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Aviation History
1939
1939 - 1184.PDF
APRIL 20, 1939 15 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 412c - 't* t- z m r 0 I - -« • $ I X - j 4 » r" * •* / / • /«• 10" *o" FLAP ANdLE Fie.5 / ^ -/ * X ' y f * / ** s£— ' 1— 11 A 30* • 40 ' / jfr /* A* / * / *•* '«V ^ • ~~/~~ / /r » * « "" * • ** 1 1 speeds will look after themselves to a large extent. Now the range of speeds in which flaps are normally used is in the region of 1.2 to 1.5 times the stalling speed, for lower speeds are avoided by pilots because of doubtful stability, while higher ones are ruled out in most machines for the sake of the flap structure. The incidences of o and 5 degrees in these tunnel tests may therefore be taken as the flap range, and it will be seen that over the flap settings between 30 and 50 degrees (which the pilot would use for gliding angle control) the balanced design involves maximum hinge moments of about half the plain slotted type at the lower speeds and of less than a quarter at the higher speeds. This is brought out more clearly in Fig. 6, where the hinge moments have been re-plotted (in terms of the maximum moment on a slotted flap at the stall) for varying speeds; the shaded areas represent the used flap ranges, the upper one being for the plain slotted flap and the lower one for the balanced design. It should be noted that the lower one is falling with speed increase whereas the upper one is rising ; also that the upper curves are based on the separate incidence curves for that flap and not on the 5-degree curve only. The flow past the under-side of the flap was studied carefully, and it was found that at all the larger flap open ings there was a standing eddy of air between the flap and the balance surface, but that only a very little of it was carried away as a wake, and that in consequence there was quite a remarkably thin layer of turbulent air passing the flap trailing edge; its velocity was measured and found to be riot much less than the full velocity just outside that region, so that the momentum loss will be small. This narrow wake was only a small fraction of the total wing wake, so that the take-off should hardly be affected. No attempt was made to measure lift, owing to the low Reynolds Number available. The third flap may perhaps best be described as an improvement on the once-well-known Zap type, and is Rhown in Fig. 7. Just why it did not occur to the Zap Corporation to allow the leading edge of the flap to move back along a suitably shaped track instead of limiting its motion to a straight line, is hard to say, for it seems a fairly obvious development—at any rate once it has occurred to one. It is clear that a track can be so shaped that the forces of air load, drag-arm tension, and track reaction all meet at one point (on the drag arm), thus leaving only the friction of the rollers and hinges to be over come by the pilot. At least that is the case at any one incidence, but the point at once arises that change of inci dence may so alter the air reaction as to quite upset the balance of forces, and some appreciable change was expected, judging by centre-of-pressure tests on plain split flaps at different incidences. Effect of Incidence-change The point that required investigation was whether the shielding of the nose part of the flap as it moved up into the wing would alter the characteristics for better or for worse, and it was to this end that a tunnel test was carried out. The wing was the same as in the previous test, and the flap had the same projected size at 75 degrees maximum setting as had the previous flaps at that angle. The track shape was unfortunately rather a bad first estimate, so that the hinge moments were far higher than they should have been; this, however, does not invalidate the main point of the test, which was to determine the change of moment with incidence. The results are given in Fig. 8, again compared with a plain split- flap of the same size and angle, and only tests at o degrees and 5 degrees were made, as these represent the limits of the used flap range. It will be seen that the effect of change of incidence is remarkably small, while even with the bad track design used the operating forces are reduced to modest propor tions. This flap, in fact, is in some ways the most attractive of the three, as it is simple and strong, while having light operating loads. Tidying-up the Ends T HE Hellermann binding system for electric cables is described in detail in a leaflet issued by Hellermaim Electric, Ltd , 1-2, Savoy Parade, London Road, Croydon. This system is a process of finishing-off the ends of covered and braided electrical conductors with elastic rubber sleeves by the aid of special tools.
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