General Principles - Cruise — Page 256, Lesson 314

All right, let's get into the next major factor that shapes how far an aeroplane can fly: the wind. We've already looked at how weight and altitude affect range, but wind is the big external one, and it changes the whole picture of what speed you should fly. Here's the core idea. Range is about distance covered over the ground for a given amount of fuel. Wind directly attacks that ground distance. A headwind pushes against you, so your ground speed drops. For the same fuel flow, you cover less ground, so range is reduced. That's the simple, unavoidable penalty. But here's the clever part, and this is what I want you to really lock in. To fight a headwind, you don't just accept the loss. You actually increase your airspeed. And the increase is by a margin slightly less than the headwind speed itself. So if you have a 20-knot headwind, you'd speed up by a little less than 20 knots. Why would you burn more fuel to go faster into a headwind? Think about the trade. Yes, increasing speed increases thrust and power required, which increases fuel consumption. That's the cost. But the benefit is time. By flying faster, you spend less time exposed to that headwind. You get through the bad air quicker. That shorter exposure time recovers some of the range you would have lost. So the net effect is that you sacrifice a little fuel efficiency to buy back distance. Now flip it for a tailwind. A tailwind boosts your ground speed, so you cover more distance for the same fuel burn. Range increases. That's the gift. But to maximise it, you do the opposite of what you might expect. For maximum range with a tailwind, you decrease your airspeed slightly. Again, the reduction is slightly less than the tailwind component. By slowing down, you reduce thrust and power required, which reduces fuel flow. And since the tailwind is already carrying you over the ground, you can afford to fly slower through the air. That lower fuel flow stretches your range a little more. So the rule of thumb is beautifully symmetric: headwind, speed up a bit less than the wind. Tailwind, slow down a bit less than the wind. Now, let's see this on the drag curve, because that's where it becomes visual and precise. We'll focus on the jet aeroplane's drag curve to keep it simple. The key tool here is the tangent line. For a jet, the speed for best range is found where a line drawn from the origin just touches the drag curve — that tangent point gives you the optimum speed. Here's what the wind does to that construction. With a 20-knot headwind, the origin of the tangent line moves 20 knots to the right. So you're no longer drawing from zero; you're drawing from a point that's shifted forward by the wind speed. And look at what happens — the tangent now meets the curve at a point corresponding to a higher speed. That's the graphical confirmation of what I just told you: in a headwind, the speed for best range is higher. Now reverse it. With a 20-knot tailwind, 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 the opposite: in a tailwind, the speed for best range is lower than in zero-wind conditions. So the graph isn't just a picture — it's the proof. The tangent line's origin shifts by exactly the wind component, and that shift forces the tangent point, and therefore your optimum speed, to move in the direction I described. Headwind pushes the optimum speed up; tailwind pulls it down. Let me just make sure the logic chain is airtight for you. The tangent method finds the speed where you get the most range per unit of fuel. When you add a headwind, the ground distance you cover per unit of fuel drops, so the optimum shifts to a higher airspeed to minimise the time spent fighting the wind. When you add a tailwind, the ground distance per unit of fuel rises, so you can afford to slow down and save fuel. The graph shows exactly that shift. That's the whole story of wind and range.