# Plain flap

> What is a plain flap and how much lift does it add?

- 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/plain-flap/ · JSON: https://xemstudio.com/api/geometry?object=plain-flap · Plain text: https://xemstudio.com/objects/plain-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 plain flap is a hinged section of the trailing edge that increases camber and lift coefficient at a given angle of attack; it greatly increases drag and moves the centre of pressure aft, pitching the nose down (FAA PHAK ch.6).
- Its section CLmax increment is about 0.9 (Arnedo, data from Franchini et al.); on a NACA 0012, a 20% chord flap at 40 deg raises CLmax from just under 1.6 to 2.4 (Virginia Tech Appendix A, NACA data).
- Wing-level gain for plain flaps scales with flapped area ratio and cos^3 of quarter-chord sweep (Arnedo).
- 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 |
|---|---|---|
| Section delta-CLmax | 0.9 | Arnedo (Franchini) |
| NACA 0012 + 20% chord flap at 40 deg | CLmax ~1.6 -> 2.4 (Re 6e6) | Virginia Tech Appendix A |
| NACA 0012 zero-lift angle with that flap | about -12 deg | Virginia Tech Appendix A |
| Wing delta-CLmax | 0.92 x dCl x (Sflapped/S) x cos^3(sweep) | Arnedo |
| Typical CLmax/cos(sweep), plain flap | 1.4-1.6 takeoff (20 deg) / 1.7-2.0 landing (60 deg) | Arnedo (Franchini) |
| Reference build | chord 100, 25% flap, 300 span, 20 deg | Sadraey/Arnedo-range example |

## Where the files are

- FAA: [Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), Ch. 6 Flight Controls](https://www.faa.gov/sites/faa.gov/files/08_phak_ch6.pdf) — web — Publisher's page; dimensions are facts
- LibreTexts (Arnedo, Fundamentals of Aerospace Engineering; data from Franchini et al.): [3.4.3 Increase in CLmax (high-lift devices)](https://eng.libretexts.org/Bookshelves/Aerospace_Engineering/Fundamentals_of_Aerospace_Engineering_(Arnedo)/03%3A_Aerodynamics/3.04%3A_High-lift_devices/3.4.03%3A_Increase_in_CLmax) — web — Publisher's page; dimensions are facts
- Virginia Tech Pressbooks (data from Abbott & von Doenhoff, NACA): [Appendix A: Airfoil Data](https://pressbooks.lib.vt.edu/aerodynamics/?p=52) — web — Publisher's page; dimensions are facts

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

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

- plain-flap-wing-segment: [STEP](https://xemstudio.com/files/hardware/plain-flap/plain-flap-wing-segment.step) · [STL](https://xemstudio.com/files/hardware/plain-flap/plain-flap-wing-segment.stl) — plain flap: wing segment 100 mm chord × 300 mm, 25 % chord surface deflected 20°

## Questions

**How far should a plain flap deflect?**

Typical design points are 20 deg for takeoff and 60 deg for landing (Arnedo).

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