# Tail moment arm (tail lever arm ratios)

> How long should the tail moment arm be on an aircraft?

- 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/tail-moment-arm/ · JSON: https://xemstudio.com/api/geometry?object=tail-moment-arm · Plain text: https://xemstudio.com/objects/tail-moment-arm/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

- The tail moment arm is the distance from the wing's aerodynamic centre (or c.g.) to the tail's aerodynamic centre; it multiplies every newton of tail lift into pitching or yawing moment (Kitplanes, Wainfan).
- Statistics of 30 transport aircraft put the horizontal-tail arm at about 46% of fuselage length and the fin arm at about 42%, with the horizontal arm about 5% longer than the vertical on conventional tails and about 23% longer on T-tails (Scholz 2021).
- For light aircraft, Pazmany's data give tail arms of 11.8-15.5 ft on wings with 4.17-5.33 ft MAC, i.e. roughly 2.5-3 MAC (EAA Chapter 62). A short arm needs a bigger tail; Sadraey notes fighters use short arms and transports long arms (quoted in HAW Hamburg thesis).
- Used on: Piper J-3 Cub, Luscombe Silvaire, Ryan Navion.
- 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 |
|---|---|---|
| Horizontal tail arm / fuselage length (30 aircraft) | 46.2% (SD 2.8) | Scholz 2021 / HAW Hamburg |
| Vertical tail arm / fuselage length (30 aircraft) | 41.8% (SD 4.8) | Scholz 2021 |
| Engines on wing: Lh / Lv | 47.0% / 44.7% of fuselage length | Scholz 2021 |
| Engines on aft fuselage: Lh / Lv | 44.6% / 36.1% of fuselage length | Scholz 2021 |
| T-tail: horizontal arm vs vertical arm | about 23% longer | Scholz 2021 |
| Conventional: horizontal arm vs vertical arm | about 5% longer | Scholz 2021 |
| Piper J-3: Lh, MAC | 13.2 ft, 5.33 ft (Lh/MAC = 2.48, derived) | EAA62 (Pazmany) |
| Luscombe Silvaire: Lh, MAC | 11.8 ft, 4.17 ft (Lh/MAC = 2.83, derived) | EAA62 |
| Ryan Navion: Lh, MAC | 15.5 ft, 5.22 ft (Lh/MAC = 2.97, derived) | EAA62 |
| Sh/Sw (J-3, Silvaire, Navion) | 13.7%, 15.5%, 23.3% | EAA62 |
| Sv/Sw (J-3, Silvaire, Navion) | 5.7%, 7.6%, 7.9% | EAA62 |

## Where the files are

- INCAS Bulletin / HAW Hamburg (D. Scholz): [Empennage sizing with the tail volume complemented with a method for dorsal fin layout (2021)](https://www.fzt.haw-hamburg.de/pers/Scholz/Aero/AERO_PUB_INCAS_TailVolume_Vol13No3_2021.pdf) — web — Publisher's page; dimensions are facts
- HAW Hamburg (Aircraft Design and Systems Group): [Empennage sizing: tail lever arm as % of fuselage length (thesis; Table 2.1 typical tail volume coefficients)](https://www.fzt.haw-hamburg.de/pers/Scholz/arbeiten/TextMcDavid.pdf) — web — Publisher's page; dimensions are facts
- EAA Chapter 62 (Stan Hall, from Pazmany 'Light Airplane Design'): [How Big The Tail](https://eaa62.org/technotes/tail.htm) — web — Publisher's page; dimensions are facts
- Kitplanes (B. Wainfan): [Design Process: Horizontal Tail (2022)](https://www.kitplanes.com/?p=177046) — web — Publisher's page; dimensions are facts

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

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

- tail-moment-arm-tail: [STEP](https://xemstudio.com/files/hardware/tail-moment-arm/tail-moment-arm-tail.step) · [STL](https://xemstudio.com/files/hardware/tail-moment-arm/tail-moment-arm-tail.stl) — conventional layout: horizontal span 400 × chord 100, vertical 140 × 114 mm on a 500 mm boom, NACA 0009 surfaces

## Questions

**What is a good tail arm for an RC plane?**

Light aircraft in Pazmany's data run 2.5-3 MAC from wing to tail (EAA62); use that with a tail volume coefficient to set tail area.

## Fits with

- [Tail volume coefficient (horizontal and vertical tail sizing)](https://xemstudio.com/objects/tail-volume-coefficient/)
- [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/)
- [V-tail (butterfly tail with ruddervators)](https://xemstudio.com/objects/v-tail/)
- [Inverted V-tail (anhedral V)](https://xemstudio.com/objects/inverted-v-tail/)