since the reservoir liquid pressure is greater than the pressure in Z, liquid is forced around the check valve from the reservoir line. When the pump handle is moved to R, a positive pressure will form in Z, causing the check valve to the reservoir side to close. Since the hydraulic system liquid is incompressible, it is forced out through the bottom check valve to the system line.
Suppose the force delivered by the pilot was F1 = 100 lb, at a distance D1 = 20 inches from the pump handle pivot, and suppose that D2 = 1 inch from the pivot to the pump piston. Then by moment equilibrium (F1D1=F2D2), the force acting on the piston would
Analysis of a D.A.D.I. Pump
Selector valves are used as (1) directional control devices to insure the movement of the hydraulic fluid flow in the proper direction, and (2) as stop-locks to lock the selector switch in a certain position.
Positions 1, 2 and 3 (shown below) are representative positions for the piston-type selector valve. Position (1) is the position of the selector valve, for example, upon the extension of the landing gear or the lowering of flaps. Position (2) is the position of the selector valve upon retraction of the landing gear or the raising of the flaps. Position (3) is the stop-locking position of this type of valve. This piston type valve uses the Vickers spool mechanism in which the piston "lands" isolate the high pressure oil from the low pressure oil.
Poppet Type - Stacked Poppet
In this type valve, any movement of the handle (at the lower right of the diagram) changes the camshaft and cam settings, thereby opening and closing the poppet valves and letting high and low pressure oil to the proper sides of the actuating cylinder and return line, respectively.
In in-line poppets, the poppet valves are set up along a cam shaft, in the same manner as for the intake and exhaust valves of a car engine. The camshaft is represented by the "sinusoidal" line in the diagram below. However, the motion of the poppets are controlled by the pilot.
These mechanical type selector valves require a fair amount of tubing. In order to reduce the amount of tubing, electric switches have been used to operate solenoids which operate the selector valves. This has the added advantage of reducing the wasteful motions of pilot. This is type of combined electronic circuits and hydraulic system is called electrohydraulics.
There are four types of restrictors:
1. one way fixed restrictor
2. one way variable restrictor
3. two way fixed restrictor
4. two way variable restrictor
The One Way Fixed Restrictor
The Two Way Fixed Restrictor
The One Way Adjustable Restrictor
The One Way Adjustable Restrictor is being used nowadays. It is the same as the One Way Fixed Restrictor but the amount of fluid passed through the drilled opening in the seat is regulated by means of an adjustment screw.
The Two Way Adjustable Restrictor
The Two Way Adjustable Restrictor is the same as the Two Way Fixed Restrictor, but it also has an adjustable screw that can be used to further restrict the amount of hydraulic fluid passing through the opening.
Another example where flow equalizers are needed is in case of air-to-air missile attack. Suppose our selector valve is set to neutral and we try to get away from a rocket missile by turning right or left. The pressure forces on the wings would be so unequal that the wing actuating cylinder (of the wing undergoing the smaller turn radius) would act as a pump, since its greater pressure loading would cause wing sweepback. The hydraulic fluid would be pushed out of one cylinder and the only path that it could take
would be to the other wing's actuating cylinder, causing that cylinderâ„¢s wing to go in the opposite direction to that of the first wing. This would be catastrophic.
If a downward force is applied to the left piston and the selector valve is closed, the oil is forced into the right cylinder causing the right piston to move up--a motion opposite to what is needed.
Thus the flow equalizer is made up of two power pumps placed side by side in which the drive gears of both pumps are connected (see the side view for the figure below). The main figure shows the two power pumps of the side view, cut along the side view's centerline and opened like a book. When one drive gear turns (2), it causes the other to turn as well (1) through the connection. Since the volumetric output per revolution is the same for both sides, we have found the right device to keep synchronization.
This kind of power pump is set between the two cylinders requiring equalization and would channel the flow to both cylinders (as shown below left). The VICKERS EQUALIZER setup is shown below right, where the VICKERS pumps are connected by a shaft at the center of the diagram. Input is at the top of the pumps and outputs are shown by the arrows.
You should never connect the actuation cylinders of the two wings is series, since this type of circuit, for it to work, would require cylinder (1) to put out twice the fluid pressure, or even more, in order to operate itself and cylinder (2) [see the figure below). The hydraulic fluid in this series type of circuit will burst the hydraulic tubing, due to the fluid pressures required to operate both actuating cylinders, and, because this type of circuit does not compensate for expansion or contraction of the hydraulic fluid. The tubing between cylinders and to the selector valve can transmit high pressure hydraulic fluid as well as return line pressure hydraulic fluid.
These circuits are used to cause certain operations to occur in a particular sequence. Sequencing circuits have been used, for example, for the complete ejection of a pilot from the plane. The sequencing valve is such that it sends hydraulic fluid through the valve to the other sequencing valves and actuating cylinders. As the piston rod (of the extreme left cylinder) moves upward, it activates the sequencing valve releasing hydraulic fluid to the next cylinder.
Sequencing Valve Operation
As the actuating cylinder piston moves upward, it hits the rod of the sequencing valve (shown in bottom). The rod, in turn, moves up into the sequencing valve pushing the poppet up and releases the hydraulic fluid from the holding side (top) to the releasing side (middle), permitting it to go to the next cylinder and sequencing valve.
By proper connection of the sequencing valves and actuating cylinders, any sequence of events can be made to occur.
A. Single Piston, Single Rod
B. Single Piston, Double Rod
The piston ends can be attached to a pulley system that can pivot a control surface. The motion of the piston to the right, due to high pressure fluid in the left side of the actuating cylinder, can be activate the pulley system and pivot the control surface upward. By reversing the flow, the opposite motion will occur.
C. Double Piston, Double Rod
Single Action Cylinders
1. The decrease in piston stroke on account of weight and size of spring or inserted rubber mass;
2. The fact that oil on the spring side of the piston must move both the piston and the spring;
3. The size and weight of the spring to do the job might be too great.
Â¢ The locking cylinder is part of the Fail-safe design philosophy: if the cylinder is malfunctioning, it should fail in a position that is safe for the rest of the flight.
Stationary Piston-Moveable Cylinder
Whenever we donâ„¢t want the piston rod to extend into an environment that can damage the rod or contaminate the hydraulic fluid (for example, a stream of sea spray or in the
path of little pebbles when landing or taking off from a rough airstrip), we make the piston cylinder extend instead of the piston rod. The type of cylinder we use is the stationary piston moveable cylinder.
Hydraulic systems determine flight worthiness, usability and reliabity
Advantages of Hydraulic Systems Over Other Systems
1. It is lighter in weight than alternate existing systems.
2. It is dead beat, that is, there is an absence of sloppiness in its response to demands placed on the system.
3. It is reliable; either it works or doesn't.
4. It can be easily maintained.
5. It is not a shock hazard; it is not much of a fire hazard.
6. It can develop practically unlimited force or torque.
The actuating cylinder can change hydraulic power to linear or rotating motion. It has a reduction gear in it to reduce rotating motion to that amount which is needed.
Previously, systems used to control motion by using steel cables connected by pulleys between the controlling mechanism (such as the pedals) and the controlled surface (such as the rudder). The cables were affected by expansion rates of the cables due to temperature changes. Hydraulic systems can control motion without worrying about the effect of temperature since it is a closed system (not open to the atmosphere) compared to a cable system. This means better control of the plane and less lag time between the pilot's movement to control the plane and the response by the control surface.
Usually there are 3 sets of hydraulic systems in a commercial aircraft.
SYSTEM A USES [ above 3100 psi]:-
1.PRIMARY FLIGHT CONTROLS [LID][LEFT, INBOARD, DOWN]
3.L/E FLAPS AND SLATS.
4.INBOARD FLIGHT SPOILERS.
6.L/G EXTENSION AND RETRACTION.
8.NOSE WHEEL STEERING.
SYSTEM B USES [below 2800 psi]:-
1.PRIMARY FLIGHT CONTROLS.
2.OUTBOARD FLT SPOILERS.
STANDBY SYSTEM USES:-
2.L/E DEVICES [FLAP + SLAT] EXTENSION OMLY.
A. Water-based MIL-O-7083 type hydraulic fluid has been used in naval aircraft because it is not flammable. However, it is still corrosive. It is a water based lubricant which is chemically treated. Its commerical name is HYDROLUBE
B. Mineral-based petroleum oil MIL-O-5606 has been used by NATO. Old name of this type fluid is AN-VV-O-366. It has a cherry red dye added to it.
C. Synthetic Oils have much appeal. They are non-flammable and work over a wide temperature range. One such synthetic oil is known as Skydroll 500 and it has a purple or green color. It is made from a phosphate-ester base.
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