A first aircraft should have a rectangular wing, because every rib is identical, the structure is simple and the wing tends to stall at the root first, which leaves the ailerons working. Tapered and elliptical planforms lose less to induced drag and carry bending more efficiently, but their ribs all differ, and a tapered wing tends to stall at the tip first unless it is twisted to prevent it. The planform changes the stall pattern as much as it changes performance.
Part of Aerodynamics and Flight
The outline of a wing seen from above, and what each outline does to drag, stall behaviour and the work of building it.
The Three Planforms And What Each Trades
| Planform | Gains | Costs |
|---|---|---|
| Rectangular | Every rib identical, simplest jig, root stalls first | More induced drag for the same span; heavier structure for the same strength |
| Tapered | Less induced drag, less weight out at the tip, stronger root | Every rib a different size; tip tends to stall first |
| Elliptical | Lowest induced drag for a given span and lift | Curved edges, every rib different, hardest to build accurately |
The carrier (Aerodynamics and Flight) recommends rectangular for a first aircraft and tapered for a second, with elliptical for advanced builders, and this agrees with the trade-offs above.
Why The Outline Changes The Stall
Lift is spread along the span in a pattern set by the planform and the twist. In a rectangular wing the section at the root carries a larger share of the lift coefficient than the tip, so the root reaches its stall angle first. In a strongly tapered wing the tip has a short chord and runs at a higher lift coefficient, so it can stall first. A tip stall takes out the ailerons and drops a wing at once (Stall Dynamics). The remedy is washout, a built-in twist that lowers the angle at the tip (Wing Twist).
Elliptical Is An Ideal, Not A Recipe
The elliptical distribution of lift gives the least induced drag for a given span; a wing of elliptical outline is one way to approach it, and a moderately tapered wing gets part of the way. The exact benefit depends on the span loading and is not given here; see Aspect Ratio.
Structure Follows The Outline
A rectangular wing’s spar must carry the full bending at the root and is the same depth all the way, which wastes material outboard. A tapered wing lets the spar thin toward the tip, as the bending falls (Structural Spars). That saving is real, and it is paid for with a template for every rib. The carrier’s rib jig, a single board with nails marking the outline, serves identical ribs; ribs of changing size need a separate jig for each.
Common Mistakes
- Choosing taper for efficiency on a first aircraft — you gain a small amount of drag and take on tip-stall risk and extra building error.
- Taper without washout — the combination is the one that drops a wing at the stall.
- Adding rib patterns without recording them — number every rib and its station; an out-of-order rib is an invisible error.
Safety
The planform decides which wing drops first
A wing that stalls tip first can roll over before the pilot has felt a warning, and at low height there is no room to recover. Do not alter a proven planform on the basis of a drag estimate.
What’s Next
- Learn next: Aspect Ratio, span against chord
- Combine with: Wing Twist, the cure for a tip stall