Along the lines of Wing's tributes to various aircraft on the Civil Aviation thread, I have decided to start a similar thread in Tech/Ops. The emphasis will be on the technical aspects of various components and sub-systems, and I would love for other A.netters of a technical bent to produce their own tour of their favorite piece of aircraft related machinery.
Adding ramblings of your experiences with these devices is encouraged. Think of it as a technical trip report from a personal perspective. Most of all, entertain and educate your fellow A.netters.
Anyhow, JetMech has a thing for the Rolls-Royce (RR) Trent 700. So here goes. The Trent 700 (T-700) series of turbofan engines are designed to produce a nominal thrust in the range of 68-72,000lbs. The engine is a triple spool design of the following layout;
N1 spool, 1 stage of Low Pressure Compressor (LPC) driven by a four stage Low Pressure Turbine (LPT).
N2 spool, 8 stages of Intermediate Pressure Compressor (IPC) driven by 1 stage of Intermediate Pressure Turbine (IPT).
N3 spool, 6 stages of High Pressure Compressor (HPC) driven by 1 stage of High Pressure Turbine (HPT).
|Moving the "C" duct / thrust reverser. Careful boys; it's VERY heavy!|
Here the 'C" duct / thrust reverser (TR) half is being opened for maintenance access. The "C" duct / TR half is forced open by a hydraulic ram, nonetheless, a fair bit of manual handling is still required to fully secure this cumbersome piece of equipment in an open position. This hydraulic ram uses pressurised oil fed to it by the green and orange cart.
The holes in the "C" duct / TR half inner wall are to allow for the compressor bleed valves. The "C" duct / TR halves remain on the strut when the engine is changed. The TR operates by pivoting doors into the cold-steam duct and vectoring air forwards.
The 524G2's and D4's were slightly different. With these RR engines, the thrust reverser was an integral part of the engine, and came away and stayed with the engine when the engine was changed. The TR mechanism was also slightly different in that the entire assembly slid backwards to operate. This backwards sliding simultaneously raised blocker doors in the cold-stream duct, and uncovered cascade vanes which vectored the bypass air forwards.
|JetMech in the midst of a RR Trent 772. JetMech's roughly hewn mug has been replaced with something far more pleasant.|
Here you can see JetMech standing in the space that is left behind when the "C" duct / TR half is opened. You are literally opening up half of the side of the engine when you open the "C" ducts / TR halves.
Behind my back you can see the exit of the fan case and various components below it. In the fore-ground is the back part of the engine cowling, which is called the Integrated Nozzle Assembly (INA) . This comes away with the engine during engine changes.
You can also see the last stage of the LPT and the exhaust cone. This exhaust cone is often an exotically beautiful blue colour on new engines. It feels weird to be both "inside" and "outside" of the engine at the same time.
|Right hand "C" duct / thrust reverser opened to reveal the innards of the beast.|
This photo is the view you get from behind the open "C" duct / TR half. You can clearly see how much of the air bypasses the core of the engine from the size of the bypass region of the fan case.
When the "C" duct / TR half is closed, it's inner wall forms a cowl around the core of the engine whilst simultaneously being part of the inner wall of the bypass duct. The outer wall of the "C" duct / TR half forms part of the outer wall of the bypass duct as well as forming part of the outer aerodynamic shape of the engine pod. This helps to give the T-700 cowling a beautiful, full skirted look.
You can also clearly see the myriad of pipes and tubing that conduct various vital fluids to and from the core and the accessories on the external gearbox and fan case. The fluids include fuel, oil, air, and various electrical and electronic signals carried by the wire bundles.
You can also see how well this area is sealed up. I think the main reason for this is to divide the engine up into fire zones, and also to insulate and protect the pipes, tubes and wire bundles.
|Turbine & compressor casings, fuel distribution manifold and injector nozzles, bleed valves and the housing for the external gearbox drive shafts.|
Here we get a nice view of what normally is covered by the inner wall of the "C" duct / TR half. I don't have much experience on the T-700, so my following description of the components is based on familiarity with other engines.
The two, big, round and protruding objects are bleed valves. They operate in various combinations according to the operating point of the engine. They bleed air off the compressor to stabilise the airflow through this region of the engine under various operating conditions. This air gets dumped into the bypass duct.
The fuel distribution manifold and fuel injector nozzles are the twisted mass of black tubing the runs behind and under the left hand bleed valve. This manifold surrounds the circumference of the combustion casing with numerous equally spaced fuel injector nozzles that pierce into the casing to deliver fuel to the combustion chamber. The basic idea is to have a homogeneous "ring of fire" around the entire circumference of the combustion chamber.
|A close up of the Trent 772's most intimate regions! All that tubing reminds me of a CF6.|
Another view of the area reveals some more components. Underneath the left hand bleed valve is a pipe. This pipe seems to be bleeding air off the compressor and ducting it off to the fan case somewhere. Does anyone know what this air is being bled off for? Nacelle anti-ice etc?
You can also see the casing that houses the drive shafts to and from the core and the external gearbox. IIRC on the G2 / D4, the external gearbox is driven from the N3 spool. Does anyone know what happens on the T-700?
You can also see just a small part of the extensive sealing that is used on a modern turbo-fan. One of the most beautiful and amazing sights on an engine is to close the "C" duct / TR half. It is just amazing how all the components move past one another without interfering. It is just amazing to see how all the various seals are made by the simple action of closing the duct. It's like a thousand closely inter-meshing fingers that never actually touch.
|The only component I am familiar with in this photo is the oil tank.|
Here we see some of the fan case mounted components. The component located in the "middle" is the oil tank. The T-700 has a stand alone oil tank. On the G2 and D4, the oil tank was an integral part of the external gearbox. I am not 100% sure what the other components are, but I will have a guess.
The large component above the oil tank is a Fuel Cooled Oil Cooler (FCOC). The FCOC acts as a heat exchanger between the oil and fuel. The cold fuel is heated up by the hot oil, and vice versa, the hot oil is cooled by the cold fuel.
Cooling the oil is self explanatory, but why would you heat the fuel?
Jet A1 often contains entrained water. At cruising altitudes, the fuel temperature will eventually go below freezing point and may possibly cause ice crystals to form in the fuel. This was a potential problem back in the day of hydro-mechanical engine control units.
These devices where computers; fantastically complex mechanical ones that used the condition of various fluids as inputs to make "decisions" about how to control a multitude of parameters such as the rate of fuelling and the scheduling of bleed valves and stator vanes.
The fear was that ice crystals in the fuel would have the potential to block any number of the fine passages in these hydro-mechanical units and cause malfunction. The solution was to heat the fuel above freezing point and melt these ice crystals back into liquid water before the fuel got to the hydro-mechanical unit.
I am not sure if this needs to be done with the modern crop of Full Authority Fuel Control (FAFC) and Full Authority Digital Engine Control (FADEC) engine types
I hazard to guess that the component below the oil tank is a supplementary oil cooler, and I suspect it uses fan discharge air as the cooling medium.
|Close-up of the stand-alone T-772 oil tank. RB211-524G2's and D4's had the oil tank as an intergral part of the external gearbox.|
Here is a closer view of the fan case components. You can see three, medium brown coloured lines going down to a component on the front of the external gearbox. One line is "large", one "medium" and the other "small" in diameter.
I suspect that these are hydraulic lines and and that the component they lead to is an engine driven hydraulic pump. The large line supplies the pump with hydraulic fluid from the hydraulic reservoir. The medium line delivers a certain flow rate of hydraulic fluid to the aircraft's hydraulic systems, whereas the small line is known as the case drain line.
The hydraulic pumps used in aircraft are of the tilting swash-plate design. Basically what happens is that the swash-plate can tilt. This tilt alters the stroke of several piston type pumps within the pump casing. These pistons are designed with a small amount of leakage for lubrication purposes.
The case drain line removes this leakage fluid from behind the pump pistons to prevent a hydraulic lock forming behind the pistons. The hydraulic pump produces a certain flow rate of hydraulic fluid. When this fluid meets an obstruction, hydraulic pressure is generated.
The tilt of the swash-plate responds to the pressure in the aircrafts hydraulic system. In low demand situations, the swash plate tilts in a manner to cause the stroke of the pump pistons to be short or zero. This results in the hydraulic pump delivering a small amount of hydraulic fluid flow to the hydraulic systems.
In high demand situations, the swash-plate tilts in a manner to maximise the stroke of the pump pistons and hence maximise the flow of hydraulic fluid delivered by the pump to the aircraft's hydraulic systems.
|The external gearbox is sandwiched between the starter motor and the IDG. Above the starter motor is the kevlar fan blade containment band.|
This picture is taken from the other side of the fan case. You can see the starter motor on the left hand side, the Integrated Drive Generator (IDG) on the right hand side (the black object), the external gearbox sandwiched in between, and a pipe the runs above all of them.
In the 5th, photo, I alluded to the purposes of a pipe coming off the compressor casing. I presume that this is the continuation of that pipe, and the purpose of bleeding off the air is to provide Nacelle Anti Ice (NAI).
The starter motor is a pneumatically driven turbine. Under normal operations, the APU will supply a large volume flow rate of air at a medium pressure to the starter. This will spin the turbine in the starter. This is geared down to the output shaft of the starter.
This output shaft enters into the external gearbox. It then spins a gear train inside the gearbox which eventually turns drive shafts. These are the drive shafts between the engine core and external gearbox, and were mentioned in the 5th photo.
These drive shafts transmit torque to one of the spools in the engine which causes it to spin. This spinning spool will induce airflow through the core of the engine which will impinge on the compressor and turbine blades of the spools not driven by the starter. This airflow will eventually cause all the spools to spin.
Once a high enough "motoring" RPM is reached by each of the spools, fuel and ignition can be provided to fire up the engine.
The IDG is so named because on modern jetliners, the drive section is integrated with the generator section; with this arrangement constituting a single device. Earlier jetliners often had these items as separate devices, with the generator piggy-backed on to the drive section.
Modern aircraft electrical systems require the generator to deliver electric current with a certain, constant frequency. This is a difficult chore as the engines that drive the generators operate at many different speeds.
This is where the drive section comes in, the drive section delivers a constant output RPM to the generator no matter what the RPM of the input from the engine is. ( In reality of course, the output will vary within a narrow band, and the range of engine RPM inputs are from ground idle to maximum power ).
The drive section is basically two hydraulic pumps back to back. One is a tilting swash-plate design similar to the hydraulic pump discussed in photo 7. The other pump is also a swash-plate design, but the tilt of the swash-plate is fixed at some intermediate angle.
IIRC, the tilt-able pump is connected to the engine, whilst the fixed pump is connected to the generator, with a fluid circuit between the two pumps. The tilt-able pump adjusts itself via the tilt of the swash-plate, such that the flow rate of fluid it delivers to the fixed pump is just enough to spin the generator at the correct constant RPM.
Imagine an airliner on the runway waiting to take off. Its engines are at ground idle. The tilt-able pump would be at maximum tilt so as to deliver maximum fluid flow to the fixed pump. This would act as a gearing up mechanism to make up for the slow RPM of the engine.
When the pilot slams the throttle to maximum RPM, the swash plate of the tilt-able pump will reduce it's tilt to reduce the fluid flow to the fixed pump driving the generator.
At maximum engine RPM, the tilt of the swash plate of the tilt-able pump will be quite small, and thus, the engine driven pump will be delivering a relatively small flow rate of fluid to the fixed pump driving the generator. This would act as a gearing down mechanism to make up for the fast RPM of the engine.
If the system is designed and developed correctly, the engine driven pump will adjust itself in such a manner that the generator RPM will remain relatively constant even during the quick transition period from ground idle to take off power.
Well A.netters, that's it, I'm spent. Don't take what I just posted as gospel for the Trent 700, as I have very limited experience with this engine type. Most of my post is with respect to generic descriptions and my much greater experience with other engine types. I have also taken many liberties in my explanations. Nonetheless, most of it should apply in a broad sense to the T-700.
P.S. I'm having a hard time reconciling the beautifully graceful exterior of the T-700 with the warts and all innards. Surely a case of beauty being skin deep?
[Edited 2006-10-27 19:53:23]