FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1913
1913 - 1093.PDF
OCTOBER II, 1913. birds partially fold their wings. There is a splendid specimen of a wing in this condition in one of the cases, and it shows how admirably Nature's design maintains a suitable aeroplane surface in spite of the difficulty that is presented by this problem. If a bat were partially to fold its wings the membrane would sag out of shape. The folding of a bird's wing is an automatic mechanical action resulting from the drawing in of the humerus towards the body. Ligaments or muscular cords attached to the humerus on either side run round the back and front of the elbow as if this latter were a pulley. They similarly pass over the wrist joint. When the elbow is moved back towards the body it pulls upon one of the ligaments, which in turn draws in the hand, and so the different sections of the wing are folded and extended in unison. The ulna and the radius bones also play a part in this movement; they behave, 1/fcWHT Goliath beetle (Goliathus giganteus) dissected to show the mode of folding of the wings when at rest. in fact, somewhat in the manner of the parallel motion mechanism that is familiar to the engineer. The method by which a bird spreads its tail is probably unknown to many of our readers. The muscular control is applied directly to the outer feathers only, but the feathers are attached to each other by an elastic ligament that acts like a spring. By pulling on the outer feathers the ligament is stretched, and the other feathers open out fan-like automatically and equally. When the controlling muscle is relaxed the ligament contracts and the tail is folded. Anything more simple could scarcely be imagined, but it is one of the fascinating features of Nature's solution of her problems that, although a past-master in the art of attaining simplicity, she is equally facile in evolving complicated motions when they are proved necessary. A positive masterpiece in complicated wing folding, for example, is that on the earwig. Many people may be un aware that the earwig can actually fly. It is not often seen in flight, and it must be admitted that if any human being had to fold his wearing apparel in the same manner that the earwig folds its wings he would be very well satisfied to use it as little as possible. The earwig's wing first folds rather like a paper fan. When partially folded it halves its length by bending the folded portion back upon itself. Having done that it still contrives to make another fold or two before it finally tucks its wings under its body. Everyone is familiar with the size of an earwig ; we leave it to the imagination to conceive the minuteness of its folded wing. In the exhibition the folding is illustrated by a paper model that is many hundreds of times larger than the original. The folding of the wings of beetles is similarly complicated, and is equally well illustrated. Taken both as a whole and in detail, the exhibition is, as we have said, one of singular interest, and our only criticism is that numerical data relating to the weights and wing areas of birds and other exhibits might with advantage be given among the enumerated facts. It is true that they are available elsewhere, and are perhaps of small interest to the student of natural history, but we venture to think that they would bring home to the aviator how great is the difference between the loading of wing surfaces on birds and on aeroplanes. It is, in fact, probably because the low standard of wing loading utilised by Nature in her most successful flight cannot be maintained with any structural convenience on a very large scale that the bigger and heavier birds, like the ostrich, are unable to fly at all. Yet how small is the ostrich compared to man's aeroplane. More attention to the relative motion of the flapping wing in the air might also with advantage be given in the course of time, when Dr. Ridewood has the leisure from his other numerous duties to attend to this phase of the subject. We notice, for instance, a model illustrating an old theory that the wing feathers operate on the principle of a Venetian blind in order to let the air through during the up stroke. It is pointed out in the catalogue and on the appended notice that this model is exhibited with reserve, but we must admit that we should more approve its absence. There is, so far as we can see, no primary reason why the wing should need to operate on this principle, for we do not accept as true the theory that supposes a wing to be necessarily subjected to down pressure during its up-stroke. On the contrary, we believe that a bird's wing in flight continues to lift all the time, although possibly with a varying force. It seems to us that those who believe otherwise cannot have given sufficient importance to the effect of the superimposed horizontal motion of the wing due to the flight speed of the bird as a whole. If the wing were to merely flap up and down in still air, with the bird stationary, it would, of course, be subjected to top pressure during the up-stroke. In flight the'bird as a whole is moving horizontally through the air. If at the same time the wing beats downwards, the direction of its motion in the air is a downward slope. During the up-stroke the direction of its motion is an upward slope. The effect of this slope is merely to alter the virtual angle of incidence of the wing to the relative wind. It causes the angle to be greater during the down-slroke and finer during the up-stroke for a constant attitude of body and absence of rocking of the wing at its shoulder. Under these conditions, there would be a superior force during the down-stroke due to the increased angle, but 1119
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
