Wings: Planform, Aspect Ratio, Wingtip Devices, High-Lift Devices, Sweep, Dihedral and Anhedral
This domain tests the finite wing: how planform sets induced drag and the stall pattern, and how sweep, dihedral and high-lift devices change the trade-offs. Items often compare configurations or ask what a device does to CLmax, drag and pitching moment.
Planform and aspect ratio
- AR = b²/S. A high aspect ratio reduces induced drag, since CDi = CL²/(πAe).
- An elliptical spanwise lift distribution gives minimum induced drag for a given span, but it tends to stall along the whole span at once.
- Taper moves the stall inboard, but with sweep and insufficient washout it can still produce tip stall and wing drop.
Wingtip devices
- Winglets recover part of the tip vortex energy, reducing induced drag. They add wetted area and structural load, and can alter directional stability.
High-lift devices
- Flaps increase camber (and, on Fowler types, area), raising CLmax. They also add drag and normally produce a nose-down pitching moment that must be trimmed out.
- Slats and slots re-energise the upper-surface boundary layer, delaying separation to a higher alpha; they increase CLmax and the stalling angle.
- Spoilers destroy lift and add drag; they are not high-lift devices.
Sweep, dihedral and anhedral
- Sweepback raises the critical Mach number, reduces the lift-curve slope and increases the stalling speed. It also produces a dihedral effect and can cause pitch-up at the stall.
- Dihedral gives lateral static stability through sideslip; anhedral is used on high wings or T-tails to reduce an excessive dihedral effect.
Worked example
A wing has span 20 m and area 40 m². AR = 400/40 = 10. With CL 0.5 and Oswald efficiency 0.8, CDi = 0.25/(π × 10 × 0.8) = 0.00995. The same wing with AR 5 gives 0.25/(π × 5 × 0.8) = 0.0199 - double the induced drag coefficient for the same lift. That is why gliders are long and narrow.
Traps
- Aspect ratio uses b², not b.
- Flap extension increases CLmax but also drag and pitching moment; approach performance depends on the whole configuration.
- Sweep reduces CLmax and raises approach speed; it is not a free improvement.
- Winglets reduce induced drag only; they do nothing for profile drag.
- Dihedral effect and lateral stability are related but not the same thing as roll control.