Citation Encore 560 Spécifications télécharger pdf (Page 53)

SECTION II

MODEL 560 AIRPLANE AND SYSTEMS

ENGINE

GENERAL

Jet engines produce thrust by accelerating air. It is the product of the mass of the air times the

increase in velocity that determines thrust output. To generate a given amount of thrust, a small volume of

air can be accelerated to a very high velocity, or a relatively large amount can be accelerated to a lower

velocity.

In a turbojet engine, incoming air is compressed, mixed with fuel, combusted and exhausted at a high

velocity. In a turbofan engine, only a portion of incoming air is combusted. The hot air then drives the fan

which accelerates a large volume of air to a lower velocity. This air is bypassed around the engine and is

not mixed with fuel or combusted. The relation of the total mass of bypassed air, to the amount of air going

through the combustion section, is known as the bypass ratio. The bypass ratio of the Citation Encore

engine is 2.55 to 1.

The PW535A, developed for the Citation Encore by Pratt and Whitney Canada Inc., is a turbofan

engine rated at 3400 pounds static thrust. A concentric shaft system supports the fan and turbine rotors.

The inner shaft connects the fan (N

) and the axial boost stage of the low pressure compressor at the front

of the engine to the two rear low pressure turbines. The outer shaft connects two axial compressors and

the centrifugal compressor (N

) and the forward high pressure turbine.

All intake air passes through the fan. Immediately aft of the fan the airflow is divided by a concentric

duct. More than two-thirds of the total airflow is bypassed around the engine through the outer duct and is

exhausted at the rear. Air entering the inner duct passes through guide vanes to the axial boost

compressor stage, then through a second set of guide vanes and is compressed by the two axial and one

centrifugal high pressure compressor. The high pressure air then passes through a diffuser assembly and

moves aft to the combustion section.

The combustion chamber is of a reverse flow design to save space and reduce engine size. A portion

of the air entering the chamber is mixed with fuel and ignited. The remainder enters the chamber liner

downstream for cooling.

Fuel is introduced by 11 dual orifice nozzles supplied by a dual manifold. The mixture is ignited

initially by two spark igniters which extend into the combustion chamber at the five and seven o'clock

positions. After start, combustion becomes self-sustaining. The hot gases expand, reverse direction and

pass through a set of turbine guide vanes to the high pressure turbine. The power generated by this

turbine is transmitted by the outer shaft to turn the N

compressors.

Only a small part of the energy available in the hot, high pressure air is absorbed by the high pressure

turbine. As the expanding gases move rearward, they pass through another set of guide vanes and enter

the two-stage, low pressure turbine. A greater portion of the remaining energy is extracted there and

transmitted by the inner shaft to the forward mounted fan and boost rotor. The hot gases then exhaust

through the mixer nozzle and the bypass duct.

The turbofan is in effect two interrelated powerplants. One section is designed to produce energy in

the form of high velocity, hot air. The other utilizes some of this air to provide the power to drive the fan.

The fan of the PW535A, pumping a high volume of cool low velocity air, produces over one-half of the total

thrust.

The Model 560 is equipped with a ground idle system which automatically allows the engines to

decelerate to an idle speed of 49.1% N

eight seconds after the landing gear squat switches have sensed a

landing. The slower idle speed allows better taxiing control at lighter weights and in very cold temperatures,

when the normal flight idle speed of 52.9% N

(at sea level) would require more use of the brakes, resulting

in reduced brake life. The ground idle function is controlled by a GRND IDLE switch on the tilt panel, which

has NORM and HIGH positions. For normal conditions NORM is selected. HIGH position will disable the

ground idle system, resulting in an idle speed of not less than 52.9% N

. A GND IDLE

annunciator is

located on the annunciator panel. The annunciator will illuminate when the airplane is on the ground and

the engine has assumed the slower idle speed, or will assume it when the throttle is reduced to idle. If the

pilot desires to prevent the engine from assuming the slower idle speed, for reasons of faster engine

acceleration time or higher electrical load, etc., the HIGH position is selected.

56OMB-01 Configuration AA 2-3

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Citation Encore 560 Spécifications Page 53

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