Gas Turbines - Introduction — Page 218, Lesson 285

Let's start with the heart of this section: propulsive efficiency. I want you to think of it as a measure of how well the engine's thrust-producing mechanism converts the energy it generates into forward motion. The key relationship, which I'll come back to, is this: the closer the aircraft's speed comes to the speed of the jet efflux — that's the exhaust gas stream leaving the engine — the higher the propulsive efficiency of the engine/propeller combination. Now, let's look at the three main engine types and how they compare on this scale. At low airspeeds, the turbo-propeller engine combination offers the highest propulsive efficiency. That's the turboprop — a gas turbine that drives a propeller. But here's the catch: above about 350 miles per hour, the propeller's efficiency drops off quite rapidly. The reason is the disturbance of the airflow at the tips of the blades. As the aircraft speeds up, the blade tips start to encounter airflow problems, and efficiency suffers. In comparison with the turboprop, the propulsive efficiency of the pure turbojet appears quite poor at the lower airspeeds. A pure turbojet is the basic jet engine — all the thrust comes from the high-velocity exhaust jet, with no bypass air and no propeller. At low speeds, it's inefficient. But as the airspeed increases in excess of 800 miles per hour, the turbojet's propulsive efficiency starts to improve beyond what the turboprop can match. From that point on, there's no comparison — the turbojet eventually reaches a propulsive efficiency close to 90%. Now, here's the practical problem. Cruising speeds in the order of 800 miles per hour are at present out of reach of most transport aircraft. So most of the world's transport aircraft operate in the mid-speed range. And that's where the bypass type of engine fits in. This type includes the ducted fan, or turbofan engine. Its propulsive efficiency sits neatly between that of the turboprop and the pure turbojet. Why does it work so well? Because it deals with comparatively larger mass airflows at lower jet velocities. Let me unpack that. The turbofan moves a large amount of air, but at a relatively modest exhaust speed. That combination — big airflow, lower jet velocity — gives it a propulsive efficiency that exceeds both the turboprop and the pure turbojet at the speeds normally associated with jet transport aircraft. So in the mid-speed range where airliners actually fly, the turbofan is the winner. Now let's move to the second topic: Modular Construction Methods. The background here is economic. The use of larger and larger aircraft has made air travel less and less expensive — that works well as long as the aircraft themselves work well. But if one restricting component on a large aircraft, such as an engine, becomes unserviceable, the expense of keeping three or four hundred passengers fed, accommodated, and happy becomes exorbitant. Think about it — a grounded airliner with a full passenger load is burning money every minute. So engine manufacturers, to minimize the financial burden on the users of their equipment in the event of failure, started using Modular Construction Methods. The idea is simple: these methods facilitate changing sections of an engine rather than the whole engine. Instead of replacing an entire powerplant when one part fails, you swap out just the faulty module. Figure 13.13 shows how the engine is split into several modules. So to tie it together: we've got the efficiency story — turboprop for low speed, turbojet for very high speed, turbofan for the mid-speed transport range — and we've got the maintenance story, where modular construction lets you replace sections instead of the whole engine, saving enormous cost when something fails.