Planform

Wing Planform

The planform is the shape of the wing seen from above. Different planforms have very different aerodynamic and structural characteristics.

Main forms

1. Rectangular planform

• Constant chord along span.
• Lift distribution: higher at root, uneven toward tip.
• Stall behaviour: root stalls first — benign stall, ailerons remain effective.
• Examples: Cessna 172, PA-28 (with mild taper), many trainers.
• Pros: simple construction, good stall behaviour.
• Cons: higher induced drag, heavier for same span.

2. Tapered planform

• Chord decreases from root to tip.
• Lift distribution: approaches elliptical → more efficient.
• Stall behaviour: tip can stall first (dangerous, aileron effectiveness lost — washout required).
• Examples: Cessna 152, Beechcraft Bonanza, many modern aircraft.
• Pros: lower induced drag, lighter structure.
• Cons: more complex; stall handling via washout.

3. Elliptical planform

• Chord follows an ellipse along the span.
• Lift distribution: mathematically ideal (Prandtl theory) → minimum induced drag for given span and weight.
• Examples: Supermarine Spitfire (WW2), De Havilland Mosquito.
• Pros: aerodynamically optimal.
• Cons: very labour-intensive manufacture (curved ribs, varying airfoils by position). In modern mass production usually replaced by tapered.

4. Trapezoidal / swept wing

• Leading edge swept rearward.
• Lift distribution: biased to tip → tip stall first.
• Examples: all modern airliners (sweep 25–35°).
• Pros: higher Mach numbers without wave drag (relevant above M > 0.3).
• Cons: tip stalls first → stick-pusher in many large aircraft.

5. Delta wing

• Triangular.
• Very high AoA possible (lift via vortex from sharp leading edge).
• Examples: Concorde, Mirage, MIG-21.
• Pros: high manoeuvrability, high Mach.
• Cons: high induced drag at low speed, poor for take-off/landing.

6. Delta with canard

• Delta with small forward control surfaces (canard).
• Examples: Eurofighter Typhoon, Saab Gripen, Dassault Rafale.
• Pros: even more agile manoeuvring.

Sweep angle

Λ (lambda) = sweep angle of the leading edge or quarter-chord.

• Λ = 0°: straight wing (Cessna).
• Λ = 5–10°: mild sweep (B737 outboard).
• Λ = 25–35°: typical transport category (A320, B787).
• Λ = 45–60°: high-speed fighters (F-104).

Lift distribution — Prandtl theory

Ludwig Prandtl (1875–1953) showed: minimum induced drag arises with elliptical lift distribution along the span.

Actual lift distribution depends on planform:

• Rectangular: lift higher at root.
• Tapered: approaches elliptical.
• Elliptical: ideal.

Stall behaviour by planform