Aircraft fuel system. Aircraft fuel systems. Checking the operation of the booster pumps and the tightness of the power supply system of the main engines is carried out by turning on the supply tank pumps one by one
0The fuel system on an airplane is designed to accommodate fuel and uninterruptedly supply it to the engines in the required quantity and with sufficient pressure at all given flight modes and altitudes.
The fuel system of a modern aircraft includes the following main elements:
tanks or compartments of the aircraft that contain the fuel supply necessary for the flight;
power control taps (tank switching); emergency shut-off valves for fuel supply to engines (fire valves);
taps for draining fuel sludge from different points of the system; filters for fuel purification;
pumps that supply fuel to engines and transfer fuel from one tank to another;
devices for monitoring the amount of fuel, its consumption and pressure; pipelines for supplying fuel to engines, connecting tanks to the atmosphere and returning separated fuel.
Bucky. On modern aircraft, fuel reserves can reach many tens of tons. When flying over long distances, fuel is placed in a large number of tanks installed in the wing and less often in the fuselage.
Currently, three types of fuel tanks are used: hard, soft and sealed compartment tanks.
Rigid tanks are made of light aluminum-manganese alloys, which allow deep stamping and hammering, are well welded, have great elasticity and resistance to corrosion. To give the tanks the necessary strength and rigidity, they have a frame made of longitudinal and transverse partitions and profiles. The transverse baffles also serve to reduce shocks resulting from the movement of fuel inside the tank during accelerated flight. Small tanks may not have internal partitions.
Currently, soft tanks are widely used. They are easier to use, more durable, and lighter in weight. Soft tanks are made of special rubber or nylon. Thin rubber tanks are glued onto blanks made of fabric and one or two layers of rubber made of synthetic polysulfide (thiokol) rubber. Rubber-metal fittings are glued into such tanks: flanges for fuel meter sensors, filling necks, connecting pipes, mounting lock sockets, etc.
Rubber thin-walled tanks are mounted in containers inside the wing or fuselage.
The tank compartment is a properly sealed internal volume of the wing part. The tank compartment is sealed with synthetic films. The rivet seam is made airtight, for which the rivets are pre-coated with sealant. Final sealing is achieved by repeatedly coating the entire internal surface with a liquid sealant that cures at room temperature.
The covers of the service hatches of the compartment tanks are mounted on bolts with rubber O-rings and sealed (blind) nuts.
Cranes, installed in the fuel supply system, allow you to control the supply of fuel to the engines from the corresponding tanks (or groups of tanks), as well as turn off the supply of fuel to a failed engine. In accordance with their purpose, all taps are divided into shut-off (shut-off) and distribution taps. According to the control method, cranes can be directly controlled or remotely controlled. By design, they can be plug, spool, valve, etc.
Remote control of the valves is carried out using electric valve closing mechanisms such as MZK or compressed air.
Filters. The need to clean the fuel supplied to engines from foreign impurities is caused by the presence in carburetors, direct injection units, and pumps of gaps ranging in size from tenths to thousandths of a millimeter, which must be protected from the ingress of solid particles. Although the fuel filled into the tanks is filtered, and the tanks are protected from the ingress of mechanical impurities into them, during operation it is possible that corrosion products of pipelines and fuel system units may form, pieces of rubber gaskets, etc. may enter. The presence of even the smallest amounts of water in the fuel sharply increases its corrosive properties and, in addition, can lead to clogging of pipelines in the event of ice formation at low temperatures. Particularly dangerous is the loss of moisture and the formation of ice in the pipelines of the fuel systems of modern high-altitude aircraft, which can gain great altitude in a short time, as a result of which the formation of condensation is sharply accelerated.
In the fuel systems of aircraft, mesh metal, silk, slotted, metal-ceramic, paper and mechanical filter devices are used.
Fuel pumps serve to supply fuel to the engines in flight at all altitudes, at any evolutions and from all tanks or groups of tanks.
Pumps are divided by purpose into booster and transfer pumps, and by type of drive - driven by an aircraft engine and with an autonomous drive, usually from an electric motor. Of the wide variety of different designs and types of pumps, the most widely used are low-pressure rotary or centrifugal pumps, and high-pressure piston and gear pumps.
Modern aircraft are usually equipped with two boost pumps, one electrically driven in the fuel supply tank or at the beginning of the fuel supply line, and the other driven by the aircraft engine at the end of the pipeline in front of the feed (high pressure) pump. This installation of pumps ensures reliable fuel supply to the engines.
Transfer pumps are designed to transfer fuel from those tanks from which it should be produced in the first place, into supply tanks, that is, into tanks from which fuel is sent directly to the engines. The production of fuel from different tanks or groups of them is dictated by the need to maintain a strictly defined alignment of the aircraft throughout the flight and to ensure the necessary unloading of the wing.
Fuel system pipelines that supply fuel to engines, communicate tanks with the atmosphere, and refuel under pressure are most often made of aluminum alloy and hoses with connecting fittings. The most common pipeline connections are: durite (flexible) with clamps and nipple (rigid).
Recently, flexible metal hoses have been widely used, which resist vibration loads well, are convenient for installation, and are relatively lightweight.
In Fig. 115 shows a diagram of the aircraft fuel system.
Fuel is produced from the tanks using aircraft booster pumps, the outlet pressure of which must be greater than the minimum permissible (usually about 0.3 kg/cm2). A check valve is usually installed behind the boost pump to prevent fuel from flowing back.
The fire hydrant shuts off the fuel supply line when the engine is not running and in flight in case of emergency.
On some aircraft, the hydraulic resistance in the line from the tank to the engine pump reaches large values. This necessitated the inclusion of an additional engine booster pump in the fuel line, which provides the required pressure to the main engine pump.
If cooling of the oil of the engine lubrication system with fuel is provided, then a fuel-oil radiator is installed in the fuel system.
As fuel is exhausted from the tank, the pressure in the latter will decrease, which can lead to the tank collapsing. To prevent this, fuel tanks communicate with the atmosphere through drainage pipelines.
On airplanes flying at altitudes exceeding 15-20 thousand m, there is a threat of a significant amount of fuel being released through the drainage. To eliminate this, excess pressure must be created in the tanks. This pressure is created by inert gases - nitrogen, carbon dioxide and others, which are also a means of fighting fire.
A characteristic feature of the fuel systems of modern aircraft is the large capacity of their tanks. Filling a large amount of fuel through the upper conventional necks of the tanks is a complex, labor-intensive task, which is why the vast majority of modern aircraft have pressurized fuel filling systems from below. These systems allow refueling to be carried out in a very short time.
The fuel refueling system of each aircraft consists of refueling necks (one or two), a refueling control panel, fuel supply pipelines to refueling tanks or groups of tanks, refueling valves with electric remote control, and float safety valves that prevent tanks from overfilling if the refueling valves fail.
To increase the flight range of combat aircraft, some types can be refueled in the air from a specially equipped tanker aircraft.
A forced landing of a modern transport aircraft immediately after takeoff, i.e., at maximum flight weight, is in some cases unacceptable due to the limited strength of the landing gear. Lightening the landing weight in these emergency cases can be achieved by draining the fuel.
The in-flight emergency fuel drain system must meet the following requirements: a certain amount of fuel (sufficiently lightening the aircraft) must be drained in a limited time of about 10-15 minutes. In this case, the alignment of the aircraft should change slightly. The drained fuel must not come into contact with hot gases.
The emergency fuel drain system consists of taps, pipelines and drain control valves.
Literature used: "Fundamentals of Aviation" authors: G.A. Nikitin, E.A. Bakanov
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The energy source for the operation of aircraft engines is hydrocarbon fuel placed in the aircraft. The larger the fuel supply on the plane, the longer the possible range and duration of the flight. Fuel on an airplane is stored in compartments of the fuselage, wings and sometimes the fin. To increase the flight range, they use the installation of outboard drop tanks, which are located under the fuselage and under the wings.
On transport aircraft, additional removable tanks are installed in the cargo compartments. Depending on the type of aircraft and the location of the tanks, their number and design vary widely.
When choosing the volume of tanks, it is necessary to take into account that when heated, the volume of fuel increases.
To ensure an emergency landing, fuel is drained from the tanks so that the landing weight of the aircraft does not exceed the permissible strength standards of the landing gear and other components of the aircraft.
In order to replenish fuel reserves and increase flight duration, in-flight refueling from special tanker aircraft is used.
When flying at high altitudes, the fuel cools significantly, so there is a fuel heating device to prevent pipelines and filters from clogging with ice crystals.
The fuel system layout is determined by:
The location of the fuel tanks in the area of the aircraft’s center of gravity so that as the fuel is consumed, the alignment of the aircraft does not change significantly;
Maximum use of volumes for fuel placement; - location of fuel lines, pumps, batteries below the bottom of the tanks so that they are always filled with fuel;
By installing the tank pressurization drainage system above the tanks so that fuel does not get into these systems.
Fuel generation procedure and aircraft alignment
When assembling an aircraft, the location of the fuel tanks is chosen such that the center of gravity of an aircraft fully filled with fuel is located near the center of gravity of an empty (unfuelled and without cargo) aircraft.
The number of supply tanks usually corresponds to the number of engines, but fuel systems with a common supply tank for several engines are used.
A fuel system with a supply tank allows you to: install high-pressure pumps for pumping fuel into engines only in the supply tank, and install light low-pressure pumps in the remaining tanks to transfer fuel to the supply tank;
simplify automatic control and manual control scheme for fuel production when a failure occurs;
to ensure, by simple design methods, stable power supply to the engines during various flight evolutions and landing fuel residue (emergency) in the consumable tanks to complete the flight;
provide filtration, degassing of fuel and, if necessary, reduce or equalize the temperature of the fuel supplied to the engines, etc.
The priority of fuel generation is determined by the following factors: acceptable alignment of the aircraft, reduction of the load on the wings, reduction of fuel heating due to aerodynamic heating from operating engines and the air conditioning system, tasks performed by the aircraft (first priority production of the drop tank on high-speed aircraft).
Engine fuel supply system
The engine fuel supply system includes a fuel tank (usually a supply tank), from which fuel is directly supplied to the engine or engines (depending on the selected circuit); high-pressure pumps that provide the required flow and boost of fuel supplied to the engine control pumps (plunger-type control pumps to create spray pressure on the nozzles in the engine combustion chambers require increased inlet pressure to avoid cavitation); a fuel line from the supply tank with a flow meter sensor, a fuel pressure indicator in front of the engine, a remote control shut-off valve for cutting off fuel from the engines in an emergency; a loop tap for powering the engines from another supply tank (in a scheme with several supply tanks).
The fuel system is characterized by the flight altitude up to which an uninterrupted supply of fuel to the engines is ensured. The main factors determining the altitude of the fuel system are:
fuel pressure in front of the engine regulator pump.
Fuel transfer system
The system for pumping fuel into the supply tank ensures that the aircraft is aligned when the engine runs out of fuel by observing a predetermined sequence and order of fuel transfer from the fuel system tanks to the supply tank.
The most widespread are systems for pumping fuel into a supply tank with centrifugal electric drive pumps. On some aircraft, due to conditions of increased pumping rates, hydraulically driven pumps or turbopumps are used. Recently, jet pumps have been widely used in pumping systems.
Transfer systems with jet pumps
Jet pumps are used for pumping fuel, pumping out remaining fuel from tanks of complex configuration with a large bottom surface, which is typical for wing tanks.
Small overall dimensions and weight, the absence of moving parts and electrical wiring determine their widespread use in fuel systems, despite their lower efficiency than other types of pumps.
Jet pumps are driven by engine pumps or electrically driven centrifugal pumps.
Fuel tank pressurization drainage system
The above-fuel space of the tanks communicates with the atmosphere using a drainage system. Communication of the above-fuel space with the atmosphere is necessary when filling tanks with fuel, especially with a closed centralized refueling to remove air from the tanks, excluding air back pressure when filling the tanks; when pressurizing tanks to release excess air into the atmosphere; when changing flight altitude to maintain a constant law of pressure difference between the above-fuel space and the external atmosphere, etc.
Fueling system
Two types of filling are used:
a) open, in which a tank or group of tanks is filled with top
drain through the openable filler neck of the tank, located
at the top of the tank.
b) centralized, which is carried out under pressure
through the fuel receiver located at the bottom of the aircraft, in a place convenient for maintenance.
The aircraft is refueled in flight from the refueling aircraft via a hose connected to the fuel receiver on the aircraft being refueled.
Fuel drain systems
The fuel system must provide:
draining fuel in flight;
draining fuel from all tanks (or individual tanks) in the parking lot by sucking it out with a fuel tanker;
draining fuel sludge on the ground.
Fuel tanks
Depending on the type of aircraft, thermal operating conditions of the structure, and location on the aircraft, soft fuel tanks, fuselage compartment tanks made of light alloys, plastics or composite materials are used.
Soft fuel tanks are made by gluing on collapsible molds from heat-resistant rubber and reinforcing fabric in sizes and configurations corresponding to the container-compartment in which the tank is placed.
The welded structure of the aircraft allows the hermetic containers of the fuselage and wing compartments to be used as compartment tanks.
Tank compartments made by assembly processes consist of outer skin panels and walls.
Drop tanks are used to increase flight range and are usually jettisonable, but if necessary, aircraft can land with drop tanks empty.
Fuel pumps
Pumps in fuel systems are necessary to create pressure in front of the plunger pump-regulators of the engine and to pump fuel from the tank to the supply tank.
Centrifugal and axial fuel pumps are driven by DC or AC electric motors, hydraulic and pneumatic turbines.
The hydraulic turbine drive pump uses high pressure fuel energy generated by a pump mounted directly on the engine to rotate a transfer pump mounted on the fuel tank. Energy is transmitted through a turbine mounted on the impeller. The fuel from the engine, giving energy to the pump turbine, rotates a low-pressure pump with high flow rate.
A fuel pump with a pneumatic turbine drive is a unit in which the pump is driven by an air turbine. Compressed air is taken from the engine compressor and supplied through a pipeline to the installation site of the unit. The compressed air rotates the turbine and, after transferring its energy to the turbine, is released into the atmosphere.
Purpose of an aircraft fuel system
Fuel system designed to place the required amount of fuel on the aircraft and supply it to the engine (engines) in all flight modes.
The fuel used on modern aircraft is high-octane gasoline for piston engines and aviation kerosene (T-1, TS-1, RT, etc.) for jet engines.
The fuel system is conventionally divided into the aircraft fuel system and the engine fuel system.
In any aircraft fuel system Three characteristic areas can be distinguished:
fuel filling system;
fuel container;
fuel supply system to the engine.
Fueling into the tanks is done either by gravity or centrally.
Fuel tanks are made in the form of separate tanks or in the form of separate sealed compartments of the aircraft airframe. Fuel tanks are placed on the aircraft so that the center of gravity of all fuel is located near the center of gravity of the empty aircraft. In order to ensure the necessary roll stability of the aircraft, fuel from the right and left tanks is produced evenly using an automatic leveling device or manually. Based on their placement, they distinguish between fuselage and console fuel tanks, and based on the nature of their use – consumable and additional.
The fuel supply system must continuously supply the required amount of fuel to the engine fuel system. The power supply system must satisfy the following requirements:
ensure reliable fuel supply to engines at all flight modes and altitudes, regardless of atmospheric conditions.
The aircraft's fuel supply must ensure the specified flight range and duration.
the possibility of normal power supply to the engines if one of the tanks or pipeline sections fails.
be easy to use and fire safe.
Fuel production must occur according to a given program and have little effect on the flight alignment of the aircraft.
complete depletion of fuel (remaining no more than 1.5% of tank capacity)
There are two types of fuel systems:
open;
closed.
In open - cavities of fuel tanks communicate with the atmosphere. In closed ones, these cavities communicate with the air intake system from the engine compressor or are pressurized with neutral gas from a special pressurization system.
Design of the fuel system of the TL-2000 aircraft (20 min.).
The fuel system of the aircraft TL – 2000 Sting carbon is open type, i.e. cavities of fuel tanks communicate with the atmosphere. Fuel is supplied to the engine by a mechanical pump or an electric pump.
The fuel supply system consists of:
fuel tanks;
pipelines;
shut-off - fire hydrant;
filter - settling tank;
electric pump;
mechanical pump;
systems for monitoring the presence and production of fuel;
fuel drain valve;
filler necks.
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Rice. 10.1. Schematic diagram of the fuel system TL – 2000 Sting carbon
Imagine that, sitting in the center of the Tu-154M cabin, there is at least 3 tons, or even 8 tons, of kerosene below you. It looks something like this:
Can you imagine 8 tons of kerosene? I agree, it's difficult. I assure you that in the wings of the aircraft there is much more room under the passenger seats than in the center section. Moreover, there is fuel on the plane Always, drains completely only in cases of special maintenance. On a Tu-154M with installed engines, it is generally prohibited to drain all fuel, otherwise it will sit on its tail. This happens, photo below ;).
Shall we refuel?
The story in this article will be about fuel on an airplane. Very much and detailed ;).
The cost of kerosene today varies from 17 to 35 thousand rubles per ton. A simple Google search gives the following sites:
http://www.riccom.ru/sale_market_r_np_12.htm
http://distoplivo.ru/prais/
You'll figure it out without me =).
We fuel at Pulkovo with two grades of aviation kerosene, which are considered interchangeable and can be mixed in any proportions: TS-1 and RT. Abroad they use Jet Fuel A, Jet Fuel A-1 (freezing point -47°C) and something else. You can also pour and mix in any proportions. The main thing is what is written in the aircraft documentation. If the crew encounters some unfamiliar brand, they need to consult with the base.
In winter, an additive, liquid “I”, is added to kerosene so that it does not freeze at lower temperatures (exactly -60°C). They add very little, 0.05% of the total. More liquid "I" Prevents thickening and waxing of diesel fuel at low temperatures. Prevents fuel filter icing. Promotes complete combustion of fuel. Removes water from the fuel system. Increases torque. Provides easy engine starting in cold weather.
http://www.masla.su/?Produkciya:Tehnicheskie_%0Azhidkosti
They say that pure kerosene can be drunk, and it helps cure diseases (blood, gastrointestinal tract, genitourinary system). BUT! You can’t drink kerosene with liquid “I”!. I don’t know why or how, but the only thing I ask is, don’t try to ask your familiar technicians or pilots to pour a jar of kerosene in winter, spring or fall. It may contain this dangerous additive. I don’t know what exactly is dangerous, but it’s better not to risk it.
So, the fuel tanks are mostly caisson tanks. This means that kerosene is simply poured into the wing cavity, there are no special containers, everything is located in a sealed compartment of the structure.
Let's see where the fuel is stored and how it is used on board? On different aircraft, the tanks are located differently, but in general the trend is the same - three tanks (the central one, which is also consumable, from which fuel is taken to the engines, and the wing ones).
Let's take a look at the A-320:
Boeing 737 Classic (the most popular type of 737 in Russia, produced in the 90s).
Well, now the highlight of the Tu-154M issue:
The "fifty kopeck" tanks are located quite cleverly. The supply tank is called: “First”, and is located in the middle, at the back. The fourth tank is filled first and is very often used to maintain alignment.
What is a supply tank? This is a fuel tank from which fuel goes directly to consumers - engines. From all other tanks, fuel is pumped into the supply tank and only then sent to the engines.
On some aircraft (for example, the A330, in my opinion, it is also allowed to be used on the latest modifications of the Tu-204) there is an additional tail fuel tank to adjust the alignment of the aircraft in flight. They can be located both in the fin (Tu-204) and in the stabilizers (A330).
Any tank must communicate with the atmosphere, in other words, be “leaky”. For what? Try drinking Duchess (Coca-Cola, whatever you like) from a glass bottle without lifting your lips (so that air does not pass inside). You won't last long. The pressure inside the bottle will drop sharply, and you will not be able to drink.
Therefore, instead of fuel leaving, air should enter the tank in its place from outside the aircraft. For this purpose, it is common practice to create drainage tanks on foreign aircraft. They are located at the end of the wing. And their exit into the atmosphere looks like this:
Such a tricky entrance (often used) so that the incoming air flow presses the kerosene in the tanks.
In the case of the Tu-154M there are no drainage tanks. They are connected directly to the atmosphere through cunning pipelines encircling the fuselage. The pipes first go up, then go around the contour of the fuselage and have an outlet at the bottom. This is done so that when the aircraft tilts (rolls), fuel does not spill out. The picture is complex, I recommend enlarging it.
I already wrote once in a magazine about refueling an airplane before a flight. I'll try not to repeat myself.
So, before refueling the plane, it is necessary to drain the fuel sediment to check for the presence of water in the tanks of the plane. It is precisely intended for draining sediment into it in the field.
The drainage of sludge by a technician is often controlled by a flight engineer (in the photo the drainage of sludge from an IL-76):
Then a fuel tanker arrives.
The technician must tell the tanker driver how much fuel needs to be filled, so while the tanker connects the hose (sometimes he connects two at once to speed up the process), the technician goes to look at the remaining fuel:
The remainder is determined by the aircraft's instruments and is also recorded in the logbook. As you can imagine, these data sometimes do not add up. The temperature outside has changed - the fuel density has changed, the instrument readings have changed. The thing is that on an airplane it is measured in kilograms, and in a tanker in liters. The arrow division price is 1 ton. Depending on the voltage in the aircraft's electrical network, the needle readings may fluctuate. The photo shows the control panel of the Tu-154M fuel system (arrow indicators show the amount of fuel in each group of tanks):
A bunch of lights and switches help control the in-flight kerosene flow from the various tanks. The lights indicate whether the pump for each tank is currently on or off. In general, it took me a long time to get used to this system; at first it was difficult to figure it out =). At the beginning of the operation of the Tu-154 aircraft, there was a disaster when the engines turned off during flight due to the fact that the fuel tank ran out, and the flight engineer forgot to turn on the pumping from others to the supply tank. The engines stopped, the plane fell =(. After this, changes were made and when a certain level in the supply tank drops, fuel begins to flow from others automatically.
If the fuel gauge readings and the entries in the logbook match at least +/- 200 kg, then refueling can begin. The main thing at this stage is not to forget to check the refueling driver so that he connects his car to the aircraft with a grounding cable (the electrical potentials must be equalized through it, and not through the refueling hose, because this can cause a strong spark of static electricity). And also another grounding cable must be connected from the machine to the grounding point on the apron (usually a piece of pipe buried in the ground).
Open the filler neck (usually in the wing):
And connect the hose:
Or hoses (photo Boeing-767):
The neck differs from a car neck in that there are check valves there. You don't have to worry about fuel spilling out. The whole process is “dry”, the valves open only when pressure is applied:
Fortunately, both on the Tu-154 and on foreign aircraft, this connection is unified everywhere and no adapters are needed. The spring presses the plate so that the fuel does not flow back.
The fuel meter at the gas station is in liters. Therefore, before refueling, we need to calculate how many liters we need. The density of the fuel depends on the ambient temperature and varies from 0.779 to 0.791 (the numbers may not be accurate, I forgot everything) and is written on the control card, which confirms the condition of the fuel. The last check must be completed no more than six hours ago, otherwise fuel cannot be refueled. All required signatures and verification hours are indicated on the ticket. If everything is in order, we count the liters and tell them to the gas station attendant.
But before you say “let’s go,” you need to perform one more procedure: checking the fuel in the fuel tank for the presence of water. We kindly ask the refiller to provide a sample in a jar. If no water is detected (in my lifetime I have never seen water in the TZ), then you can refuel.
We open the taps of the tanks we need (where we will refuel at the moment). All is ready.
Go!
Kerosene rushes into the aircraft tanks at great speed. This all happens before the passengers arrive. In Russia there are special procedures for refueling an aircraft with passengers, but everyone tries not to do this. Why take the risk?
Of course, the tanks have overpressure protection to prevent them from bursting when refueling. The protection is a small valve that opens when the pressure is exceeded and releases the air. A simple and common mechanism.
You can control refueling on foreign aircraft right next to the hose connection point, and you can also control the taps there (to select the filling of the tanks we need):
The A330 refueling panel is located on the fuselage at the rear:
A320x family:
Sometimes the panel is located directly on the wing, sometimes on the fuselage, at the request of the aircraft customer.
On the Tu-154M, you can only control the cranes from the outside, while all the indications are inside, in the cockpit. Only. This always irritated me; I had to run from the cockpit under the wing and back.
You can, of course, use measuring rulers from the outside, but their minimum value is sometimes not sufficient to show the desired level. Pulled straight out of the wing:
It turns out that a magnet floats in the tank, which at the right moment picks up the ruler and does not allow it to fall below the kerosene level. This way you can determine how full the tank is without any electronics. To be honest, I have never used this method. It was always safer for me to look in the cockpit.
Is it possible to refill the tank, fill too much? It is forbidden. The automation will close the taps and prevent you from filling the plane with more than is allowed. But automation tends to fail. The mechanics work for this case:
There are valves in the wing that at a certain point begin to drain excess kerosene directly onto the ground. They open during refueling from the pressure of incoming kerosene:
Aviation has everything covered ;).
In the cockpit, pilots always have the opportunity to view instrument readings about the fuel level. For example in 737:
Pump control in flight:
On Airbuses everything is simpler; fuel information is generally displayed on one of the pages of the multifunction display:
Compare with the fuel system control panel 154 =). That's where the power is =).
Actually I'm kidding. Of course, this is why a flight engineer does not fly on new foreign aircraft as part of the crew. It's simply not needed there. The plane does everything itself.
Especially on larger aircraft, refueling must be carefully monitored to ensure that one wing does not end up with much more fuel being pumped into it than the other. This is called a "fork". You understand that if there is several tons more fuel in one wing, this can not only affect the pilot’s comfort (the plane will pull to the side), but also flight safety.
The worst part is that the situation is very difficult to correct. If you end up with a large fork, you need to drain the excess fuel and refill it in the other wing. And this is at least an hour (if everything coincides successfully and the necessary ground equipment is at hand, which never happens) of time. Accordingly there is a delay. And for delays due to personal fault, the technical staff will not be patted on the back... Drained fuel is no longer refilled into airplanes. It goes to airfield equipment, tractors and something else.
So, the refueling is complete. The TK driver issues a request, which states the liters of kerosene on the meter. This is a very crucial moment when all the calculations need to come together, otherwise there will be problems. We convert the liters in the requirement into kilograms and add them to the balance before refueling. If this value is equal to the one required for the flight, then everything is fine, we put the necessary markings (and as you thought, everyone answers with their head, especially in the matter of fuel).
How much kerosene does a plane take? I will not give specific numbers for flights, because I have already begun to forget them. I can say that the Tu-154M usually filled 25-35 tons. B-737-500 no more than 15 tons. A320 approximately 15-25 tons. These data are given for approximately the same routes. It’s better to ask the pilots how fuel is calculated; I’ve never done this and wasn’t particularly interested. I know that the refueling includes an aeronautical reserve that allows the aircraft to fly for several more hours and is calculated differently for each type.
15 minutes after refueling, you need to drain the fuel sediment from the aircraft again. During this time, possible water should have sunk to the bottom of the tanks, where we check it through the drain points:
We bring the jar and check the condition of the kerosene. Everything is fine?
And now I’d better say a few words about how it is consumed in flight. So, fuel is supplied from the supply tank by pumps. Typically these pumps are centrifugal:
This type of pump is simpler than others and allows it to operate at idle speed, even if there is nowhere to pump fuel (the fuel supply valves to the engines are closed). There are transfer and booster pumps. Some help move fuel through the tanks, while others send it to the engine power supply line.
But to start the engine, it is not enough to turn on the pumps. It is also necessary to open the “fire valves” (as they are called on domestic equipment, because they are closed first in the event of an engine fire). When the taps are opened, the fuel enters the engine, where it is filtered and heated (usually there is a radiator that cools the oil circulating in the engine and at the same time heats the fuel) and is supplied to the injectors. This is already the motor part, so we will talk about it in detail in the following posts. I can only say that there are several degrees of filtration and even if all the filters become clogged, the fuel will bypass. The main thing is to maintain smooth operation so that the plane can be landed.
Finally, I would like to show you what happens when mistakes are made with refueling on the Tu-154:
Photos from the Internet
Yes, yes, the plane can just land on its tail!
Photos from the Internet
In fact, this is the worst dream of every Tu-154 technician and flight engineer. The tail of the plane is very heavy. Passengers should preferably exit in order - the second cabin, the first cabin, especially if there is little fuel left in the fourth tank.
Photos from the Internet
They recently wrote about how fuel is stored at the airport here: http://community.livejournal.com/sky_hope/180444.html#cutid1
I highly recommend watching it.
Compartments for fuel tanks are placed on the aircraft during aircraft configuration, and the mass of fuel in the compartment is determined as
M t =ρ(W 0 -W st -W a -W st -W m.b.),= ρ W t
W 0 - the volume of the compartment in the aircraft structure for the tank;
ρ - fuel density at a given temperature;
W St- the free volume of the space above the fuel, necessary for the expansion of the fuel when its temperature changes;
W a- volume of in-tank fittings, pumps, fuel meters, etc.;
W st - volume of tank walls;
W m.b - the amount of space between the outer surface of the tank and the structural elements of the aircraft;
W t – volume of fuel filled.
Conventionally taking the fuel density at a temperature of 20 °C as the initial one and introducing the concept of the compartment filling factor to z.o., the use of aircraft compartment volumes for fuel storage can be assessed and compared. This coefficient is the ratio of the volume filled with fuel to the volume of space inside the aircraft structure allocated for it: to z.o. = W t / W 0 .
Depending on the type of aircraft, location, purpose and design of the tank, this coefficient can vary within fairly wide limits. It has the greatest value, close to unity, for tanks made in the form of sealed aircraft compartments, from which fuel is displaced by compressed gas. The smallest value of the compartment fill factor ( to z.o.= 0.8-0.9) occurs in consumable protected tanks with a large number of automatic control devices for fuel production, pumps and other equipment.
An increase in the required fuel reserves causes certain difficulties in its placement on aircraft. On transport aircraft, passengers and cargo are placed in the fuselage, and fuel can generally only be placed in the wing consoles. In this regard, the choice of the height of its profiles is made not only from aerodynamic requirements, but also from the condition of placing the necessary fuel reserves in them. To make the most efficient use of the internal volumes of the wings and increase the capacity of the fuel system, on modern aircraft compartments formed by the wing structure are used for fuel tanks. They are coated from the inside with sealant and are called caisson tanks.
Typically, only part of the wing volume is allocated for fuel, and the rest of the volume contains pumps, wing mechanization, landing gear and elements of the aircraft control system. When the wing is positioned at the top, its center section can be used to store fuel, which is not permissible for a low-wing aircraft (fuel may ignite during an emergency belly landing).
It should be noted that the mass of fuel in flight unloads the wing, resulting in a certain gain in the mass of its structure. During landing, the mass of fuel increases the load acting on the wing attachment, but landings are usually made with a small amount of fuel in the wing tanks. In emergency landings, after a short period of time after takeoff, fuel is drained from the tanks, for example on Tu-104, Tu-114, etc. aircraft.
To replenish fuel reserves and increase flight duration, combat aircraft use in-flight refueling from special tanker aircraft. For safety reasons, passenger aircraft do not provide in-flight refueling.
On fighter aircraft, due to the limited volume of the aircraft structure, the bulk of the fuel is located in the fuselage and additionally in the wing. Fuselage tanks have a complex shape, which is determined by their location. They have a relatively large height, which contributes to more complete fuel production. On these aircraft, the fuselage has a relatively small free volume for fuel due to the placement of special equipment in it. Therefore, suspended fuel tanks are used to increase fuel reserves.
Outboard fuel tanks on swept-wing aircraft are installed under the fuselage and consoles. On aircraft with low wing sweep angles, drop tanks are installed at the ends of the wing, which is explained by the smallest increase in drag, an effective increase in wing area and unloading of the wing.
The capacity of external fuel tanks ranges from 500 liters to 5000 liters, and on some types of aircraft, for example the B-58 bomber, where the external fuel tank is made in the form of a container suspended under the fuselage, it reaches 10,000 liters.
Drop tanks have a negative impact on the flight characteristics of the aircraft (maneuverability and acceleration characteristics deteriorate, drag increases, altitude decreases, etc.).
The volume of outboard drop tanks for a particular aircraft is determined by fuel consumption in non-critical sections of the flight path (launch, testing, taxiing, takeoff, climb, flight over home territory, etc.). If necessary, in critical sections of the flight path (evolution, air combat), the outboard tanks are dropped, regardless of the presence of fuel in them.
In-flight refueling has become widespread on combat aircraft, which makes it possible to increase the endurance and combat effectiveness of the aircraft. The placement of fuel in all free volumes of the wing and fuselage, and in some cases in the vertical tail, leads to a large number of fuel tanks located in various places along the longitudinal axis of the aircraft. Therefore, as fuel is exhausted from the tanks, the position of the aircraft’s center of mass changes.
When configuring an aircraft, the location of the fuel tanks is chosen such that the center of mass of the aircraft, fully filled with fuel, is located near the center of mass of the aircraft not filled with fuel. Depending on the aircraft layout, there may be two options for fuel placement on the aircraft. Symmetrical arrangement, when the centers of mass of fully filled tanks are at the same distance X from the center of mass of the aircraft and fuel volumes W 1 And W 2 the front and rear tanks (relative to the center of mass of the aircraft) are equal to each other. Non-symmetrical arrangement, when the volumes of the tanks and their distance to the center of mass of the aircraft are not equal, but only the moments of mass of the tanks are equal:
ρW 1 X 1 = ρW 2 X 2.
In the first case, fuel consumption, if it is necessary to maintain constant alignment of the aircraft, must be carried out while maintaining equal flow rates from the front and rear tanks ( Q 1 = Q 2). In this case, the fuel consumption from each tank must be proportional to the fuel consumption of the engine:
Q 1,2 = ,
Q door- fuel consumption for the engine;
n- number of engines powered from one supply tank;
k- the number of simultaneously produced tanks in the supply tank.
In this case, uneven production of the front and rear tanks, i.e., a change in the alignment of the aircraft, can occur due to different fuel consumption by the engines and instability of the hydraulic characteristics of the pumping lines.
On aircraft where fuel must be produced asymmetrically, fuel is pumped with a predominant flow of fuel from the front or rear tanks.
With an asymmetrical arrangement of fuel, if alignment compensation is not required to maintain equality of moments, for example when landing cargo, fuel consumption is carried out or is continuously proportional to the law
Q 1= or Q 1 = Q 2
or in separate portions within the boundaries of a given alignment field.
In general, the alignment of the aircraft when fuel is consumed from the tanks is estimated:
= /b sah,
Where G i– reserve (or production of part of the fuel) i fuel tank;
x i– coordinate of the center of mass of the corresponding fuel tank relative to the toe of the average aerodynamic chord;
b sah, average aerodynamic chord.
The position of the center of mass during flight determines the necessary characteristics of stability and controllability with minimal fuel losses due to balancing resistance in all sections of the flight path.
For aircraft with different wing sweeps, the following alignment ranges are recommended:
aircraft with straight wings 0.20...0.25;
aircraft with swept wings (χ=35 0 ...40 0) 0.26...0.30;
aircraft with swept wings (χ=50 0 ...55 0) 0.30...0.34;
delta wing aircraft
small elongation 0.32…0.36.
According to their functional purpose, fuel tanks, which are part of the aircraft structure, are divided into consumable and main ones. The main fuel tanks are designed to accommodate the largest volume of fuel on board. These tanks can be placed in various "free" places on the aircraft (subject to the necessary requirements), resulting in a significant number of them.
Consumable fuel tanks related to the main fuel system serve both to accommodate part of the fuel and to provide the engines with fuel. In addition, the automation installed in them allows you to control the order of fuel production within the entire fuel system. Consumable tanks are usually located near the center of mass of the aircraft so as not to significantly affect the change in the alignment of the aircraft after the fuel is exhausted from them.
It is most advisable to place intake compartments or negative overload compartments in consumable fuel tanks, which ensure an uninterrupted supply of fuel at any possible position and overload of the aircraft.
In addition, the use of a system of consumable tanks allows:
a) by simple constructive methods, ensure the landing fuel balance (fuel reserve) in the supply tanks;
b) for complex pumping schemes, simplify crew control of the automation and provide a reserve of time in the event of a failure in the pumping lines;
c) reduce and equalize the temperature of the fuel supplied to the engine;
d) constructively ensure degassing of fuel entering the supply tank from the next tanks, and improve the cavitation characteristics of booster pumps;
e) ensure partial settling of fuel supplied to the engines;
f) install powerful pumps for supplying fuel to engines only in supply tanks; install low-pressure and, therefore, lighter pumps in all other tanks.
The number of supply tanks usually corresponds to the number of engines, but in some cases schemes with a common supply tank for several engines can be used.
The layout of the pumping lines depends on the number of fuel tanks, their location on the aircraft, minimum weight and operational reliability
The implementation of a given fuel transfer program on maneuverable aircraft requires the system of fuel tanks, pipelines and units to have stable hydraulic characteristics, regardless of the evolution of the aircraft in space.
From all main tanks, fuel is pumped into consumable tanks. In this case, the order of fuel transfer is determined by the necessary alignment of the aircraft in flight and the requirements, the fulfillment of which is necessary for the normal functioning of the fuel system itself:
The fuel transfer procedure must ensure that the service tank(s) are kept full or nearly full until all other tanks are emptied;
In all cases, the remaining fuel in the supply tank(s) by the time all other containers are emptied should not be less than the fuel reserve,
The procedure for pumping fuel into the supply tank must prevent fuel from entering the already exhausted main tanks, since after the end of fuel depletion from the tank, the transfer pump is exposed, goes into off-design mode and must be turned off by the crew or automatically. The same requirement remains when fuel is supplied to the supply tank from other tanks under air pressure (extrusion). In this case, after the end of fuel consumption from the tank, the boost is turned off and the fuel that has again entered the tank will remain unused.
On fighter aircraft, in the absence of external drop tanks, fuel transfer to the supply tank should begin from the wing tanks. This is explained by the low height and large area of the wing fuel tanks, which makes it difficult to completely and uniformly exhaust fuel from them, especially during aircraft evolutions. The rate of fuel transfer from wing tanks is usually low, since laying large-diameter pipelines in thin wings is difficult. In the wing tanks of fighter aircraft, transfer pumps are usually not used due to their large dimensions, and fuel is supplied under air pressure, the increase of which is associated with an increase in the weight of the structure and difficulties in ensuring the tightness of the tank compartments.
It should be noted that on some types of fighter aircraft, in order to unload the wing structure, during flight, fuel is initially partially generated from the fuselage tanks, and then from the wing tanks.
5.6. METHODS OF SUPPLYING FUEL TO ENGINES
Scheme
The choice of a rational scheme for supplying fuel to the engines is influenced by: the purpose and layout of the aircraft, its flight modes, the type and number of engines, the type of fuel used, measures to ensure safety and altitude of flights. The difficulty of creating a rational scheme for supplying fuel to engines is due to the need to place a large amount of fuel in a limited volume, ensure uninterrupted operation of engines in a wide range of flight speeds and altitudes, turn on automatic devices that provide a given fuel production program and control the operation of the fuel system.
One of the most important parts of the fuel supply lines to the engines is the production of fuel from the tanks. To ensure fuel production, the following methods are used: gravity, displacement, booster pump
Producing fuel from tanks by gravity (Fig. 5.4 a) is used on aircraft with relatively low-power engines, where fuel consumption and the required pressure at the inlet to the engine pump are low. On an aircraft with engines that develop high thrust (power), the generation of fuel from tanks by gravity is used to transfer fuel from tank to tank, like communicating containers (either within one group, or as an emergency fuel transfer).
Fuel production from tanks by displacement (Fig. 5.4 b ) carried out with compressed air or neutral gases. The above-fuel space of the tank is isolated from the surrounding atmosphere. The advantages of such a design are: the ability to fly at high altitudes, the absence of fuel pumps on the aircraft, the ability to regulate pressure, the absence of drainage, losses due to fuel evaporation and energy consumption to drive pumps. However, there are significant disadvantages: the large mass of tanks loaded with internal pressure and their low survivability if damaged.
On modern civil aviation aircraft, the extraction of fuel from tanks only by displacement is not used, but in some cases it is possible to pressurize the fuel tank with a slight excess pressure (15...30 kPa). This excess pressure is obtained from the engine compressor (through a reducing device) or due to the high-speed pressure.
The production of fuel from tanks by a booster pump (Fig. 5.4 c) leads to the fact that the tanks are less loaded, their walls can be made thinner, and the tanks can be made lighter. The tank can be located below the booster pump; automation of pump control is possible. Pumping allows you to create sufficient pressure at the inlet to the main engine pump, providing the required altitude. The disadvantage of this method is that it makes the fuel system heavier. Electrically driven booster pumps have an increased fire hazard. The height of the pumps themselves is insufficient. To increase reliability, sometimes two parallel operating pumps are installed in the fuel line.
Fuel transfer systems on an aircraft perform various functions and can be divided into main and auxiliary.
The main fuel transfer system participates directly in the fuel supply chain from the next tanks to the supply tank with the supply of fuel necessary to power the engines.
Auxiliary systems provide pumping of fuel from drain tanks, generation of residual fuel from tanks and pipelines, etc.
The balancing pumping system ensures the creation of the necessary balancing moment of the aircraft. The most widely used systems are systems for pumping fuel into supply tanks with centrifugal electric drive pumps. Such systems are used on almost all domestic and foreign aircraft.
In Fig. Figure 5.5 shows a schematic diagram of the aircraft fuel system. It represents a multi-tank system that ensures an uninterrupted supply of fuel to the engine in all permissible operating modes of the aircraft. This diagram, consisting of a number of lines, reflects the presence of the main, necessary units and devices that ensure reliable operation of the power plant. Depending on the purpose, type of aircraft and its operating conditions, the composition of the fuel system can vary not only in the nomenclature of the subsystems themselves, but also in the units included in them. Therefore, the presented diagram should be considered as functional.
The scheme under consideration includes:
Supply line (fuel supply from the supply tank to the engine);
A transfer line that supplies fuel from the wing and fuselage main and external fuel tanks;
Drainage line.
Let's consider the fuel supply according to the proposed scheme (see Fig. 5.5). Fuel from supply tank 1 enters the fuel intake of the negative overload compartment 8. Under negative overload conditions, the fuel, occupying the upper position, will freely flow into the intake pipe until the compartment is completely exhausted. Its filling occurs when the aircraft returns to normal flight through valves 9. The latter prevent spillage
| Fig.5.5 Schematic diagram of the aircraft fuel system 1 - service fuel tank, 2 - fuselage fuel tank, 3 - wing fuel tanks, 4 - external fuel tank, 5 - booster line, 6 - transfer line, 7 - emergency overflow line, 8 - negative overload compartment, 9 - negative overload compartment valve, 10 - booster centrifugal pump (HCP), 11 - motor centrifugal pump (MCP), 12 - check valve, 13 - fuel accumulator, 14 - fuel and oil accumulator, 15 - thermal valve, 16 fine filter, 17 - shut-off (fire-fighting) valve, 18 - flow meter sensor, 19, 21 - float hydraulic valves, 20 - transfer centrifugal pump, 22 - fuel valve with actuator, 23 - fuel production hydraulic valve, 24 - hydraulic valve drainage of wing fuel tanks, 25 - drainage line, 26 - safety valve, 27 - command pressure line for fuel production, 28 - command pressure line for drainage of wing fuel tanks, 29 - pressure indicator, 30 - emergency fuel remaining sensor. |
fuel from the compartment during some aircraft evolutions. It should be noted that negative overload compartments are installed on aerobatic machines, and their volume ensures engine operation for (15...30) seconds of negative overload.
Fuel is supplied to the engine by booster pump 10. To increase the reliability of operation, as a rule, two pumps are installed in the supply tanks with the obligatory installation of check valves at their outlet. If one of the pumps fails, its check valve will block the flow of fuel back into the tank from the operating pump. The backup pump operates either in parallel with the main one, or has autonomous control and is switched on in the event of failure of the main pump.
The same type of pumps are usually used as backup pumps, but systems with backup pumps that have a non-electric drive (ejector or turbo-driven pumps) are known. In the latter case, fuel transfer can also be ensured in an emergency when the aircraft power supply system fails.
On aircraft with high fuel consumption, in some cases, centrifugal pumps driven by an air or hydraulic turbine are used as the main fuel transfer pumps.
Recently, jet pumps have become widespread in fuel transfer systems (especially in after-treatment mode).
On modern aircraft, to ensure a reliable supply of fuel to the engines (including to eliminate cavitation at the inlet to the main engine pump), multi-stage pumping is used. Usually they get by with one first stage booster pump (NP1) 10 and one second stage booster pump on the engine (NP2) 11 . In this case, NP1 creates the required pressure at the inlet to NP2, and the latter provides the required pressure at the inlet to the main engine pump (ONP) . The advantages of such a two-stage pumping system are the lower total mass of NP1 and NP2 and also lower power for their drive compared to a single boost pump, which provides the required pressure at the inlet to the OND. In addition, this scheme for switching on the pumps allows fuel to be supplied from the supply tank at lower pressures, which relieves the pressure on the pumping line pipelines and eliminates the occurrence of fuel leaks.
Fuel accumulator 13 can perform a dual function: to provide fuel supply from the supply tank (in the absence of a negative overload compartment) under the influence of negative overloads and damping fluctuations in fuel flow and pressure in transient modes.
The fuel accumulator consists of two cavities separated by a flexible rubber membrane - an air cavity and a fuel cavity. Air (or gas) pressure is supplied to the air cavity, somewhat less than the pressure created by the fuel pump of the supply tank. The fuel cavity is connected to the engine supply line. Behind the supply tank pump 10, a check valve 12 is installed, allowing fuel to flow only towards the engine. When the pump operates, the accumulator is filled with fuel due to a flexible rubber membrane and the fuel pressure is maintained in a filled state. When the pressure behind the pump drops (fuel supply decreases or stops), the fuel accumulator compensates for its supply from its cavity. After the pressure behind the supply tank pump is restored, the accumulator is filled with fuel again. The duration of negative overloads and their magnitude depend on the purpose of the aircraft and its flight conditions.
On aircraft with turbojet engines, the fuel systems include a fuel-oil radiator 14, which cools the oil of the aircraft oil system with leaking fuel. In this case, the heated fuel is better atomized in the engine injectors and protects the filter 16 from possible freezing. If fuel consumption to power the engine is less than to cool the oil in the fuel-oil radiator, then part of the fuel, having passed the radiator, is bypassed through thermal valve 15 , back to the tank. Fine fuel filter 9 is required in all fuel systems. The filtration fineness is about 15 microns. If it is possibly clogged, the fuel, bypassing the filter element, enters the engine through the bypass channel provided in the design of the filter itself.
Shut-off (fire) valve 17 is designed to stop the fuel supply to the engine in emergency situations (fire, belly landing, etc.). It has a remote servo for closing. Opens only on the ground. The control and measuring equipment is represented by an emergency fuel remaining sensor 30 , pressure gauge or pressure switch 29 , flow meter 18.
If there is a significant amount of fuel, large tanks are required to accommodate it. Difficulties in installing such tanks force the use of relatively small tanks, but their number increases accordingly. To organize a rational supply of fuel to engines with low hydraulic pressure losses, a small mass of lines and to ensure the required alignment range, the tanks are combined into groups of 2, usually by connecting them in series according to the scheme of communicating vessels.
Moreover, there can be several such groups, and fuel production from each group is carried out by its own transfer pump 20.
The filling level of the supply tank is controlled by valve 22. If there are several groups, each of them is connected to its own valve, and the order of fuel production among the groups will depend on the installation level of these valves.
The float valve (Fig. 5.6) serves to protect the supply tank from overfilling when pumping fuel from the main fuel tanks.
The valve is installed inside the supply tank in its upper part. The valve assembly is placed in housing 1. The connector between the housing and cover 5 is sealed with a rubber gasket 4. Inside the housing there is a damper valve 2, which blocks the access of fuel to the tank. It consists of a mushroom valve 20 and a number of parts assembled into one unit. During a hydraulic shock, valve 2 moves downward in the piston, moves away from the housing seat and releases excess pressure into the tank. When a certain level of fuel in the supply tank is reached, the damper valve 2 blocks the access of fuel to the tank under the action of the spring 3 and fuel pressure at the moment the valve 6 closes the hole in the lid 5. When the fuel level in the tank decreases, the lever with the float valve 6 opens, which causes a decrease pressure under the piston 18. Under fuel pressure, damper valve 2, compressing spring 3, moves away from the seat, opening the flow area and fuel
through the windows in building 1 it pours into the tank and fills it. When filling the tank, when the float occupies the upper position, valve 6 closes the hole in the cover 5. Through the nozzle in valve 20, fuel flows into the internal cavity of the valve and its pressure, together with spring 3, presses the damper valve to the seat, blocking the flow of fuel into the tank. From the wing tanks 3 and the outboard tank 4, fuel is removed under excess pressure, taken either from the engine or compressed gas cylinders.
According to the scheme, production from tank 4 is carried out primarily using the float hydraulic valve 19 and the fuel production hydraulic valve 23, their circuit diagrams are given in Fig. 5.7 and 5.8, respectively.
When the fuel level in tank 1 decreases, float 4 (see Fig. 5.7) goes down and ball valve 2 shuts off the fuel discharge (the latter is taken from pump 10). This causes an increase in pressure in the command line 6, which is connected to the membrane box 1 of the hydraulic valve (see Fig. 5.8). Under the influence of excess pressure, membrane 4, overcoming the force of spring 3, opens valve 6, which ensures the supply of fuel to the supply tank. When the required fuel level in the supply tank is reached, float 4 (see Fig. 5.7) will open the ball valve, the pressure in the command line will drop and valve 23 (see Fig. 5.5) will shut off the fuel supply from the hanging tank. After emptying the hanging tank, the hydraulic valve of excavation 23 will be in the open state.
The production of fuel from the wing tanks is controlled by hydraulic valve 21 and its float is installed at a lower fuel level in the supply tank. When the fuel level decreases below a predetermined level, the pressure in the command line 28 increases, which closes valve 3 (see Fig. 5.9), cutting off the cavities of the wing tanks from the general drainage system. The pressure in the wing tanks increases, under the influence of which it is forced out through the open valve 23 and increases the fuel level in the supply tank 1. After which the hydraulic valve 22 relieves the pressure in the command line 28. The command pressure relief valve 24 connects the cavities of the wing tanks with drainage and the fuel supply stops.
5.7 FUEL PUMPS.
Pumps used in aircraft fuel systems must provide, depending on the type of aircraft, a fuel supply from 0.3 to 100 m 3 /h or more at a relatively low pressure (no more than 200 ... 250 kPa) and small inlet pressure. They must be reliable in operation, have low weight and overall dimensions and a long service life. In addition, special requirements are imposed on fuel pumps due to the temperature of the fuel and ambient air, the magnitude of overloads, the position of the unit in space, etc. Of the large number of pump types currently existing, vane and jet pumps most fully meet these requirements.
Vane (centrifugal) pumps have a number of advantages compared to volumetric pumps:
They operate at a significant speed of rotation of the impeller;
They have high productivity;
Characterized by small dimensions and low weight;
The connection of the impeller to the drive is simplified (usually directly), which eliminates complex transmission mechanisms;
Ensure free flow of fuel with a stationary impeller.
All these advantages and relatively high efficiency. make vane pumps reliable and easy to use.
Jet pumps, compared to all the listed types of pumps, have the smallest weight and greater reliability, but do not always have satisfactory economic characteristics due to low efficiency values.
Centrifugal fuel pumps are driven by various types of drives. Direct drive from the aircraft engine shaft is the most reliable and economical, but can only be used for pumps installed directly on the aircraft engine, for example, second-stage fuel pumps. For all other fuel pumps, various drives are used: electric, hydraulic motor and pneumatic turbine drives.
Electric motor driven fuel pumps.
In-tank electrically driven centrifugal pumps (ECP) have become widespread (Fig. 5.10). The main advantage of these pumps is that they can be placed inside a tank using fuel to cool the electric motor.
The reliability and service life of in-tank ESPs largely depends on the degree of tightness and, consequently, on the perfect design of the seals of the rotating parts. The sealing collar is cooled by fuel leaking between the seal and the pump shaft. Leaking fuel hitting the centrifugal deflector 4, fixed to the shaft, thrown towards the drainage channel 10, to which a tube is connected, the free end of which is led outside the aircraft into an area of low pressure.
Pumps driven by electric motors have fairly high reliability. In booster and transfer fuel pumps, in case of drive failure, the fuel supply is ensured by gravity (due to suction by the subsequent pump stage) through the internal channels of the impeller.
The most widely used drives for centrifugal pumps are DC electric motors with mixed excitation and three-phase asynchronous AC motors. It should be noted that the service life of a DC electric drive is determined by the reliability of the brush-commutator assembly.
The great advantage of AC electric motors, due to the absence of a commutator and brushes, is trouble-free operation in a highly rarefied atmosphere with low humidity (high altitudes). The disadvantages of an AC electric motor are strictly regulated rotation speeds and a lower starting torque than DC motors, which in some cases limits their use.
Fuel pumps with pneumatic turbine drive. The required drive power of pumping units in some cases may exceed (7... 10) kW.
The pneumatic turbine drive has a low weight and overall dimensions with high power, high reliability and no influence of the drive on the thermal balance of the fuel. This explains the widespread use of this type of drive on supersonic aircraft with high fuel temperatures at the engine inlet.
The use of pumps driven by an air turbine makes it possible to reduce the power of units installed directly on the engine. At the same time, the midsection of the power plant and its weight are reduced.
Jet pumps. On aircraft with gas turbine engines, when there is high-pressure fuel on board from the bypass line of the main and afterburner pumps of the engine, jet pumps, due to their simplicity of design, ease of operation, operational reliability and practically unlimited service life, are becoming increasingly widespread.
A schematic diagram of the installation and power supply of the jet pump of the first stage of fuel pumping is shown in Fig. 5.11. In this scheme, fuel from the supply tank enters the jet pump and is then supplied to the centrifugal pump of the second stage of pumping. High-pressure fuel enters the jet nozzle of the pump through pipeline 6 from the constant bypass circuit of the turbojet engine regulator pump. The electric drive pump, located in the fuel tank, is connected by pipeline 7 to the line between the jet pump and pump P of the boost stage and provides fuel supply in engine acceleration modes.
Power supply schemes for jet transfer pumps are possible using the reserve power of stage 1 booster pumps installed in the supply tank, since their full capacity is used only for a short time during the aircraft’s climb mode.
In Fig. 5.12. ejector efficiency data are given for various values, mixing coefficient q cm and various dimensional ratios m. As can be seen from these graphs, the maximum possible efficiency of a jet pump is 27% at q 0= 2.25 and m = 7.75.
Efficiency values of a jet pump (25...27)% can only be obtained with constant values of the mixing coefficient q c m and dimensional ratio coefficient m, which can be implemented in some cases only for transfer pumps. Obtain high efficiency values for jet pumps of the first pumping stage, which are characterized by variable values of the mixing coefficient q cm, is possible only when using special systems for regulating the ratio of the cross-sectional dimensions of the nozzle and the mixing pipeline (with a variable value of the coefficient m).
5.8. CAVITATION
Cavitation (from the Latin cavitas - emptiness) is an arbitrary transition of the liquid phase of fuel into the vapor phase, when the static pressure in the liquid is compared with the pressure of saturated vapors.
In the lines of aircraft power plants, cavitation can occur due to a decrease in external pressure with an increase in flight altitude. In the initial stage, the vapor phase is represented by small bubbles; then the enlargement of the bubbles occurs, which move in the horizontal pipe in the upper part of the section and, finally, separation of the vapor and liquid phases and rupture of the jet is possible.
The highest pressure of vapor above a liquid, which is established when steam is released in a closed vessel at a given temperature, is called saturated vapor pressure (p t). For a one-component liquid, the value p t depends only on the temperature and physical properties of a given liquid and does not depend on the volumetric ratio of the vapor and liquid phases; for a multicomponent liquid - not only on temperature, but also on the ratio of the vapor and liquid phases (with a decrease in the volume occupied by the vapor phase, the saturated vapor pressure increases ). When testing aviation fuels in laboratories, the standard ratio of vapor and liquid phases is 4/1. . On the graphs in Fig. 5.13 the values are given p t for various fuels.
With increasing temperature, the saturated vapor pressure of single- and multicomponent liquids increases, but to different degrees for different liquids. To characterize the saturated vapor pressure of a liquid in one number, a temperature of 37.8°C = 100°F is conventionally adopted, at which the pressure is called the Reid pressure and is designated pRid. This value is a physical characteristic of a specific fuel and is found from reference data.
With increasing flight altitude, decreasing atmospheric pressure leads to a drop in pressure in tanks and fuel lines, while more air and gas inclusions are released into the space above the fuel, which carry fuel vapors with them. If the external pressure is higher than the pressure of saturated fuel vapor, then the evaporation of fuel from the surface does not significantly affect the size and intensity of the release of air bubbles; if the external pressure is lower than the saturated fuel vapor pressure, then internal evaporation (boiling) of the fuel begins, which begins the earlier, the higher the saturated fuel vapor pressure.
In the initial stage, a slight decrease in pressure leads to the release of air dissolved in the fuel, which appears in the fuel flow in the form of small bubbles, approximately evenly distributed throughout the volume of the moving liquid (Fig. 5.13a, A).
With a further decrease in pressure, vapors of low-boiling fuel fractions are released from the liquid. The bubbles mainly consist of fuel vapors, and the liquid flow becomes two-phase; followed by enlargement of bubbles. In a horizontal pipe they move mainly in the upper part of the section (Fig. 5.13a, b). Finally, cases of complete separation of the vapor and liquid phases are possible and their movement is carried out by independent flows (Fig. 5.13a, V).
With a deep decrease in pressure, the entire liquid turns into a vapor state, which leads to a violation of the continuity of the flow and the appearance of vapor locks. This causes the fluid supply to stop (Fig. 5.13a, G).
Negative consequences include a decrease in the throughput of the line (up to a complete interruption of supply), the occurrence of oscillatory processes in the fuel flow and cavitation destruction of fuel system elements.
Fluctuations in flow rate are caused by the fact that a vapor lock entering the pump impeller almost completely stops its performance. This leads to a decrease in flow velocity and an increase in the static component of pressure, which exceeds the fuel vapor pressure. This causes them to condense, the liquid becomes single-phase, the fuel supply is restored and the process repeats.
Cavitation destruction of surfaces is explained as follows. During turbulent fuel flow, the existing vapor bubbles end up on the wall surface in the boundary layer, where the static pressure exceeds the vapor elasticity. As a result of bubble condensation, a local water hammer is created at the point of contact, leading to the removal of the surface protective oxide film. Over time, this area oxidizes again and the process repeats again. Thus, the surface is subject to erosion-corrosion destruction.
5.9. CAVITATION CHARACTERISTICS OF CENTRIFUGAL PUMPS
The cavitation characteristic of a centrifugal pump (Fig. 5.14) is the dependence of the actual performance Q d depending on the pressure at its inlet P in. Cavitation phenomena most often occur at the inlet of pumps.
Cavitation characteristics are determined experimentally and measured at a constant speed of rotation of the pump impeller and a constant pressure difference at its outlet and inlet ∆Р us =Рout. - P input =const. These specifications are based on specific fuel and operating temperature.
Cavitation phenomena most often occur at the inlet of pumps. The cavitation properties of the pump are determined by cavitation characteristics, which are determined by tests and establish the relationship between inlet pressure and pump flow (Fig. 5.14) . This characteristic is given for a given liquid at some constant rotation speed
| Fig.5.14 Cavitation (height characteristic of a centrifugal pump) |
pump shaft and temperature. To determine the pump flow rate, a constant pressure drop is maintained during testing, and vice versa, to determine the pressure drop created by the pump, a constant flow rate is maintained.
Calculation of the fuel system for altitude involves determining the conditions for cavitation-free operation of the fuel system. The main quantity that determines the normal operation of the fuel system is pressure. at the inlet to the fuel pump r in, which, in order to avoid cavitation, must exceed the fuel vapor pressure p t by some amount.
Required pressure at the pump inlet p input input is uniquely determined by the existing cavitation characteristic at a given minimum permissible fuel consumption Qmin.
In the absence of cavitation characteristics p input input determined by calculation:
r in ≥ r t + Δ r kav . (5.4)
Here Δ r kav- cavitation pressure reserve exceeding the fuel vapor pressure can be determined in two different ways - calculated and using experimental data.
The calculated option is estimated using the formula S.S. Rudneva:
Δ r kav =ρg 10, (5.5)
Where Q- pump supply, m 3 /s;
n – impeller rotation speed, rpm;
With - cavitation coefficient: for pumps with poor cavitation properties With=600…700, conventional pumps With=800…1000; and pumps with good properties With=1000…1500.
This condition must be met in all aircraft flight modes at all overloads and fuel temperatures. The amount of required cavitation reserve for different pumps varies within a very wide range from hundredths of an atmosphere to several atmospheres, depending on the type of pump, its operating mode, speed, etc.
Even for the same pump, depending on the flow rate, operating conditions and the requirements placed on it, the required NPSH can vary significantly.
From the point of view of the requirements for the performance of the transfer pump and the pressure it creates, its operation even in a zone of noticeably developed cavitation can be satisfactory. However, a reduced suction pressure for booster pumps is unacceptable, since this results in sharp fluctuations in pressure in the system, leading to disruption of the automation, etc. In addition, sudden pressure fluctuations can cause erosive wear of engine pumps and, in particular, plunger pairs.
In some cases, the required cavitation reserve should exclude even small signs of cavitation that do not affect the basic characteristics of the pump.
Transfer pumps can operate with fairly low suction pressures, that is, in the area of significant inlet cavitation, provided that they must provide the required fuel flow.
The amount of pressurization of fuel tanks is mostly determined by the requirements for the main booster pumps installed in the supply tanks, although according to the operating conditions of the transfer pumps, the booster pressure of the tanks in most cases could be less.
Required cavitation pressure reserves Dr kav for various pumps, in most cases they are determined experimentally.
Below are averaged statistical (experimental) data on cavitation pressure reserves for fuel system pumps.
For low pressure pumps (100…150) kPa and moderate performance (tank pumping and transfer pumps) Dr kav =(10…25) kPa. For DCP (intermediate boost pumps installed on the engine) - Dr kav =(60…80) kPa.
For high pressure pumps (regulator pumps) - Dr kav = (150…250) kPa.
To reduce the release of air from fuel for aircraft with a high rate of climb, increase the cavitation reserve (pressure reserve in the tanks) by approximately (70 ... 100) mmHg Art.
To improve the cavitation characteristics of booster pumps (and other centrifugal pumps), an internal booster pump (prepump) is installed in front of the impeller (impeller) in the form of an axial or screw stage (Fig. 5.6).
The prepump, due to the low pressure it creates and the reduced loads on the blades, does not require such high suction pressures as the main, more loaded stages. At the same time, the impeller of the prepump creates a swirl of fuel behind it, which ensures a decrease in the relative speed of liquid entry into the centrifugal stage, which mainly determines the local vacuum at the inlet to the wheel and thereby the required cavitation reserve.
The separating impeller installed as a pre-pump allows a flow rate greater than the main centrifugal stage, and along with the excess fuel discharged between the stages back into the tank, air and gas bubbles released from the fuel are also removed. All this improves the cavitation characteristics of the pump.
In these cases, pumps require absolutely negligible cavitation reserves, up to satisfactory operation of the booster pump on boiling and, especially, air-releasing liquid.
All these qualities of separating impellers are fully manifested only in cases where the excess performance of the prepump, together with the steam and air bubbles captured by it, can be freely separated into the cavity of the tank. If this possibility is not available or it is difficult, then often the installation of such an axial step turns out to be even harmful.
5.10. BASICS OF FUEL SYSTEM CALCULATION
The calculation of the fuel system comes down to the following:
Determining the required tank capacity;
Determining the required volumes of air cushions, especially for closed systems;
Calculations related to the procedure for discharging fuel from tanks and ensuring aircraft alignment;
Calculation of the fuel system for altitude.
The altitude of the fuel system is the maximum flight altitude up to which an uninterrupted supply of fuel to the main engine pumps is ensured with the required pressure and required flow rate.
Since the first to third points are completely resolved in the process of preliminary preliminary layouts of the aircraft, these issues are not considered further and it is considered that in the calculation of the fuel system, the tank capacities, their configuration and placement on the aircraft, as well as the required order of fuel generation are specified.
Requirements for calculating the height of the fuel system
The technical conditions must provide for the most unfavorable possible operating conditions for the aircraft:
Schematic and wiring diagrams of the fuel system with their geometric and hydraulic characteristics;
Maximum fuel consumption Q;
Highest (and sometimes lowest) design fuel temperature conditions t(RaTU);
Maximum flight altitudes H races;
Highest rate of climb;
Maximum overload nx, ny And n z .
Pressure and cavitation characteristics of aircraft vehicle pumps.
Additionally you should know:
physical characteristics of fuel - density r, coefficient kinematic viscosity n, saturated fuel vapor pressure at operating temperature P t.
The sections of the pipeline that are in the least favorable conditions for fuel supply are calculated (according to the length of the pipelines and the relative height of one object above another). Therefore, the design diagram of the fuel system should give an idea of the length of the lines and the relative position of the units. Based on the least favorable conditions, the case is taken when the fuel in the tank is running low (i.e., the fuel level in the tank should be neglected).
In general, calculations are performed for a number of modes. It is necessary to check the operation of the fuel supply lines under the most severe operating conditions. These are the take-off run and acceleration of the aircraft to take-off speed, take-off and climb at maximum speed, horizontal flight at the altitude of a given flight level. Overload P determined from aerodynamic calculations. If this data is not available, then for civil aviation aircraft you can accept:
p y =(+4…-0.5); n x =±0.3; n z =0.
The dependence of the volumetric fuel consumption of the engine on the flight altitude (Fig. 5.15) is indicated in its characteristics.
The required engine operating modes are determined by aerodynamic calculations. To calculate the altitude of civil aviation aircraft with operating NP1, it is recommended to accept the nature of the change in volumetric fuel consumption along the line a B C, corresponding to the maximum mode, and to calculate the height with non-working NP1 - along the line abgd, where is the plot gd- cruising mode.
Vehicle calculations can be divided into two options: design and verification.
5.10.1. Design calculation of vehicle height.
It comes down to assessing the sources of pressure (the amount of boost in the fuel tank Δр b. and pressure behind the booster pump r us.), which, having overcome all hydraulic losses along the fuel line path, would provide the required pressure at the inlet to the main fuel pump of the engine.
The calculation of the vehicle is based on the Bernoulli equation, written for two sections 1-1 and 11-11, the height of the levels of the corresponding sections y 1-1 And y 11-11 is evaluated relative to an arbitrarily taken reference plane 0-0. All designations are given in the design diagram Fig. 5.16.
р 1 +y 1 ρg+ =p 11 +y 11 ρg+ +Δp,(5.6)
Where p 1- pressure in the space above the fuel;
V 1- vertical speed of liquid movement in the tank;
V 11- speed of fuel movement at the outlet of the fuel system;
Δp- pressure loss along the pumping line path.
Here you can accept V 1, based FVρ= const, That , and F 1 >>F 11 And V 1<
Then (5.6) can be written:
p 1 =p 11 +(y 11 - y 1) ρg + +p training. +p local +p in. , (5.7)
Where p tren. , p local , p in. respectively, pressure loss from friction, from overcoming local resistance and inertial pressure.
Static pressure in section 1-1 is determined by atmospheric pressure pH, corresponding to the given flight altitude H, and the amount of pressurization of the fuel tank Dp b . : p 1 =p H +Dp b..
Tank pressurization (Dp b.) You should not do more than the minimum necessary, as this causes an unjustified increase in the weight of the tanks (or containers - in the case of soft tanks), especially if the design contains tanks with more or less flat walls.
For load-bearing tanks, the boost can be taken to be slightly increased, since the influence of internal pressure on the weight of the tanks in this case is significantly reduced. There are even cases with very thin-walled tanks or when the tank wall transmits engine thrust, when an increase in internal pressure improves the operating conditions of the supporting tank structure and even leads to a reduction in its weight.
Generally for pump fed aircraft it is adopted Dp b max 30 kPa. In case of displacement feeding - Dp b.= 80 kPa.
Pressure p 11 is nothing more than the required pressure at the inlet to the pump (DCS or main engine pump) p input input and can be determined from expression (5.4) or from the available cavitation characteristics.
Expression (5.7) will be written in the following form, if we consider the left side of the equation as pressure sources, and the right side as losses:
pH +Dp b. = p input input ± yrg + p training + p places +r in. + ,(5.8).
Hydrostatic pressure. In the case of horizontal flight, hydrostatic pressure yrg determined by height y(see Fig. 5.16). The “+” sign is taken if the fuel level in the tank is low relative to the vehicle outlet pipeline, and otherwise the “-” sign is taken.
In flight at some angle j to the horizon y is found as the excess of the fuel surface in the supply tank above the end of the aircraft fuel system and will be written in the following form:
y= -h fuel ± , (5.9).
Where h fuel-exceeding the height of the fuel above the tank intake pipe;
l x And l y– projections of pipeline lengths (with a complex spatial diagram) onto the corresponding coordinate axes of the aircraft.
The signs in front of the amount are determined according to the following rule: if the fuel in the pipelines flows in the direction of gravity, then the “-” sign is taken, and otherwise, the “+” sign is taken.
Hydraulic losses. Travel pressure loss p tren. are caused by friction of the liquid against the walls of the pipeline and is expressed:
p tren. = , (5.10)
Where l– pipeline length,
d- hydraulic diameter pipeline diameter.
Here, for a turbulent flow, the coefficient. friction , Reynolds number Re=Vd/ν, Where ν – coefficient kinematic viscosity of the fuel at the operating temperature of the fuel.
In design calculations V is taken to be equal to (1…2) m/s when fuel moves by gravity and (4…7) m/s when pumped. Required diameter d at a given fuel pumping Q will be determined:
d=, (5.11)
Received value d rounded to standard value, then evaluated p tren(formula 5.10) based on true values Vn
In the direction of the axes X And z overloads are usually small, but pipeline lengths can be large. As a rule, the most significant overload is in the direction of the axis y, reaching in some cases the calculated values p y= (10… 12)
For the calculation, it is necessary to take the extremely unfavorable case, when all pressures fall into the category of losses.
Now that all components of losses have been determined, from (5.8) we can find the value of the pressure source:
Dp b. = pin. consumption ± yrg + p training + p places +r in. + -pH. (5.14)
If the received value Dp b > 30 kPa, then the system must include a booster pump with outlet pressure r us.
In this case, expression (5.14) will take the form:
r us. = pin. consumption ± yrg +(p training) 1 +(p local) 1 + +() 1 -(p H +Dp b) .(5/15)
In (5.15) the values (p training) 1, (p local) 1 And () 1 determined at new speed values corresponding to pumping fuel supply [accepted V=(4...7) m/s]. Received value r us. corresponds to one design operating mode of the power plant.
5.10.2. Check calculation of vehicle altitude (flying on the ceiling).
Flight on the ceiling involves uniform and level flight. In this case, the inertial pressure loss r in. are equal to zero.
A special case of calculating the fuel system is the verification calculation of its height at altitudes significantly higher than the static ceiling of the aircraft due to the fact that for high-speed aircraft with a high power supply, the dynamic ceiling can differ significantly from the static one.
For some (for example, experimental) aircraft, stopping the engines at extreme altitudes is in some cases acceptable, since after completing the task the aircraft can descend to moderate altitudes, at which the launch system allows the engines to reliably start and continue flight. For combat aircraft, the need to significantly reduce altitude to start the engines can completely destroy any benefits gained by exceeding the static ceiling by using the accumulated kinetic