# Ground effect: height/span ratio and induced-drag reduction

> How close to the ground does a wing need to be for ground effect?

- 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/ground-effect-height-span-ratio/ · JSON: https://xemstudio.com/api/geometry?object=ground-effect-height-span-ratio · Plain text: https://xemstudio.com/objects/ground-effect-height-span-ratio/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

- Near the ground the wingtip vortices and downwash are suppressed, cutting induced drag. At constant CL the reduction is about 1.4% at one span height, 23.5% at a quarter span and 47.6% at a tenth of a span (FAA-handbook values as reproduced by The Wise Pilot).
- NASA's tests of low-AR wings found Wieselsberger's h/b theory agreed with experiment for h/b of about 0.03-0.25, but underestimated the benefit below 0.03; the percentage gain in L/D at a given height is the same regardless of aspect ratio (NASA TN D-926).
- Lift-curve slope also rises near the ground while lift at 0 deg stays nearly constant; at h/c = 1 the wings were essentially out of ground influence (NASA TN D-926).
- 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 |
|---|---|---|
| Induced drag reduction, h = 1 span | about 1.4% | The Wise Pilot (FAA values) |
| Induced drag reduction, h = 1/4 span | about 23.5% | The Wise Pilot (FAA values) |
| Induced drag reduction, h = 1/10 span | about 47.6% | The Wise Pilot (FAA values) |
| Wieselsberger theory valid range (tests) | h/b ~0.03-0.25 | NASA TN D-926 |
| Out of ground influence (chord basis) | h/c = 1 | NASA TN D-926 |
| Reference build | span 600, chord 300 (AR 2) endplated | computed |

## Where the files are

- The Wise Pilot (pilot training page; values match FAA PHAK ground-effect figure): [Ground Effect](https://thewisepilot.com/?p=66220) — 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

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

Index: https://xemstudio.com/files/hardware/ground-effect-height-span-ratio/index.json · license: https://xemstudio.com/files/hardware/ground-effect-height-span-ratio/LICENSE.md

- ground-effect-height-span-ratio-wing: [STEP](https://xemstudio.com/files/hardware/ground-effect-height-span-ratio/ground-effect-height-span-ratio-wing.step) · [STL](https://xemstudio.com/files/hardware/ground-effect-height-span-ratio/ground-effect-height-span-ratio-wing.stl) — endplated wing: span 600, chord 300 mm, anhedral 0°, endplates 30 mm (scale to your craft)

## Questions

**When does ground effect start?**

It is small (about 1.4% induced-drag cut) at one span and strong (47.6%) at a tenth span (FAA values via The Wise Pilot).

## Fits with

- [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/)
- [Endplates on ground-effect wings](https://xemstudio.com/objects/wig-endplates/)