Aviation is awash in weird terms and expressions that non-aviation people just don’t use much. For instance, consider the term, “relative wind.” Use the term around a pilot and they’ll nod and give every impression they understand entirely. Use it with a non-aviation enthusiast on the other hand, and they’ll scratch their head and start to wonder about your sanity.
Relative wind is critical to flight, so if you want to be a pilot, it’s important that you know what it is, and what it isn’t. So here’s the long and the short of it. Relative wind is the apparent speed and direction of the wind, based on your perspective. It is not the actual speed and direction of the wind.
Think of it this way. It’s a beautiful summer day, and you’re sitting ata stop light in a super-sharp, mega powerful sports car. The sun is shining, the wind is calm, and the birds are singing in the trees. The light turns green, your foot hits the floor, and before you know it you’re screaming down the street at 100 mph. When you stick your arm out the window (which I don’t advise under the circumstances), you find you’re having difficulty holding it still because the wind is so strong. In fact, you’re barely able to keep the howling wind from tearing your arm out of its socket and pinning it back along the length of the car.
Then you hit another stop light. You jam on the brakes, screech to a halt, and find the wind is nonexistent again.
What’s up with that? Well, that’s the relative wind, my friend.
When you’re stopped the wind is calm. Because the wind truly is calm. When you’re ripping along the road at breakneck speed however, the wind appears to be howling. In fact, that air is standing still, but you’re passing through it so quickly it feels to you as if there is a powerful wind. Hence the term, relative wind. Because the speed and direction of the wind are relative to your own movement.
Now what if the wind isn’t calm? Well that’s simple, too. You either add, or subtract the wind speed from the speed you’re moving, and that results in the relative wind. Say for instance the wind is out of the north at 20 mph, and you just happen to be headed north at 100 mph. Well that makes the relative wind 120 mph – for you. You add the wind speed to your speed, and that’s the relative wind for you. On the other hand, if your best friend is travelling south at the exact same time, in the exact same place, the relative wind they see is only 80 mph, because he (or she) had to get up to 20 mph on a southerly heading before the wind become calm. Until then they had a tail wind (literally, a wind blowing from behind you).
You see, if the wind is going south at 20 mph, and you’re going south at 20 mph, then as far as you can tell the wind is calm, even though somebody standing on the street corner thinks it’s blowing at 20 mph. You’re both right. For them it is blowing at 20 mph, but for you it’s calm, and for your friend travelling in the opposite direction the wind clocks in at 40 mph.
Since the wing of an airplane generates lift as a result of the speed and angle of the air moving over it, it stands to reason that something as weird and unknown outside of aviation, like relative wind, would be important to those fun-loving few who fly. And since you read this blog, you’ll now know what they’re talking about when the term, “relative wind,” comes up.
Next time we’ll get into that angle thing. Specifically, we’ll talk about, “angle of attack.” Trust me, it’s easier than it sounds. But once you’ve got it, people are going to start thinking you’re sharp as a tack. Get used to it. There’s going to be plenty more compliments coming your way as you learn more and more about how airplanes and flight really works.