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Aviation History
1913
1913 - 0087.PDF
the feature most emphasized in the Dunne patent. Those who may be interested to try the principle would thus be well advised to take stock of what Mr. Dunne has to say on the subject, for to Mr. Dunne belongs the credit of having developed the principle, not in thought only, nor in models only, but in actual full-sized machines ; and to the best of my knowledge he is the first to employ definitely down- pressure wing tips in aeroplane construction. That the principle does in fact break down the need for the rudder is shown by the fact that the Dunne aeroplanes have no such organ ; moreover, they are capable of steering closed circuits with fixed controls. In the steering of his machine, it will be observed that Mr. Dunne differentiates the permanently negative tips, both tips remaining negative after the warp. In modern systems of control, one wing tip at least is positive after the warp. The value of the negative angle for steering has also been recognised by Mr. H. S. Wildeblood, Superintending Engineer, Indian Public Works Department, of Mount Abu, Rajputana, who filed a patent, No. 6,642, of 1910, containing the following claim: "Balancing planes, which lie normally in the direction of flight, and in such a position that their efficiency will not appreciably be interfered with by the main planes, worked independently, and so that their angle of incidence is always below and never above the direction of flight." Mr. Wildeblood's patents are the outcome of some exceedingly interesting experiments at Rajputana, which he described in FLIGHT of June 24th, 1911. In the main this work relates to the balancing qualities of flexible trailing tips, which have an interest of their own as a principle related to the recovery of balance rather than to inherent absolute stability. It is, however, interesting to quote the following sentence that occurs as an incidental remark towards the end of the article : " A slight initial dip to both the balancing planes increases the automatic stability." In connection with the subject of stability generally, nothing is of greater interest, I think, than the observations that have been made in different parts of the world on bird flight. Not all of the observers were qualified for their work, but I think it will be agreed that the observations of Dr. Hankin, at Agra, certainly rank with those of Marey and Lilienthal as a record of first-class importance. At the moment there is only space to refer to his remarks on steering during soaring flight. (FLIGHT, p. 739, August 26th, 1911.) " I first obtained a clue to the nature of steering movements by observing the flights of the black vulture . . . Occasionally the tip of one wing will be seen to be depressed downwards momentarily and then raised at once to its original position . . . After the movement there is almost time to formulate in words which way the bird is going to turn before the commencement of the turn can be recognised. In my notes I originally described this movement as a dipping downwards of the wing tip. This phrase was soon abbreviated to dip, by which term I propose to refer to the move ment in future. " It is necessary to consider how the dip is brought about. The first possibility that suggested itself to me was that it was caused by some intrinsic muscles of the wing. But on examining the wing of a dead bird, it appeared to me that the range of possible movement at the carpal-joint was less than my observations had led me to expect. It then occurred to me that perhaps what really happened was that the whole of the wing was rotated until the air pressed on its upper surface instead of on its under surface ... In order to decide between these two possibilites I dissected the wing of a black vulture and found that neither of the above suggested explana tions is an adequate statement of the facts of the case. "None of the intrinsic muscles of the wing have any power of making a dip movement by any direct action. But, on the other side of the ulna, I found three muscles that have the power of rotating the front edge of the outer part of the wing. Supposing the wing is extended horizontally, then if these three muscles come into action, the front edge of the wing tip becomes depressed. That is to say, the wing tip is rotated round the axis of the wing. The rotation is in such a direction that the air ceases to press on the underside of the wing tip feathers. Instead, it presses or tends to press on their upper surfaces. Hence the tips of these feathers are bent downwards, producing the appearance of the dip movement. From the dorsal aspect of the wing, two muscles may be seen that have the power of rotating the front edge of the wing tip in the opposite direction. These muscles come into action to return the wing tip to its original position. I have also found these muscles in the wings of the common vulture, the adjutant, and the sarus." It seemed to me, when I first read this remark, that the dip was a temporary movement of a kind permanently represented in the wing of a Duane aeroplane, and when Dr. Hankin read his pap:r on soaring flight before the Aeronautical Society, Mr. Dunne during the discussion himself referred to the subject Aeronautical Journal, 1912, p. 39) : " If the bird is rolling down its wing tips in the way that Dr. Hankin points out, and which I suppose most of you know is in my own particular machine, there is undoubtedly something in that. If I was to use a vertical tail in my machine I should upset it altogether, because the outer part of the wing is travelling at a much faster rate on turning than the inner part, and as that is the part that is connected with the angle of incidence it would tend to press that wing down rather than to raise it. It is a matter of nicety how you get your balance." It would appear that the natural state of the extremities of a bird's wing is not one in which they have a permanently negative angle. In general the camber seems to be washed out, and under particular circumstances the wing tip nuiy apparently flex upwards by local pressure. From these eon siderations it seems to me probable that Nature's design has succeeded in combining efficiency with security. Under ordinary conditions of flight in fine weather, the safeguard afforded by the permanently negative tip would be unnecessary, and, therefore, merely a handicap to one thoroughly accustomed to the ways of the air and dependent on self-exertion for propulsion. For instance, birds that pursue flapping flight could more often make use of a highly efficient wing than a wing that is dead stable. The flexible tip, it would seem, may automatically come into action as a virtual fin to make the recovery from side-slip more sensitive. On the other hand, birds that habitually soar in high winds and travel long distances from the land might have occasion to appreciate the absolute security of negative tips, which they can apply to their wings at will by the muscular action described by Dr. Hankiu. Moreover, the inefficiency is a matter of less concern to the bird if the wind is doing the work. Other practical work that has always seemed to have an especial interest, is that of Weiss, Handley Page, Etrich and those who have sought stability in special wing forms embodying an up-turned tip as a characteristic feature in common. The natural balance of the Weiss models was remarkable, as I was able to see from a demonstration that Mr. Weiss very courteously made for me some years ago. Gordon England, too, who ventured to pilot the first Weiss man-carrier without engine and, as I under stand, without controls, has often told me of personal experiences that make it essential to believe in the presence of some especial virtue in its design. It appears to me of importance first to know, however, whether the up-turned wing tips on the above-mentioned machines are or are not normally causing a down pressure on the wing. The fact of them appearing to be negative does not necessarily imply more than a " wash-out." When warped, it seems especially likely that one of the tips may become positive. In any case, the deciding factor is the use of the rudder to promote a bank, and in this connection some notes on his experiences as a pilot by the late Lieut. Parke definitely record the automatic banking of the Handley Page machine when steered with the rudder alone. Whether, therefore, the tip pressure is normally zero or downwards on the wing, the fact remains that its magnitude does not neutralise the increasing lift of the positive part of the wing in a relative spin. Much light was also thrown on the subject of up-turned wing tips, dihedral angles and the like, by Mr. E. H. Harper when he gave his synopsis of Prof. Bryan's mathematical treatment of stability as a paper to the Aeronautical Society recently. Up-turned wing tips in this case primarily relate to the extremities as a whole being up-turned, the up-turned portion being parallel with the line of flight. According to that theory, all such modifications of straight wings project virtual fins on the plane of symmetry, and may be regarded as such. The fin of the up-turned portion is projected in a position at right angles to the plane of the up-turned surface so that if the wings are retreated the effect is to throw the virtual fin forwards. A retreated wing with up-turned tips, therefore, projects a virtual fin somewhere above and ahead of the propeller. Together with a tail fin, such as a neutral rudder, this elevated and forward position of the balancing fin constitutes, so I understood from Mr. Harper, the best combination for still air stability that he and Prof. Bryan have as ytt investigated. Their work on this particular case being unfinished, there is no reference to it in Pro!. Bryan's book. The foregoing theory, unless I misunderstand it, supposes the up-turned tip either to be contributing to the support of the system or to be neutral. The first to investigate by the same process the consequences of making this balancing tip a down-pressure surface is Mr. Hume-Rothery, whose entirely independent yet coincident investigation arrives at the same conclusions as those obtained from my own different line of thought. The much greater importance of Mr. Hume-Rothery's work is, of course, that it puts the matter on a quantitative basis, and so enables designers to study the subject from a numerical standpoint. Mr. Hume-Rothery's definition of a " partial horizontal gust " appears in effect to be the same as my own treatment of a hori zontal gust as a " sudden angular acceleration of the wind producing relative spin." ( To be continued.) 87 C
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