Department of Biology, Lund University, Sölvegatan 37, 223 62 Lund, Sweden.
J R Soc Interface. 2012 Feb 7;9(67):292-303. doi: 10.1098/rsif.2011.0238. Epub 2011 Jun 15.
Many small passerines regularly fly slowly when catching prey, flying in cluttered environments or landing on a perch or nest. While flying slowly, passerines generate most of the flight forces during the downstroke, and have a 'feathered upstroke' during which they make their wing inactive by retracting it close to the body and by spreading the primary wing feathers. How this flight mode relates aerodynamically to the cruising flight and so-called 'normal hovering' as used in hummingbirds is not yet known. Here, we present time-resolved fluid dynamics data in combination with wingbeat kinematics data for three pied flycatchers flying across a range of speeds from near hovering to their calculated minimum power speed. Flycatchers are adapted to low speed flight, which they habitually use when catching insects on the wing. From the wake dynamics data, we constructed average wingbeat wakes and determined the time-resolved flight forces, the time-resolved downwash distributions and the resulting lift-to-drag ratios, span efficiencies and flap efficiencies. During the downstroke, slow-flying flycatchers generate a single-vortex loop wake, which is much more similar to that generated by birds at cruising flight speeds than it is to the double loop vortex wake in hovering hummingbirds. This wake structure results in a relatively high downwash behind the body, which can be explained by the relatively active tail in flycatchers. As a result of this, slow-flying flycatchers have a span efficiency which is similar to that of the birds in cruising flight and which can be assumed to be higher than in hovering hummingbirds. During the upstroke, the wings of slowly flying flycatchers generated no significant forces, but the body-tail configuration added 23 per cent to weight support. This is strikingly similar to the 25 per cent weight support generated by the wing upstroke in hovering hummingbirds. Thus, for slow-flying passerines, the upstroke cannot be regarded as inactive, and the tail may be of importance for flight efficiency and possibly manoeuvrability.
许多小型雀形目鸟类在捕捉猎物时经常会缓慢飞行，在杂乱的环境中飞行或降落在栖木或巢上。在缓慢飞行时，雀形目鸟类在下降过程中产生大部分飞行力，并且在“有翼的上升过程”中，它们将翅膀靠近身体收回，并将主翼羽展开，使翅膀处于非活动状态。这种飞行模式与蜂鸟的巡航飞行和所谓的“正常悬停”在空气动力学上的关系尚不清楚。在这里，我们展示了时间分辨的流体动力学数据以及三只斑胸草雀在从近乎悬停到计算出的最小功率速度的一系列速度下的翅膀运动学数据。食虫鸟适应低速飞行，它们习惯在捕捉飞行中的昆虫时使用这种飞行方式。从尾流动力学数据中，我们构建了平均翅膀挥动尾流，并确定了时间分辨的飞行力、时间分辨的下洗分布以及由此产生的升阻比、展弦比效率和襟翼效率。在下降过程中，慢速飞行的食虫鸟产生单个涡环尾流，与鸟类在巡航飞行速度下产生的尾流相比，与悬停的蜂鸟中的双环涡旋尾流更为相似。这种尾流结构导致身体后面的下洗相对较高，这可以通过食虫鸟相对活跃的尾巴来解释。因此，慢速飞行的食虫鸟的展弦比效率与巡航飞行中的鸟类相似，可以假设比悬停的蜂鸟更高。在上升过程中，慢速飞行的食虫鸟的翅膀没有产生显著的力，但身体-尾巴的配置增加了 23%的重量支撑。这与悬停的蜂鸟中翅膀上升过程中产生的 25%的重量支撑惊人地相似。因此，对于慢速飞行的雀形目鸟类，上升过程不能被视为非活动的，尾巴可能对飞行效率和机动性很重要。
