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Aviation History
1921
1921 - 0633.PDF
SEPTEMBER 22, 1921 SOME REMARKS CONCERNING SOARING FLIGHT By L. PRANDTL. TN our issue of September 8, we published a report ofthe results of the Rhon soaring flight competition, dealing mainly with the sporting side of the competition, and givingthe winners of the different sections of the competition. Reliable figures relating to what is, perhaps, from the scientificpoint of view, the most interesting side of gliding and soaring the rate of descent—were not available, and hence little ornothing could profitably be said concerning this aspect of the competition. That German scientists are fully aware of thepossibilities of extracting energy from the air so as to prolong a glide and, possibly, turn it into a soaring flight, i.e., a glidewithout loss of height, is amply demonstrated by the interest taken in the subject by many of Germany's leading aero-dynamic scientists. That soaring is possible was demonstrated repeatedly at the Wasserkuppe, by machines attaining,during glides,' heights considerably above that of their starting point. Indeed, it did not require the Rhon competitionto demonstrate this, as Wilbur Wright did that as long ago as 1911, when he remained in the air for ten minutes on aglider, hovering in a wind blowing up the hill from which he had started. On the Wasserkuppe, however, a stepforward has undoubtedly been made, since flights have been successfully carried out during which turns, figures-of-eightetc., were made, and considerable portions of the flights were made in a side wind and even down wind, after themachine had risen above its starting point while flying into the wind. As it appears highly probable that the successes attainedin the Rhon mountains may encourage other countries in taking up the subject—which certainly offers a vastly moreuseful field than the " Aviette," for instance—it seems well to establish from the beginning the fundamental aerodynami-cal laws upon which the theory of soaring is based, and to attempt to deduce from these the aerodynamic featureswhich motorless aeroplanes—as distinct from power-driven machines—should have to enable them to extract the maxi-mum of energy from the air, or, more correctly speaking, from the wind. As being one of the clearest and mostconcise of modern expositions of the subject, we have trans- lated below an article by Professor L. Prandtl, which appeared in Zeitschrift fur Plugtechnik und Motorluftsckiffahrtof July 30. Professor Prandtl, as is of course well known, is head of the Gottingen Laboratory and is one of Germany'sleading aerodynamic scientists. His work on the theory of air-flow around aerofoils is, perhaps, the most advancedyet produced, and his remarks on the subject of soaring there- fore carry more than ordinary weight. After acknowledging his indebtedness to Herren G.Madelung and A. Betz for consenting to the publication of these notes, Professor Prandtl says :— By soaring flight—in contrast with gliding flight—ismeant motorless flight without loss of height. According to the laws of the mechanics of flight, two sources of energyare available for soaring flight. One is, air currents having an upward trend ; and the other is, irregularities in the naturalwind. Under the • latter head we can distinguish between two effective forms : (1) Great fluctuations in the strength(and also direction) of the wind, lasting several seconds ; (2) The rapid fluctuation in wind direction which is commonlydescribed as " turbulent wind." The fluctuations of long duration can only be utilised by effecting considerable changesin the velocity and height of the machine. For instance, by, in one single gust, climbing at the expense of relativevelocity, and then, in the calm which follows, gliding with a downward acceleration. The procedure to follow in orderto extract energy from the wind can be condensed into a simple rule : One must attempt to equalise the fluctuations inthe wind. In so doing, the energy in the wind fluctuations is obviously reduced, and the energy of the machine increasedby a corresponding amount. Thus, for instance, one must present great resistance against a gust, small resistance againsta lull; in an upward current the machine must be gradually elevated to increase the pressure on the wings, in a downwardcurrent depressed to decrease the pressure. The fluctuations of short duration, on the other hand, can beutilised without any substantial change in velocity of the e.g. of the machine. As A. Betz has shown in a very instruc-tive article in this journal in 1912,* useful effect is available in sufficiently great fluctuations in wind direction even withrigid and non-warped aeroplane wings. With elastically mounted or flexible wings, the force obtained from changes in • A BeU : " A contribution to the Explanation of Soaring Flight," Z.F.M.,ia p. 269. wind direction, if these be sufficiently great, may assumeperceptible proportions. The effect may be imagined as resembling that of the so-called fish-tail propellers. Thisname has been used to designate an arrangement by which the waves of the sea have been utilised for the production ofpower. The arrangement consists of a number of flexible plates, rigidly attached with their leading edge to the sidesof a ship. As the waves rise and fall the plates bend like the tail of a fish, and produce power both when bent downwardsand when bent upwards. When we now come to the question of which of these sourcesof energy do soaring birds make use, the answer can—at least as regards our domestic birds—only be that, if notexclusively at any rate mainly, they take advantage of rising air currents. Rising air currents are always to be found inuneven country when a wind is blowing. They are also caused over the plains by meteorological influences. For theirflying practice birds naturally seek the rising air currents, and as these are frequently not of very great extent in space,the birds have to circle in order to remain in the rising air currents. By observing the soaring of birds of prey one mayfrequently see that they suddenly lose height and then commence to flap their wings, continuing in flapping flightuntil they are seen suddenly to rise. From this moment onwards they recommence to soar. They have again foundtheir rising current which they had previously lost. The soaring seagulls near a steamer make use of the air deflectedby the steamer and commence to flap as soon as, for some reason or other, they have to leave their favourable position.That a bird intentionally makes use of gusts does not appear to occur generally. In order to do so it would have constantlyto make jumps up and down, which, so far as I am aware, it has not been observed to do. On the other hand, it doesnot appear unlikely that many birds utilise the rapid fluctua- tions in the wind besides, after the manner of the fish-tailprinciple. Probably, however, these forces suffice in no case to cover the entire work of flying, so that the axiom" no soaring without a wind with an upward trend " can probably be accepted as correct. It might be mentionedthat in regard to sea birds, each individual wave gives the wind an upward deflection, of which the birds take advan-tage.! In the case of human soaring flight, all the previouslymentioned possibilities of making use of the energy in the air hold good in principle. The fish-tail effect, as I will callit for the sake of brevity, may possibly be utilised by a suitable form of flexible wing-sections, although I should adviseexperiments with crewless (unbemannten) models. It is quite probable that the wind fluctuations, which such wingswould be designed to utilise, might frequently not be of such magnitude as to have a perceptible effect. In view of theprobably inconsiderable gain to be expected one would not like to risk the uncertainty of the flying qualities of such aflexible aerofoil, which might easily lead to a useless, or even dangerous, machine. A crewless model, on the other hand,could more easily be sacrificed. The utilisation of great wind fluctuations is a question ofthe skill of individual fliers. It is conceivable now and then to extract a slight gain from these, but too much should notbe expected from this source. There then remains, as the most important help to human soaring flight, air currentswith an upward trend. If it is desired to utilise to the fullest extent rising aircurrents, one must strive to build machines with a slow rate of descent (sinkgeschwindigkeit). One is then in a positionto utilise all rising air currents whose vertical component is greater than the rate of descent of the machine, providedthat, at the same time, the upward slope of the wind is greater than the best gliding angle of the machine. On aslope which is steeper than the best gliding angle of the machine, it is moreover possible to soar in winds whosevelocity is smaller than that of the lowest gliding speed of the machine, provided the rate of ascent of the wind is greaterthan the rate of descent of the machine. The machine will in this case soar out horizontally into the free air and willobviously rise in doing so. It would, however, in time get outside the region of the ascending wind, but if the slopehas sufficient breadth the machine can be pointed diagonally t Since writing this article I find in Physihalischen BericUen, 19 21, some notes by Everting on a work on soaring flight by E. H. Haukin (Proc. Cambr.Phil. Soc, 20 (1921), pp. 219-227), according to which soaring by turbulent wind only is said to be possible when the earth is strongly heated, as occurs in the tropics. Dr. Hankin describes observations of birds, dragon-flies and flying fish.
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