# WIG pitch and height stability (moving centre of pressure)

> Why are ground-effect craft hard to keep stable in pitch?

- Category: Aircraft design › Ground-effect design
- Kind: Standard part
- Basis: Design rules and figures from the cited sources; example geometry by Xem Studio
- Page: https://xemstudio.com/objects/wig-pitch-height-stability/ · JSON: https://xemstudio.com/api/geometry?object=wig-pitch-height-stability · Plain text: https://xemstudio.com/objects/wig-pitch-height-stability/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

- In ground effect the centre of pressure moves with both pitch and height, so a craft can be stable in one and not the other. The Irodov criterion requires the aerodynamic centre in height to lie upstream of the centre in pitch (Rozhdestvensky).
- Remedies: a large tail kept out of ground effect (KM up to 50% of wing area), a reverse-delta planform that limits c.p. shift, or for flying wings, profiling and automatic damping, which have a small stable range (Rozhdestvensky).
- NASA found ground-effect wings had height stability at positive and instability at negative angles of attack, favouring positive-AoA cruise (NASA TN D-926); over waves, ram pressure raised lift at the cost of pitch stability (Frontiers 2022).
- Used on: KM (Caspian Sea Monster), Lippisch X-113.
- 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 |
|---|---|---|
| Irodov criterion | centre in height upstream of centre in pitch | Rozhdestvensky 2006 |
| Height stability sign | stable at +AoA, unstable at -AoA | NASA TN D-926 |
| Tail size for stability (KM) | up to 50% of wing area | Rozhdestvensky 2006 |
| Reference build | reverse delta 700 x 400, anhedral 15 | computed |

## Where the files are

- Progress in Aerospace Sciences (K.V. Rozhdestvensky, 2006): [Wing-in-ground effect vehicles (review)](https://www.hassanhameed.com/wp-content/uploads/2016/01/wingingroundeffectvehicles-130120003005-phpapp01.pdf) — web — Publisher's page; dimensions are facts
- NASA Langley (Fink & Lastinger): [Aerodynamic Characteristics of Low-Aspect-Ratio Wings in Close Proximity to the Ground, NASA TN D-926 (1961)](https://ntrs.nasa.gov/api/citations/19980231058/downloads/19980231058.pdf) — web — Publisher's page; dimensions are facts
- Frontiers in Aerospace Engineering (2022): [Review of ground-effect experiments on rectangular, reverse-delta and delta wings](https://www.frontiersin.org/journals/aerospace-engineering/articles/10.3389/fpace.2022.975158/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/wig-pitch-height-stability/index.json · license: https://xemstudio.com/files/hardware/wig-pitch-height-stability/LICENSE.md

- wig-pitch-height-stability-wing: [STEP](https://xemstudio.com/files/hardware/wig-pitch-height-stability/wig-pitch-height-stability-wing.step) · [STL](https://xemstudio.com/files/hardware/wig-pitch-height-stability/wig-pitch-height-stability-wing.stl) — Lippisch reverse delta: straight leading edge, trailing edge swept forward to the tips, span 700, root 400 mm, anhedral 15°, endplates 40 mm (scale to your craft)

## Questions

**Why does my WIG model pitch up near the ground?**

Its centre of pressure moves with height; Irodov's criterion and a large out-of-effect tail address this (Rozhdestvensky).

## Fits with

- [Ground effect: height/span ratio and induced-drag reduction](https://xemstudio.com/objects/ground-effect-height-span-ratio/)
- [Ground effect: height/chord ratio (ram / chord-dominated regime)](https://xemstudio.com/objects/ground-effect-height-chord-ratio/)
- [Lippisch reverse-delta ground-effect craft](https://xemstudio.com/objects/lippisch-reverse-delta-wig/)
- [Ekranoplan layout (low wing + high tailplane out of ground effect)](https://xemstudio.com/objects/ekranoplan-wing-tail-layout/)
- [Tandem-wing ground-effect craft (Jorg type)](https://xemstudio.com/objects/tandem-wig-configuration/)
- [Power-augmented ram (PAR)](https://xemstudio.com/objects/power-augmented-ram-par/)