Flight (zoology)

Organisms employ two methods of movement through a fluid environment: rowing and flying. Organisms that row use the viscosity (stickiness) of the fluid to propel themselves. These generally have limbs that are used as oars. These limbs push up against the fluid (usually water), and the fluid exerts a force back onto the oars that drives the animal forward. Flying organisms use the fluid's inertial qualities (usually air). Essentially, they will use differential flow rates over the body surfaces to create thrust and lift. Flying is necessary when animals are large, and the drag on the body becomes too high for rowing to be effective. The choice between rowing and flying concerns the organism’s Reynolds number (Re), which is a ratio between length and inertial viscosity forces. Viscosity dominates at low RE, whereas inertial forces dominate at high RE. Drag is any force that degrades the forward movement of an object in a fluid environment. Drag can be caused by friction on the body surface. Pressure drag occurs when fluid does not adhere to the shape of the object and peels away as a wake. The size of an organism capable of achieving airborne flight by means of rowing is quite small. One example is the mymarid or fairy wasp, which is smaller than a fruit fly. In aquatic environments, larger forms can row due to the greater viscosity of water.

How Flight Works

All objects in flight are subjected to four forceslift, thrust, drag, and gravity. Lift is created by the flow of fluid over a surface such as an aircraft or bird wing. A fluid can be a gas, such as air, or a liquid, such as water. Lift will act opposite to the force of gravity (or the weight of the object). When the force of lift is greater than the force of gravity, the object will rise. Thrust is a force that moves an object forward in the air. This force can come from various means, such as from an aircraft engine or by the flapping of a bird’s wings. Thrust will also help create lift. Drag is a force that acts opposite to thrust. It is created from the contact, or friction, between the object’s surface area and the fluid as the object moves forward. For an object to achieve and sustain flight, lift must be greater than gravity, and thrust must be greater than drag.

Regardless of the organism, flow must take on different velocities above and below the body or wing, according to Bernoulli’s principle, whereby flow is understood in terms of conservation of energy. This means that in an ideal fluid with no friction (that is, no viscosity), if the fluid accelerates, its pressure goes down. Conversely, when the velocity is reduced, the pressure goes up. While this may be counterintuitive, it nonetheless provides a basis for understanding flight. Thus, wings are not necessary to fly, but rather an organism needs to create a surface that is longer on top than it is on the bottom. A concave body form might possibly provide lift by its shape alone in an air streamline. Wings can assume such a shape by twisting themselves, as in insect wings, or creating a curvature with feathers, as in birds. In this situation, air flows faster above than below the structure, and lift is produced perpendicular to the upper surface.

This only works because fluids follow the law of continuity: if a streamline of air is split by an airfoil, the streamline will flow both over and under the structure. Both the upper and lower streamline must theoretically meet at the end of the structure. Thus, a curved structure or a structure that is inclined at an angle to the oncoming flow will create a longer upper surface than a lower surface, hence the differential velocity as the streamlines race to the rear edge of the structure. If the aerodynamic structure is inclined downward, the lifting force can now be separated into a force that drives the structure forward and upward.

This picture of flow is very basic, and it is more complicated in practice. The flow over the wings circulates around the wing in circles. Bernoulli’s principle may still be applied to the circulating flow to understand how lift is generated. This circulation is shed off the wings at the tips, particularly patterns based on the movements of the wing or (in the case of most fish) the tail. This circulation of fluid around the wing must be generated for lift and thrust to be generated. These circulating rings of fluid are shed off the wings or oscillating tail as vortex rings. This is why aircraft are spaced apart during landings, as these vortex rings may cause such turbulence that the aircraft flying behind may experience a loss of lift.

Laminar flow is a flow whose constituent pattern is in one direction, an even flow. Turbulent flow occurs when areas within a flow become chaotic, causing heat production and a loss of velocity. Worse than this is a condition when the turbulence becomes so great that the flow no longer adheres to the aerodynamic surface and departs from the structure, causing a loss of lift. This can happen for a variety of reasons. If the angle of attack of a wing becomes too steep, the flow may become turbulent and separate from the structure. If there is no circulation on the wing, there is no lift, a stall develops, and gravity takes over, with dire consequences. In addition, if the velocity is too great, the flow will become turbulent and unable to stay on the surface.

Shape also may contribute to the production of turbulence. Surface roughness may cause turbulence to occur through collisions with the streamlines. In addition, convexities and concavities can cause turbulence to form and prevent the adherence of the fluid on the surface of the object. While this has practical applications in design, it points to the fact that aerodynamic structures are streamlined, having tapered ends that preclude the necessity of fluids having to adhere to abrupt curvatures and creating potentially turbulent flows.

This description applies to large-scale turbulence where separation of flow from the object’s surface occurs. Small-scale turbulence can actually benefit the lift on the aerodynamic structure since the longer the flow adheres to the airfoil, the more lift and thrust the structure can generate. Small-scale turbulence may actually maintain the flow on a surface, and thus, the lift produced far outweighs the drag force produced by the turbulence.

Flight in Animals

The evolution of flight in birds is centered mainly on the evolution of feathers. Feathers are complex epidermal structures that may have been derived through an evolution from scales. Feathers are the most complex epidermal structure found in vertebrates. They have a central strut, called a rachis, and a series of barbs emanating from both sides of the rachis in a pinnate fashion to form the feather vane. These barbs are hooked together by a series of hooklets that act very much like Velcro. Feathers form the major aerodynamic surface of a bird. They are lightweight but very strong. Contour feathers maintain a uniform surface for the bird so that there are no abrupt curvatures that may create turbulence. The long-flight feathers, known as primaries, secondaries, and tertiaries, form the wing. Long, narrow wings termed high aspect-ratio wings, consume less power when flapped than broad or low aspect-ratio wings. The bird wing is cambered, and so creates lift in an airflow. Shorebirds, such as gulls, are seen lifting out of the water just by holding out their wings in a breeze without the need of flapping. Flapping the wings faster than the velocity of the air increases the lift and thrust imparted to the air. Birds are able to fly because they can generate more lift than their body weight, and it has taken them considerable evolutionary time to perfect the weight reduction necessary for flapping flight. Penguins use their wings to fly underwater. They are extremely maneuverable and use their agility and speed to catch fish. The wing beat does not proceed through the same distance as aerial flying birds because of the greater viscosity of water.

Large birds are typically heavier than smaller birds. They must have larger wings and wing spans to generate more lift to compensate for their greater weight. The size of these wings also adds to the weight of the birds and the gravity that must be overcome to achieve lift. For example, the wings of a vulture can account for a quarter of its weight. Unlike the relatively large mass animals can attain on both the land and sea, the size of birds of flight is limited. The largest bird on Earth is the male ostrich. It can weigh hundreds of pounds and grow upwards of nine feet tall. Its mass, however, renders it flightless.

Bats are also evolved from a quadrupedal ancestor. In their case, a skin membrane is stretched between the elongated fingers. In this way, the camber and aspect ratio of the wing can change quickly. Many bats retain a membrane between the legs, the uropatagium, that increases the lifting surface of the body. Bats are agile and rapid flyers and, equipped with ultrasonic pulses, can locate a variety of prey, from insects to frogs.

The only animals capable of true flight, also called winged or powered flight, are most birds, bats, and insects. These animals' evolutionary success is evident in their species' number and diversity. Because of the evolutionary advantage flight offers, other animals began evolving and gained gravitational gliding and soaring abilities. Mammals have evolved gliding forms in much the same way across other taxa. A wing membrane is stretched between the fore and hindlimbs in marsupials such as sugar gliders and among placental mammals such as flying squirrels (rodents) and the two colugos or so-called flying lemur species, including the Sunda flying lemur (Galeopterus variegatus) and the Philippine flying lemur (Cynocephalus volans).

Pterosaurs, the first recorded flying vertebrate that lived around 215 million years ago, also used a skin membrane stretched between the elongated fourth digit and the hindlimb. This design worked well for over a hundred million years, but a tear in the membrane would destroy the lift-generating ability of the wing. While most pterosaurs were small, some giant forms evolved with wing spans of over forty feet. Lizards have never evolved powered flight but have generated gliders and parachutists. In the Triassic and Cretaceous periods, as well as in the present day, lizards evolved a flight membrane supported by the ribs. The ribs can be folded against the body when the lizard is climbing among the branches in search of insect prey. Other lizards, such as geckos, have evolved body fringes of skin and the ability to flatten themselves to create an aerodynamic surface. Some species of snakes launch themselves into the air, and some frogs use their expanded webbed hands and feet for gliding.

Although fishes essentially fly with the tail, and many flap the pectoral fins like bird wings, some fish have taken to the air. Flying fish will gain speed underwater using their tail and then leap out of the water, spreading their large pectoral fins. These fishes can glide for some distance before reentering the water. The flight strategy is associated with escape behavior from predaceous fish.

There is controversy surrounding the origin of insect flight. Some postulate that insect wings developed for temperature control, as wing beating would heat the organism. Moreover, wings could be used to collect or dissipate heat depending upon exposure to the sun or wind. Others contend that insect wings evolved from lateral extensions of the exoskeletal tergites to aid in the stabilization of jumps. However, it began, insects are the only invertebrates to evolve aerial powered flight. Normally, insects have two pairs of wings, the first pair essentially acting as covers for the posterior wings involved in lift and thrust generation. Some insects have two functional pairs of wings, such as dragonflies. The wings are operated by muscles that have short contraction lengths to obtain rapid wing movements. As is the case in birds, small insects have the highest wing beat frequencies, reaching over one thousand beats per second. Muscles attach either directly to the wing bases, or indirectly to a specialized part of the dorsal carapace termed the notum. The deflection of the notum moves joints that connect to the wings, allowing them to beat in a twisted fashion and create aerodynamically shaped surfaces.

Principal Terms

Drag: a force that acts in the opposite direction of the movement of a body through a fluid medium; sources of drag vary but include friction and pressure suction

Fluid: a substance, either liquid or gas, that flows or conforms to the outline of its container

Inertia: the property of an object with kinetic energy to move in a straight line unless acted upon by an outside force

Lift: an aerodynamic force created through differential flow above and below a structure

Reynolds number (RE): the results of a formula that takes into account the velocity of an object, its characteristic length divided by the dynamic viscosity of the fluid

Turbulence: flow that is chaotic and may create stall conditions through the loss of lift

Viscosity: the stickiness of a fluid created by internal forces as molecular attractions

Bibliography

Chari, N., et al. Biophysics of Insect Flight. Springer, 2022.

Ellington, C. P. “Aerodynamics and the Origin of Insect Flight.” Philosophical Transactions of the Royal Society of London, vol. B305 1991 pp. 1-181.

Feduccia, Alan. The Origin and Evolution of Birds. 2nd ed. New Haven, Yale University Press, 1999.

Pennycuick, C. J. Animal Flight. London, Edward Arnold, 1972.

Renken, Elena. "Unraveling the Evolution of Flight." Nautilus, 16 Feb. 2024, nautil.us/unraveling-the-evolution-of-flight-515836. Accessed 10 Sept. 2024.

Tarsitano, S. F., et al. “On the Evolution of Feathers from an Aerodynamic and Constructional Viewpoint.” American Zoologist vol. 40 2000, pp. 676-86.

"The Flight of Birds." Ornithology, 2023, ornithology.com/ornithology-lectures/flight. Accessed 11 July 2023.

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"What is Drag?" The National Aeronautics and Space Administration, 21 July 2022, www1.grc.nasa.gov/beginners-guide-to-aeronautics/what-is-drag. Accessed 11 July 2023.

"What is Lift?" The National Aeronautics and Space Administration, 13 May 2021, www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/lift1.html. Accessed 11 July 2023.