Gas Turbines - Thrust — Page 319, Lesson 410

Let's start with the ground idle picture, because it sets the stage for everything else in this section. For a twin spool engine — that's an engine with two rotating assemblies, a high-pressure spool and a low-pressure spool — at engine idle on the ground, the HP rpm, the high-pressure spool speed, will be of the order of 50 to 60 percent. The LP rpm, the low-pressure spool speed, will be about 25 percent. Those are the ground idle values. Now, in flight those values will be higher, and the reason is the power take-offs from the engines — the accessories, the generators, the hydraulic pumps, all the things drawing power off the engine — they demand more, so the idle speed rises in flight. Here's a useful relationship to hold onto: in a high bypass ratio turbofan, 25 percent N1 — and N1 is the low-pressure compressor speed, the fan speed — 25 percent N1 is approximately equivalent to 5 percent of the take-off thrust. So you can see, at idle you're producing very little thrust relative to take-off. Now, engine thrust is rated by a set of terms, and I want to walk you through how thrust varies with altitude, with temperature, and with aircraft speed. Let's take altitude first. As aircraft altitude increases, both temperature and pressure decrease. The fall of pressure causes a reduction in air density, and therefore a loss of thrust as altitude increases. Think about it — the air is thinner, so less mass flows through the engine. As the mass flow of air decreases, the altitude sensing capsule of the fuel control unit adjusts the fuel flow to match the reduced airflow. The purpose is to maintain a constant engine speed for a fixed throttle position. So the fuel control unit is constantly trimming fuel to keep the engine speed steady as the air gets thinner. But here's the subtlety. The fall of temperature increases the air density — cold air is denser — so the mass flow of air into the engine increases, and that pushes thrust up. So you have two opposing effects: pressure falling reduces thrust, temperature falling increases thrust. The combined effect of the temperature and pressure reduction is that thrust will decrease, but at a lower rate than if the pressure alone were reducing. The temperature effect partially offsets the pressure effect. That holds until the aircraft reaches the tropopause. At the tropopause, any increase in altitude will cause the pressure to keep reducing, but the temperature remains constant at minus 56 degrees Celsius. So the temperature effect stops helping, and the thrust will reduce at a greater rate above the tropopause. One more point on altitude: the SFC — that's specific fuel consumption, the fuel flow per unit of thrust — will remain essentially the same as the thrust decreases, because fuel burn decreases along with thrust as altitude increases. So the ratio stays roughly constant. Now let's look at temperature variation. As temperature decreases, air density increases, and the mass of air for a given engine speed increases, therefore thrust increases. But to maintain the compressor speed, more fuel must be added — otherwise the compressor will slow down. The denser air loads the compressor more, so you need more fuel to keep it spinning at the same speed. The opposite happens in warmer air. Warmer air is less dense, so thrust will decrease because of the reduced mass flow, and the compressor will speed up unless fuel flow is reduced. So in hot conditions, you have to cut fuel to prevent the compressor from overspeeding. Now here's a really important operational concept. In cold weather, the denser air allows the engine to develop the required take-off thrust before the limiting temperature has been reached — and the limiting temperature is tied to the maximum available pressure ratio across the compressor, which is the power limiter. These engines are called part throttle or flat rated engines. What that means is the take-off rated thrust can be achieved at throttle settings below the full throttle position. In cold, dense air, you don't need full throttle to get your rated take-off thrust — you're flat rated, so the thrust is limited by the rating, not by the throttle position. Finally, let's look at thrust variation with aircraft speed. Theoretically, as aircraft speed increases, thrust decreases. If you look at the thrust equation, assuming the exit velocity remains the same, then if the inlet velocity increases, it follows that the thrust will decrease. The faster you fly, the faster the air enters the intake, and that reduces the net thrust. But in reality, the forward speed generates extra pressure in the intake — that's the ram effect. The increase in Ram Ratio increases the mass flow, therefore fuel flow has to be increased, causing an increase of SFC as the net thrust decreases. So even though the ram effect helps push more air through, the net thrust still falls with speed, and the specific fuel consumption rises because you're burning more fuel for less net thrust. So to tie it all together: thrust falls with altitude, with a slower rate of fall below the tropopause and a faster rate above it; thrust rises in cold air and falls in hot air, with flat rating protecting the take-off thrust in cold conditions; and thrust falls with aircraft speed, with the ram effect partially offsetting but SFC rising. That's the complete picture of how thrust behaves across the operating envelope.