Gas Turbines - Air Inlets — Page 218, Lesson 293

Let’s start with the air inlet itself. The engine air inlet is built into the airframe, or into the forward part of the nacelle — that’s the streamlined housing that wraps around the engine. Its whole job is to deliver a relatively turbulent-free supply of air to the face of the low-pressure compressor, or the fan, depending on the engine type. And the design of that intake duct is absolutely vital to engine performance at all airspeeds and all angles of attack, because a bad intake can lead to compressor stall. Now, the simplest form of intake is the single-entrance, circular cross-section ‘pitot’ type. In wing-mounted engines it’s normally straight, but for tailcone-mounted engines — like the Boeing 727 or the TriStar — it can be shaped into an ‘S’ shaped duct. And here’s a real operational point: unstable airflow in an S duct is a common occurrence, particularly during crosswind take-offs. So that’s a known weakness you have to anticipate. Why is it called a pitot intake? Because it maximises the use of ram effect — that’s the pressure you gain just by pushing air into the intake as the aircraft moves forward. It also suffers the minimum loss of ram pressure as altitude increases. But there’s a trade-off: the efficiency of this type of intake reduces as the aircraft approaches sonic speed, because a shock wave forms at the intake lip. That shock wave is what degrades the performance. Now let’s talk about the shape of the duct. In a subsonic intake, the air inlet is usually divergent — meaning the cross-section widens as you go downstream. That divergence does something important: it allows a reduction of velocity and an increase of pressure at the compressor face as the airspeed increases. So the duct is deliberately slowing the air down and recovering pressure from it before it hits the compressor. Here’s a subtle but crucial point about pressure. When the engine is running on a stationary aircraft — say, on the ground with the brakes on — the pressure inside the intake is actually below ambient pressure. Why? Because the high-velocity airflow through the intake creates a low-pressure region. But as the aircraft begins to move, the pressure inside the inlet starts to rise. The point at which inlet pressure returns to ambient is called ram pressure recovery. That’s the moment the ram effect has fully compensated for the suction of the engine. That point is usually reached at about Mach 0.1 to Mach 0.2. And as aircraft speed increases even further, the inlet produces more and more ram compression. That allows the engine compression ratio to increase — and here’s the beautiful part — which in turn generates more thrust without costing any increase in fuel flow. So you get free thrust from speed, purely from the ram effect. That’s the whole point of the intake design. Let me show you that relationship. — that’s Figure 13.12, the propulsive efficiencies of the gas turbine, which ties into how efficiently you convert that ram pressure into useful thrust. So, to pull it all together: the intake is a divergent duct that slows the air, raises its pressure, recovers ram pressure by about Mach 0.1 to 0.2, and then keeps compressing the air as speed builds — all without burning extra fuel. That’s the core of what the air inlet does for you.