We’re starting a brand-new chapter: Gas Turbines – Gearboxes and Accessory… — Page 338, Lesson 424

We’re starting a brand-new chapter: Gas Turbines – Gearboxes and Accessory Drives. So let’s get straight into the heart of it. The first thing I want you to meet is the auxiliary gearbox, which is also called the accessory drive. This is the unit that provides the power for the hydraulic, pneumatic, and electrical mechanisms used both on the engine and in the aircraft. It also drives the fuel pumps, the oil pumps, the tacho-generators, and various other devices that are necessary for efficient engine operation. Now, where does the drive for this accessory unit come from? Usually it’s taken from the high pressure compressor shaft. The power path goes from that HP compressor shaft, through an internal gearbox, and then out to an external gearbox. That external gearbox is what provides the mountings for the accessories, and in the majority of cases, it also mounts the starter motor. Let me make sure you’ve got that flow clearly: high pressure compressor shaft → internal gearbox → external gearbox → accessories and starter motor. Now, here’s an interesting design consideration. On modern turbofan engines, there’s much less of a problem about where to conceal the accessory drive unit. Why? Because the engine itself is so massive that even the largest accessories can be fitted into the cowling that forms the air intake fairing. So space isn’t the issue anymore. The real problem on a modern turbofan is how to get the drive shaft through the engine from the high pressure compressor shaft. And here’s the key engineering point: if the drive were taken from the hot end of the engine, the losses incurred would be very high. Also, the type of material used for the shafts would have to be fairly exotic, because of the temperatures involved. So that’s why the drive is taken from the high pressure compressor shaft instead — it’s the practical, efficient choice. Let me pause there. That’s the core of the auxiliary gearbox: what it powers, how the drive is routed, and why the HP compressor shaft is the chosen source. When you’re ready, we’ll move on to how the compressor shaft expansion is allowed for, and then the stub shaft drive and idler gear drive methods.