The physics of powered flight

Once a means for detecting and avoiding obstacles was developed in a bat ancestor, the lineage was free to expand into the nocturnal flier niche. Powered flight allows access to flying insects. Because gliders do not have the maneuverability to pursue flying insects, this feeding niche was wide open during early bat evolution. The difference between powered flight and other modes of traveling through the air is maneuverability. Gliders such as the colugo have extra skin at the body's sides, which can both stretch out and change angle during flight to control both the rate and the angle of descent. Therefore, gliding has both a downward and a horizontal component of motion. However, the starting point is always higher than the final position of the animal. This is because gravitational potential (the energy determined by a body's position in a gravity field) is the only source of kinetic energy (energy of motion) in this mode of traveling through the air. To obtain a greater height above the starting position, gliders must utilize other means (e.g., tree climbing). Power flyers can oppose the force of gravity and increase their height above the ground by using wings and the power generated by their own muscles. They are also capable of controlling the magnitude and direction of their forward speed without depending on gravity or air currents.

Glaucomys Volans
The southern flying squirrel (Glaucomys volans) uses a thin membrane that extends from its hands to its feet to glide up to 80 yd (73 m). (Photo by © Joe McDonald/Corbis. Reproduced by permission.)
These little red flying foxes (Pteropus scapulatus) hang upside down and as soon as they drop, they are in flight. (Photo by B. G. Thomson/Photo Researchers, Inc. Reproduced by permission.)

Powered flight is possible because air is a fluid. In everyday usage, the word "fluid" brings to mind a liquid such as water or gasoline. But technically, a fluid obeys the law that the faster an object moves through it, the greater the force exerted on the object. In the terminology of fluid mechanics, the force exerted on an object in a direction perpendicular to the direction in which the object moves through a fluid is called dynamic lift, which is generated when an object moving through a fluid changes the direction of the fluid flow.

Another fluid force exerted on an object is dependent on the shape of the object. This is called Bernoulli lift, which may be involved in natural selection pressure for the wing and body shape of the bat. The Bernoulli principle in fluid mechanics states that the faster a fluid flows over a surface, the lower the pressure on that surface perpendicular to the fluid flow. Therefore, the pressure is lower on the top than it is on the bottom. This pressure difference results in Bernoulli lift upward. Experimentally it has been determined that Bernoulli lift alone is not sufficient for power flying, but most likely provides a selection pressure favoring a particular wing shape, body streamlining, and flight style.

In summary, the forces that must be overcome in powered flight are inertia (the resistance to change in motion that is a property of all masses), weight (the force exerted on the mass by gravity), and drag (the fluid force exerted by air on any object moving through it). To change the height from the ground and the speed and direction of forward motion, the bat has to use its wings to manipulate the airflow to generate the forces of lift and thrust. The wing structures themselves must also be able to withstand the stresses of moving through the air.

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Essentials of Human Physiology

Essentials of Human Physiology

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