# Gurney flap / wickerbill

> What is a Gurney flap (wickerbill) and how tall should it be?

- Category: Car design › Car aero
- Kind: Standard part
- Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
- Page: https://xemstudio.com/objects/gurney-flap-wickerbill/ · JSON: https://xemstudio.com/api/geometry?object=gurney-flap-wickerbill · Plain text: https://xemstudio.com/objects/gurney-flap-wickerbill/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files Xem Studio Design License (free to use and sell from, not to redistribute as files: https://xemstudio.com/license/)
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- 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.

## Sizes

| 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 |

## Where the files are

- NASA (NTRS): [A Water Tunnel Study of Gurney Flaps, NASA TM-4071 (Neuhart & Pendergraft, 1988)](https://ntrs.nasa.gov/api/citations/19890004024/downloads/19890004024.pdf) — web — Publisher's page; dimensions are facts
- Wikipedia: [Gurney flap](https://en.wikipedia.org/wiki/Gurney_flap) — web — Publisher's page; dimensions are facts
- High Performance Academy: [Aerodynamic performance devices for club level cars: rear wing theory](https://www.hpacademy.com/courses/aerodynamics-fundamentals/aerodynamic-performance-devices-for-club-level-cars-rear-wing-theory) — web — Publisher's page; dimensions are facts
- High Performance Academy: [Aerodynamic performance devices for club level cars: front splitters](https://www.hpacademy.com/courses/aerodynamics-fundamentals/aerodynamic-performance-devices-for-club-level-cars-front-splitters) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, Xem Studio Design License, Xem Studio)

Index: https://xemstudio.com/files/hardware/gurney-flap-wickerbill/index.json · license: https://xemstudio.com/files/hardware/gurney-flap-wickerbill/LICENSE.md

- gurney-flap-wickerbill-rear-wing: [STEP](https://xemstudio.com/files/hardware/gurney-flap-wickerbill/gurney-flap-wickerbill-rear-wing.step) · [STL](https://xemstudio.com/files/hardware/gurney-flap-wickerbill/gurney-flap-wickerbill-rear-wing.stl) — gurney rear wing: span 1600 × chord 250 mm at 6°, endplates 300 mm, inverted NACA 4412 section (mounts from below)

## Questions

**How tall should a Gurney be?**

Liebeck used 1.25% of chord and expected drag penalties above that; 1-2% chord is the commonly cited range.

**Why is it called a wickerbill?**

Wickerbill is another name for the same trailing-edge tab (Wikipedia).

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- [Wing endplates / side dams](https://xemstudio.com/objects/wing-endplates/)
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