(Credits to Bold Method and Colin Cutler) Re-edited Julian Smith.
At first glance, it looks like the wings are straight on most light aircraft. But that’s not actually the case. Almost all aircraft have something called washout built into their wings, and it makes them more stable in a stall.
What is washout?
So what is washout? It’s a change in an aerofoil’s angle of incidence, measured from the root to the tip. If you look closely at a wing with washout, you’ll see that it twists from the root to the tip, with the root having a higher angle of incidence than the tip.
What does this mean when you’re flying? It means that at any airspeed, at any attitude, the root of the wing root will fly at a higher angle-of-attack than the wing tip.
Why do wings need washout?
Why would engineers want to have an airplane flying around with twisted wings? It has everything to do with a stall. When the root (inboard section) of a wing flies at a higher angle-of-attack, it also means the root will reach the critical angle-of-attack sooner than the wingtip and it will stall first.
A root stall is what you want to happen in nearly all airplanes.
When an aircraft stalls at the root first, it means there’s enough airflow over the tips of your wings to prevent any rapid rolling motion during a stall, which makes the airplane more stable. It also makes your plane more resistant to entering a spin. On top of that, a root stall also guarantees some aileron effectiveness during the stall, giving you greater control of your plane, especially if the stall occurs in a banked turn.
If your plane didn’t have washout, in most cases it would mean the entire wing would stall at once, or worse yet, the wing tip could stall first if your ailerons were deflected. And if that happened your plane could aggressively roll left or right during the stall, and possibly enter an incipient spin.
How much washout is built in?
The amount of washout built into a wing is highly dependent on the type of aircraft, but in most cases, washout from the root to the tip of a wing is only a few degrees.
Take the F-18 for example. Its wing washout is approximately 4 degrees:
That means if the root of the F-18’s wing is flying at 10 degrees angle-of-attack, the tip is flying at only 6 degrees AOA. The Cessna 172’s wing is pretty similar, with a washout of approximately 3 degrees.
Other types of washout
While wing twist is the most common type of washout, it’s not the only way to do it. Some aircraft, like the Cirrus SR-20 and SR-22, use a “double cuff” wing design to prevent the wing tip from stalling first.
On a Cirrus wing, the outboard cuffed portion of the wing has a lower angle of incidence, meaning it flies at a lower angle of attack. When the airplane stalls, the inboard portion of the wing stalls first, and the outside cuffed portion continues to have non-separated airflow over the wing tips and ailerons, allowing aileron authority and more stability throughout the stall.
More stability in a stall
No matter what you fly, it’s likely that your wing has washout built into it.
It also means the next time you’re out practicing stalls, you’ll know your aircraft is designed to help you keep the wings level, even when the stall warning horn goes off.