General Principles - Cruise — Page 256, Lesson 314

Good, we're moving into the wind section of cruise performance. This is the next major factor that affects range, right after weight and altitude. Let's get into it. Wind is a big one. A headwind will cause the aircraft to travel slower over the ground. That means for a given level of fuel consumption, you cover less distance. So, in a headwind, range is reduced. That's the fundamental problem. Now, to minimize this effect, we don't just accept the loss. We increase the speed of the aeroplane. But here's the key detail: we increase it by a margin that is slightly less than the amount of the headwind. So if you have a 20-knot headwind, you might increase your speed by, say, 15 or 18 knots — not the full 20. Why do we do that? Because increasing speed increases thrust and power required, which increases fuel consumption. That's the downside. But the positive side is that the aeroplane is exposed to the headwind for a shorter time period if it's flying faster. So this higher speed recovers some of the range loss caused by the headwind. It's a trade-off — you burn a bit more fuel per hour, but you spend less time fighting the wind. Now the opposite case. A tailwind will increase your ground speed. For a given level of fuel consumption, you cover more distance, so range increases. For maximum range with a tailwind, we do the reverse: we decrease the speed for best range slightly. This reduces thrust and power required, which reduces fuel flow, and that increases range a little more. And again, note the symmetry: the reduction in speed is slightly less than the speed of the tailwind component being experienced. Let me show you this graphically. — actually, let me use the right figure. — no, the one we need is Figure 5.24, which is the headwind effect on the speed for maximum range for a jet aeroplane. Here's how to read it. We're looking at the drag curve for the jet aeroplane. The key concept is the tangent line. In zero wind, the speed for best range is where the tangent from the origin touches the drag curve. Now, with a 20-knot headwind, the origin of the tangent line moves 20 knots to the right. The tangent then meets the curve at a point corresponding to a higher speed. That confirms what I said: in a headwind, the speed for best range is higher. The opposite happens in a tailwind. With a tailwind of 20 knots, the origin of the tangent line moves 20 knots to the left. The tangent meets the curve at a point corresponding to a lower speed. That confirms: in a tailwind, the speed for best range is lower than in zero-wind conditions. So the whole logic hangs on that tangent-line construction. The origin shifts by the wind component — right for headwind, left for tailwind — and the tangent point slides along the drag curve to give you the new best-range speed. Higher in a headwind, lower in a tailwind. That's the core of wind's effect on range. Ready to move on when you are.