Bishop Kristin L
Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA.
J Exp Biol. 2006 Feb;209(Pt 4):689-701. doi: 10.1242/jeb.02062.
Gliding is the simplest form of flight, yet relatively little is known about its mechanics in animals. The goal of this study was to describe the body position and performance of a gliding mammal and to identify correlates between kinematics and aerodynamic performance. To do this, I used a pair of high-speed digital cameras to record a portion of the middle of glides by southern flying squirrels, Glaucomys volans. The squirrels launched from a height of 4 m and landed on a vertical pole. Reflective markers were applied to anatomical landmarks and the 3-D coordinates of these points were computed to describe the kinematics of the glides. From these data I estimated the lift and drag generated during the glide, and correlated these variables with gliding performance as measured by glide angle, glide speed and stability. In the majority of the glide sequences the squirrels accelerated in the downward direction and accelerated horizontally forward as they moved through the calibrated volume in the middle of the glide trajectory, rather than exhibiting a steady glide in which the body weight is balanced by the resultant aerodynamic force. Compared to human engineered airfoils, the angles of attack used by the squirrels were unexpectedly high, ranging from 35.4 degrees to 53.5 degrees , far above the angle of attack at which an aircraft wing would typically stall. As expected based on aerodynamic theory, there was a negative correlation between angle of attack and lift coefficient, indicating that the wings are stalled, and a positive correlation between angle of attack and drag coefficient. Also as expected, there was a negative correlation between lift-to-drag ratio and angle of attack, as increasing angle of attack produced both less lift and more drag. Within glides, there was a strong correlation between nose-down pitching rotations and limb movements that tended to increase the angle of attack of the wing membrane, suggesting that the animals actively control their pitch by moving their limbs. The squirrels used much steeper glide angles than those reported for other gliding animals, ranging from 40.4 degrees to 57.4 degrees . It is likely that this is because they did not launch from a great enough height to reach their minimum glide angle. In some trials the glide angle increased over the captured portion of the glide, whereas in others it decreased, and the magnitude of the changes varied substantially, rendering it difficult to ascertain which portion of the glide trajectory was represented. Decreases in glide angle were strongly correlated with increases in lift coefficient, but were uncorrelated with drag coefficient.
滑翔是飞行的最简单形式，但人们对动物滑翔机制的了解相对较少。本研究的目的是描述一种滑翔哺乳动物的身体姿势和性能，并确定运动学与空气动力学性能之间的相关性。为此，我使用了一对高速数码相机，记录了南部飞鼠（Glaucomys volans）滑翔过程中部的一部分。松鼠从4米的高度起飞，降落在一根垂直的杆子上。在解剖学标志点上粘贴反光标记，并计算这些点的三维坐标，以描述滑翔的运动学。根据这些数据，我估计了滑翔过程中产生的升力和阻力，并将这些变量与通过滑翔角度、滑翔速度和稳定性衡量的滑翔性能相关联。在大多数滑翔序列中，松鼠在向下方向加速，并在滑翔轨迹中部的校准空间中向前水平加速，而不是呈现出一种稳定的滑翔状态，即体重由合成空气动力平衡。与人类设计的机翼相比，松鼠使用的攻角出人意料地高，范围从35.4度到53.5度，远高于飞机机翼通常失速的攻角。根据空气动力学理论，攻角与升力系数之间呈负相关，表明机翼失速，攻角与阻力系数之间呈正相关。同样如预期的那样，升阻比与攻角之间呈负相关，因为攻角增加会导致升力减小和阻力增加。在滑翔过程中，机头向下的俯仰旋转与肢体运动之间存在很强的相关性，肢体运动往往会增加翼膜的攻角，这表明动物通过移动肢体来主动控制俯仰。松鼠使用的滑翔角度比其他滑翔动物报道的要陡得多，范围从40.4度到57.4度。这可能是因为它们起飞的高度不够高，无法达到最小滑翔角度。在一些试验中，滑翔角度在捕获的滑翔部分增加，而在另一些试验中则减小，变化幅度差异很大，因此很难确定所代表的是滑翔轨迹的哪一部分。滑翔角度的减小与升力系数的增加密切相关，但与阻力系数无关。
