General Principles - Cruise — Page 241, Lesson 285

Let’s pick up right where the graph left us, because there’s a second critical point on those thrust and drag curves that I want you to see clearly. Look at the graph again. We already found the point on the right where thrust and drag are equal — that’s the maximum level-flight speed. But there’s another point on the curves where thrust and drag are equal, and this one lies to the very left of the graph. The speed at this point represents the slowest speed that can be maintained. In level flight, it would be impossible to fly any slower at this thrust setting, because the rearward force of drag would exceed the force of thrust. So you have two intersections of the thrust and drag curves — one on the right giving you the fastest speed, and one on the left giving you the slowest speed. Between those two points is your entire usable speed range for level flight at that thrust setting. Now, here’s the altitude effect. At high altitude, the thrust produced by the engine decreases, and therefore the green thrust line moves downwards on the graph. Look at Figure 5.6 — it’s clear to see that at high altitude, the maximum achievable level flight speed is slower, and the minimum achievable level flight speed is faster than at lower altitudes. So the range of speeds for the aeroplane is narrower. The two intersection points move closer together, squeezing your usable speed envelope. That’s a key performance limitation you’ll deal with at altitude. Now, this next part of the chapter focuses on the various expressions of aeroplane speed. Different expressions of speed are used for different purposes, depending on whether we’re concerned with aerodynamics, operations, navigation, or even performance. So let’s start with the first one. Indicated Airspeed (IAS). In most cases, aeroplane speed is measured using certain types of probes called the total pressure and static pressure probes. These probes help us isolate dynamic pressure, on which indicated airspeed is based. You can feel total pressure by simply putting your arm out of the window of a moving car — that’s the pressure you feel pushing against your hand. However, total pressure probes — sometimes called pitot probes — and static probes suffer from errors. Without any correction to those errors, the speed obtained from the probes when dynamic pressure is being sensed is called the indicated airspeed, abbreviated to IAS. Indicated airspeed is the speed that is displayed on the airspeed indicator. So IAS is the raw, uncorrected reading straight off the instrument. Calibrated Airspeed (CAS). Now, if the pressure probes are corrected for instrument and position errors, the speed is called calibrated airspeed, abbreviated to CAS. This speed is also known as rectified airspeed, or RAS, but that expression will not be used in this book. Calibrated airspeed is more accurate — it’s the IAS with those instrument and position errors removed. So the progression is: you start with raw IAS from the probes, you correct for instrument and position errors, and you get CAS. That’s the foundation. We’ve got the two speed limits from the thrust and drag curves, the altitude effect narrowing the speed range, and then the first two expressions of speed — IAS as the raw probe reading, and CAS as the corrected version.