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Downloaded from https://www.everyspec.com FM 1-506 FUNDAMENTALS OF AIRCRAFT POWER PLANTS DISTRIBUTION RESTRICTION: Approved for public release; distribution is unlimited. HEADQUARTERS, DEPARTMENT OF THE ARMY
United Technologies Corporation · FM 1-506 · 142 pp.
Fundamentals of Aircraft Power Plants (FM 1-506) PDF Download
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Preface
Downloaded from https://www.everyspec.com FM 1-506 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. iv
Chapter 1 - Basic Requirements
Downloaded from https://www.everyspec.com FM 1-506 PART ONE. FUNDAMENTALS OF POWER PLANTS CHAPTER 1 BASIC REQUIREMENTS Aircraft power plants must meet exacting require- ments for dependability and endurance. Many difficult engineering problems have been overcome in an effort to satisfy these requirements with further advances being made each day. Requirements imposed on aircraft power plants in an effort to obtain engines suitable for aircraft include – The overall form an engine takes is determined to a high degree by the compactness required. The degree of compactness that may be achieved is limited by the physical requirements of the engine.
For example, com- pactness is limited on radial air-cooled engines duet to the frontal area required for sufficient cooling of engine cylinders. Reliabtity. Durability. Compactness. Low weight per horsepower. High specific power output. Reasonable cost. High thermal efficiency. Freedom from vibration. Ease of maintenance. Operating flexibility. Low Weight per Horsepower Minimum weight per horsepower (HP) is a primary requirement in aeronautics. The weight of a power plant must be kept as low as possible. This allows the aircraft to carry a large useful load with a satisfactory margin of safety in proportion to gross weight.
The larger modern aircraft reciprocating engines have attained a horsepower- to-weight ratio of 1 horsepower to 1 pound weight. Gas turbine engines currently used by the Army have a greater horsepower-to-weight ratio. A good example is the T-55-L-712, which develops 4500 shaft horsepower (SHP) and weighs 750 pounds(dry) (6.0 HP per pound). DEFINITION OF TERMS Reliability Power output is based on engine size, RPM, and weight for the fuel-air mixture. Size and RPM are limited in the reciprocating engine. Therefore, an increase in the effective working pressure in the cylinders is one of the most valuable ways to increase the specific power output.
Greater pressure increases are possible by supercharging the engine (comprising the mixture before it enters the cylinders). The limiting factors in increasing cylinder pressure are resistance of the fuel to detonation and the maximum allowable cylinder pressure. High Specific Power Output Reliability is the most important fundamental power plant requirement. In the air each working part, no matter how small is important. Only by careful attention to the smallest detail can aircraft power plant manufac- turers and mechanics assure power plant reliability. Durability Durability is the measure of reliable engine life.
The durability realized by an engine depends largely on the type or condition of operation. Intelligent application of operation and maintenance procedures results in greatly improved power plant durability. Reasonable Cost Compactness Despite perfection of design and quality of workmanship, no power plant will be desirable if it is too costly in a competitive market. A primary factor deter- mining the usefulness of an engine is its cost. Because of Compactness is essential to power plant design in order to lower parasitic drag and to attain higher speeds. 1-1
Chapter 1 - Basic Requirements
Downloaded from https://www.everyspec.com FM 1-506 the raw materials and the great number of man-hours involved, complex designs drive up the cost. The most satisfactory design is generally the simplest that will meet requirements. to the engine. Some significant properties of aviation fuels are discussed below. Heat Energy Content or Net Heating Value. The energy content or heating value of a fuel is expressed in heat units (British thermal units [BTUs]). A fuel satisfac- tory for aircraft engines must have a high heat energy content per unit weight. A high heat energy content causes the weight of fuel carried to be lower than a low heat energy content.
Then more of the load-carrying capacity is available for the payload Aviation gasoline and JP fuels are very desirable from this standpoint. The heat energy content for aviation gasoline is about 18,700 BTUs/pound, and for JP fuels about 18,200 BTUs/pound. The various alcohols, which have maxi- mum energy content of about 12,000 BTUs/pound, do possess some other desirable characteristics as an inter- nal combustion engine fuel. High Thermal Efficiency Thermal efficiency is a measure of the losses suffered in converting heat energy in the fuel into mechanical work; it is the ratio of the heat developed into useful work to the heating value of the fuel.
High thermal effciency, therefore, means high fuel economy-something of great importance in aircraft engines. The less fuel re- quired for a military mission, the greater the military load that can be carried and the lower the fuel cost. Freedom From Vibration A power plant that is free from vibration is important in the light, somewhat flexible aircraft structure since severe engine vibration will in some cases reduce the life of certain structural parts. The need for freedom from vibration is met usually by using a large number of cylinders to offset the vibration torque delivered by the individual cylinders. Counterweights are installed on crankshafts to balance rotating masses.
These are usually hinged to provide dynamic damping of vibration which results from power impulses and to counteract un- desirable torsional or twisting vibration. Also, flexible engine mount isolators are used to permit certain move- ments of the power plant that are harmful to aircraft structures. Ease of Maintenance Volatility. A volatile liquid is one capable of readily changing from a liquid to a vapor when heated or when contacting a gas into which it can evaporate. Since liquid fuels must be in a vaporous state to burn volatility is an important property to consider when choosing a suitable fuel for an aircraft engine.
Volatility determines the start- ing accelerating vapor-locking and distribution charac- teristics of the fuel. Gasoline and JP fuels are very satisfactory because they can be blended during the refin- ing process to give the desired characteristics. Because of the nature of constant pressure combustion in gas turbine engines a highly volatile fuel is not necessary. JP fuels are of rather low volatility while aviation gasoline is highly volatile. Comparing a highly volatile fuel like aviation gasoline to a less volatile one like JP fuel the following effects become apparent.
The highly volatile fuel – The requirement of ease of maintenance is espe- cially important to military operations in the field. Simplicity of design and use of standard parts, when possible, assist in keeping maintenance at a low level. Starts easier in cold temperatures. Has a slightly better combustion efficiency. Leaves less deposit in the combustion chamber and on the turbine blades. Is a greater fire hazard. Creates a greater danger of vapor lock of the fuel system. Has high evaporation losses through the breather of the fuel tank at high altitudes.
Operating Flexibility Flexibility is the ability of a power plant to run smoothly and provide the desired performance at all speeds from idling to maximum power output. The wide range of operating requirements demanded of aircraft engines presents difficulties rarely encountered in other power plant fields. In addition to the requirement of unfailing reliability, the engine must operate in widely varying positions, altitudes, and atmospheric conditions. NOTE: The last two difficulties are practi- cally nonexistent with fuels having low volatility. FUELS Requirements Stability. The fuels used in aircraft engines must be stable.
Because aviation fuels are sometimes stored for long periods, they must not deposit sediment. The gums An engine fuel must be tailored to an engine and vice versa since there must be enough quantities of fuel available 1-2