# Gurney flap

> What is a Gurney flap?

- Category: Aircraft design › Wing features
- Kind: Standard part
- Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
- Page: https://xemstudio.com/objects/gurney-flap/ · JSON: https://xemstudio.com/api/geometry?object=gurney-flap · Plain text: https://xemstudio.com/objects/gurney-flap/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 is a small strip perpendicular to the trailing edge on the pressure side. Liebeck's 1978 tests used about 1.25% chord and found significant lift and drag improvement, but expected drag penalties above that height (NASA TM-4071).
- On the S1223 high-lift airfoil a 1% chord Gurney raised CLmax from 2.2 to 2.3 with similar stall (Selig & Guglielmo). On an advanced airfoil at Re 3e6 a 1.25% flap raised lift substantially but with very large drag penalties at low and moderate CL (NASA TM-4071).
- A 5% chord strip gave more max lift but a significant drag increase (Roesch & Vuillet in NASA TM-4071).
- The file is a reference example built by Xem Studio from the numbers above. Sizes the sources do not fix are illustrative; scale the example to your own vehicle.

## Sizes

| parameter | value or range | source |
|---|---|---|
| Liebeck height | about 1.25% chord | NASA TM-4071 |
| S1223 + 1% Gurney | CLmax 2.2 -> 2.3 (Re 2e5) | Selig & Guglielmo 1997 |
| 0.0125c strip on helicopter stabiliser | drag unchanged, lift up | NASA TM-4071 (Roesch & Vuillet) |
| 0.05c strip | more CLmax, significant drag increase | NASA TM-4071 |
| Reference build | chord 100, 1.25 mm tab, 90 deg | Liebeck height |

## Where the files are

- NASA Langley (Neuhart & Pendergraft): [A Water Tunnel Study of Gurney Flaps, NASA TM-4071 (1988)](https://ntrs.nasa.gov/api/citations/19890004024/downloads/19890004024.pdf) — web — Publisher's page; dimensions are facts
- University of Illinois (Selig & Guglielmo), Journal of Aircraft 1997: [High-Lift Low Reynolds Number Airfoil Design (S1223)](https://m-selig.web.engr.illinois.edu/pubs/GuglielmoSelig-1997-JofAC-S1223.pdf) — 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/index.json · license: https://xemstudio.com/files/hardware/gurney-flap/LICENSE.md

- gurney-flap-wing-segment: [STEP](https://xemstudio.com/files/hardware/gurney-flap/gurney-flap-wing-segment.step) · [STL](https://xemstudio.com/files/hardware/gurney-flap/gurney-flap-wing-segment.stl) — Wing segment with a Gurney flap 2 mm high (2 % chord) at the trailing edge

## Questions

**How tall should a Gurney flap be?**

Around 1-1.25% chord; taller adds drag (Liebeck via NASA TM-4071).

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