The Propulsion System

Unit 1

The Propulsion System

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The Propulsion System

In order to propel an airplane through the air the force is required. This force must be rather intensive and is provided by the propulsion system.

Every propulsion system consists of:

- the fuelwhich is the energy source from which the force is produced for the required time;

- the engine which is the mechanical device by which the energy of the fuel is transformed into the propulsive force. The word " engine" originally meant any “ingenious” device, and came from the Greek word " ingenious", clever.

Any aircraft propulsion system must meet certain requirements. First is low weight; second, the system must be reliable; third, the cost of the system must be sufficiently low.

The total weight of the airplane can be divided among the airframe, the propulsion system and the payload. The payload consists of the crew, equipment and cargo. The weight of the propulsion system must be so divided between fuel and engine as to permit the airplane to fly a desirable distance without landing.

It must be stated that engines present one of the most interesting groups of problems considered in the engineering field. The problems dealt with are different. One of the main problems worked at by the designers is receiving the maximum possible power or thrust for minimum weight.

Another important prob­lem dealt with is that of fuel. Both in the past and today the desig­ners work at the problem of getting lower specific fuel consumption. Specific fuel consumption is obtained by dividing the weight of the fuel burned per hour by the horse power developed.

Probably the reduction of fuel consumption can be solved when a nuclear reactor is used. If used as a source of power the nuclear pro­cess gives the most energy per unit of mass of any source known. It must be noted that controlled atomic energy has already produced electric power and marine propulsion power.

Another possible problem considered in any engine is its flexibi­lity. Flexibility is the ability of the engine to run smoothly and perform properly atall speeds and through all variations of atmo­spheric conditions.

One more important problem worked at by the designers is the engine reliability. The engine is to have a long life, with maximum of time between overhaul periods. Overall propulsion system reliability is increased by using more than one engine. At present the preferred number of engines varies from one engine on small airplanes to eight on large military bombers. In transports the use of four engines predominates. Reliability is determined by the length of time the engine can run without major overhaul. The time varies with the use of the airplane.

It must be emphasized that in some cases the problem of balance is one of the main. Balance has several possible meanings but the principal factor is freedom from vibration.

Besides any engine must be started easily and carry its full load in a few minutes.

The necessity of carrying away excess heat developed by the engine has always been a problem of first importance too.

Nowadays there exist many types of engines used for various purposes. There are gasoline engines, diesel engines, gas turbines, steam engi­nes, steam turbines, jet engines and rocket engines.

Each of them has certain advantages and disadvantages over other forms of power plants.