FUNDAMENTALS OF AIRCRAFT POWER PLANTS

141 Pages·2002·2.57 MB·English

FM 1-506

FUNDAMENTALS OF
AIRCRAFT POWER PLANTS

DISTRIBUTION RESTRICTION: Approved for public release; distribution is unlimited.

HEADQUARTERS, DEPARTMENT OF THE ARMY
P
FM 1-506
FIELD MANUAL HEADQUARTERS
NO 1-506 DEPARTMENT OF THE ARMY
Washington, DC, 30 November 1990
FUNDAMENTALS OF AIRCRAFT POWER PLANTS
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PREFACE
This manual provides information on the operation, components, and systems of aircraft power plants. The
turbine engine is relatively new to the aviation field and its technology is growing rapidly. This manual will bring
users up-to-date on new developments in the field.
Propellers have been deleted from this manual since the Army no longer works on them.
This manual is for use by Army aviation mechanics worldwide.
For specific instructions on the aircraft power plants in particular types and models of aircraft, refer to applicable
maintenance manuals. Should the information in this field manual and that in a specific aircraft maintenance
manual conflict, the latter takes precedence.
The proponent of this publication is HQ TRADOC. Submit changes for improving this publication on DA
Form 2028 (Recommended Changes to Publications and Blank Forms) and forward it to Assistant Comman-
dant, US Army Aviation Logistics School, ATTN ATSQ-LTD-L, Fort Eustis, Virginia 23604-5421.
Unless otherwise stated, whenever the masculine gender is used, both men and women are included.
This manual contains copyrighted material as follows:
Pages 3-7 thru 3-8,3-11 thru 3-12,3-14 thru 3-15,3-17 thru 3-20; 4-17; 8-1,8-5 thru
8-8; 9-1,9-3, thru 9-4,9-6, 9-7, thru 9-12; Fig 3-19, Fig 3-20: Copyright ©, 1951,
1974, United Technologies Corporation.
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PART ONE. FUNDAMENTALS OF POWER PLANTS
CHAPTER 1
BASIC REQUIREMENTS
Aircraft power plants must meet exacting require- The overall form an engine takes is determined to a high
ments for dependability and endurance. Many difficult degree by the compactness required. The degree of
engineering problems have been overcome in an effort to compactness that may be achieved is limited by the
satisfy these requirements with further advances being physical requirements of the engine. For example, com-
made each day. Requirements imposed on aircraft pactness is limited on radial air-cooled engines duet to the
power plants in an effort to obtain engines suitable for frontal area required for sufficient cooling of engine
aircraft include – cylinders.
Reliabtity. Low Weight per Horsepower
Durability.
Minimum weight per horsepower (HP) is a primary
Compactness.
requirement in aeronautics. The weight of a power plant
Low weight per horsepower. must be kept as low as possible. This allows the aircraft
High specific power output. to carry a large useful load with a satisfactory margin of
Reasonable cost. safety in proportion to gross weight. The larger modern
High thermal efficiency. aircraft reciprocating engines have attained a horsepower-
Freedom from vibration. to-weight ratio of 1 horsepower to 1 pound weight. Gas
Ease of maintenance. turbine engines currently used by the Army have a greater
horsepower-to-weight ratio. A good example is the
Operating flexibility.
T-55-L-712, which develops 4500 shaft horsepower
DEFINITION OF TERMS (SHP) and weighs 750 pounds(dry) (6.0 HP per pound).
Reliability High Specific Power Output
Reliability is the most important fundamental power Power output is based on engine size, RPM, and
plant requirement. In the air each working part, no weight for the fuel-air mixture. Size and RPM are limited
matter how small is important. Only by careful attention in the reciprocating engine. Therefore, an increase in the
to the smallest detail can aircraft power plant manufac- effective working pressure in the cylinders is one of the
turers and mechanics assure power plant reliability. most valuable ways to increase the specific power output.
Greater pressure increases are possible by supercharging
Durability
the engine (comprising the mixture before it enters the
Durability is the measure of reliable engine life. The cylinders). The limiting factors in increasing cylinder
durability realized by an engine depends largely on the pressure are resistance of the fuel to detonation and the
type or condition of operation. Intelligent application of maximum allowable cylinder pressure.
operation and maintenance procedures results in greatly
Reasonable Cost
improved power plant durability.
Despite perfection of design and quality of
Compactness
workmanship, no power plant will be desirable if it is too
Compactness is essential to power plant design in costly in a competitive market. A primary factor deter-
order to lower parasitic drag and to attain higher speeds. mining the usefulness of an engine is its cost. Because of
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the raw materials and the great number of man-hours to the engine. Some significant properties of aviation fuels
involved, complex designs drive up the cost. The most are discussed below.
satisfactory design is generally the simplest that will meet
Heat Energy Content or Net Heating Value. The
requirements.
energy content or heating value of a fuel is expressed in
High Thermal Efficiency heat units (British thermal units [BTUs]). A fuel satisfac-
tory for aircraft engines must have a high heat energy
Thermal efficiency is a measure of the losses suffered
content per unit weight. A high heat energy content
in converting heat energy in the fuel into mechanical
causes the weight of fuel carried to be lower than a low
work; it is the ratio of the heat developed into useful work
heat energy content. Then more of the load-carrying
to the heating value of the fuel. High thermal effciency,
capacity is available for the payload Aviation gasoline
therefore, means high fuel economy-something of
and JP fuels are very desirable from this standpoint. The
great importance in aircraft engines. The less fuel re-
heat energy content for aviation gasoline is about 18,700
quired for a military mission, the greater the military load
BTUs/pound, and for JP fuels about 18,200
that can be carried and the lower the fuel cost.
BTUs/pound. The various alcohols, which have maxi-
Freedom From Vibration mum energy content of about 12,000 BTUs/pound, do
possess some other desirable characteristics as an inter-
A power plant that is free from vibration is important
nal combustion engine fuel.
in the light, somewhat flexible aircraft structure since
severe engine vibration will in some cases reduce the life Volatility. A volatile liquid is one capable of readily
of certain structural parts. The need for freedom from changing from a liquid to a vapor when heated or when
vibration is met usually by using a large number of contacting a gas into which it can evaporate. Since liquid
cylinders to offset the vibration torque delivered by the fuels must be in a vaporous state to burn volatility is an
individual cylinders. Counterweights are installed on important property to consider when choosing a suitable
crankshafts to balance rotating masses. These are usually fuel for an aircraft engine. Volatility determines the start-
hinged to provide dynamic damping of vibration which ing accelerating vapor-locking and distribution charac-
results from power impulses and to counteract un- teristics of the fuel. Gasoline and JP fuels are very
desirable torsional or twisting vibration. Also, flexible satisfactory because they can be blended during the refin-
engine mount isolators are used to permit certain move- ing process to give the desired characteristics. Because of
ments of the power plant that are harmful to aircraft the nature of constant pressure combustion in gas turbine
structures. engines a highly volatile fuel is not necessary. JP fuels are
of rather low volatility while aviation gasoline is highly
Ease of Maintenance
volatile. Comparing a highly volatile fuel like aviation
The requirement of ease of maintenance is espe- gasoline to a less volatile one like JP fuel the following
cially important to military operations in the field. effects become apparent. The highly volatile fuel –
Simplicity of design and use of standard parts, when
Starts easier in cold temperatures.
possible, assist in keeping maintenance at a low level.
Has a slightly better combustion efficiency.
Operating Flexibility Leaves less deposit in the combustion chamber
and on the turbine blades.
Flexibility is the ability of a power plant to run
Is a greater fire hazard.
smoothly and provide the desired performance at all
Creates a greater danger of vapor lock of the
speeds from idling tomaximum power output. The wide
fuel system.
range of operating requirements demanded of aircraft
Has high evaporation losses through the
engines presents difficulties rarely encountered in other
breather of the fuel tank at high altitudes.
power plant fields. In addition to the requirement of
unfailing reliability, the engine must operate in widely
NOTE: The last two difficulties are practi-
varying positions, altitudes, and atmospheric conditions.
cally nonexistent with fuels having low
FUELS volatility.
Requirements
Stability. The fuels used in aircraft engines must be
An engine fuel must be tailored to an engine and vice stable. Because aviation fuels are sometimes stored for
versa since there must be enough quantities of fuel available long periods, they must not deposit sediment. The gums
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that are normally formed are insoluble in gasoline and JP Grades
fuels and may cause restrictions in fuel strainers and
Turbine fuels are high-quality fuels covering the
liners. Aviation fuel must also retain its original proper-
general heavy gasoline and kerosene boding range. They
ties during storage.
do not contain dyes or tetraethyl lead.
Purity. Aviation fuel must be free from water, dirt,
One of the major differences between the wide-
and sulfur. Small amounts of water will not usually cause
boiling and kerosene types is the fuel volatility. JP-4
any difficulty because water can be removed from the fuel
fuels have a wider boding range with an initial boiling
system by draining. Large amounts of this impurity, how-
point considerably below that of kerosene. As a group
ever, can cause complete engine failure. It is very impor-
these fuels have lower specific gravities than kerosene
tant that corrosive sulfur be eliminated from fuel. The
types. Wide-boil-range fuels have Reid vapor pres-
sulfur content of fuel may form corrosive acids when
sures of 2 to 3 pounds and flash points below room
brought in contact with the water vapor formed in the
temperature. Kerosene-type fuels have Reid vapor pres-
combustion process.
sures of less than 0.5 pound and flash points higher than
Flush Point. The flash point is the lowest tempera- l00oF (38oC). Wide-boiig-range fuels generally have
ture at which fuel will vaporize enough to forma combus- lower freezing points than kerosene fuels.
tible mixture of fuel vapor and air above the fuel. It is
The fuel authorized for Army aircraft gas turbine
found by heating a quantity of fuel in a special container
engines is JP-4. The letters “JP" stand for jet propulsion;
while passing aflame above the liquid to ignite the vapor.
the number 4 indicates fuel grade.
A distinct hash of flame occurs when the flash point
temperature has been reached. Military specification MIL-T-5624 covers JP-4, JP-5,
and JP-8 fuels. Jet A, Jet Al, and Jet B are commercial
Fire Point. The fire point is the temperature which
fuels which conform to the American Society for Tinting
must be reached before enough vapors will rise to
Materials specification ASTM-D-1655.
produce a continuous flame above the liquid fuel. It is
obtained in much the same manner as the flash point. Jet B is a JP-4 type fuel; its freezing point is -56°F
(-49oC) instead of -72oF (-58oC) for JP-4.
Reid Vapor Pnessure. Reid vapor pressure is the
approximate vapor pressure exerted by a fuel when JP-5, Jet 4 and Jet A-1 are kerosene-type fuels.
heated to 100% This is important because it is used to
ASTM Jet A and A-1 differ primarily in their fuel
determine when a fuel will create a vapor lock.
freezing points. Jet A is considered suitable down to fuel
Specific Gravity. Specific gravity is the ratio of the temperatures of -36oF (-38oC); Jet A-1, to -54oF (48oC).
density (weight) of a substance (fuel) compared to that
JP-4 is a fuel consisting of approximately 65 percent
of an equal amount of water at 60oF. Specific gravity is gasoline and 35 percent light petroleum distillate, with
expressed in terms of degrees API. Pure water has a rigidly specified properties. JP-4 is currently the Army
specific gravity of 10. Liquids heavier than water have a standard fuel for turbine engines.
number less than 10. Liquids lighter than water have a
JP-5 is a specially refined kerosene having a mini-
number greater than 10. An example is JP-4, whose
mum flash point of 140oF and a freezing point of -51oF
specific gravity in degrees API is 57. The American
(-46°C).
Petroleum Institute (API) has chosen pure water by
JP-8 is a specially refined kerosene with a minimum
which to measure the specific gravity of fuels.
flash point of 110oF and a freezing point of -54°F (-48°C).
This fuel is being classified as a total replacement fuel for
all of NATO. It will replace all fuels currently used in
NOTE: Both flash and fire points give a military equipment from generators to tanks to aircraft
relative measure of the safety properties of and even to trucks. This classification will ease logistics in
fuel a high flash point denotes that a high combat. Having only one fuel for all equipment prevents
accidentally mixing or using the wrong fuels. To date
temperature must be reached before
testing of JP-8 is proceeding well with the total single-fuel
dangerous handling conditions are en-
concept on its way to full fielding.
countered. The minimum flash point per-
mitted in a fuel is usually written into the JP fuels vary from water white to light yellow; color
specifications. coding, however, does not apply to these fuels.
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Additives in JP fuels include oxidation and corrosion 1500oF (815oC); therefore, the gases at entrance to the
inhibitors, metal deactivators, and icing inhibitors. Icing turbine vanes cannot exceed about 1600oF (898oC) for
inhibitors also function as biocides to kill microbes in more than brief periods.
aircraft fuel systems.
Fuel-Air Ratio
Should mixing of JP fuels become necessary, there
The fuel-air ratio is so low in gas turbine engines that
is no need to drain the aircraft fuel system before adding
if fuel and air were uniformly mixed, the mixture would
the new fuel. Due to the different specific gravities of
not ignite. Complete combustion of both fuel and air is
these fuels, mixing them will affect the turbine engine’s
obtained with isooctane at a fuel-air ratio of 0.066. This
performance. Be sure to consult appropriate technical
is the theoretical value which would produce complete
manuals for additional information and procedures.
combustion if given sufficient time to reach equilibrium.
When changing to a fuel with a different specific In practice, complete combustion of either fuel or air
gravity, externally adjusted fuel controls and fuel flow requires an excess of one or the other. Thus, if all of the
dividers on some engines may require retrimming or air is to be burned excess fuel must be present and a
readjustment for optimum performance. fuel-air ratio of about 0.080 is required. Control of power
output is largely determined by means of fuel-air ratio.
COMBUSTION
Increasing fuel-air ratio increases the quantity of air and
Mixing fuel with air and burning it would seem to be the temperature at which it is discharged at the exhaust
a very simple process, but this apparent simplicity is jet pipe.
deceptive. Problems encountered are with distribution
Rich-Mixture Blowout
knot.kj ignition timing, and so forth. In an internal com-
bustion engine, the combustion process is the rather rapid Combustion efficiency in a well-designed combus-
reaction between fuel and oxygen. This process liberates tion chamber using the most favorable fuel may be as high
the potential energy contained in the fuel supplied to the as 98 percent at sea level. On the other hand, it may fall
engine. In a gas turbine engine atmospheric air is taken to as low as 40 percent at extremely high altitudes with a
in and compressed; fuel is then burned in the compressed badly designed combustion chamber and unsuitable fuel
air, which then expands through a turbine that drives a As combustion efficiency is reduced, a point is reached
compressor. when the turbine does not develop enough power to drive
the compressor. Increasing the fuel supply in order to
The combustion problem would not be so great if
maintain or increase engine speed may not result in
weight and space were not so important in aircraft gas
increased engine RPM, and the unburned fuel may ex-
turbine engines. Without such limitation the air supply
tinguish the flame. This is known as rich-mixture blowout.
for the compressors could be divided. A portion suitable
to the desired power output could be burned at ap- Lean-Mixture Die-Out
proximately the chemically correct fuel-air ratio in a
In contrast to rich-mixture blowout, lean-mixture
low-velocity combustion chamber. Design of the com-
die-out occurs when the mixture is too lean to bum under
bustion chamber is such that less than a third of the total
volume of air entering the chamber is permitted to mix conditions of efficient combustion. When combustion
with the fuel. The excess air bypasses the fuel nozzles and efficiency is very low, the mixture may be ignited from an
is used to cool the hot surfaces and to mix with and cool external source (such as an igniter plug). It will then
the burned gases before they enter the turbines. either be extinguished when the spark ceases or may bum
so slowly that the flame is carried out through the turbine.
Temperature Limitations
Lean-mixture die-out can also occur when the fuel supply
Gas turbine engines produce work in proportion to is reduced in order to decrease engine speed.
the amount of heat released internally. Most of this heat
Rich mixture blowout and lean mixture die-out have
is obtained by burning fuel although some heat originates
when air is compressed in the compressor. Low fuel-air been virtually eliminated through the refinement of fuel
ratio is required to keep the temperature of the gases delivery systems.
delivered to the turbine down to a value which the turbine
LUBRICATION
wheel can tolerate. With present materials of construc-
tion, the highly stressed blades or buckets of the turbine Lubrication is a very important part of power plant
wheel cannot stand a temperature of more than about operation. An engine allowed to operate without
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lubrications certain to fail. Lubrication not only combats oil flow reduces operating temperatures of internal parts
friction but also acts as a cooling agent. not directly cooled by the engine cooling system.
Sealing Action
WARNING Oil helps seal mating surfaces in the engine, and the
Never mix reciprocating engine oils and gas turbine film of oil on various surfaces is an effective pressure seal.
engine oils; they are not compatible. Mixing them In reciprocating engines the oil film between the cylinder
causes engine failure!. wall and piston and piston rings is important in retaining
the high gas pressure in the cylinder.
The primary purpose of a lubricant is to reduce Cleaning Effect
friction between moving parts. Because liquid lubricants
Oil cleans the engine by picking up carbon and other
(oils) can be circulated readily, they are used universally foreign particles as it passes through and around engine
in aircraft engines. In theory, fluid lubrication is based on
parts. It carries these particles through the system to a
actual separation of the surfaces so that no metal-to-
strainer where they are filtered from the oil.
metal contact occurs. As long as the oil film remains
LUBRICATING OIL REQUIREMENTS
unbroken, metallic fiction is replaced by the internal
fluid friction of the lubricant. Under ideal conditions
The conditions which the engine operates under
friction and wear are held to a minimum. In addition to
determine the requirements for lubricating oil. Condi-
reducing friction, the oil film acts as a cushion between
tions like temperature, contact pressure, and type and
metal parts. This cushioning effect is particularly import-
rate of motion vary so much that one lubricant cannot
ant for such parts as reciprocating engine crankshaft and provide ideal lubrication for all components. Using a
connecting rods, which are subject to shock loading. As lubricating oil with all the desirable properties in*
oil circulates through the engine, it absorbs heat from the degrees will provide satisfactory results. TB 55-9150-200-24
parts. Pistons and cylinder walls in reciprocating engines
specifies engine oils for use in Army aircraft. Some desirable
especially depend on oil for cooling. Oil also aids in lubrcating oil qualities are –
forming a seal between the piston and cylinder wall to
Viscosity.
prevent gas leaks from the combustion chamber. Oils also
reduce abrasive wear by picking up foreign particles and Antifriction ability.
carrying them to a filter to be removed Cooling ability.
Chemical stability.
Friction Reduction
Viscosity
Lubricating oil decreases friction by preventing
metal-to-metal contact at bearing points throughout the The degree of resistance of an oil flow at a specified
engine. Separating mating surfaces of moving parts by a temperature indicates its viscosity. An oil that flows slowly
thin film of oil changes dry or solid friction to fluid friction. is described as a viscous oil or an oil of high viscosity. An
The result is less heat generated in the moving parts and oil that flows readily is said to possess a low viscosity. The
decreased wear on the parts. viscosity of all oils is affected by temperature. AS the
temperature increases, oil becomes thinner. The rate at
Cushioning Effect
which an oil resists viscosity changes through a given
Lubricating oil cushions bearing surfaces by absorb- temperature range is called its viscosity index The vis-
ing the shock between them. cosity of aircraft engine oil is fairly high because of high
operating temperatures high bearing pressures, and
Cooling
relatively large clearances inside an aircraft engine. Since
It has been noted that reducing friction results in less aircraft engines are also subjected to a wide range of
heat being generated. Also, as oil is circulated through temperatures, an oil with a high viscosity index is re-
bearings and splashed on various engine parts, it absorbs quired.
a great amount of heat. Lubrication is particularly impor-
Antifriction
tant in reciprocating engines to cool the piston and
cylinder. An efficient lubrication system will absorb as The theory of fluid lubrication is based on the actual
much as 10 percent of the total heat content of fuel separation of metallic surfaces by an oil film. Lubricants
consumed by the engine. By carrying away this heat, the should have high antifriction characteristics to reduce
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