General Principles - Cruise — Page 241, Lesson 289

Let’s pick up with the next speed in our cruise discussion: true ground speed, abbreviated TGS, or just GS. I want you to think of it this way. True airspeed is your speed through the air mass. True ground speed is your speed over the ground — your velocity within a fixed ground reference system. That’s the definition to hold onto: it’s the aeroplane’s velocity over the ground. Now, how do we get it? True ground speed equals true airspeed plus or minus the wind component. If you have a tailwind, your speed over the ground increases by a value equal to the speed of that tailwind. Example: true airspeed 250 knots, tailwind 20 knots — true ground speed is 270 knots. Conversely, a headwind reduces your speed over the ground by a value equal to the headwind’s speed. Example: true airspeed 250 knots, headwind 30 knots — true ground speed is 220 knots. If the headwind or tailwind components aren’t already known, you can work them out using a flight navigation computer, or by using graphs or tables in the aeroplane flight manual. And there’s a specific reference I want you to note: CAP 698, Section 4, Page 4, Figure 4.1 can also be used for this. Now let’s move to a different physical effect that becomes important at higher speeds. At speeds higher than about 220 knots, some of the aeroplane’s energy goes into compressing the air ahead of it, locally increasing the density of that air. This is compressibility. Compressibility affects the amount of drag force on the aeroplane, and the effect becomes more important as speed increases. Here’s the picture I want you to build. As the aeroplane moves through the air, it makes noise simply by disturbing the air. That noise emanates outwards in the form of pressure waves. These pressure waves stream out away from the aircraft at the speed of sound, in all directions — just like the ripples through water when a stone is dropped in the middle of a still pond. But here’s the key change. As the aeroplane approaches the speed of sound, it actually starts catching up with its own pressure waves in front of it. Those pressure waves turn into one big pressure shock wave, which causes a loud bang — that’s the sonic boom. And this shock wave doesn’t just make noise. It actually buffets the aeroplane and decreases the lift force. That’s a critical performance consequence: as you approach the speed of sound, the shock wave generation reduces your lift. So to summarise what we’ve covered: true ground speed is your velocity over the ground, equal to true airspeed plus or minus the wind component. And at speeds above about 220 knots, compressibility starts to matter, and as you approach the speed of sound, pressure waves pile up into a shock wave that causes a sonic boom, buffets the aircraft, and reduces lift.