First, we have the Types of Compressor — Page 226, Lesson 300

This is the start of a brand-new chapter — Gas Turbines: Compressors. So let's set the stage properly, because everything in a gas turbine engine hinges on this component. The compressor's job is to take the air coming in and squeeze it to a much higher pressure before it reaches the combustion chamber. That pressure is what makes the whole engine work. So this chapter is all about how we do that squeezing, the different ways to design it, and the problems that can happen when the airflow misbehaves. Let me walk you through the roadmap of what we're covering, because the structure of this chapter tells you what matters. First, we have the Types of Compressor. There are two fundamentally different designs used in gas turbines: the centrifugal flow compressor and the axial flow compressor. We'll look at both. Then we get into the Pros and Cons of the Centrifugal Compressor — why it was used early on, what its strengths are, and why modern large engines moved away from it. After that, the Principles of the Centrifugal Flow Compressor — how it actually works. The key idea here is that air is thrown outward by centrifugal force as the impeller spins, and that radial motion is what builds the pressure. Then the Principles of the Axial Flow Compressor — this is the design used in virtually all modern jet engines. Here the air flows straight through, roughly parallel to the engine axis, and pressure is built up stage by stage using alternating rows of rotating and stationary blades. Next, Maintaining the Axial Velocity of the Airflow. This is a subtle but critical point. As the air gets compressed, its density increases, so its volume shrinks. If we let the velocity change in the wrong way, the flow can break down. The chapter explains how the design manages this. Then Airflow Control — the systems we use to keep the airflow stable across the whole operating range of the engine. And then we get to the two big failure modes, which you absolutely must understand as a pilot: Stall and Surge. Stall is when the airflow separates from the blades and stops flowing smoothly. Surge is the more violent event — a complete breakdown of flow, sometimes with a loud bang and even flame out of the intake. We'll cover both in detail. Then Prevention of Stall and Surge — the engineering solutions. This includes Variable Inlet Guide Vanes, which are adjustable vanes at the front of the compressor that direct air into the first stage at the correct angle. And Variable Stator Vanes, which do the same job but for the stationary vanes between the rotating stages deeper inside the compressor. We also have Compressor Bleeds — valves that dump some compressed air overboard or to other parts of the engine to keep the flow stable at low power settings. Then Multi-spool Compressors — this is where the compressor is split into two or three sections, each spinning on its own shaft at its own speed. This is a major design feature of modern engines. Next, Active Clearance Control — a system that manages the gap between the blade tips and the casing, because that gap changes with temperature and affects efficiency. Then the Compressor Surge Envelope — the operating boundary on a graph that shows where the compressor is stable and where it will surge. Finally, we get into Construction — the physical hardware. We look at Rotor Blades, the rotating blades that do the work. Then Stator Vanes, the stationary blades that straighten and slow the flow. Then Fan Blades — the big front fan on a turbofan, which is really the first stage of the compressor. And finally Compressor (and Turbine) Contamination — what happens when dirt, oil, or other contaminants build up on the blades and degrade performance. That's the full arc of the chapter. We're going to work through it in order, starting with the types of compressor and the pros and cons of the centrifugal design. Let's begin with the fundamentals. A compressor has to raise the pressure of the air, and it does that by doing work on the air — adding energy to it. The two designs do this in completely different geometric ways. The centrifugal compressor is the older design. Think of it like a paddle wheel spinning very fast. Air enters at the centre, near the axis, and is flung outward by centrifugal force as the impeller rotates. The air gains velocity and pressure as it moves radially outward, and then it's slowed down in a diffuser, which converts that velocity into even more pressure. It's a compact, robust design, and it was used in early jet engines and still appears in smaller engines and auxiliary power units. The axial compressor is the one you'll find in essentially every modern large turbofan. Here, the air flows straight back along the axis of the engine. The compressor is made up of many stages, and each stage has two rows of blades. The first row is the rotor — it spins and accelerates the air. The second row is the stator — it's stationary and it slows the air down, converting that kinetic energy into pressure, and also straightens the flow so it enters the next rotor at the correct angle. Each stage adds a little bit of pressure, and you stack many stages together to get the total compression you need. So the key contrast is: centrifugal throws the air outward radially, axial pushes it straight back along the axis. Both achieve the same goal — raising pressure — but through completely different flow paths. Now, why did the industry move to axial? That's the pros and cons discussion. The centrifugal compressor is very robust, simple, and can produce a high pressure ratio in a single stage. But its big drawback is that it has a large frontal area — it's fat. For a given airflow, it takes up a lot of space at the front of the engine, which creates drag on the aircraft. The axial compressor, by contrast, is long and slim. It has a much smaller frontal area, which is better for high-speed flight. The trade-off is that it's more complex, more expensive, and more sensitive to airflow disturbances — which is exactly why we spend so much of this chapter on stall and surge prevention. So that's the big picture. We have two fundamentally different ways to compress air, each with its own strengths and weaknesses. The centrifugal is simple and robust but bulky. The axial is slim and efficient but delicate. And the rest of the chapter is largely about how we keep that delicate axial compressor happy across the entire flight envelope. That's where we're headed. Let's keep going and I'll take you through the detailed principles of each design.