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1. When a gas turbine engine is installed in an aircraft it usually requires a number of accessories fitting to it and connections made to various aircraft systems. The engine, jet pipe and accessories, and in some installations a thrust reverser, must be suitably cowled and an air intake must be provided for the compressor, the complete installation forming the aircraft power plant.
POWER PLANT LOCATION
2. The power plant location and aircraft configura-tion are of an integrated design and this depends upon the duties that the aircraft has to perform. Turbo-jet engine power plants may be in the form of pod installations that are attached to the wings by pylons (fig. 23-1), or attached to the sides of the rear fuselage by short stub wings (fig. 23-2), or they may be buried in the fuselage or wings. Some aircraft have a combination of rear fuselage and tail-mounted power plants, others, as shown in fig. 23-3, have wing-mounted pod installations with a third engine buried in the tail structure. Turbo-propeller engines, however, are normally limited to installation in the wings or nose of an aircraft.

Power plant installation

 

3.
The position of the power plant must not affect the efficiency of the air intake, and the exhaust gases must be discharged clear of the aircraft and its control surfaces. Any installation must also be such that it produces the minimum drag effect.

4.
Power plant installations are numbered from left to right when viewed from the rear of the aircraft.

5.
Supersonic aircraft usually have the power plants buried in the aircraft for aerodynamic reasons. Vertical lift aircraft can use either the buried installa-tion or the podded power plant, or in some instances both types may be combined in one aircraft (Part 18).

AIR INTAKES
6.
The main requirement of an air intake is that, under all operating conditions, delivery of-the air to the engine is achieved with the minimum loss of energy occurring through the duct. To enable the compressor to operate satisfactorily, the air must reach the compressor at a uniform pressure distributed evenly across the whole inlet area.

7.
The ideal air intake for a turbo-jet engine fitted to an aircraft flying at subsonic or low supersonic speeds, is a short, pitot-type circular intake (fig. 23-4). This type of intake makes the fullest use of the ram effect on the air due to forward speed, and suffers the minimum loss of ram pressure with changes of aircraft attitude. However, as sonic speed

Power plant installation

is approached, the efficiency of this type of air intake begins to fall because of the formation of a shock wave at the intake lip.
8. The pitot-type intake can be used for engines that are mounted in pods or in the wings, although the latter sometimes require a departure from the circular cross-section because of the wing thickness (fig. 23-5).

Power plant installation

9. Single engined aircraft sometimes use a pilot-10. The disadvantage of the divided type of air type intake; however, because this generally involves intake is that when the aircraft yaws, a loss of ram the use of a long duct ahead of the compressor, a pressure occurs on one side of the intake, as shown divided type of intake on each side of the fuselage is in fig. 23-7, causing an uneven distribution of airflow often used (fig. 23-6). into the compressor.

Power plant installation

11.
At higher supersonic speeds, the pitot type of air intake is unsuitable due to the severity of the shockwave that forms and progressively reduces the intake efficiency as speed increases. A more suitable type of intake for these higher speeds is known as the external/internal compression intake (fig. 23-8). This type of intake produces a series of mild shock waves without excessively reducing the intake efficiency.

12.
As aircraft speed increases still further, so also does the intake compression ratio and, at high Mach numbers, it is necessary to have an air intake that has a variable throat area and spill valves to accommodate

 

and control the changing volumes of air (fig. 23-9). The airflow velocities encountered in the higher speed range of the aircraft are much higher than the engine can efficiently use; therefore, the air velocity must be decreased between the intake and the engine air inlet. The angle of the variable throat area intake automati-cally varies with aircraft speed and positions the shock wave to decrease the air velocity at the engine inlet and maintain maximum pressure recovery within the inlet duct. However, continued development enables this to be achieved by careful design of the intake and ducting. This, coupled with auxiliary air doors to permit extra air to be taken in under certain engine operating conditions, allows the airflow to be controlled without the use of variable geometry intakes. The fuselage intakes shown in fig. 23-10 are of the variable throat area type.
　
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