Washout is a built-in twist that sets the wing tip at a lower angle of attack than the root, so the root reaches its critical angle first and the outer wing, where the ailerons are, is still flying when the stall begins. The amount is a design value for a specific wing and planform and must be taken from a proven design, not guessed. It is checked by laying a straightedge on the ribs and reading each rib’s angle with an inclinometer, and both wings must match.
Part of Aerodynamics and Flight
Deliberate and accidental twist along the span of a wing, why it governs which part stalls first, and how it is measured.
A Stall Starting At The Tip Takes Away The Ailerons
If the tip stalls first, the outer wing loses lift on one side before the other and the aircraft rolls, and the ailerons, which sit on the stalled part, cannot correct it. If the root stalls first, the nose drops and the ailerons still act on air that is flowing. Washout arranges that order by making the tip meet the air at a smaller angle than the root. It rests on the fact that stall is a matter of angle of attack (Stall Dynamics), not of how curved the wing is (Airfoil Geometry).
Washout can be geometric, with the tip rib set at a lower incidence than the root, or aerodynamic, with a different section at the tip. The first is simpler for a builder. The amount needed depends on the planform (Wing Planforms); the carrier names the purpose and gives no figure, and this article does not supply one.
What You Need
- A straight, stiff straightedge longer than the chord, or a flat bar.
- An inclinometer or a bevel with a protractor. Angle Measurement covers the workshop versions and their resolution; the field-survey and compass versions of that title are different articles.
- A level jig or stand that holds the wing in its flying attitude.
- A written rib station list.
Method: Measure The Twist At Every Rib
Step 1: Support the wing on its fittings in the attitude it will fly. A wing resting on its tips will sag and twist.
Step 2: Choose a datum. The root rib’s chord line is the usual reference.
Step 3: At each rib, lay the straightedge along the chord line and read the angle. Record the station, the angle and the date.
Step 4: Compare left to right. The two wings must match at every station. A difference rolls the aircraft one way at all speeds.
Step 5: Compare to the design. The twist along the span should change smoothly, not step.
Worked Example
If the root reads 3.0° and the tip 1.0° from the same datum, the washout is 3.0° − 1.0° = 2.0°. The values are illustrative, not a recommendation. If the left wing’s tip reads 1.0° and the right 1.6°, the aircraft carries 0.6° of built-in asymmetry, which will have to be trimmed out with a permanent control offset, an error to fix in the structure rather than in flight.
Twist Also Happens Without Being Built In
- Fabric tension: doping shrinks the covering and can wind a light structure out of true (Fabric Covering).
- Aerodynamic load: a wing twists under lift, and a weak or open section twists more. Torsional stiffness comes from the skin and the rear spar.
- Drying and moisture: timber changes shape; re-measure after the first season.
- Rigging: wires or struts that are tightened unevenly twist the wing.
Common Mistakes
- Washout measured with the aircraft resting on its gear — flex in the structure falsifies the reading.
- One wing measured, the other assumed — a symmetrical wing is a measurement, not a hope.
- Adding washout to cure a tip stall without checking the structure — twist moves the loads.
Safety
Wing asymmetry and tip stall are both flight-test discoveries nobody wants
Check twist and rigging on the ground before every flight series. If the aircraft shows any tendency to roll at the stall, stop flying and re-measure.
What’s Next
- Learn next: Control Surfaces, what the washout protects
- Combine with: Fabric Covering, the usual cause of unplanned twist