section is used quite frequently on direct-drive, low-powered  engines, because extra space is not required to house the  propeller reduction gears. Crankcase nose sections are usually  cast of either aluminum alloy or magnesium. The crankcase  nose section on engines that develop from 1,000 to 2,500  hp is usually larger to house reduction gears and sometimes  ribbed to get as much strength as possible. The governor is used to control propeller speed and blade  angle. The mounting of the propeller governor varies. On  some engines, it is located on the rear section, although this  complicates the installation, especially if the propeller is  operated or controlled by oil pressure, because of the distance  between the governor and propeller. Where hydraulically  operated propellers are used, it is good practice to mount  the governor on the nose section as close to the propeller  as possible to reduce the length of the oil passages. The  governor is then driven either from gear teeth on the periphery  of the bell gear or by some other suitable means. This basic  arrangement is also used for turboprops. On some of the larger radial engines, a small chamber is located  on the bottom of the nose section to collect the oil. This is called  the nose section oil sump. Since the nose section transmits  many varied forces to the main crankcase or power section,  it must be secured properly to transmit the loads efficiently. The machined surfaces on which the cylinders are mounted  are called cylinder pads. They are provided with a suitable  means of retaining or fastening the cylinders to the crankcase.  The general practice in securing the cylinder flange to the  pad is to mount studs in threaded holes in the crankcase. The  inner portion of the cylinder pads are sometimes chamfered  or tapered to permit the installation of a large rubber O-ring  around the cylinder skirt, which effectively seals the joint  between the cylinder and the crankcase pads against oil leakage. Because oil is thrown about the crankcase, especially on  inverted inline and radial-type engines, the cylinder skirts  extend a considerable distance into the crankcase sections to  reduce the flow of oil into the inverted cylinders. The piston  and ring assemblies must be arranged so that they throw out  the oil splashed directly into them. Mounting lugs are spaced about the periphery of the rear  of the crankcase or the diffuser section of a radial engine.  These are used to attach the engine assembly to the engine  mount or framework provided for attaching the powerplant  to the fuselage of single-engine aircraft or to the wing nacelle  structure of multiengine aircraft. The mounting lugs may  be either integral with the crankcase or diffuser section or  detachable, as in the case of flexible or dynamic engine mounts. Gear trains, containing both spur- and bevel-type gears, are  used in the different types of engines for driving engine  components and accessories. Spur-type gears are generally  used to drive the heavier loaded accessories or those requiring  the least play or backlash in the gear train. Bevel gears permit  angular location of short stub shafts leading to the various  accessory mounting pads. On opposed, reciprocating engines,  the accessory gear trains are usually simple arrangements.  The accessory (rear) section usually is of cast construction  and the material may be either aluminum alloy, which is used  most widely, or magnesium, which has been used to some  extent. On some engines, it is cast in one piece and provided  with means for mounting the accessories, such as magnetos,  carburetors, fuel, oil, vacuum pumps, starter, generator,  tachometer drive, etc., in the various locations required  to facilitate accessibility. Other adaptations consist of an  aluminum alloy casting and a separate cast magnesium cover  plate on which the accessory mounts are arranged. Accessory  drive shafts are mounted in suitable drive arrangements  that are carried out to the accessory mounting pads. In this  manner, the various gear ratios can be arranged to give the  proper drive speed to magnetos, pumps, and other accessories  to obtain correct timing or functioning.  The mounting arrangement supports the entire powerplant  including the propeller, and therefore is designed to provide  ample strength for rapid maneuvers or other loadings. Because  of the elongation and contraction of the cylinders, the intake  pipes which carry the mixture from the diffuser chamber  through the intake valve ports are arranged to provide a slip  joint which must be leak proof. The atmospheric pressure on  the outside of the case of an un-supercharged engine is higher  than on the inside, especially when the engine is operating at  idling speed. If the engine is equipped with a supercharger  and operated at full throttle, the pressure is considerably  higher on the inside than on the outside of the case. If the  slip joint connection has a slight leakage, the engine may  idle fast due to a slight leaning of the mixture. If the leak is  quite large, it may not idle at all. At open throttle, a small  leak probably would not be noticeable in operation of the  engine, but the slight leaning of the fuel/air mixture might  cause detonation or damage to the valves and valve seats. On  some radial engines, the intake pipe has considerable length  and on some inline engines, the intake pipe is at right angles  to the cylinders. In these cases, flexibility of the intake pipe  or its arrangement eliminates the need for a slip joint. In any  case, the engine induction system must be arranged so that it  does not leak air and change the desired fuel/air ratio. Accessory Section Accessory Gear Trains 1-7