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NSF PR 03-12 - January 22, 2003
Multiple Factors Affect Flight Power Curves Among Species
ARLINGTON, Va. -- Researchers using three dimensional computer modeling and wind tunnels have made the first accurate comparative measurements of muscle power output of birds in-flight to establish that physical structure, body mass, force and flight style all have major effects upon the magnitude and shape of a species' power curve.
The research by Harvard integrative physiologist Andrew A. Biewener and fellow researchers was publicly funded through the National Science Foundation (NSF) and published in the Jan. 23 edition of Nature.
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The research has broad-ranging impact since the power curves provide graphical insight into how muscles are used to produce power for flight at different speeds. Knowledge of flight capabilities would be useful in natural environment studies of issues such as bird migration and a bird's flight ecology. The data might also be useful in the development of more efficient robotic aerial vehicles, where factors such as thrust and forward velocity are paramount.
Birds were used in the study because they rely primarily upon a single pectoralis muscle in each wing to fly. It is difficult to measure mechanical power output in other animals since most use multiple, distributed muscles for locomotion. In comparison, humans have 45 muscles in the thigh, leg and foot region alone, most of which are active during locomotion.
The researchers established mechanical power curves for cockatiels (Nymphicus hollandicus) and turtledoves (Streptopelia risoria) by measuring the wing and body movements as the birds flew at various speeds in a wind tunnel. They also used high-speed video to record from multiple angles the length changes of the birds' pectoralis muscle. These data were applied to a computer model of aerodynamic power output. When analyzed with other data collected, the researchers were able to calculate minimum and maximum power outputs for the birds.
The cockatiel and turtledove data were compared to the power curve of magpies, which had been studied previously. It was found that cockatiels and turtledoves generate more power in linear flight than do magpies, which achieve maximum power output during hovering.
The researchers attribute the difference to the individual physical structure and shape of the birds' tails and wings. Compared to cockatiels and turtledoves, magpies have broad, rounded wings and a longer tail that likely increases drag and prevents it from flying faster than 14 meters per second (31.36 mph) despite it having sufficient pectoralis power to fly at faster speeds. Instead, its wings are better suited for flight at lower speeds and likely allow it to maneuver more effectively.
In contrast, the researchers concluded that turtledoves and cockatiels, which have pointed wings and proportionally smaller tails, can sustain much faster flight speeds. Top speed for a cockatiel is about 14 meters per second (31.36 mph), while turtledoves can achieve 17 meters per second (38.08 mph). In comparison, Tim Montgomery, the current "world's fastest human," achieved a top speed of about 22.9 mph in the 100-meter dash. The differences in the power curves of the three studied species are most apparent at faster speeds.
The researchers said that flight style may explain power curve differences between the birds studied and magpies. Magpies combine a fluctuating wing beat gait, speed and altitude for an intermittent flight style. Cockatiels and turtledoves use more regular wing motions to fly over a range of speed.
The researchers noted that the maximum mass-specific power output of cockatiels and doves, which can fly for hours before tiring, is 60 percent less than estimated for "galliforms." Galliforms are pheasant-like birds with short, broad wings. Their pectoralis muscles have greater mass and are capable of providing power "bursts," which the birds use to take flight quickly to escape danger before rapidly tiring and returning to the ground to run or hide.
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