Low Reynolds numbers not only reduce the maximal lift coefficient, C L, they also increase the minimal drag coefficient, C D, because of complex flow separation phenomena associated with low Reynolds number effects.
Consequently, as flyers get smaller and are thus slower, they have to operate at lower Reynolds numbers. The velocity of a flyer in equilibrium diminishes with size, because velocity is proportional to the square-root of weight divided by wing surface area,, the square-root of length. For small animals and robots,, in which U is the flight velocity, L is the chord length and 68 000 is calculated by taking the inverse of the kinematic viscosity of air at sea level. The smaller the flyer, the lower its aerodynamic efficiency, which decreases with the Reynolds number ( Re). This helps explain why current aerial robots are typically limited to mission times of about half an hour.
This requirement is especially important for small flying robots as compared to larger aircraft, because energy efficiencies associated with aerodynamics and on-board energy conversion greatly reduce with size. Regardless, flying animals and robots need to land frequently if they cannot acquire energy mid-flight. Further, generalist birds, bats and many flying insects can fly for much longer than small aerial robots. In contrast to flying robots, animals can land on and take off from a diverse range of complex natural and engineered surfaces. Similarly, the pitch-up landing manoeuvres and surface attachment techniques of animals can offer an evolutionary design guide for developing robots that perch on more diverse and complex surfaces. Biomechanists can use the current variety of specialized robots as inspiration for probing unknown aspects of bimodal animal locomotion. The maximal size of both perching animals and robots is limited by scaling laws for both adhesion and claw-based surface attachment. While current aerial robots show promise, flying animals, including insects, bats and birds, far surpass them in versatility, reliability and robustness. To accomplish such novel robot behaviours, recent efforts have included advancing our understanding of the aerodynamics of surface approach and take-off, the contact dynamics of perching and attachment and making surface locomotion more efficient and robust. These opportunities have prompted an interest in bimodal aerial and surface locomotion on both engineered and natural surfaces. Beyond perching, small robot flyers benefit from manoeuvring on surfaces for a diverse set of tasks, including exploration, inspection and collection of samples. One way to extend mission times is to perch, as biological flyers do.
Small aerial robots are limited to short mission times because aerodynamic and energy conversion efficiency diminish with scale.
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