People flying in airplanes invariably
think of birds in flight and often draw analogies between the aerodynamic
design of birds and that of aircraft. The study of bird flight and of the form
and structure of birds had supported the development of human flight. But the
birds are not altogether appropriate models for aeronautical design. Birds use
beating wings to defy gravity and to propel themselves through the air, while
airplanes use fixed wings for lift and engines for propulsion.
To be truly analogous with aircraft,
birds-or other flying animals- would have to generate their propulsion from a
source other than their wings, and those wings would have to remain fixed.
Decades ago, as the field of aviation was quite literally getting off the
ground, researchers had looked at a different group of animal flyers with fixed
wings as a possible analog to the airplane.
In the October 1908 issue of The
Aeronautical Journal, between an article titled “The Probable Cause of the
Explosion of Count Zeppelin’s Airship” and another called “The Wright Bros’
Flying Machine,” was one about the flight of flying fish. The article was
beautifully examined the steering and handling mechanisms of the fish for
application to the airplane. But the subject is complex.
A decade after Lindbergh flew across
the Atlantic in the Spirit of St. Louis, researchers could not fully grasp how
fish could fly above the surface of the ocean. Airplane designs were used to
understand flying fish, rather than the converse. I first observed some flying
fish from a boat off Key Largo in Florida. As the boat cruised along, the fish
exploded from the waves produced as the bow sliced through the water.
They took off in a direction
perpendicular to the boat’s course. With outstretched wings they sailed into
the air about two feet above the water, traveling in a lazy banking arc. At the
end of the flight, they descended gradually to the surface of the water before
falling back into the sea and disappearing into the swells as magically as they
had appeared. Years later, interest in flying fish-unconnected in any way to
emerge from studying the variation in wing design among genera of flying fish
and how such design influences aerodynamic capabilities.
How to Fly Fish?
Flying fish are part of a single
taxonomic family known as the “Exocoetidue” and are most closely related to
needlefish and halfbeaks. The family includes eight genera, eight groups, which
frequent the tropical oceans of the world. Most are small, approximately six
inches long, although the California flying fish can grow to 18 inches.
The “wings” of flying fish are
enlargements of the pectoral and pelvic fins, the paired fins of the body. the
shoulder and located just behind the gills, and the pelvic fins are located
toward the rear, on the underside of the body. When outstretched, both sets of
fins furnish a broad surface to generate an upward lift force for flight.
The aerodynamic shape of the pectoral
fins is remarkably like that of some birds’ wings. Like the curved upper surface
on the wings of any bird or any commercial jetliner, the numerous fin rays
supporting the wings of flying fish produce a curved or arched profile that
helps generate lift for flight.
Flying fish are gliders, not true
flyers like birds, bats, and insects, all of which fly by beating their wings.
They had misunderstood the fluttering of wings from fish in flight. The
wing flutter is not as with birds-an oscillatory mechanism to generate lift,
but the result of air rushing by a flexible.
The pectoral fins are borne by
structure. It is like a flag waving in the breeze. To take to the air, a flying
fish leaps from the water or rises to the surface continually beating its tail
to generate propulsion as it starts to taxi. The taxiing run lets the fish accelerate
at the water surface and build momentum for takeoff. Once the fish reaches its
top speed of 20 to 40 miles per hour it spreads its elongate fins and becomes
airborne.
This action was captured beautifully in
a series of high-speed elements flash photographs taken at night by the
father of stroboscopic photography, Harold Edgerton. The animals are
impressive, with typical glides of 50 to 300 feet and flight. The flight
performance of these times of 30 seconds. The fish can reach altitudes of 20
feet. There are accounts of them landing on the decks of ships.
Experts hypothesize that the fish can
increase distance and time aloft by using updrafts from the windward face of
waves. One study claimed that when flying into the wind a fish could travel
over a quarter of a mile! flying fish is a flat arc, like that of some
missiles. The French Exocet (the word means “flying fish” in French) is a
missile that skims just above the water surface before striking its target,
usually a ship.
Such a sea-skimming weapon caught the
world’s attention in 1982 when the British ship HMS Sheffield was sunk by an
Exocet launched from an Argentinean naval aircraft. At the end of the glide
when speed and altitude are decreasing, flying fish can either fold up their
wings and fall back into the sea or drop their tail into the water and re
accelerate for another flight.
This capacity for successive flights
greatly increases the possibilities for air time. The record reported is 12
consecutive flights covering 1200 feet. The key to this version of touch-and-go
is the unique design of the flying fishtail. The usual tail fin of a modem bony
fish has equal upper and lower lobes. The tail of a flying fish has an extended
lower fin lobe.
Except for flying fish, this type of
fin is found only in the fossilized remains of ostracoderm, an early group of
jawless fishes. The elongated lower lobe in flying fish lets the tail
oscillations generate enough speed for flight without the entire body becoming
submerged. The tail lobe works like an outboard motor powering a boat. why
flying fish take wing. They may be escaping from predators.
By leaving the water, they may be
fleeing the jaws of death and confusing pursuers by splashing down in
unpredictable places. They may also be saving energy. By moving through the
air, a less dense medium than water, they may be reducing the amount of energy
they need for locomotion. Scientists think energy conservation may explain why
We don’t understand very well dolphins leap from the water when swimming at
high speed.
Flying fish can be divided into two
morphologically distinct groups based on their wing design-Cypselurus and
Exocoetus. The first group has long broad wings derived from the pectoral fins,
the paired fins at the shoulder. Enlarged pelvic fins aid these pectoral fins,
representing as much as 25 percent of the overall wing surface. The Exocoetus
have narrower pectoral fins than those of Cypselurus, and the pelvic fins make
up only about eight percent of the overall wing surface.
The difference in wing design between
Cypselurus and Exocoetus has been understood since 1930 when one researcher
compared Cypselurus to an advanced biplane with a main (pectoral) top wing and
a staggered (pelvic) underwing and Exocoetus to a monoplane with long narrow
main wings.
Aerodynamics
To understand how the wing design of
flying fish affects flight performance, let’s examine the aerodynamics of
gliding. As a glider moves through still air at a constant speed, it is acted
upon by gravity and pulled downward by a force equal to its weight. That force,
in turn, is resisted by an equal and opposite force directed upward. This
upward aerodynamic force represents the balance between the “lift” force
generated by the wings and the “drag” force resisting the movement of the body
and wings through the air.
The balance of these forces is
all-important in-flight dynamics. The proportion of lift relative to drag is
called the glide ratio. If drag rises relative to lift, the rate at which a
glider loses height-its sinking speed-will increase. A loss of almost all lift
is called stall. Anyone who has ever watched a paper airplane stop in mid-air
before plummeting to earth is aware of the stall. To study how wing design
affects flight performance, the aerodynamic quantities of wing loading and
aspect ratio, looking at the gliding ratio and the sinking speed.
Wing loading is the weight supported by
the area of the wings. Large flying fish with high wing loading's must fly
faster to remain aloft with the same rate of the sink as smaller fish with low
wing loading's. The aspect ratio is the square of the wingspan divided by the
wing area. A high aspect ratio means the wings are long and thin with high lift
and low drag characteristics, wings that reduce sinking speed, like those of an
albatross.
Low aspect ratio wings are short and
broad with low lift and high drag, like those of a flying squirrel. Although
Cypselurus and Exocoetus are similar in size, Cypselurus has a lower wing
loading and aspect ratio. The four broad wings of Cypselurus help with
increased lift at slow flight speeds. The underside of certain species is
flattened, increasing the total lifting surface of the fish.
Exocoetus has two high aspect ratio
wings, which means a substantial reduction of the drag on the wings for rapid
flight. Although accurate flight speeds for the two groups are hard to come by
it is difficult to make measurements at sea without a fixed frame of reference
and with variants in wind direction and speed-Cypselurus can glide farther than
Exocoetus.
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