General Principles - Climb — Page 182, Lesson 210

Let’s start with the core idea of this section: how turbojet thrust behaves as speed changes, and then how it behaves as altitude and temperature change. I’m going to walk you through this exactly as it appears in the syllabus. First, look at Figure 3.20. For a given engine rpm and a given operating altitude, the variation of turbojet thrust with speed is shown. The key phrase here is "essentially constant with IAS." IAS is Indicated Airspeed — the speed shown on your airspeed indicator. So, for a turbojet, once you fix the rpm and the altitude, the thrust available stays essentially the same regardless of how fast you fly. That’s why the graph shows a straight, flat line labelled "Thrust Available — Turbojet." Now, there’s one exception noted: unless the take-off run is being considered. During the take-off run, thrust behaviour is different, but for all other phases — climb included — future illustrations will display turbojet thrust available as a straight line. So remember that simplification: constant thrust with speed, except during take-off. Next, let’s move to the variation of thrust with density altitude. A turbojet engine is un-supercharged. That’s a critical point. Because it’s un-supercharged, it has no compressor stage forcing extra air in — it relies on the natural density of the air flowing through it. So, if you increase density altitude, which means lower air density, the mass flow through the engine decreases. Mass flow is simply the amount of air passing through the engine per unit time. Less air flowing through means less thrust available. So thrust available decreases as density altitude increases. This matters in two situations. First, during a climb — as you go higher, density altitude increases, so thrust falls off. Second, when operating at airfields with a high pressure altitude and/or a high outside air temperature — OAT. Both of those conditions reduce air density, so they reduce thrust available even on the ground. Let me clarify pressure altitude for you, because it’s a precise definition. Pressure altitude can be determined on the ground by setting 1013 hPa on the altimeter subscale. hPa is hectopascals, the unit of pressure. So, if you set 1013 on the subscale and the altimeter reads 1000 ft on the ground, then the pressure altitude is 1000 ft — irrespective of the actual height of the airfield above sea level. Why? Because the aeroplane will experience the air pressure that corresponds to 1000 ft in the International Standard Atmosphere. So pressure altitude is the altitude that corresponds to the current pressure, referenced to the standard atmosphere — not your true elevation. Now, Figure 3.21 shows this graphically. It plots IAS on the horizontal axis and thrust available on the vertical axis, for a turbojet. There are two lines: one labelled "Low Density Altitude" and one labelled "High Density Altitude." The high density altitude line sits lower — meaning thrust available has a lower value with increasing density altitude, i.e., lower air density. So the whole constant-thrust line shifts downward as density altitude increases. Finally, let’s look at the variation of take-off thrust with air temperature, OAT. This is Figure 3.22. Here we have a graph with outside air temperature in degrees Celsius on the horizontal axis and thrust available on the vertical axis, for a given pressure altitude. The thrust line is not straight — it has a "kink" in it. Let me explain what that kink represents. At lower OAT, the thrust is "flat rated." That means the thrust stays constant — flat — as temperature increases, up to a certain point. At higher OAT, the thrust is "EGT limited." EGT is Exhaust Gas Temperature — the temperature of the gases leaving the turbine. So, at higher outside air temperatures, the thrust is limited by EGT, and the thrust line drops off. The point where the flat portion meets the dropping portion is the "kink" — that’s the temperature at which the engine transitions from flat-rated to EGT-limited. Why does this happen? Generally, the thrust of any turbojet engine is restricted by the maximum temperature the turbine blades can withstand. The turbine blades are the rotating blades at the back of the engine that extract energy from the hot gas. If the gas is too hot, the blades can be damaged. So the hotter the gas, the more the thrust must be limited to protect the blades. Now, here’s the engineering trade-off: the more heat-resistant the material from which the turbine blades are made, and the more efficient the blade cooling, the higher the maximum turbine inlet temperature can be. Turbine inlet temperature is the temperature of the gas just before it enters the turbine. And the higher that maximum turbine inlet temperature, the greater the thrust the engine can safely develop. So blade material and blade cooling directly determine how much thrust you can safely extract. So, to summarise the whole picture: turbojet thrust is essentially constant with IAS for a given rpm and altitude. It decreases as density altitude increases, because the engine is un-supercharged and mass flow drops. And at a given pressure altitude, thrust is flat-rated at low OAT, then EGT-limited at high OAT, with the kink marking the transition — all governed by the maximum temperature the turbine blades can withstand.