Gas Turbines - Compressors — Page 237, Lesson 317

Let’s pick up right where the compressor hardware gets interesting — the actual fixing of the blades to the disc. The most common way to attach a rotor blade is the dovetail joint. The root of the blade is shaped like a dovetail — wider at the base, narrower at the neck — and it slides into a matching slot in the disc. Then it’s secured with a pin or a locking tab. That pin or tab stops the blade from sliding out along the slot. Now, here’s the design problem. On smaller engines, it gets harder and harder to design a practical fixing method while still keeping the disc weight to a minimum. The dovetail, the pin, the locking tab — all of that adds weight and complexity. So on small engines, one solution is to machine the blades integral with the disc — one single piece. That blade-and-disc combination is called a blisk. Remember that term — blisk, from "blade" and "disc." Now let’s talk materials, because the compressor is not uniform. The compressor casing is built from aluminium alloy at the front stages. The intermediate stage casing is steel alloys. And in the high pressure section, temperatures get so high that only nickel based alloys can withstand them. So the casing material changes as you move from front to back — aluminium, then steel, then nickel. Now the rotor blades themselves. They are aerofoil section — that means they have the same cross-sectional shape as a wing. They’re normally made from drop forged stainless steel, machined to a close tolerance before being attached to the rotor disc. And here’s a key point: the blades reduce in size from the front to the rear of the compressor. Why? To accommodate the convergent shape of the air annulus — the annular passage the air flows through gets narrower as you go back, so the blades shrink to match. Some of the low pressure stages may use titanium blades, where the temperatures of compression are not too high. Titanium is lighter and strong, but it can’t take the heat further back. Now, an important operational detail about the dovetail fixing. The dovetail system does not hold the blade immovable in the disc. In fact, the blades are quite loose until they are firmly seated by centrifugal force during engine operation. When the engine spins up, centrifugal force throws the blades outward and seats them firmly in their slots. But when the engine is windmilling on the ground — that is, the rotor is turning slowly from airflow, not from combustion — the blades rattle loosely. And it sounds, as the text puts it, "somewhat like a bag of nails being shaken." So if you ever hear that rattle on the ground, that’s normal — the blades are loose until centrifugal force seats them. Now the stator vanes. These are also aerofoil shaped, and they are fixed to the compressor casing — either directly, or into stator vane retaining rings, which are themselves fastened to the casing. So there are two mounting options: direct to casing, or via retaining rings. The vanes may be assembled in segments in the earlier stages. And the longer ones are shrouded at their inner ends — that means they have a shroud, a band, at the inner end — to prevent vibration which can be induced by the velocity flow over them. Long vanes are prone to flutter, so the shroud stiffens them. Materials for stator vanes: early engines used aluminium alloys, but aluminium did not withstand foreign object ingestion damage at all well. Foreign object ingestion — that’s when debris, birds, stones, anything gets sucked into the engine and hits the vanes. Aluminium just couldn’t take that impact. So the modern choice: steel or nickel based alloys have a high fatigue strength and are less easily cracked or eroded by impact. Titanium is sometimes used for the vanes in the early stages, but it is not suitable further down the engine where the high temperatures can affect it. And one more problem that can occur: rub. That’s when the rotating blades or vanes contact the casing — an excess of rub might occur through some misalignment or distortion. That’s a wear condition you watch for. Let me show you the blade fixing and the bleed valve operation, because these are the two figures that go with this material. So to tie it together: the compressor is a series of rotor blades and stator vanes. The rotor blades are loose in their dovetail slots until centrifugal force seats them. The stator vanes are fixed to the casing, either directly or via retaining rings, and the longer ones are shrouded to stop vibration. Materials progress from aluminium at the front, to steel, to nickel-based alloys at the high-pressure rear, with titanium used where temperatures allow. And the whole assembly is designed to handle the convergent air annulus, with blades shrinking from front to rear.