# V-tail (butterfly tail with ruddervators)

> What is a V-tail and how do you size the ruddervators?

- Category: Aircraft design › Tail configurations
- Kind: Standard part
- Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
- Page: https://xemstudio.com/objects/v-tail/ · JSON: https://xemstudio.com/api/geometry?object=v-tail · Plain text: https://xemstudio.com/objects/v-tail/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 V-tail replaces the stabiliser and fin with two slanted surfaces; their ruddervators move together for pitch and differentially for yaw (FAA PHAK ch.6). FAA notes it is more complex, more prone to Dutch roll, and the 'total reduction in drag is minimal'.
- Sizing rule (McCombs, Model Aviation): effective horizontal area = 2S cos^2(a) and effective vertical area = 2S sin^2(a), where S is one panel and a the dihedral. Because cos^2 + sin^2 = 1, total V-tail area equals the sum of the conventional horizontal and vertical areas it replaces, which is also Wainfan's conclusion (Kitplanes: no net area saving).
- Yaw input creates a roll opposite to the commanded yaw (Kitplanes), though on RC sailplanes Stackhouse calls it 'truly negligible'; ruddervators can flutter, so balance them (Kitplanes). NACA TR-823 (Purser & Campbell) is the classic test-verified V-tail theory.
- Used on: Beechcraft Bonanza V35, Fouga CM 170 Magister, Antonov A-15.
- 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 |
|---|---|---|
| Typical dihedral per panel (models) | about 40 deg | McCombs, Model Aviation 1994 |
| Typical dihedral (full-size) | usually about 45 deg | Kitplanes V-Tails |
| Effective horizontal area | 2S cos^2(a) | McCombs 1994 |
| Effective vertical area | 2S sin^2(a) | McCombs 1994 |
| Angle from required areas | tan^2(a) = Sv_req / Sh_req | derived from McCombs formulas |
| Worked example (McCombs) | 40 deg -> 31 deg cut effective fin area from 12.4 to ~7.96 in^2 | McCombs 1994 |
| Reference build (computed) | Sh_req 40,000 + Sv_req 16,000 mm^2 -> a = 32.3 deg, each panel 28,000 mm^2 (250 x 112 mm) | computed |
| Pitch coupling fix | small differential: more travel on the down-going ruddervator | Stackhouse, CRRC |

## 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
- Model Aviation (AMA), W.F. McCombs: [Wing and Tail Dihedral for Models (Dec 1994)](https://library.modelaviation.com/article/wing-and-tail-dihedral-models) — web — Publisher's page; dimensions are facts
- Kitplanes (B. Wainfan): [Design Process: V-Tails (2022)](https://www.kitplanes.com/design-process-v-tails/) — web — Publisher's page; dimensions are facts
- Charles River Radio Controllers (D. Stackhouse): [V-Tail Adverse Yaw (1999)](https://charlesriverrc.org/articles/design-and-construction/aircraft-design/tail-design-and-structure/vtail-adverse-yaw/) — web — Publisher's page; dimensions are facts
- NACA (Purser & Campbell): [Experimental Verification of a Simplified Vee-Tail Theory, NACA TR-823 (1945)](https://ntrs.nasa.gov/citations/19930091901) — web — Publisher's page; dimensions are facts
- engrXiv (Bhattacharya et al., ICMMM 2023): [Design and Computational Analysis of Y-tail Configuration for Maritime UAV](https://engrxiv.org/preprint/download/6582/10812/9153) — web — Publisher's page; dimensions are facts

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

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

- v-tail-tail: [STEP](https://xemstudio.com/files/hardware/v-tail/v-tail-tail.step) · [STL](https://xemstudio.com/files/hardware/v-tail/v-tail-tail.stl) — v tail layout: horizontal span 250 × chord 112, vertical 0 × 0 mm on a 500 mm boom, NACA 0009 surfaces

## Questions

**What angle should a V-tail be?**

Pick it from the areas you need: tan^2(a) = vertical area / horizontal area (McCombs). Models typically land near 40 deg (McCombs), full-size near 45 deg (Kitplanes).

**Does a V-tail have less drag?**

Not much: there is no wetted-area saving and FAA calls the total drag reduction minimal (Kitplanes; PHAK ch.6).

## Fits with

- [Tail volume coefficient (horizontal and vertical tail sizing)](https://xemstudio.com/objects/tail-volume-coefficient/)
- [Tail moment arm (tail lever arm ratios)](https://xemstudio.com/objects/tail-moment-arm/)
- [Conventional tail (low-mounted stabiliser and single fin)](https://xemstudio.com/objects/conventional-tail/)
- [T-tail (stabiliser on top of the fin)](https://xemstudio.com/objects/t-tail/)
- [Cruciform tail (stabiliser part-way up the fin)](https://xemstudio.com/objects/cruciform-tail/)
- [Inverted V-tail (anhedral V)](https://xemstudio.com/objects/inverted-v-tail/)