AIRCRAFT HYDRAULIC SYSTEM

Hydraulics is based on a very simple fact of nature - you cannot compress a liquid. You can compress a gas (think about putting more and more air into a tire, the more you put in, the higher the pressure). If you're really strong you can compress a solid mass as well. But no matter how much pressure you apply onto a liquid, it isn't possible to compress it. Now if you put that liquid into a sealed system and push on it at one end, that pressure is transmitted through the liquid to the other end of the system. The pressure is not diminished.

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. This power can be distributed throughout the airplane by means of tubing that runs through the aircraft. Hydraulic power may be reconverted to mechanical power by means of an actuating cylinder, or turbine.

(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

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 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

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.

Pascal’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
1. thermal contraction of oil.
2. 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.
3. emergency supply of oil -  this case occurs only when the hand pump is used.
4. 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.

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

Poppet type valve is the preferred type that is used in hydraulics now. The front of the poppet (left side of the picture above) sits snugly on the hard seat (darker shaded areas on the left side). The poppet works on the following principle. When high pressure fluid (with pressure P₁ ) comes in on the left, it forces the poppet open. Since P₁>P₂ , the force on the left side of the poppet (F₁) is greater than the force due to the spring (F₂ ) and is just enough to open the poppet. But, when flow stops, or there is a high pressure flow from the right side of the poppet, then P₂>P₁ and the pressure forces the poppet against the valve seat, closing off the opening. Thus the fluid is allowed to flow through in one direction only.

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.

7.Pressure Control

(Pressure limiting device-relief valves)

Function

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

8.Hydraulic System Accumulator

Principle of Operation

At the bottom of the accumulator is a gas valve. Compressed gas at about one half the system pressure is let into the accumulator through the gas valve. This forces the diaphragm that separates the oil side from the gas side to "pop" up towards the oil side. Then oil is sent through the system. When the system pressure reaches a point when it

is greater than the pressure of the accumulator, the diaphragm will deploy (inflate). Using Boyle’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 ½ 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.