Abstract
Energy expenditure is a critical characteristic in evaluating the flight performance of flying insects. To investigate how the energy cost of small-sized insects varies with flight speed, we measured the detailed wing and body kinematics in the full speed range of fruitflies and computed the aerodynamic forces and power requirements of the flies. As flight speed increases, the body angle decreases and the stroke plane angle increases; the wingbeat frequency only changes slightly; the geometrical angle of attack in the middle upstroke increases; the stroke amplitude first decreases and then increases. The mechanical power of the fruitflies at all flight speeds is dominated by aerodynamic power (inertial power is very small), and the magnitude of aerodynamic power in upstroke increases significantly at high flight speeds due to the increase of the drag and the flapping velocity of the wing. The specific power (power required for flight divided by insect weigh) changes little when the advance ratio is below about 0.45 and afterwards increases sharply. That is, the specific power varies with flight speed according to a J-shaped curve, unlike those of aircrafts, birds and large-sized insects which vary with flight speed according to a U-shaped curve.
Highlights
Hovering and level-forward flight are the most common flight modes of insects
The body motions in freely forward flight have been tracked when studying the flight behaviors of fruitflies under visual control [24,25,26], there is still no measured wing kinematics of fruitfly flying at high speed; the measured largest flight speed for fruitfly with detailed wing motion obtained was about 1.0 m/s, which is only a medium flight speed for fruitfly
To acquire the forward flights of fruitflies, experiments were conducted with a low-speed open-jet wind tunnel, which is customized for insect flight tests
Summary
Hovering and level-forward flight are the most common flight modes of insects. To accomplish various flight tasks in different scenarios, insects adjust the generation mechanisms of aerodynamic forces by sophisticatedly controlling the flapping motion of their wings [1]. Present experimental measurements were mostly focused on hovering or single-speed flight [8,9,10,11,12,13,14,15,16,17,18,19]; there only exist three studies that measured the wing and body kinematics of an individual insect in multiple-speed forward flight [20,21,22]. Dudley and Ellington [20] filmed and measured the forward flight of three bumblebees using a wind tunnel and each individual had 3–4 flight speeds; since only one high-speed camera was used, the pitching angle of flapping wings could just be estimated. The body motions in freely forward flight have been tracked when studying the flight behaviors of fruitflies under visual control [24,25,26], there is still no measured wing kinematics of fruitfly flying at high speed; the measured largest flight speed for fruitfly (fruitfly F1 in Meng and Sun [14]) with detailed wing motion obtained was about 1.0 m/s, which is only a medium flight speed for fruitfly
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