Gas Turbines - Thrust — Page 319, Lesson 412

Let's start with ram recovery, because it's the first thing that happens to the air before it ever reaches the compressor. As the aircraft speeds up, the intake — the duct at the front of the engine — is shaped so that it converts some of that extra forward velocity into pressure. That's what we call ram recovery. The faster you fly, the more the intake slows the incoming air down and squeezes it, trading kinetic energy for pressure energy. Why does that matter? Because it raises the pressure right at the face of the compressor. Higher pressure at the compressor face means more air is being pushed through for a given compressor speed — so the mass flow increases. And more mass flow means more thrust. In other words, ram recovery restores some of the thrust that would otherwise be lost as speed increases. Without it, the engine would lose thrust as you accelerated; with it, you recover a portion of that loss. Now let's move to altitude, and here we switch to the turboprop. As the aircraft climbs, density drops, and the turboprop suffers a similar loss of power — just like the jet, but we measure it differently. For a turboprop we talk about SHP, shaft horsepower, and ESHP, equivalent shaft horsepower. As altitude increases, both the shaft horsepower and the net jet thrust reduce — so ESHP reduces too. Here's the interesting part: as density reduces, the fuel flow also reduces, but the specific fuel consumption — the fuel burned per unit of power produced — stays essentially the same. So you're burning less fuel per hour, but you're also producing less power, and the efficiency ratio doesn't improve with altitude. Then we look at the effect of aircraft speed on the turboprop. As airspeed increases, the ram effect into the intake causes the SHP to increase — that's the same ram recovery idea working for you. But at the same time, the net jet thrust decreases. So you gain on the shaft side and lose on the jet side. Fuel burn increases in line with the additional mass flow — more air coming in means more fuel being burned — but here's the payoff: the sfc, specific fuel consumption, goes down. So at higher speed, you're burning more fuel overall, but you're burning it more efficiently per unit of thrust or power produced. That figure shows the variation of thrust with altitude — you can see how thrust falls off as you climb. Now, these are the book's practice questions — let's try them one at a time.