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I recently came into the posession of a Lucas Aerospace CR.201 APU. The unit is a small twin shaft gas turbine engine which provides both electrical power and starting horsepower for the Rolls-Royce Pegasus engine fitted to Harrier aircraft.
The specifications for this unit are as follows.
Type of engine
Electric start single stage gas generator with back to back centrifugal compressor and radial inflow turbine. Fully self contained fuel and oil system with twin sscavenge on oil system. Annular reverse flow combustion chamber with five flat fan pattern fuel injectors. Seperate, axial flow, free power turbine driving through a twin planetary gear system and solenoid operated dog clutch to initiate output shaft drive. The engine speed is governed internally in one of 3 modes, 'Start' mode which allows full rpm and power to be transferred to the Pegasus main engine to start it via power turbine driven output shaft, idle mode which is governed by an inline restrictor in the fuel system and forces the rpm into the >40krpm region, and apu mode which governs the generator drive plate to give correct generator output frequency.
Together with Roger Marmion, I made a 600mile round trip to a secret location in Ipswich to take delivery of three of these engines. Two of the engines were complete and in servicable condition, one each for me and Rog. and the third had thrown it's power turbine making it unusable as a running engine but a good supply of spare parts. Over the last couple of weeks I have systematically stripped the blown engine and photographed the various engine components. Major components like the rotor/diffuser/ngv assemily have been left as assembled units for possible replacement if one of our engines should fail. I have been very impressed by the build quality of these motors, and how compact they are.
This is the 'gas generator', which is a small gas turbine unit, the exhast gas of which blows onto the free power turbine causing it to rotate. The compressor and turbine wheels are in a back to back configuration on a shaft which overhangs the gearbox on the right. The silver cylinder seen at the right of the image contains the shaft bearings and oil seals. This set up keeps all the bearings out of the hot section of the engine. The aerofoil section blades seen around the top of the combustion chamber straighten out the rotating airflow from the compressor and diffuse it to increase the pressure ratio.
This view shows the gas generator turbine, which is a radial inflow type as seen in a turbocharger. The five holes in the combustion chamber are for the fuel injectors which atomise the fuel flow into a flat fan pattern. The casing which encloses this assembily contains a centeral pipe which seals against the turbine housing to carry the exhaust gas via a set of guide vanes onto the power turbine.
This shot shows the combustion chamber removed. The row of blades just to the left of the turbine housing are the guide vanes which force the combustion chamber gasses onto the turbine at the correct angle. At the left, just to the right of the sealing gasket, are the compressor diffuser outlet holes where the fresh charge air enters the air casing.
This is another view of the gas generator turbine, you can see where the edges of some blades have been removed to balance the assembily. Without proper balancing the rotating assembily would not be able to withstand the 77,000rpm it will run to.
These images show the air casing and power turbine guide vanes on a damaged engine, the left image has the turbine wheel sat in the middle, almost all of the turbine blades have been destroyed. The lump in the middle is where the turbine has sheared itself free of it's connecting shaft. I would not have liked to be in the aircraft when this happened!
This is the fuel pump, it is driven by the accesory gearbox at the front of the engine, which is driven by the gas generator rotor. It incorporates both a fuel filter and a governor mechanism to limit the top speed of the engine.
This is the oil pump. It is also driven by the accesory gearbox. It also scavenges oil from various parts of the engine and gearboxes.
This gear engages the starter motor to the accesory gearbox via an internal sprag clutch mechanism. The sprag acts like a ratchet but is far superior. It allows free rotation in one direction but locks up solid in the other with zero backlash.
This running engine belongs to Ian Bennett, hopefully ours will be like this soon!
This is what my engine currently looks like. It needs a good clean, a starter motor, a couple of modifications to prevent it running in 'start mode' which could seriously damage the power turbine if not attatched to a Pegasus engine.
**CURRENT STATUS OF ENGINE**
The engine currently needs a starter motor, a high energy ignition module, a clean, a stand to allow it to be run safely.
The job both me and Roger are working on now is modifying a couple of motorcycle starter motors to produce much higher levels of power and rpm than they were originally designed for! This means a lot of modified wiring and new ballraced end covers to prevent vibration and allow the motor to mate with the APU.
The starter motors have been made, this involved fabricating new end caps for the motorcycle starters which incorporate ballraces, and the rotor has been balanced and strengthened with Kevlar to cope with the high rpm it will experience. The power turbine in my engine was damaged, the blades had small chunks missing from the outer edge, this is due to the matereal suffering from creep. The turbine has been removed and the engine is going to be run without a power turbine fitted.
The engine is now complete on it's test stand with fuel and oil tanks fitted. All electronic control systems are complete and the engine runs nicely.
Click and save here to hear the first breath of life from my rebuilt engine!
Macintosh system 7 sound format, stuffit hqx'd to reduce file size.