If you look closely at the wings on most aircraft, they're tilted up slightly. Why would they ever do that? It's not because you pulled too many Gs on your last flight. It's because of a design feature called dihedral.
First Off, What's Dihedral?
Dihedral sounds like one of those words you cringed at in math class, but it's actually pretty simple. Dihedral is the upward angle your aircraft's wings. Here's a great example of wing dihedral on a Boeing 777:
Why Do You Need Dihedral?
It all comes down to stability. If you didn't have dihedral, you'd spend more 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.
How Dihedral Fixes The Problem
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 slightly 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.
It's Not All Good News: Dihedral Comes At A Cost
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....
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