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If you look closely at the wings on most aircraft, they're tilted up slightly. So why would they ever do that? It's not because you pulled too many Gs on your last flight. It's because of something called dihedral.
Dihedral sounds like one of those words you cringed at in math class, but it's actually really simple. Dihedral is the upward angle your aircraft's wings. Here's a great example of wing dihedral on a Boeing 777:
See how the 777's wings angle upward? That dihedral makes the jet more laterally stable, or in other words, more stable when it rolls left or right. And it's not just large jets that have dihedral like this. It's found on almost every aircraft out there.
It all comes down to stability. If you didn't have dihedral, you'd spend a lot of time keeping your wings level. Here's why:
When you bank an airplane, the lift vector tilts in the same direction as the bank. And when that happens, your airplane starts slipping in the same direction, in this case, to the right.
The problem is, if you have a straight-wing aircraft, there's no force that will bring the airplane back to wings-level flight without you intervening. And while that may be good for an aerobatic aircraft or fighter jet, it's not something you want in your general aviation aircraft or airliner.
When you add dihedral, you add lateral stability when your aircraft rolls left or right. Here's how it works: let's say you're flying along and you accidentally bump your controls, rolling your plane to the right. When your wings have dihedral, two things happen:
1) First, your airplane starts slipping to the right. That means the relative wind is no longer approaching directly head-on to the aircraft, and instead is approaching slightly from the right. This means that there is a component of the relative wind that is acting inboard against the right wing.
2) Second, because the relative wind has the inboard component, and because the wings are tilted up slightly, a portion of the the relative wind strikes the underside of the low wing, pushing it back up toward wings level. What's really happening here is the low wing is flying at a higher AOA, and producing more lift.
The more dihedral your aircraft has, the more pronounced the effect becomes. But for most aircraft, they only have a few degrees of dihedral, which is just enough to return your wings to level during small disturbances, like turbulence, or bumping your flight controls in the cockpit.
Dihedral isn't always good, and like almost every design factor, it comes with a cost. In this case, there are two costs: increased drag, and decreased roll rate.
Wings with dihedral don't produce lift completely vertically. There's a vertical component, and a horizontal component. So when you're flying straight and level, your lift is not 100% vertical.
And, the same dihedral effect that keeps your wings level in turbulence, works against you when you try to roll right or left. When you put an aircraft into a bank, the dihedral effect constantly tries to return your wings to level. And with enough dihedral, your roll rate can be dramatically decreased. While that's good for stability, it's bad for maneuvering.
Dihedral lets you fly more hands off, even in turbulence. And a more hands off, stable airplane is good for everybody. Especially when you're trying to manage multiple things in the cockpit, like your checklist, your iPad, and maybe even your coffee.
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at firstname.lastname@example.org.