FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsMedium
An older-generation swept-wing transport is equipped with wing fences—thin, chordwise vertical plates mounted on the upper wing surface. What is their primary aerodynamic purpose?
- ATo impede spanwise (outboard) boundary layer flow and delay wingtip stall
- BTo re-energize the boundary layer in the same manner as vortex generators
- CTo reduce structural bending loads on the wing during turbulence
- DTo reduce induced drag by acting as endplates similar to winglets
Show answer & explanationAnswer & explanation
Correct answer: A. To impede spanwise (outboard) boundary layer flow and delay wingtip stall
On swept wings, spanwise pressure gradients cause the boundary layer to migrate outboard toward the tips, thickening it and promoting early tip stall. Wing fences act as physical barriers to block this spanwise flow, helping the wing stall root-first, which preserves aileron effectiveness longer.
Why the other options are wrong
- B. Vortex generators add energy to the boundary layer via small vortices, unlike fences which act as barriers.
- C. Fences are aerodynamic, not structural, devices and don't reduce bending loads.
- D. Winglets reduce induced drag by managing tip vortices, a different mechanism than fences.
Wing Fences
Chordwise plates on a swept wing's upper surface that block spanwise boundary-layer flow to delay wingtip stall.
- Combat spanwise flow caused by wing sweep
- Promote root-first stall progression
- Common on older swept-wing designs (e.g., MiG-15, early jets)
Memory trick: Fences build a fence to stop the boundary layer from running to the tip.