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Aviation History
1955
1955 - 0041.PDF
14 January 1955 41 HIGH-LIFT GENERATION PART 2 By A. R. WEYL, A.F.R.Ae.S. AS stated in the concluding paragraph of Pan I of thisarticle, to develop aeroplanes which are able to" compete with the helicopter, three requirements will have to be satisfied: (1) High specific lift; (2) abolition of the stall, or, at least, incidence of incipient stall remote from operational maximum lifts; and (3) lift generation at zero for- ward speed. It can be shown that these three requirements are not beyond our reach. Aeroplanes with such features will, of course, be rather different from the customary high-speed type; and, in many ways, basic research is involved. High-lift Realities.—Substantially increased specific-lift forcesare quite possible so long as we do not insist upon low drag. Some comparative figures, though without claim to accuracy, will beindicative. A modern high-speed and low-drag ("laminar") aerofoil sectionhas a maximum lift coefficient of the order of 1.2, at about 13 deg critical incidence. Good plain cambered sections reach 1.6, at16 deg. Highly cambered aerofoils (like those of turbine blades) give 10 to 20 per cent more.A leading-edge slat, with a slot to energize the boundary-layer flow and to intensify the low-pressure region over the slat, mayadd 80 per cent to the basic lift maximum. Split flaps and trailing- edge flaps increase the effective camber and /or induce vigorousboundary-layer flow near the trailing edge; they may increase the basic-lift maximum by half or decrease the zero-lift incidence,but, unlike the slat, they do not increase the incidence of stall. More effective are extending-chord flaps, if (as is usual) the liftis referred to the original wing area; thus values of nearly 3.0 for the Cunaxhave been reached, at incidences of 13 deg. Slatsand slotted trailing-edge flaps, used in combination roughly double the basic lift. Flaps of all sorts are in practical use butslotted wings have become rare indeed, except for specific slow- flying aircraft. In a wind-tunnel test, a completely slotted aerofoilof eight slats and seven slots has produced a CLmax of 3.92 at 45 deg incidence. Wind-tunnel tests with mechanically more complicated double-flaps of the extending-chord type, combined with leading-edge slats, have given CLmax values of the order of 4.4, at 28 degincidence; no aircraft has as yet operated with such an arrange- ment and at such high lift. All these high-lift gadgets do nothing more than postpone flow-separation to higher effective incidences, i.e., they extend the use- ful lift range and, in some cases, make the lift-curve approximatemore closely to that predicted from theory. Power-assisted high-lift devices (Fig. 11) generally expend mechanical energy upon manipulation of the boundary layer, bymeans of suction or blowing, or by both in combination. About a dozen research aircraft have been flown with equipment of thiskind. Usually, ihe initial reports are enthusiastic, but subsequently nothing more is heard of any intended application. An exceptionmight have been an Arado 232 prototype with a combined suction and discharge arrangement and a wide, extending-chord flap. Thisshould have attained, in actual flight, a maximum lift coefficient of 3.8 at 10.5 deg incidence, and the handling properties werereputed to be tolerable in spite of very high wing-loading. The same high-lift value was achieved in flight with a modified Storchresearch aeroplane which had a suction flap of the Schrenk type. To the author's belief, these are the highest lift values everrecorded in steady flying.* In laboratories, higher Cuna* values have been observed withassisted devices. The operation of distributed suction slits on the dorsal surface of a cambered aerofoil of 30 per cent thickness gave(with substantial end discs) a value of 6.1 at 70 deg incidence. Very high lift coefficients can be produced with rotatingcylinders (Figs. 12 and 13). A. G. von Baumhauer reported measured values of up to 15, and theoretically higher values canbe expected. This "rotor" lift is entirely independent of in- cidence, and thus free from stall. Circular cylinders, however, arenot very practical as lifting aerofoils. The circulation depends on peripheral velocity; hence, the rotors are clumsy and very heavyon account of their great diameter; or, if they are kept slender, very high rotational speeds are required. The installation of rotorsin aerofoil shapes has proved a dismal disappointment, on account of disregard for boundary-layer effects; with a leading-edge in-stallation, as the optimum arrangement, a CLmax of only 2.4 was recorded, and the profile drag was exceptionally high. An interesting suction wing by B. Regenscheit, of Gottingen(Fig. 14) produced particularly high lift. The trailing edge of a normal N.A.C.A. 23015 section was obliquely cut so as to accom-modate a suction slot instead of the trailing edge, and a flap was deflected by 45 deg aft of it (Fig. 15). With this, a CLmax of notless than 12.5 was reached, at 21 deg incidence. Unfortunately, the power required to operate this supercirculation at a full-scalewing would amount to about 6,300 b.h.p. It is interesting to speculate upon what such an enormousexpense of mechanical energy is expended. Suction is, of course, less economical to produce than pressure energy (discharge *The Supermarine 322 "Dumbo" of ten years ago recorded 3.9in controlled flight. Further Details of the Arado and other systems appeared in our issue of November 12th last.—Ed. iiiiiiUniiiiiniiiiiiiliiiiim ii iluiuiu i nui III in 11 in i miii MMTTT77 Fig. 11. A short history of pneumatic high-lift devices. (A) L Prondtl, 1904: Suction slit in water channel. (B) De Bonnechose, 1913: Discharge (pressure) jet to accentuate low pressure over dorsal surface. (C) A. Baumann, 1921: Discharge. (D) J. Ackeret, 1926: Multiple suc- tion jets. <E) K.A.E. (Perring and Douglas), 1927: Suction and discharge. (F) C. C. Jones, 7936: Suction and discharge for very low speed. ADJUSTABLE DEFLECTOR SLAT
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