. Animal flight; a record of observation . ing when in flap-glidingflight in unsoarable air. Let us imagine that the bird P. Lift. W. Weight. DCG. Centre of Drag centre. Vulture flap-gliding in unsoarable air at commencement of a glide. is travelling horizontally. The bird is being acted onby the four chief forces. Of these the lift and the weight act at points one vertically above the otheras elsewhere explained. The other two forces arethe pull and the drag. The pull consists ofthe momentum of the bird. This acts in a horizontaldirection at the centre of gravity (
. Animal flight; a record of observation . ing when in flap-glidingflight in unsoarable air. Let us imagine that the bird P. Lift. W. Weight. DCG. Centre of Drag centre. Vulture flap-gliding in unsoarable air at commencement of a glide. is travelling horizontally. The bird is being acted onby the four chief forces. Of these the lift and the weight act at points one vertically above the otheras elsewhere explained. The other two forces arethe pull and the drag. The pull consists ofthe momentum of the bird. This acts in a horizontaldirection at the centre of gravity (see fig. 70). The drag consists of the resistance of the body of thebird to passage through the air, plus the resistance dueto the action of the air on the wings. The drag must therefore act in a horizontal direction probably acts at a point slightly above the level ofthe centre of gravity. But its exact position, that isto say the position of the drag centre, is at least in slow-flapping flight the head end. ANIMAL FLIGHT. 209 of the bird rises during the down-stroke and falls duringthe up-stroke. The transverse axis round which thisrotation occurs probably passes through the dragcentre. In flapping flight there is an increase oftransverse axis and dorso-ventral axis stability. Thesefacts suggest that the drag centre is situated pos-teriorly to the centre of gravity. Just as the lift and the weight act at two points some distanceapart one above the other, so, in like manner, it is pro-bable that the pull and the drag act at differentpoints, one behind the other, thus conferring a smallmeasure of natural stability. Since the drag consists not only of the resistance dueto the action of the air on the wings, but also to resist-ance due to the action of the air on the body, there canbe little doubt that, under the conditions described, itis situated on a slightly higher level than the centre ofgravity. If this is the case., there must be a couplebetw
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