Gurney flap / wickerbill — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/gurney-flap-wickerbill/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
Catalog version: 2026-10-20

Sizes: see table below
- A Gurney flap, also called a wickerbill, is a small tab at the trailing edge set at right angles to the pressure side (Wikipedia, citing Van Dam and Storms & Jang). It raises lift by changing the trailing-edge Kutta condition and creates a pair of counter-rotating vortices behind it.
- Liebeck tested a 1.25%-chord flap (0.0125c) and found better lift and, unexpectedly, lower drag; he stated that flaps much taller than about 0.0125c would probably show drag penalties (NASA TM-4071).
- Dan Gurney first used it in 1971 on Bobby Unser's car; HPA describes it as a way to add downforce once wing angle is maxed out.
- Used on: Eagle Indy cars (Dan Gurney, 1971), Sikorsky S-76B horizontal stabiliser, Bell JetRanger.
- The file is a reference example built by Xem Studio from the numbers above (gurney_h = 1.25% of 250 mm chord (Liebeck).). Sizes the sources do not fix are illustrative; scale the example to your own vehicle.

parameter  value or range  source
Liebeck test height  0.0125c (1.25% chord)  NASA TM-4071
Height above which drag penalty expected (Liebeck)  > ~0.0125c  NASA TM-4071
Typical height  1-2% of chord  Wikipedia (Storms & Jang 1994)
Club-car wing gurney  about 10-20 mm tall  HPA rear wing
Splitter edge gurney/upkick  about 20-30 mm  HPA splitter
First use  1971 (Dan Gurney, Bobby Unser)  Wikipedia
Liebeck paper introducing it  AIAA 76-406 (1976)  Wikipedia

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- NASA (NTRS): A Water Tunnel Study of Gurney Flaps, NASA TM-4071 (Neuhart & Pendergraft, 1988) — web — Publisher's page; dimensions are facts — https://ntrs.nasa.gov/api/citations/19890004024/downloads/19890004024.pdf
- Wikipedia: Gurney flap — web — Publisher's page; dimensions are facts — https://en.wikipedia.org/wiki/Gurney_flap
- High Performance Academy: Aerodynamic performance devices for club level cars: rear wing theory — web — Publisher's page; dimensions are facts — https://www.hpacademy.com/courses/aerodynamics-fundamentals/aerodynamic-performance-devices-for-club-level-cars-rear-wing-theory
- High Performance Academy: Aerodynamic performance devices for club level cars: front splitters — web — Publisher's page; dimensions are facts — https://www.hpacademy.com/courses/aerodynamics-fundamentals/aerodynamic-performance-devices-for-club-level-cars-front-splitters

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/gurney-flap-wickerbill/index.json
API: https://xemstudio.com/api/geometry?object=gurney-flap-wickerbill