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Aviation History
1939
1939 - 1181.PDF
SUPPLEMENT TO FLIGHT 4126 14 THE AIRCRAFT ENGINEER APRIL 20, 1939 FIG.4. which was badly under balanced ; test runs had, however, been made with the drag arms fur ther back and the flap had become overbalanced when fully open, as in Irving's tests. Fig. 2 gives force-deflection curves for the various arrangements of flap, and the forces are given in terms of that on the plain split flap at 900, since it is the reduc tion and not the absolute values that it is desired to emphasise ; the deflection represents distance-moved by the rear edge of the flap and not projection from the wing under-surface, as plotted by Mr. Irving—for the whole point is variable'drag, and this plotting gives a truer picture. The curve for the split flap is the average of readings at 5, 9 and 14 degrees incidence, while the balanced flap curves are for 4 degrees incidence only, as there seemed to be a little variation with incidence on the modified Irving flap. The upper Irving flap curve is with the unsuitable hinge, and the lower full curve is the estimated optimum that could have been obtained. The curve for the modified Irving design lies on the zero line for the first half of the opening and then falls below the line, since it is in closing the flap that the operating work is done. Ease of Operation The areas within the curves represent work done, of course, and the figure for the modified Irving flap is 0.7 per cent, of the plain split flap work. This of course is an absurdly low value, and great care had been" taken to avoid friction in the design ; the result was that at 30 m.p.h. it- was difficult to feel the forces at all by hand, although the flap when fully open projected nearly 5 in. from the wing, and a number of independent experts who tried it found it difficult to believe. Translated into practice it means that the pilot could comfortably apply the flaps on an Armstrong Whitworth Ensign by means of a simple hand lever. Fig. 3 gives the variation of flap opening with setting of the controlling surface, and it will be seen that the curves for different incidences show no drastic or sudden changes—even beyond the stall. From the behaviour of the flap on the car model under these fairly comprehensive tests one would not have the least hesitation in flying an aeroplane fitted with this indirectly-operated flap. The drag arm length was 8.6 in. and the flap was hinged to the wing at a point 0.9 in. above the under-surface. For fuller details reference should be made to British Patent No. 476,673. * Improving the Slotted Flap The second flap, which is shown in Fig. 4, is an attempt to improve upon the well-known Handley Page type of slotted flap. Three objects were aimed at in this design : first, to obtain closer balance while permitting the flap to be mounted on a simple hinge within its contour ; secondly to reduce profile drag by giving a flush and reasonably airtight finish to the under-surface of the wing ; and, thirdly, to give a better slot shape by permitting easier entry of air to the slot, in the hope of giving more lift. It will be seen that the method is to hinge a forwardly- projecting balancing and fairing surface to the nose of the slotted flap, and to control this balance surface by linking it suitably to the wing. As the main flap is depressed the nose of the balance surface also goes down, and so permits air to flow through the slot; so that for take-off a good flow of air through the slot should be obtained with little drag, while for landing the balance surface enables the flap to be set to large angles with greatly reduced hinge moments. Points of Doubt Two points were originally in some doubt, namely, whether the downwardly projecting nose of the balance surface would cause a wake that would add to the drag in the take-off condition, and, secondly, whether the shielding of the flap by the balance surface as they approached each other would make it difficult to obtain a close degree of balance in the system ; on the other hand, the wide choice of size and of possible gearing arrangements seemed to offer scope for designing a good working compromise. Since the lift with a slotted flap begins to fall off after about 50 degrees flap angle the design was limited to the range of settings up to that angle. The results of the tunnel tests on hinge-moment are given in Fig. 5, and again for purposes of comparison a plain split flap of the same size was also tested ; that is to say, that at 90 degrees each flap projected equally from the wing. The wing section was, as used on the car tests already described, a Clark Y increased to 18 per cent, thickness but keeping the under- surface flat, and this will have a zero lift angle of about -8 degrees. To get as wide a flap chord as possible a short length of wing was fixed right across the tunnel, with the result that the stall began to develop at just over 10 degrees; due allowance must be made for these facts in interpreting the incidence values, and 5 degrees therefore represents the case of flying at approximately 1.2 times stalling speed while o degrees incidence represents a value of 1.5. Hinge Moments As before, the hinge moments are given in terms of the moment on a corresponding plain split flap at the same maximum angle—in this case 50 degrees. The slotted flap was tested without its balance surface and with the hinge at 0.25 of the flap chord and 0.075 AaP chord below the surface, and it will be seen that the hinge moments are about half those of the split flap, especially at the bigger angles ; the incidence of this test and of the plain split flap test was 5 degrees, and although tests were also made at o and 10 degrees the curves have been omitted, as in both flap cases they fell close to the 5-degree curves. Several balance surface variations were tried, and the best one is given in the three curves for o, 5 and 10 degrees incidence in Fig. 5, It was found that wide variations of balance or over-balance could be easily obtained, but in all cases there was a comparatively large change in balance with incidence ; this appears inevitable, as the balance sur face is obviously more effective at small incidences than near the stall, while at very high angles beyond the stall it would ultimately contribute nothing. In a way it is fortunate that the closer balance is obtained at the higher speeds, for this case can be designed for while the lower
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