on factors other than those that can be discussed from an  analytical point of view. For that reason, a general discussion  of powerplant selection follows. For aircraft whose cruising speed does not exceed 250 mph,  the reciprocating engine is the usual choice of powerplant.  When economy is required in the low speed range, the  conventional reciprocating engine is chosen because of  its excellent efficiency and relatively low cost. When high  altitude performance is required, the turbo-supercharged  reciprocating engine may be chosen because it is capable  of maintaining rated power to a high altitude (above 30,000  feet). Gas turbine engines operate most economically at high  altitudes. Although in most cases the gas turbine engine  provides superior performance, the cost of gas turbine  engines is a limiting factor. In the range of cruising speed  of 180 to 350 mph, the turboprop engine performs very  well. It develops more power per pound of weight than does  the reciprocating engine, thus allowing a greater fuel load  or payload for engines of a given power. From 350 mph  up to Mach .8–.9, turbofan engines are generally used for  airline operations. Aircraft intended to operate at Mach 1 or  higher are powered by pure turbojet engines/afterburning  (augmented) engines, or low-bypass turbofan engines.  Aircraft engines can be classified by several methods. They  can be classed by operating cycles, cylinder arrangement,  or the method of thrust production. All are heat engines that  convert fuel into heat energy that is converted to mechanical  energy to produce thrust. Most of the current aircraft engines  are of the internal combustion type because the combustion  process takes place inside the engine. Aircraft engines  come in many different types, such as gas turbine based,  reciprocating piston, rotary, two or four cycle, spark ignition,  diesel, and air or water cooled. Reciprocating and gas turbine  engines also have subdivisions based on the type of cylinder  arrangement (piston) and speed range (gas turbine).  Many types of reciprocating engines have been designed.  However, manufacturers have developed some designs that  are used more commonly than others and are, therefore,  recognized as conventional. Reciprocating engines may be  classified according to the cylinder arrangement (in line,  V-type, radial, and opposed) or according to the method  of cooling (liquid cooled or air cooled). Actually, all  piston engines are cooled by transferring excess heat to the  surrounding air. In air-cooled engines, this heat transfer is  direct from the cylinders to the air. Therefore, it is necessary to  provide thin metal fins on the cylinders of an air-cooled engine  in order to have increased surface for sufficient heat transfer.  Most reciprocating aircraft engines are air cooled although  a few high powered engines use an efficient liquid-cooling  The opposed-type engine has two banks of cylinders directly  opposite each other with a crankshaft in the center  Figure 1-1 .  The pistons of both cylinder banks are connected to the single  crankshaft. Although the engine can be either liquid cooled  or air cooled, the air-cooled version is used predominantly  in aviation. It is generally mounted with the cylinders in  a horizontal position. The opposed-type engine has a low  weight-to-horsepower ratio, and its narrow silhouette makes  it ideal for horizontal installation on the aircraft wings (twin  engine applications). Another advantage is its low vibration  characteristics. The inline engine has a small frontal area and is better  adapted to streamlining. When mounted with the cylinders  in an inverted position, it offers the added advantages of a  shorter landing gear and greater pilot visibility. With increase  in engine size, the air cooled, inline type offers additional  problems to provide proper cooling; therefore, this type of  engine is confined to low- and medium-horsepower engines  used in very old light aircraft. An inline engine generally has an even number of cylinders,  although some three-cylinder engines have been constructed.  This engine may be either liquid cooled or air cooled and has  only one crank shaft, which is located either above or below  the cylinders. If the engine is designed to operate with the  cylinders below the crankshaft, it is called an inverted engine. system. In liquid-cooled engines, the heat is transferred  from the cylinders to the coolant, which is then sent through  tubing and cooled within a radiator placed in the airstream.  The coolant radiator must be large enough to cool the liquid  efficiently. The main problem with liquid cooling is the added  weight of coolant, heat exchanger (radiator), and tubing to  connect the components. Liquid cooled engines do allow high  power to be obtained from the engine safely.  Inline Engines Types of Engines Opposed or O-Type Engines Figure 1-1. A typical four-cylinder opposed engine. 1-4