Powered-gliding/climbing flight performed by bats for saving fuel.

Results of recent research show that bats perform flights with continual altitude changes rather than flying at a constant altitude. However, the current state of knowledge suggests that the reason for these altitude changes is not known, and it is stated in the literature that further study is necessary in order to understand this behaviour. The goal of this paper is to provide an explanation by showing that flights with continual altitude changes constitute a fuel-saving flight mode in bats. The descents in the altitude changes - which were analysed using flight measurement data - show a power support by flapping the wings to yield a powered glide. Accordingly, this flight mode may be termed powered-gliding/climbing flight. Corresponding to the described flight characteristics, powered-gliding/climbing flight can be seen as an extension of flap-gliding flight, which is a flight mode known in the research on animal flight. This paper shows that the powered glide enables a decrease in aerodynamic drag, as well as an explanation of the underlying physical mechanism. I also developed a flight mechanics model of powered/gliding climbing flight in bats. Results based on this model show that fuel consumption can be reduced. Thus, a substantial fuel saving can be achieved when compared with the best flight at constant altitude, which is classically considered as the flight mode requiring the lowest fuel consumption.