Understanding Washout
Understanding the twist in your wing
by David Andersen [email protected]
Washout is a built-in twist in a wing that reduces the angle of attack from root to tip (typically 1° to 2°). The intent is to make the root stall before the tip, softening the stall and keeping the ailerons effective deeper into the stall.
Why It Happens
Washout causes the wing root to stall before the wingtips. The resulting loss of lift at the root gently lowers the nose or prevents it from rising, which helps avoid a sudden, violent stall and an unexpected snap roll. Note that stalls can occur at high airspeeds too — pylon racers, for example, can stall in high‑G turns.
At high angles of attack the ailerons lose effectiveness because both are producing lift; the differential in lift decreases as angle of attack increases. Washout keeps the ailerons meeting the airflow at a lower angle, improving their effectiveness, especially at low speeds.
In a banked turn the down aileron increases lift and drag while the up aileron reduces lift and drag. The difference in wingtip drag tends to yaw the airplane opposite the turn (adverse yaw). Washout reduces adverse yaw for the portions of the ailerons that remain near zero angle of attack, but it is a partial cure. Other measures include aileron differential and engine offset; the best technique is for the pilot to counter adverse yaw with rudder to achieve a coordinated turn (even when inverted).
Washout also reduces wingtip vortices and the associated drag. While overall wing efficiency is often unimportant in models, reduced wingtip drag improves lateral (yaw) stability and rudder effectiveness — important at low speeds and high angles of attack. Because wingtip vortices cannot be eliminated, ailerons are rarely taken all the way to the tip. In highly swept wings, washed‑out wingtips can act like horizontal stabilizers, increasing pitch stability; when carried far enough this concept can eliminate the tail (some flying wings, such as the Northrop N‑9M, exploit this).
Why Not?
Too much washout can be detrimental, especially in inverted flight. Problems include loss of aileron effectiveness, nonuniform roll rate, adverse yaw, surprise snap rolls, and aileron reversal or snatch. For these reasons, full‑scale aerobatic aircraft usually avoid washout so their inverted behavior matches upright behavior and snap rolls remain predictable.
Constant‑chord wings (for example the J‑3 Cub or many STOL aircraft) benefit least from washout because they need all available lift; instead they often use stall strips to soften the stall and shaped wingtips to reduce vortex drag. Biplanes typically set wing incidences so the forward wing stalls before the rear wing; because the ailerons are usually on the rear wing, aileron control remains when the other wing is stalled, so washout is often unnecessary.
Leading‑edge slats can prevent tip stalls and are frequently combined with washout for extra low‑speed control margin. Flaps increase local angle of attack by rotating the chord line and also increase effective washout, improving pitch stability and aileron control at low speeds.
Washout should be avoided in lightweight, flexible wings that cannot resist additional twisting in flight. In such wings the root may produce positive lift while the tip produces negative lift, causing a twisting force that increases washout (aeroelastic divergence). In a dive this can suddenly reverse the twist and lift the entire wing, possibly causing structural failure. Many RC glider failures are attributed to this principle.
A pilot’s instinct to add more aileron in a developing tip stall usually worsens the situation; the correct remedy is to use rudder, not more aileron. Beware of this in inverted climbing turns and victory rolls, particularly in warbird models.
How Much?
The optimum washout varies from zero to several degrees depending on:
- High aspect ratio wings (span/chord) need more washout because thin wingtips tend to stall.
- Tapered wings need more washout proportional to the taper.
- High wing loading requires more washout because of tip‑stall tendency.
- Underpowered aircraft need more washout because they fly at higher angles of attack.
- Thin wings need more washout because they can stall abruptly at low angles.
- Multiengine airplanes need plenty of washout for rudder effectiveness in an engine‑out condition.
- Biplanes generally need less washout (see “Why Not?”).
- Aerobatic airplanes generally need none so upright and inverted behavior match.
- Washout effectiveness decreases as dihedral increases.
For scale models, use the full‑scale aircraft’s washout if known. In general, RC warbirds use roughly 1°–2° of washout, adjusted up or down by the factors above. An RC airplane rarely needs more than about 4°.
Where?
In most designs the wingtip angle of attack should be close to zero in level flight, producing little or no lift in cruise; thus the washout angle is measured relative to the root angle. Washout is usually distributed uniformly from root to tip, but there are exceptions:
- The three‑piece wings of the Mitsubishi Ki‑15, North American AT‑6, and Junkers Ju 87 Stuka have no twist in their center sections; twist begins outboard of the landing gear.
- The Focke‑Wulf Ta 152H high‑altitude fighter has 2° of washout concentrated in the aileron area of its high‑aspect wing.
- Near‑constant‑chord designs (for example the Howard Pete) need little or no washout; a small amount may be introduced by shaping the leading edge of the outermost rib bay.
Methods for adding washout during assembly include:
- Temporary tabs on each rib to hold the required angle.
- Shims of varying height under the spars.
- Tapered, full‑span sticks upon which the ribs rest during assembly.
- Building ribs and spars flat, then raising the trailing edge of the end ribs to twist the structure before sheeting.
- Building open‑structure wings, covering them with heat‑shrink film, then twisting the wing while reheating with a hot‑air gun.
If you discover in flight testing that more washout is needed, a temporary fix is to raise the trailing edge of the aileron panels slightly to add about 1° of washout. For a typical giant‑scale model this is usually less than 3/16 inch. After further testing, lower the ailerons in small increments until neutral if the extra washout is no longer required.
Questions or Comments
I welcome comments and questions. You can contact me via the website listed below or by email.
Thanks to Joe Grice, Scott Russell, Tony Paladino, and Jon Bomers for technical assistance.
—David Andersen [email protected]
Sources
- Minnesota Scale and Giant Scale R/C — www.mnbigbirds.com
Transcribed from original scans by AI. Minor OCR errors may remain.
