Avian surface reconstruction in free-flight with application to flight stability analysis of a barn owl and peregrine falcon

Birds primarily create and control the forces necessary for flight through changing the shape and orientation of their wings and tail. Their wing geometry is characterised by complex variation in parameters such as camber, twist, sweep and dihedral. To characterise this complexity, a multi-stereo photogrammetry setup was developed for accurately measuring surface geometry in high-resolution during free-flight. The natural patterning of the birds was used as the basis for phase correlation-based image matching, allowing indoor or outdoor use while being non-intrusive for the birds. The accuracy of the method was quantified and shown to be sufficient for characterising the geometric parameters of interest, but with a reduction in accuracy close to the wing edge and in some localized regions. To demonstrate the method's utility, surface reconstructions are presented for a barn owl (Tyto alba) and peregrine falcon (Falco peregrinus) during three instants of gliding flight per bird. The barn owl flew with a consistent geometry, with positive wing camber and longitudinal anhedral. Based on flight dynamics theory this suggests it was longitudinally statically unstable during these flights. The peregrine flew with a consistent glide angle, but at a range of airspeeds with varying geometry. Unlike the barn owl, its glide configuration did not provide a clear indication of longitudinal static stability/instability. Aspects of the geometries adopted by both birds appeared to be related to control corrections and this method would be well suited for future investigations in this area, as well as for other quantitative studies into avian flight dynamics.

鸟类主要通过改变翅膀与尾翼的形状与姿态，产生并控制飞行所需的各类作用力。其机翼几何特性（wing geometry）表现为弯度（camber）、扭转（twist）、后掠角（sweep）与上反角（dihedral）等参数的复杂变化。为表征这一复杂特性，研究团队开发了多立体摄影测量系统（multi-stereo photogrammetry setup），可在鸟类自由飞行过程中以高分辨率精准测量其表面几何形态。研究依托鸟类天然的体表斑纹作为基于相位相关（phase correlation）的图像匹配基准，既支持室内外多场景部署，又对鸟类无侵入性干扰。该方法的测量精度经量化验证，足以满足目标几何参数的表征需求，但在机翼边缘与部分局部区域的精度存在一定程度的下降。为验证该方法的实用性，研究展示了仓鸮（Tyto alba）与游隼（Falco peregrinus）在单次滑翔飞行三个时刻的表面重建结果。仓鸮飞行时保持了稳定的几何构型，机翼弯度为正且纵向呈下反角（anhedral）；根据飞行动力学理论，这表明其在该飞行阶段处于纵向静不稳定状态。游隼则保持了稳定的滑翔角，但在不同空速下呈现出多样的几何构型；与仓鸮不同，其滑翔构型无法明确反映纵向静稳定性的状态。两种鸟类所采用的部分几何特性似乎与飞行控制修正行为相关，该方法非常适用于该领域的后续研究，以及其他鸟类飞行动力学的定量分析工作。