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