The Aerodynamic Characteristics of Flaps

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Notation--continued A C,~ (st) Increment in value of C,, at the stall, due to a slat A(dC,~/dCr)o.SCrmax Increment in gradient dC,,/dCL measured at CL ---- 0"SCLmax due to a slat F(A) K Function representing variation of lift-curve slope with aspect ratio (see Fig. 3) Function required to determine induced drag of flapped wings (see Figs. 18, 19, 20) t~ Normal distance from wing surface to chord line in plane of flap hinge l Maximum wing thickness s~ Tail semi-span y~ Distance of inboard end of flap from centre-line of aircraft Wing incidence ~0 .Incidence for zero lift /~ Flap angle Angle of sweepback F Tan 1, $ 2, ~ ~ Functions required to determine the profile drag coefficient increment of a flap (Figs. 10, 11, 12) 21, ~, ~, ~2~ Functions required to determine the lift coefficient increment of a flap (Figs. 5, 6, 7, 8, 9) /~1, ~2, #3 Functions required to determine the pitching moment coefficient increments of a flap (Figs. 13, 14, 27) s Downwash Suffices 1 and 2 refer to the first and second flaps, respectively, of a flap combination, suffix w refers to the wing alone. 3
1. Introductio~.--The invention of the flap dates back to the very early days of aeronautics. It may be said to have been born with the aileron when the latter was first used to replace wing warping as a means of providing lateral control. From then on, the possibilities of the flap as a device for increasing lift or drag must have been apparent to many. As early as 1914 quite extensive model tests were made at the National Physical Laboratory by Nayler and others 6 on a number of aerofoils equipped with a form of large chord plain flap, and plain flaps were used on a number of types of aeroplanes during the first World War. Since then, various forms of flaps have been developed and tested all over the world, ranging in complexity from the simple split flap to large chord high lift flaps of two or more components. A detailed historical summary of the early developments of flaps will be found in Ref. 5. The flap only became a device of practical importance, however~ in the early 1930's Very rapid advances had by then been made in the aerodynamic efficiency and cleanness of aircraft, accompanied by rapid increase in wing loadings. In consequence, gliding angles had decreased, and landing and take-off speeds had increased, until landing and take-off difficulties threatened to offset the advantages of further aerodynamic improvements. The flap was then adopted as the obvious solution, as it provided both lift to reduce the stalling speed and drag to increase the gliding angle. For take-off, the advantage of the extra lift due to the flap had to be weighed against the disadvantages of the drag, but it soon became clear that even with the simple split flap there was something to be gained at take-off when a moderate setting of the flap was used. The development of the slotted flap was then stimulated since it promised a relatively small drag increase at moderate settings for take-off, with a drag increase that could be made comparable to that of a split flap at large settings for landing. The demands of the Fleet Air Arm, with its stringent landing and take-off conditions, have hastened the development of flaps giving large lift increments. To distinguish such flaps from the more normal and less ambitious varieties, they will be referred to as high-lift flaps. In recent years many forms of high-lift flaps have been produced, some of which are in current use. They all involve some degree of effective wing-area extension and, in consequende, a certain amount of mechanical complication and extra weight, which must be taken into account in estimating their value for any particular design. The more ambitious of these flaps promise maximum lift coefficients of the order of 4.0, and in combination with wing leading-edge slats lift coefficients of the order of 4.5 are possible 3. During recent years much of the data accumulated in research laboratories all over the world on flaps of various kinds has been sifted and analysed (see, for example, Refs. 3, 4, 39, 60). Enough order has been extracted from these data to make it possible to predict with reasonable accuracy the aerodynamic characteristics of flaps, at least of the simpler kind. The main purpose of this report is to collect in a convenient form the results of such analyses, and, in addition, to cover a few aspects of the subject that have so far not been adequatel3; dealt with. With the acquisition of war-time German research data it has become clear that sweepback may offer considerable advantages for high-speed designs. It is, therefore, desirable that some reference to the characteristics of flaps on sweptback wings should be made in a review of this kind. Unfortunately, the available data are neither systematic nor Comprehensive enough to enable adequately reliable quantitative rules to be deduced from them. The discussion given in section 13 is offered as an interim summary of these data, with the reservation that future research may render the conclusions out of date and possibly incorrect. Apart from this section, the report is confined entirely to flaps on wings without sweepback. 2. Brief Descriptions of Types of Flaps Considered.--2.1. Plain Flaps.--If a portion of the rear of a wing is simply hinged, then that portion' is defined as a plain flap (see Fig. la). The plain flap is the starting point for all forms of control, and its aerodynamic characteristics will therefore be dealt with very fully, at least for angular movements up to about 20 deg, in the monographs dealing with controls. It is not, therefore, proposed toconsider plain flaps in any great detail over that range of angular movement in this monograph.
Page 1 and 2: "'% ~ ~.:~1 _~ ~,-:~.~-,~ ~,~, L',t
Page 3: A Aspect ratio Lift-curve slope Not
Page 7 and 8: m Since the optimum slot shape is a
Page 9 and 10: In the case of a split flap set for
Page 11 and 12: Hence ACre' = AC,,, CLc --2 -- C ..
Page 13 and 14: here estimated for an aspect ratio
Page 15 and 16: partially retracted positions, the
Page 17 and 18: the type of chordwise lift distribu
Page 19 and 20: (2) With the arrangements illustrat
Page 21 and 22: Kruger's nose ftap is illustrated i
Page 23 and 24: Generally speaking, a lower surface
Page 25 and 26: 13.3. Maximum Lift Coefficient Incr
Page 27 and 28: formula for A C,,o is generally cor
Page 29 and 30: 14.10 Flaps on swept-back wlngs.--O
Page 31 and 32: Main TABLE 2 Characteristics of Hig
Page 33 and 34: BIBLIOGRAPHY No. Name 1 Alston :. .
Page 35 and 36: No. N~,lytg B IB L I OG RAPHY--cont
Page 37 and 38: No. Name 77 Wenzinger and Harris ..
Page 39 and 40: N~me Purser and Turner .... Davis .
Page 41 and 42: PLAIN WING . . . . WING WITH FLAP~
Page 43 and 44: 3-0 2.O ' I / ~-- ¢/~= . i/ , / TH
Page 45 and 46: 17"/C ---- O-'l~ / t/.. ,~ 0- 30 3.
Page 47 and 48: I*~- m I,O Zt, J I TAPER RATIO oo
Page 49 and 50: ,55 ° • 0"2 (o.~ BLACKBURN SLOTT
Page 51 and 52: NOSE FLAP (CL) ILLUSTRATION OF KRU(
Page 53 and 54: , +o.,~ +~1 t .A~/l I I F~A'~- . I.
Page 55 and 56:
O,0~5 - - ~ I O RECTANGULAR WINrm X
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The Aerodynamic Characteristics of Flaps

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