Why can birds fly so well?

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Original author: Heidi Ledford

To understand how wing shapes affect the flexibility of birds in flight, parameters that can relate mass and geometry to aerodynamic performance are needed. An analysis of the nature of flight inertia fills this gap.

An early and critical stage of aircraft design is the development of equations of motion. These equations relate the inherent properties of the aircraft, such as its mass and geometry, to its aerodynamic performance (the forces and moments involved). The ratio of force and torque to mass and inertia (resistance to motion change) provides information for designers to calculate the potential acceleration ability of the aircraft. The equation of motion also helps to evaluate the basic characteristics of the aircraft, such as its potential speed, orientation and maneuverability (changing the direction and speed of its flight path).

Developing such a framework is a standard way to discover design limitations and create opportunities. By contrast, although bird flight has fascinated scientists and engineers since Leonardo da Vinci's time, a standardized framework for studying bird flight and mobility has been lacking, in part because of limited data related to the nature of inertia.

Harvey and his colleagues took an important step towards establishing such a framework. They developed an analytical method to determine the inertia properties of 22 species of birds by measuring the full range of motion of the wing elbow and wrist (figure 1). The author assumes that a bird can be modeled as a combination of several simple geometric shapes, by which the contribution of wings, body and tail to inertia is determined by the angle function of elbow and wrist. These inertia can be used as input parameters of the theoretical framework to study the maneuverability of birds when changing the shape of their wings (called wing deformation) during flight.

Compared with the experimental methods limited to observable flight behavior, the use of analytical methods to study maneuverability provides a method to test the evolutionary hypothesis about the limits of maneuverability. This method can also be used to reveal how geometric properties and range of motion affect stability (the tendency of birds to restore balance after interference) without the need for a large number of flight tests and observations.

The key findings of this study are to provide insights into the effect of wing deformation on the position of birds' center of gravity; to propose a method to estimate the contribution of various parts of the bird's body to rotation characteristics in different directions of motion (called pitching, yaw and rolling); and to be able to assess birds' ability to fly from stable to unstable flight, and vice versa.

Harvey et al showed that the center of gravity remained almost unchanged within the range of motion of the entire elbow and wrist joint. Compared with the contribution of tail, torso and neck, the contribution of wing deformation to pitch rotation inertia is very small. In contrast, wing deformation, especially the angle of the elbow, strongly affects roll and yaw inertia. This may mean that birds tend to move their elbows during flight to trigger changes in roll angle and speed.

By studying the full range of motion of the elbow and wrist, Harvey and colleagues proposed a large parameter space that allows conclusions about how the overall shape of the wing affects inertial characteristics. But what is important is that it relates the relevant insights on aerodynamic efficiency, stability and maneuverability to the parameter space.

However, it is not clear whether birds have evolved to have a more stable or mobile ability to fly. The index used to evaluate the pitching stability of aircraft or birds is called static stability margin. It is defined as the distance between the center of gravity and the neutral point, and the neutral point is the point where the aerodynamic force acts. The positive static stability margin indicates that the neutral point is behind the center of gravity, which means that the configuration is more stable and the mobility is lower, while the negative static stability margin (the neutral point is in front of the center of gravity) indicates that the configuration is unstable, but the mobility is high.

The wing shape of birds is studied, and this feature is associated with flight-related characteristics, which provides insights for controlling the physical and evolutionary pressures of bird flight. From which people may refine the key principles and apply them to the development of bionic drones with excellent performance. Such aircraft can adjust their wing shape in a variety of missions to maximize efficiency, stability or maneuverability under various operating conditions.

References:

1. Harvey, C., Baliga, V. B., Wong, J. C. M., Altshuler, D. L. & Inman, D. J.?Nature?603, 648 Murray 653 (2022).

2. Harvey, C., Baliga, V. B., Lavoie, P. & Altshuler, D. L.A. J. R. Soc. Interf.?16, 20180641 (2019).

3. Smith, J. M.?Evolution?6, 127mur129 (1952).

? Nature

Doi:?10.1038/d41586-022-00638murx

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