General Principles - Cruise — Page 241, Lesson 285

Let’s pick up right where the graph left off. You’ve seen the high‑speed end of the thrust‑and‑drag curves, where the two forces balance at the maximum level‑flight speed. Now look at the very left of that same graph. There’s another point where thrust and drag are equal, and that point marks the slowest speed you can maintain in level flight at that thrust setting. Fly any slower, and the rearward force of drag would exceed the forward force of thrust, so the aeroplane would decelerate and lose height. That’s your minimum level‑flight speed. Now, here’s the altitude effect. At high altitude, the engine produces less thrust, so the green thrust line on the graph moves downwards. Look at Figure 5.6 and you’ll see the consequence: the maximum achievable level‑flight speed becomes slower, and the minimum achievable level‑flight speed becomes faster. So the whole range of speeds available to the aeroplane narrows as you climb. That’s a key performance point — your speed envelope shrinks with altitude. Now I want to move into the different expressions of aeroplane speed, because we use different ones for different purposes — aerodynamics, operations, navigation, performance. Let’s start with the raw measurement. Aeroplane speed is usually measured with two types of probes: the total pressure probe and the static pressure probe. These probes isolate dynamic pressure, and it’s dynamic pressure that indicated airspeed is based on. You can feel total pressure yourself — stick your arm out of the window of a moving car and the force you feel is total pressure. The total pressure probe is sometimes called the pitot probe. But both the pitot probe and the static probe suffer from errors. When you take the speed obtained from these probes, sensing dynamic pressure, without any correction for those errors, that speed is called the indicated airspeed, abbreviated to IAS. And IAS is simply the speed displayed on the airspeed indicator. Now, if you correct those pressure probes for instrument errors and position errors, the speed becomes the calibrated airspeed, abbreviated to CAS. It’s also known as rectified airspeed, or RAS, though this book won’t use that term. Calibrated airspeed is more accurate than IAS, because you’ve removed those two sources of error. That’s the next step up in the speed ladder — and we’ll build on it from here.