Washout is a built in twist in an aircraft wing that makes the wing tip meet the airflow at a smaller angle than the wing root. This matters because stalls often become dangerous when the tip stalls first and the ailerons lose their grip on the air. By making the root stall first, washout helps the pilot keep roll control during slow flight, landing, and stall recovery.
The idea is simple, but it is one of the most important safety features in wing design.
Understanding Aviation: Washout and Wing Twist
A wing does not experience one single airflow condition from root to tip. The fuselage changes the air near the inner wing. The propeller slipstream can change it further on many light aircraft.
Near the tip, air tends to curl around from the high pressure lower surface toward the low pressure upper surface. This creates a wingtip vortex and changes the local direction of the flow.
The local angle of attack depends on that flow direction, not just on how high the aircraft nose appears. Wing designers therefore need to consider the whole span, especially when the aircraft is slow and producing a large amount of lift.
The pattern of lift across the wing matters as much as the total lift. A highly loaded outer wing section can reach separation suddenly. On swept wings, air has a tendency to move outward along the wing.
That movement can make the outer sections more likely to lose smooth airflow. Twist helps counter this tendency, but it is not a complete solution by itself.
Designers may combine it with a carefully chosen airfoil, a change in wing shape, stall strips near the root, or small fences that limit outward airflow. A well behaved stall gives warning through buffet and a noticeable change in control feel before the aircraft loses too much lift.
Roll control becomes especially important close to the ground. During a crosswind landing, the pilot may need to lower one wing with the ailerons while flying slowly. The downward moving aileron increases lift demand on its wing section.
It effectively asks that section to operate at a higher angle of attack. If that section is already near its limit, the extra demand can start separation there. The result can be a sudden roll in the wrong direction.
A twisted wing gives the outer section more operating margin, so the aileron can still change the lift on that part of the wing. This does not make slow flight risk free, since abrupt control inputs can still cause a stall.
Wing twist involves tradeoffs. Reducing the angle at the tip usually reduces the lift made near the tip during normal flight. That can change drag, structural loads, and the size of the tail needed to trim the aircraft.
Some wings use geometric twist, meaning the physical structure is rotated along the span. Others use aerodynamic twist, where the shape of the airfoil changes even if the structure looks nearly straight. Students should keep angle of attack separate from pitch attitude.
An aircraft can have a low nose attitude and still stall in a steep turn because increased load factor requires more lift. They should also remember that a published stall speed assumes particular conditions. Bank angle, weight, turbulence, flap setting, and control use can all change how close each wing section is to its limit.
Key Facts
- Washout means the wing tip has a lower angle of incidence than the wing root.
- Effective angle of attack can be estimated as αeffective = αaircraft + i, where i is local wing incidence.
- Geometric washout angle is Δi = iroot - itip.
- A wing section stalls when its local angle of attack exceeds its critical angle of attack.
- With washout, the root reaches αcritical before the tip, so the stall begins near the fuselage.
- Ailerons are usually near the wing tips, so keeping tip airflow attached helps maintain roll control.
Vocabulary
- Washout
- Washout is a deliberate wing twist that lowers the angle of incidence from the wing root to the wing tip.
- Wing root
- The wing root is the part of the wing closest to the fuselage.
- Wing tip
- The wing tip is the outer end of the wing, farthest from the fuselage.
- Angle of incidence
- Angle of incidence is the fixed angle between a wing chord line and a reference line of the aircraft.
- Tip stall
- A tip stall occurs when the outer part of the wing stalls before the root, often reducing aileron effectiveness.
Common Mistakes to Avoid
- Confusing washout with dihedral is wrong because washout is twist along the wing span, while dihedral is the upward angle of the wings when viewed from the front.
- Assuming the whole wing stalls at once is wrong because each section has its own local angle of attack and can stall at a different time.
- Thinking washout increases lift everywhere is wrong because the lower tip incidence usually reduces tip lift at a given aircraft attitude.
- Ignoring aileron location is wrong because ailerons are commonly near the tips, so a tip stall can remove much of the pilot's roll control.
Practice Questions
- 1 A wing root has an incidence of 4 degrees and the wing tip has an incidence of 1 degree. What is the geometric washout angle Δi?
- 2 An aircraft is flying at an angle of attack of 10 degrees. The root incidence is 3 degrees and the tip incidence is 0 degrees. Using αeffective = αaircraft + i, find the effective angle of attack at the root and at the tip.
- 3 Explain why a wing with washout can be easier to control during the beginning of a stall than a wing with no twist.