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Aviation History
1922
1922 - 0690.PDF
NOVEMBER 23, 1922 a consequent decrease in the pressure diminution or suction. The trailing edge remains practically unaffected. Thus we obtain here, near the leading edge and mainly on top, a decrease in pressure. The strong suction (or negative pressure) is diminished, so that here also there is a flatter pressure increase, as shown by the dash line. By increasing the angle of attack we return approximately to the original curve, while the lift of the rear wing remains practically unchanged. Hence, in this combination the two wings produce a greater maximum lift than when separate, the gain being principally on the front wing. The phenomena described will perhaps be more intelligible if we take for comparison the perfectly analogous phenomena of a simple and a compound Venturi tube. Fig. 6 shows a double Venturi tube, such as is often used on aeroplanes for measuring air speed. If we first imagine the small inside tube removed, we have a simple Venturi tube. The air flows through the constricted section with increased velocity and correspondingly diminished pressure. In the diverging cone behind it the kinetic energy is again largely transformed into pressure, so that at the rear end the external and internal pressures are again equal. Exactly the same causes which limit the lift in a wing here make it impossible to obtain, by narrowing the throat, a pressure diminution of any desired value. In this case, however, it has long been known how to increase the suction bv a suitable combination of tubes. •£= Fig. 7. Measured pressure distributions in a slotted wing. Such an instrument is shown in the figure. The exit of the inner tube is at the point where a diminished pressure is already produced by the outer tube. The latter now forms the starting-point for the further pressure diminution in the inner tube, just as in the case of the front part of the slotted wing section, which we have already considered. We assumed in our discussion that the two parts of the wing were of about the same size. In practice, however, the front part is usually much narrower than the rear part. Our assumption that the pressures on the leading edge of the front part were not noticeably affected by the rear part no longer holds true. Here the whole of the front section lies in a field of increased velocity, and is thus able to produce a greater lift, since the lift is proportional to the square of the velocity. For the rear section, however, our previous remarks hold good. The disturbance due to the front section is felt principally on the leading edge, which therefore has approximately its normal lift. Accordingly, we even here obtain increased lift for the whole combination. That the actual pressure distribution is approximately as described follows from the data published by Handley Page in Engineering, March 4, 1921. These data are given in Fig. 7. For small angles of attack the rear section shows about normal lift distribution. The auxiliary wing in front gives only a small lift, since its angle of attack is much too small. Only from 12° up does the lift of the auxiliary wing show any considerable increase, while the pressure distribution of the main wing remains almost the same. The increased suction on the trailing edge of the auxiliary wing is also evident. At about 20° the flow separates from the main wing and the lift of the auxiliary wing diminishes. The above conception of the phenomena renders the occurrence of an increased lift-coefficient somewhat more comprehensible, and even offers the prospect of making it possible to compute the relations. On the other hand, the following consideration may be presented. If the slot is continuously narrowed, the arguments pointing to a higher maximum lift continue to hold good, but the phenomena vanish when the slot is closed altogether. This was to be expected from the first, since the wing is transformed by closing the slot into one of a practically normal shape. In any event, a slot of a certain minimum width is essential. Since the theory just presented says nothing about this, the phenomena must also be considered from a different stand point. In explaining the phenomena of lift production, I called attention to the fact that the clinging of the air stream to, or its separation from, the upper surface of the wing depends on whether the dead air is carried off fast enough. When we consider this phenomenon on the rear section of a slotted wing, it is obvious that the work (which may be called pump- work or suction) must be performed at the expense of the kinetic energy of the thin air stream flowing through the slot. If the latter is made too narrow, the ribbon of air finally becomes so thin that its kinetic energy no longer holds out to the trailing edge of the rear section, but is itself transformed into dead air by mixing with the dead air above and below it. When considered from this stand point the phenomena of the slotted wing appear in quite another light. We can now think of this wing section as an entity derived from an ordinary wing section by connect ing its upper and lower surfaces by slots, which is, in fact, the conventional conception. The slots convey new energy to the marginal layer of air retarded by friction on top of the wing, thereby increasing its velocity and thus preventing the accumulation of dead air. The air stream flowing out of the slot acts like the jet from a syringe and reinforces the air stream on top of the wing in carrying away the dead air. Since the production of lift depends on the efficiency of this pump-work and the maximum lift is conditioned by the limited pos sibility of carrying off the dead air, it is apparent that any increase in the pumping efficiency increases the maximum lift. We are now inclined to ask which of these two theories is the right one. The answer is that both are equally correct, since they both explain the same phenomena, but from different standpoints. We should rather ask which view point is the more practical. To this question I would reply that we have use for both, according to what we wish to learn. The conception of the slotted wing as a biplane whose wings mutually influence each other has the advantage of enabling computation to a certain extent. With its help, we may succeed in constructing formulae which will enable the determination, in some measure, of the quantitative relations. The second view-point is essential when it is desired to form an idea of the requisite width of the slot. I would add a word of warning against too great optimism. The relations are much more complicated here than, for example, in the theory of the monoplane or biplane. Much work mu.it still be done before these theories are developed into practical rues. With the limited means now available, much time will be required for this work. The immediate task is to determine whether the theories just presented really explain the essential features of the phenomena, or whether other circumstances of decisive influence will come in. This cannot be conclusively determined from the experi mental data now available. If the theory, however, agrees with the facts, this is already a great gain, even though we do not succeed in working out convenient computation formula?. We then know, at least, what the essentials are for obtaining the right shapes, and can thus save ourselves much useless work. FI6.7. 690 \
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