Flying animals are able to control their movement through the air, in addition to supporting their weight. Moreover, since it appears that animals do not have to learn how to fly (usually they only need to develop an innate ability by practice) it must follow that the nervous and sensory systems of such animals have evolved, by gradual degrees of increment, to ensure that the animal can correctly respond to disturbances in flight.
If an aircraft is to be successfully controlled by a pilot, it must be stable. An aircraft (or a flying animal in the biologist’s case) can be considered stable if, when disturbed from its course, the forces acting on it tend to restore it to its initial flight path, without the need for pilot intervention (or muscle contractions in the animal’s case). Defining stability for flapping flight is also possible; a flying animal that does not glide but flaps its wings can be considered stable if the forces acting on it tend to restore it to its original cycle of continuous muscle contractions.
In aeronautics, static stability is of high importance. Static stability is a term used to denote stability for rotation about the pitching axis (the pitching axis is a horizontal axis normal to the flight path).
If an aircraft is to be successfully controlled by a pilot, it must be stable. An aircraft (or a flying animal in the biologist’s case) can be considered stable if, when disturbed from its course, the forces acting on it tend to restore it to its initial flight path, without the need for pilot intervention (or muscle contractions in the animal’s case). Defining stability for flapping flight is also possible; a flying animal that does not glide but flaps its wings can be considered stable if the forces acting on it tend to restore it to its original cycle of continuous muscle contractions.
In aeronautics, static stability is of high importance. Static stability is a term used to denote stability for rotation about the pitching axis (the pitching axis is a horizontal axis normal to the flight path).
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| http://upload.wikimedia.org/wikipedia/commons/4/48/Aptch.gif |
If an adequate horizontal surface is placed behind a flying object’s centre of gravity, stability in pitch can be increased. Directional stability also plays a role in aeronautics. Directional stability is a term used to denote stability for yawing rotations (that is rotations about a vertical axis).
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| http://upload.wikimedia.org/wikipedia/commons/c/c5/Ayaw.gif |
It is worth noting that instability in pitch usually renders an aeroplane more completely uncontrollable than instability in yaw.
Now then, as far as I know, only four animal groups have mastered flight; bats, birds, insects and pterosaurs. There are good reasons to suppose that the earliest forms of at least three of the aforementioned groups (namely the birds, the insects and the pterosaurs) were stable in the sense defined above.
Now then, as far as I know, only four animal groups have mastered flight; bats, birds, insects and pterosaurs. There are good reasons to suppose that the earliest forms of at least three of the aforementioned groups (namely the birds, the insects and the pterosaurs) were stable in the sense defined above.
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| http://www.fossilmuseum.net/fossilpictures-wpd/Archaeopteryx/Archaeopteryx.jpg |
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| http://www.nhm.ac.uk/nature-online/virtual-wonders/images/artist_archaeo.jpg |
The Archaeornithes were early ancestors of modern birds. The Archaeopteryx pictured above possessed an elongated tail boarded with a row of feathers on either side, thus allowing the tail to function as an effective stabilizing surface.
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| http://www.scientific-web.com/en/Biology/Dinosaur/images/ExRhamphorhynchus1.jpg |
The earliest pterosaurs from the lower Jurassic belong to the suborder Rhamphorhynchoidea. Creatures within this suborder were known to have long stiff tails which, within at least one group, Rhamphorhynchus (pictured above), were boarded by a stiff fluke of skin at their tip. These tails probably had a stabilizing function (either for stability in pitch – if the fluke of skin was disposed in a horizontal plane, or stability in yaw – if the fluke of skin was disposed in a vertical plane).
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| http://www.windsofkansas.com/kulem.JPG |
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| http://www.kendalluk.com/FOSSIL01.gif |
The earliest order of winged insects is the Palaeodictyoptera (pictured above) from the Upper Carboniferous. They had a long abdomen with each segment bearing prominent lateral lobes, hence forming an effective stabilising surface. Moreover, these creatures possessed a pair of slim and often dramatically elongated cerci. These structures would be pretty ineffective stabilizers on an aeroplane, but they are probably quite effective on an insect (because of the increased significance of air viscosity on a small scale).
Given the above examples, I think it is reasonable to conclude that primitive flying animals tended to be stable, probably because in the absence of a highly evolved sensory and nervous system they would not have been able to fly unless they exhibited such stability. It is however worth noting that instability does offer certain advantages, for example, unstable aircraft are able to turn more rapidly. Moreover, they can achieve lower stalling speeds (since the elevators on an unstable aircraft can be lowered at slower speeds hence allowing the tailplane to support more of the aircraft’s weight).
Although instability does offer certain advantages, engineers are usually restrained by practical limitations. However, these practical limitations do not apply to the animal kingdom and there is good evidence that birds do not need to be stable in order to fly. For instance, many modern birds do not have a tail that is even remotely aerodynamic. In fact, most modern bird tails do not seem to act as stabilisers at all; they are instead used as an accessory lifting surface when flying slowly, this can be easily observed in the case of gulls.
When flying slowly, or turning sharply, gulls tend to open their tails, the slower they fly the more their tails are lowered, as mentioned earlier in the post, this is typical of the unstable state.
A low stalling speed (a characteristic trait of the unstable state) can be advantageous, especially for larger animals. The minimum speed needed to keep an aeroplane, or an animal, of a given shape in the air (the stalling speed) varies as the square root of its linear dimensions. Therefore larger animals may well require a low stalling speed in order to land successfully (the gull tail example above illustrates this point nicely). In fact, a recent study demonstrates that the evolution of a pterosaur as large as Pteranodon (pictured below) is dependent upon the prior evolution of instability.
A low stalling speed (a characteristic trait of the unstable state) can be advantageous, especially for larger animals. The minimum speed needed to keep an aeroplane, or an animal, of a given shape in the air (the stalling speed) varies as the square root of its linear dimensions. Therefore larger animals may well require a low stalling speed in order to land successfully (the gull tail example above illustrates this point nicely). In fact, a recent study demonstrates that the evolution of a pterosaur as large as Pteranodon (pictured below) is dependent upon the prior evolution of instability.
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| http://media.nowpublic.net/images//83/4/834d87b4ddf5bd4191a5178be20faf32.jpg |
Since evolution is a gradual and incremental process involving the random mutation of genes followed by their differential survival and reproduction, a dramatic change (such as the transition from stability to instability) almost certainly could not have occurred in a single step. Such a transition almost certainly must have occurred gradually. Any reduction in stability would have been advantageous provided that there was a parallel increase in the animal’s efficiency of control (which is presumably dependent upon its nervous and sensory systems). It is useful to once again draw upon the analogous situation in aeronautical engineering. Passenger airliners are usually designed with a reasonably high degree of stability because safety is obviously paramount. However, when designing fighter jets, manoeuvrability takes precedence hence the stability margin is reduced to a bare minimum. If we think about the problem in that way, it becomes possible to see how instability could have evolved by gradual degrees of increment.
