Flying birds often form flocks, with social, navigational and anti-predator implications. Further, flying in a flock can result in aerodynamic benefits, thus reducing power requirements, as demonstrated by a reduction in heart rate and wingbeat frequency in pelicans flying in a V-formation. But how general is an aerodynamic power reduction due to group-flight? V-formation flocks are limited to moderately steady flight in relatively large birds, and may represent a special case. What are the aerodynamic consequences of flying in the more usual 'cluster' flock? Here we use data from innovative back-mounted Global Positioning System (GPS) and 6-degrees-of-freedom inertial sensors to show that pigeons (1) maintain powered, banked turns like aircraft, imposing dorsal accelerations of up to 2g, effectively doubling body weight and quadrupling induced power requirements; (2) increase flap frequency with increases in all conventional aerodynamic power requirements; and (3) increase flap frequency when flying near, particularly behind, other birds. Therefore, unlike V-formation pelicans, pigeons do not gain an aerodynamic advantage from flying in a flock. Indeed, the increased flap frequency, whether due to direct aerodynamic interactions or requirements for increased stability or control, suggests a considerable energetic cost to flight in a tight cluster flock.

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Flying in a flock comes at a cost in pigeons.

Design, performance evaluation and optimization of a UAV

Bats evolved the ability of powered flight more than 50 million years ago. The modern bat is an efficient flyer and recent research on bat flight has revealed many intriguing facts. By using particle image velocimetry to visualize wake vortices, both the magnitude and time-history of aerodynamic forces can be estimated. At most speeds the downstroke generates both lift and thrust, whereas the function of the upstroke changes with forward flight speed. At hovering and slow speed bats use a leading edge vortex to enhance the lift beyond that allowed by steady aerodynamics and an inverted wing during the upstroke to further aid weight support. The bat wing and its skeleton exhibit many features and control mechanisms that are presumed to improve flight performance. Whereas bats appear aerodynamically less efficient than birds when it comes to cruising flight, they have the edge over birds when it comes to manoeuvring. There is a direct relationship between kinematics and the aerodynamic performance, but there is still a lack of knowledge about how (and if) the bat controls the movements and shape (planform and camber) of the wing. Considering the relatively few bat species whose aerodynamic tracks have been characterized, there is scope for new discoveries and a need to study species representing more extreme positions in the bat morphospace.

/pdf/bat-flight-aerodynamics-kinematics-and-flight-morphology-4zgouurerg.pdf

Bat flight: aerodynamics, kinematics and flight morphology.

Airplanes and helicopters use high aspect ratio wings to reduce the power required to fly, but must operate at low angle of attack to prevent flow separation and stall. Animals capable of slow sust...

https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2015.0051

Power reduction and the radial limit of stall delay in revolving wings of different aspect ratio

A series of water-tunnel experiments were conducted to determine the effect of sinusoidal leading-edge protuberances on the aerodynamic characteristics of finite span wings. The models consisted of seven rectangular planform wings, two swept-leading-edge wings, and two wings with a planform resembling humpback-whale flippers. All models had an underlying NACA 634-021 profile with protuberance amplitudes of 0.025–0.12 times the chord length. The models were examined at Reynolds numbers up to 4.5×105 and angles of attack up to 30 deg. The lift and drag coefficients were nearly independent of Reynolds numbers above 3.6×105. Specific rectangular-planform models had appreciably greater lift coefficients over a limited angle-of-attack range when compared to the baseline model. However, with the exception of the planform that resembled the humpback-whale flipper, the lift-to-drag ratio of all leading-edge modified models was comparable to or less than the equivalent baseline model. The flipper model had a slight...

Aerodynamic Characteristics of Finite Span Wings with Leading-Edge Protuberances

Elegant and inviscid analytical theory can predict the induced drag on lifting wings of finite span. The theoretical prediction is then often modified by multiplication with a dimensionless coefficient for which the departure from a value of 1 is used as a way to incorporate realistic and necessary departures from the idealized model. Unfortunately, there are conflicting definitions of these dimensionless coefficients, often known as span efficiencies, so that even if numerical values are assigned in a clear and transparent fashion, their application and validity remain unclear. Here, the differences between two commonly used definitions of span efficiency are identified and it is shown that for the case of airfoil sections and finite wings at chordwise Reynolds numbers less than 10 5 , neither one has values close to those commonly assumed in the aeronautics literature. The cause of these significant viscous modifications to inviscid theory is traced to the movement of separation points from the trailing edge of real airfoils. A modified nomenclature is suggested to reduce the likelihood of confusion, and appropriate formulations for the drag of streamlined bodies in viscous flows at moderate Reynolds number are considered, with application to small-scale flying devices, both natural and engineered.

J. McArthur

Papers

Span Efficiencies of Wings at Low Reynolds Numbers