Wash-In
Wash-in is the opposite of washout: the angle of attack at the tip is greater than at the root. It is a rarer design measure.
Definition
Tip α > root α — the wing has a positive geometric twist (tip nose-up relative to root).
Purpose — when used?
1. Gliders with elliptical lift distribution
With elliptical lift distribution, the tip is more loaded than with rectangular. Wash-in can deliberately boost tip lift to achieve an ideally elliptical distribution — for specific wing geometry.
2. Propeller-effect compensation (P-factor)
The slipstream of a propeller aircraft hits the right wing (in most US/EU configurations) with higher flow than the left. The right wing generates more lift → unbalanced roll moment.
Wash-in on the left wing compensates: left wing has higher α → more lift → balances slipstream effect on the right.
Examples: some WW2 fighters (e.g. Hurricane, Spitfire — slight wash-in on inner left wing).
3. Special purpose
- Aerobatic trainer: precise roll character partly requires wash-in.
- High-speed aircraft with sweep: wash-in can prevent tip stall.
Risks of wash-in
Negative stall characteristic without further measures:
- Tip stalls earlier than root → aileron effectiveness lost.
- Spin risk elevated.
- Hence very rare in general aviation — washout preferred.
When is wash-in acceptable?
- When stall behaviour is otherwise controlled:
- Leading edge slats at tip (extending α_stall).
- Wing fences for flow control.
- Very early stall warning systems.
- Gliders with specific design goals.
Practical PPL relevance
For standard trainers (Cessna 172, PA-28, DA-40, DR400): washout, not wash-in. This lesson is mainly for completeness and to understand the terms.
Aerodynamic mathematics
With wash-in, the local lift coefficient at the tip rises → lift distribution shifts outboard → induced drag can be reduced (if lift distribution is closer to elliptical).
However:
- Stall behaviour endangered without compensation.
- Design advantage minimal for low to medium aspect ratio.
Comparison Washout / Wash-In
| Aspect | Washout (−twist) | Wash-In (+twist) |
|---|---|---|
| Tip α | < root α | > root α |
| Stall starts | root | tip |
| Aileron effect at stall | retained | lost |
| Use in PPL aircraft | standard | very rare |
| Lift distribution | less ideal | closer to ideal (elliptical) |
| Spin risk | low | elevated without compensation |
Summary
- Wash-in = tip α > root α.
- Rare in normal GA.
- Specific applications: gliders, propeller-effect compensation, special designs.
- Risk: tip stalls first → aileron loss → spin.
- Compensation required: slats, fences, or wash-in only as part of a coordinated concept.