Abstract
Here, we analyse the energetics, performance and optimization of flight in a moving atmosphere. We begin by deriving a succinct expression describing all of the mechanical energy flows associated with gliding, dynamic soaring and thermal soaring, which we use to explore the optimization of gliding in an arbitrary wind. We use this optimization to revisit the classical theory of the glide polar, which we expand upon in two significant ways. First, we compare the predictions of the glide polar for different species under the various published models. Second, we derive a glide optimization chart that maps every combination of headwind and updraft speed to the unique combination of airspeed and inertial sink rate at which the aerodynamic cost of transport is expected to be minimized. With these theoretical tools in hand, we test their predictions using empirical data collected from a captive steppe eagle (Aquila nipalensis) carrying an inertial measurement unit, global positioning system, barometer and pitot tube. We show that the bird adjusts airspeed in relation to headwind speed as expected if it were seeking to minimize its aerodynamic cost of transport, but find only weak evidence to suggest that it adjusts airspeed similarly in response to updrafts during straight and interthermal glides.This article is part of the themed issue ‘Moving in a moving medium: new perspectives on flight’.
Highlights
The moving atmosphere presents opportunities and challenges alike for the animals that fly through it
This mode of presentation is convenient for identifying theoretical optima, it does not lend itself to displaying multiple solutions simultaneously, because every point (U, Us) on the aerodynamic glide polar is optimal for multiple combinations of updraft and headwind (Wu, Wh)
We have shown here how empirical data obtained from onboard instrumentation can be used to test and develop the underlying theory of soaring and gliding flight
Summary
The moving atmosphere presents opportunities and challenges alike for the animals that fly through it. Even a constant wind field will affect the optimization of heading and airspeed, which presents further challenges for sensing the wind, and adjusting velocity appropriately in response [2,3,4,5,6] None of these themes is new, but we look at them here with fresh eyes, through the lens of some new theoretical development, and with the aid of empirical data collected using state-of-the-art onboard instrumentation. In the second part of the paper (§3), we test our theoretical predictions using empirical data collected using onboard instrumentation carried by a captive steppe eagle Aquila nipalensis (§3a) This allows us to test whether and how airspeed is adjusted in relation to headwind speed (§3b), updraft speed (§3c) and the combination thereof (§3d). As these sections are quite self-contained, we conclude with only a brief discussion at the end (see §4)
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More From: Philosophical Transactions of the Royal Society B: Biological Sciences
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