---
title: "Why Do Birds Sweep Their Wings? A Gull in Flow360"
description: "A gull sweeps its hand back to set its own pitch stability, not to cut drag. Flow360 on a real gull wing through one recorded fold, checked against NASA and nine wind-tunnel models."
canonical_url: "https://engineering.flexcompute.com/articles/why-birds-sweep/"
markdown_url: "https://engineering.flexcompute.com/articles/why-birds-sweep.md"
image: "https://engineering.flexcompute.com/images/og/why-birds-sweep.jpg"
publish_date: "2026-10-21"
kind: "Case Study"
authors:
  - "Harry Smith"
tags:
  - "CFD"
  - "Flow360"
---

import WhyBirdsSweep from '../../components/why-birds-sweep/WhyBirdsSweep.astro';

Scroll through the story below. The gull on the right is a fit to the Flow360 runs, rebuilt in your browser, so at the end you can set the fold and angle of attack yourself and cut through the flow anywhere. The written summary follows it.

<WhyBirdsSweep />

## The short version

A common claim is that birds sweep their wings back for the same reason as jets, or to reduce drag. The better explanation is that a gull sweeps its hand back to move its neutral point aft and so set its own pitch stability, as a tailless aircraft does with sweep. Harvey et al. showed in 2019 that gulls tune their pitch stability by morphing the wing; this post is a way of seeing it, in Flow360, through one whole recorded fold.

I ran Flow360 on a real gull wing (glaucous-winged gull 20_0341, from Harvey et al.'s measured landmarks) through one recorded fold, from tucked to fully open, after validating the solver at bird Reynolds numbers. For the wing and body with no tail:

- Tucked, the neutral point sits about 120 mm behind the shoulder: a static margin of about +27 % of the mean chord.
- Fully open, it comes forward to about 77 mm: about +19 %.
- The neutral point travels 43 mm and the centre of gravity 13 mm, so folding and sweeping the hand works as a stability dial: very stable for a steady glide, less stable and quicker to manoeuvre when open.

## Validation

- **Bird-scale aerofoil.** Eppler 387 at $Re = 10^5$ against NASA TM-4062. RANS follows the lift curve to within 30 lift counts from 0° to 8°; drag is 47 counts low at 4°, from slow recovery after the laminar bubble. A wall-resolved LES of the same slice is 15 counts above NASA (0.0247 against 0.0232).
- **Nine wind-tunnel gull models.** Harvey et al. (2021) tested nine wing-body models of a second gull (17_0285). Rebuilt from their published landmarks with their own geometry code (area, span, mean chord and tip twist match their records), Flow360's neutral point is within 4.4 mm RMS of the tunnel (−3.3 to +7.8 mm); the tunnel's own runs at two speeds differ by up to 7.1 mm. Lift slope is 11 % below the tunnel, most on the models with the most twisted-down hands.

## Method

- 15 fold states, each at $\alpha$ = 0°, 4° and 8°, on a mesh refined behind the wing: 45 runs. The neutral point at each fold state comes from the slope of pitching moment against lift, and a Gaussian-process fit gives it across the fold.
- The surface pressure and the 3-D flow field are fitted across fold and $\alpha$ (POD plus Gaussian processes) so the page can draw any fold. The flow is described in coordinates attached to the wing, so a vortex sits in the same place at every fold. I checked the fit on two fold states left out of it.

## Limits

- Pitch only: no sideslip, so nothing here on yaw or roll.
- The wing and body have no tail; a real tail would add margin.
- The specimen's measured hand washout separates the flow over the hand at some fold states, so I don't report lift-to-drag.
- One mesh level for the tunnel models, and $\alpha$ only from −4° to +4° there.
- Harvey et al. (2019) found extending the elbow made the wing more stable, the other way round from here. They tested dried wings without a body, with the wrist held open, and took the pitching moment about the shoulder, with no centre of gravity. Their 2021 wing-body tunnel models, which Flow360 matches above, show the neutral point moving forward as the elbow extends, as it does here.

## Sources

- Gull geometry, centre of gravity and tunnel data: Harvey, Baliga, Wong, Altshuler and Inman, _Nature_ 603, 648 (2022), and Harvey, Baliga, Goates, Hunsaker and Inman, _J. R. Soc. Interface_ 18, 20210132 (2021), CC BY 4.0; data [doi:10.6084/m9.figshare.c.5275490](https://doi.org/10.6084/m9.figshare.c.5275490).
- Gulls tuning pitch stability by morphing the wing: Harvey, Baliga, Lavoie and Altshuler, _J. R. Soc. Interface_ 16, 20180641 (2019), [doi:10.1098/rsif.2018.0641](https://doi.org/10.1098/rsif.2018.0641).
- Gulls controlling dynamic pitch stability by morphing the wing: Harvey and Inman, _PNAS_ 119, e2204847119 (2022), [doi:10.1073/pnas.2204847119](https://doi.org/10.1073/pnas.2204847119).
- Eppler 387: McGhee, Walker and Millard, NASA TM-4062 (1988).
- All CFD: [Flow360](https://www.flexcompute.com/flow360/).