Gas Turbines - Thrust — Page 319, Lesson 408

Right, let's get into the heart of gas turbine thrust. We've covered the basics of how the engine works; now we're looking at how we actually rate and measure that output. We'll start with the Thrust Ratings. These are the different power settings we use, and each has a specific job and a specific limit. First, Take-off thrust. This is the maximum thrust the engine can produce, and it's normally time limited. That means you can't run the engine at this setting indefinitely—it's for the brief, high-power phase of getting airborne. Next is Go-around thrust. This is what you'd use if you have to abort a landing and climb away. It can be take-off thrust, but it's normally a lower value. It's a compromise—enough power to safely climb out, but not so much that you stress the engine unnecessarily. Then we have Max continuous thrust. This is the key one. As the name suggests, this setting can be used continuously. It's the highest power you can hold indefinitely without damaging the engine. Below that is Max climb thrust. This setting is equivalent to max continuous, and it gives the best angle of climb speeds. So it's the sweet spot for getting altitude efficiently. Finally, Max cruise thrust. This is a value below max continuous, and its purpose is to prolong engine life. You don't need full power in the cruise, so you back off to save the engine. Now, for turboprop and some turboshaft engines, we don't just talk about thrust. We use a combined unit called Equivalent Shaft Horsepower, or ESHP. This is the unit of power output for those engines. The formula is simple: ESHP = SHP + HP from jet thrust. So it's the shaft horsepower the turbine produces, plus the equivalent horsepower you get from the jet exhaust. And here's a handy conversion: under static conditions, one shaft horsepower equals approximately 2.5 pounds of thrust. This brings us to a fundamental concept. The gas turbine engine can either give a small mass of air a large acceleration—that's your low bypass ratio turbojet—or it can give a large mass of air a small acceleration—that's a high bypass ratio turbofan, or a turboprop. The merits of each relative to propulsive efficiency were discussed in chapter one, but the key point is this: the thrust or shaft horsepower developed depends on the mass of air entering the engine and the acceleration given to that mass as it passes through. And that's affected by changes in altitude, temperature, and airspeed, which all have a bearing on the engine's efficiency and therefore the gas energy available for conversion into thrust or SHP. Next, we have Specific Fuel Consumption, or SFC. To maintain an economical engine, the ratio of fuel consumption to thrust or SHP must be as low as possible. That ratio is the SFC, and it's measured in pounds of fuel used per hour per pound of thrust or SHP. The thermal and propulsive efficiency of the engine determine the SFC. Finally, we have the Thrust to Weight Ratio. In a similar way to piston engines, which produce power, a gas turbine engine's thrust output can be compared to its weight. This ratio is used to compare one engine against another. Let's look at the example: an engine producing 53,000 pounds of force of static thrust, with a weight of 10,400 pounds, would have a thrust/weight ratio of 53,000 divided by 10,400, which equals 5:1. And one last point on how thrust varies with rpm. The amount of thrust produced by a turbojet is proportional to its rpm, because increased rpm increases the mass flow. But here's the crucial bit: the higher proportion of the thrust is produced at compressor speeds higher than 80-85% of HP rpm. So the engine really comes alive at the top end of the speed range.