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Aviation History
1911
1911 - 1056.PDF
fjjGHT] DECEMBER Q, ion* B8 O&o*. ^l.3tu^o£.$)irdTligRf By Dr E.HHankin. MA. DSc. (Copyright Reserved) . oOO CHAPTER XXXVIII.—The Position of the Centre of Gravity under different Conditions of Flight. THE outer third of the wing of a vulture consists of the wing tips. The inner two-thirds of the wing are cambered (when the wing is extended), and are concerned with lifting effort in unsoarable air and with lifting and tractive effort in soarable air. The centre of lifting and tractive effort of each wing may therefore be taken as being near the junction of its inner and middle thirds. The wing sections shown in the following figures may be regarded as taken at this point. The following diagram (Fig. 57) represents a side view of a vulture when gliding in a straight line in soarable air. It will be seen that the position of the centre of effort of the wings is vertically above the centre of gravity. Supposing the vulture advances its wings, thus (Fig. 58), then a couple is produced tending to rotate the bird upwards round its transverse axis. Such rotation, as we have seen, actually occurs. The bird rotates until it assumes the following position (Fig. 59). Thus the centre of lifting effort re-acquires its position vertically above the centre of gravity. Conversely if the bird retires its wings, a couple originates that rotates the bird in the opposite direction, thus (Fig. 60), and again the centre of effort is vertically above the centre of gravity (Fig. 61). Therefore, so long as the bird is gliding in a straight line in unsoarable air, the centre of effort of the wings is vertically above the centre of gravity. If the centre of effort is displaced, rotation round the trans verse axis at once occurs until the centre of effort is again vertically above the centre of gravity. We have already seen that when a bird is gliding in an ascending current the same law holds. Under these conditions the centre of effort is near the centre of area of the wing, and in order to retain gliding horizon - tally the bird advances its wings until the lift is again vertically above the weight. Does the same relation hold when the bird is subjected to a propelling force, as in flapping flight or when soaring ? Let us first consider the case of flapping flight. We have already seen that if the bird while flap ping changes its wings from the " straight " to the " retired " position, it rotates round its trans verse axis, and the direc tion of its flight is in a downward direction. Conversely, if, as in stop flapping, the bird ad vances its wings it rotates upwards round the trans verse axis. These facts suggest that the law holds Fig. 57.—Section of a vulture gliding in unsoarable air. Fig. 58.—Effect of advancing wings, first position. Fig. 59.—Effect of advancing wings, second position. Fig. 60.—Effect of retiring wings. Fig. 61.—Section of vulture in flapping flight. good. But if this is the case, why is it that the wings are advanced in slow horizontal flight, and why does the amount of advancing diminish as speed increases ? Supposing a bird is gliding horizontally in calm air, and someone momentarily catches hold of its tail, so as to check speed ahead. Supposing, in consequence, the bird was to flap its wings up and down in order to regain speed ahead. Then, at first, as the air strikes the surface of the wing nearly at a right angle, the "lift" is at a paint near the centre of area. Hence the bird has to advance its wings in order to bring the " lift" over the " weight." Hence the wings can be seen to be advanced in slow flapping flight. But as speed ahead increases, the angle of incidence diminishes. Consequently the " lift" approaches the Fig. 62.—Outline of a vulture circling in air not fully soarable, or circling in fully soarable air without effort to gain height. Fig. 63.—Outline of a vulture circling in fully soarable air and with effort to gain height. Fig. 64.—Outline of a vulture slow flex-gliding (8 metres per second). Fig. 65.—Outline of a vulture flex-gliding at medium speed (12 metres per second). Fig. 66.—Outline of a vulture fast flex-gliding (22 metres per second). IO58
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