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Projects - Rolls-Royce VIPER 522 TurboJet

Viper Main    Viper Spec's    Viper Background    Theory of Operation

 

Theory of Operation:

Description-

The Viper 522 is a straight through axial flow engine. Air entering into the engine is directed into the compressor by entry guide vanes. As the air moves through the stages of the compressor its pressure is increased.  When the flow exits the compressor a set of two stage flow straighter blades feed the compressed air into the combustion section. The combustion chamber is annular and uses 12 "walking stick" type vapourising burners as the primary method of burning fuel. Six starting "atomising burners" are used when starting the engine and these are shut down by the engine control when primary fuel pressure is reached by the Pressure Increasing Valve (PIV).

 

Fuel:

Fuel is pumped from the accessory gearbox driven fuel pump via the Barometric Flow Control Unit (BFCU) and the Air Fuel Ratio Controller (AFRC) and limited by the Top Temperature Control unit (TTC) and the Automatic Thrust Limiter (ATL). Fuel burns in the combustion chamber where the velocity of the gas is increased and travelling via the nozzle guide vanes feeds the impulse/reaction type turbine. The turbine's job is to extract some of the energy in the gas flow to drive the main shaft and transmit power to use for the compressor and accessory gearbox.

The rest of the gas flow expands in the jet pipe and is somewhat straightened by the turbine nozzle cone where it is forced to atmosphere via a convergent jet pipe nozzle, thereby developing thrust.

There is a diagram below of the operation of the Viper 522 fuel and control system.

It looks complex, but remember two things :

1) In the 1960's when this engine was built there were no computers or silicon chips to control complex machines such as this so control of the engine had to be done mechanically.

2) The control systems were designed to allow stable operation over the aircrafts temperature and altitude range. Remember that as altitude increases this will affect the pressure of the fuel so this has to be compensated for.

The way the system works is a little like what you experience when you're having a shower and someone turns on a tap in the house. The shower temperature changes because the water pressure drops as the other tap is turned on. The same thing happens on the Viper 522. As the sensors detect a change they bypass fuel or dump fuel from a rail to control the final pressure reaching the burners. Also the fuel pump is a piston type pump. Its bore is constant but its stroke is adjusted by feeding control fuel into the bottom of the pumps cylinder thereby raising or lowering its floor, therefore its stroke.

So most of that stuff we won't need.  We had got the idea that if any of the fuel and control systems didn't work then for ground running we could just bolt on our own hydraulic pump and feed the burners with our own system.

 

Air:

The air system of the Viper falls into two main categories ; engine and services. As you'd expect the bulk of the air passing through is used for combustion.  But some of the air is bled off at different stages to be used for engine services and aircraft services.

P1 air, atmospheric air, is sensed at the intake assembly for the BFCU and Pitot head.

There is a complex air bleed system that is designed to bleed off 4th stage compressor air under starting and low RPM conditions. It's a valve that is pneumatically controlled by an actuator which senses P1 and P2 (compressor delivery air) pressures and activates the bleed valve when correct conditions are met.  The reason for this seems to be to allow better control at low engine speeds.

Final compressor air is bled off to supply aircraft services and also via a butterfly valve to supply anti-icing hot air to the inlet cowling.

Air is also bled internally from the 6th and 7th compressor stages to supply cooling air to the front and rear faces of the turbine wheel and to provide anti-icing air to the inlet nose cone. This is fed in internal passages and down the hollow main shaft.

Finally some of that internal air is used to help provide a seal on the internal shaft labyrinth seals.

 

Lubrication:

 

The lubrication system of the Viper comprises of a storage tank, holding nearly 8 litres of oil, a pump and filters.  The system is part scavenge / part total loss.

The oil from the centre and rear main roller bearings have no return as that would make for a complex manufacturing process and the oil would have to be cooled. So integral in the oil pump are two "micro-pumps".  Their job is to provide a metered, measured spray of oil to feed the centre and rear main bearings. The used oil then adds itself to the gas flow and exits via the jet pipe.  This means that the engine uses oil and therefore has a limited time of running, but in reality it is longer than any flight would be.

The front main bearing, which is a ball bearing, and the accessory gearbox are provided with lubrication via the same pump but the front of the engine has return lines and channels via a scavenge filter where the oil can return, cleaned and filtered to the tank.