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Flight efficiency in animals

Efficiency of animal flight can be considered at various levels. First, there is an overall energy conversion efficiency of energy intake to mechanical energy output. This is usually measured as metabolic rate during flight (power input) and the mechanical power output (mainly as aerodynamic work on the air surrounding the animal). The energy that is not converted into useful mechanical work is lost as heat. It has proven quite hard to actually measure the mechanical power output from flying animals, but there us hope that this will soon be possible by measurements of animal wakes. The power input part is somewhat easier to measure by various methods such as respirometry, doubly labeled water or related methods. The energy conversion efficiency is probably in the range 20-40% in animal flight.

The overall aerodynamic efficiency of an animal is measured as the ratio between lift (L) and drag (D), where a large L/D is associated with good overall flight performance and low energy cost of flight. For flapping flight we refer to the effective L/D, while in steady gliding flight the L/D is synonymous with the ratio between forward airspeed and the sink rate.

There are also other efficiencies of animal flight of interest. One is the aerodynamic efficiency of a wing to generate a lift. The ideal (except some arcane exceptions) is a wing that generates a uniform downwash along the span and where the lift distribution is elliptic between the wing tips. In many animals the body is not as good as the wings in generating lift and therefore the downwash is often reduced behind it, which represent a departure from the optimum and hence a reduction of efficiency. The figure shows an example of downwash distributions between two bird and two bat species, showing that in these cases the birds are somewhat more efficient than the bats.

Yet another efficiency parameter is the induced power factor, k, which indicates departures from the ideal (k=1) in generating induced drag. In animal flight k is often assumed to be 1.1-1.2, while some birds having slotted wing tip may beat the ideal and have k<1.

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Downwash distributions

Spanwise downwash distributions at mid-downstroke and mid-upstroke at 7 m/s, for the pied flycatcher (yellow), blackcap (red), Pallas' long-tongued bat (blue), and lesser long-nosed bat (green). The dotted lines show the average downwash for all measurements and the bars around the dotted lines are the sliding 95% confidence interval.