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OVERVIEW of POWERED-LIFT TECHNOLOGY John P. Campbell

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OVERVIEW of POWERED-LIFT TECHNOLOGY John P. Campbell https://ntrs.nasa.gov/search.jsp?R=19780016104 2020-03-22T04:38:55+00:00Z . -. 4 OVERVIEW OF POWERED-LIFT TECHNOLOGY John P. Campbell The George Washington University, Joint Instftute for Acoustics and Flight Sciences - 1 SUMMARY l i il This introduc:ory paper is intended to set the stage for the conference by ! reviewing progress to date in the powered-lift'field. The concept and applica- tion of powered lift and the effects of some fundamental design variables are discussed. A brief chronology of significant developments in the field is also , i presented and the direction of research efforts in recent years is indicated. All powered-lift concepts are included, but emphasis is on the two externally ! blown schemes which involve blowing either above or below the wing and which 1 are now being utilized in the YC-14 and YC-15 airplanes. 'This review deals / i primarily with aerodynamics and vehicle design, and only touches briefly on the I areas of acoustics, propulsion, and loads. I INTRODUCTION It is perhaps appropriate to start this review with a bit of historical backg-ound which illustrates one of the factors that spurred interest in powered lift back in the 1950's. Richard E. Kuhn brought out this point very well by the use of figure 1 which is a history of maximum lift development from the Wright Brothers to the present day. The upper solid line shows that with the introduction of trailing-edge flaps and with the continuing refinement and sophistication of these flaps, the maximum lift coefficient CL,~~~obtained in wind-tunnel tests increased at a rapid rate up until the 1940's but at a much more modest rate afterward. Of course, the values of CL,,,, attained with operational aircraft lagged well behind the wind-tunnel progress at first, but it later became apparent that airplanes would soon be using up most of the mechanical-flap hizh-lift technology developed in le winds tunnel. This trend was foreseen by researchers in the early 1950's who recognized that the ceiling on CL,rnax obtainable with mechanical flaps could be bypassed by making full use of the energy of the turbojet propulsion engines to augment wing lift, as indicated by the dashed line. Exploratory research on the jet-flap principle was therefore started in an effort to realize this potential. In this jet-flap concept, a high-velocity jet sheet is turned downward by a trailing-edge flap and effectively increases the chord of the flap to produce higher lift. The total lift produced is made up of the three compone,lts shown in figure 2: the power-off lift produced by the wing and flap, the lift due to thrust deflection (that is, the vertical component of the thrust), and powered cir1:ulatlon lift which is the additional circulation lift induced on the wing ;;it\l hv the presence of the jet sheet. The proportions of the tl~reecorn- !:kx,1 LI?++ :, ~rtvary quite a bit, depending on the type of flap and the particular pof.t, l.c.,\-l lft concept used. POWERED-LIFT CHRONOLOGY 'i k~unb~rof different concepts have been studied as indicated by the ~?i~;~t~r~il-liftchro~lology presented in figure 3. Dates are shown for the first :iA:.t :.irc!l conducted on a given concept an3 For the first flight of an airplane incorporating the concept. The blowing boundary-layer-control (BLC) scheme illustrnted at the top is not usually considered a true powered-lift concept since it only xses engine bleed air and hence does not make full use of the available engine thrust. It is included here, however, because'of its basic similarity to the jet flap and because some of the work on blowing BLC provided useful information in the 1 devclopmtmt of the jet flap. Exploratory studies of blowing BLC were carried out as carly iis the 1920's but it was not until the 1940's and 1950's that sys- tematic research was condr~ctedthat lead to application of the concept. Some of the most impressive work was dons on the Navy's F9F-5 airplane in the early 1950's under the directioc of John Attinello (ref. 1). A number of other air- craft with blowing BLC have been flown, including the Boeing 367-30 airplane which was used by NASA for low-speed flight research in the early 1960's. (See The principle of the jet flap was proposed and verified by Schubauer in 1932, but very little attention was given to the c.otlct3pt until 20 years later when Attinello's studies in the United States (ref. 1) and Davidson's studies in England (ref. 2) showed great promise for the jet flap. This work led to extensive research programs on the concept in England, France, and the United States. (For example, see refs. 1 to 4.) The Hunting jet flap research air- plane (fig. 5) was built in the early 1960's to study the flight characteristics associated with the jet flap. (See ref. 5.) Unfortuuntoly, the airplane had a number of deficiencies which limited its usefulness as a research aircraft. In the late 1950's De Havilland of Canada initiated research on a variation . , of the jet flap called the augmentor wing. This concept incorporates a shroud assembly over the flap to create an ejector system which augments the thrust of the nozzle by entraining additional air. The augmentor wing was the subject of a comprrhensive rese~rchprogram carried out jointly by NASA and the Canadian governmrnt starting in 1965. This program culminated in the design and con- struction of the C-8 augmentor wing research airplane by Boeing and De Havilland. (See fig. 6.) The aircraft was first flown in 1972 and since th.it time has been used in n joint NASA-Ames and Canadian flight research frogram. (See ref. 6.) Both tr~?augmentor wing and the jet flap proved to bc very efficient aero- dynamically in that they produced a large increase in wing lift with a givc?n amount of engL~lethrust. But tbcy are intcrnolly blown systems and hence suffer the disadvnntilge of requiring iuternal ducting which adds to the weight, cotit, and complexity of the wing structurc. In an effort to eliminate internal ducting and to provide much simpler powered-lift systems, NASA Langley Research Center started work in the 1950's on the so-called "externally blown systems" - the externally blown flap rlsed with conventional pod-mounted engines, and the uppci- surface blown flap. Exploratory research was first carried out on the externally blown flap in 195b (ref. 7); research on the upper surface blown flap started about a year later (ref. 8). Initial results appeared to be promising for both concepts. A fairly extensive research program was carried out to develop the technology for the externally blown flap; but there were no indications of serious int~erestby the industry in applying the concept until Boeing incorporated it in its proposal for the C-5 competition. Although Boeing's entry did not win, this show of interest accelerated the research 011 the externally blown flan and led to an earlier build-up of the technology base required for application of the concept. The culmination of all this research is, of course, the McDL>nnell-Douglas YC-15 AMST (fig. 7) which his been flying since August 1975. As pointed out earlier, the initial results obtained on the concept for the upper surface blown flap in 1957 appeared to be prom!\-ing. The aerodynamic per- formance was comparable with that of the externally blown f lap, and preli1:linary noise studies showed it to be a potentially quieter concept beca:lse of the shielding effect of the wing. (See ref. 9.) However, since the upper surface blowing arrangement involved a change in engine location away from the generally accepted underslung pods and since there was at that time no special concern with the noise problem, research on the upper surface blown flap was dropped after the initisi studies. Research was resumed in the early 70's when it was becoming apparenL that the externally blown flap might have difficulty meeti,lg increasingly st rirl2er r noise requirements. Since that time, of course, research on the upper surface blown flap has been carried out at an accelerated pace; this research lead to che Boeing YC-14 AMST (fig. 8) wh ch will make its iirst flight within a few months and to the NASA quiet short haul research aircraft (fig. 9) which should be flying in about 3 years. As the conference procePds, you will note that there is special emphasis on the upper surface blown flap, for this is the concept which has been researched most extensively since the last NASA powered-lift conference held in 1972, PERFORMANCE Now, let us turn to some general performance considerations for powered- lift aircraft. The landing performance will be considered since it is generally more critical than take-off performance for these aircraft. Some of the factors irivolved in landing-field length are illustrates in figure 10. On this plot of wing loading against appmach speed and the corresponding operational field length, there is a family of curves representing different approach lift coeffi- cients. The band of valdes for 1.5 to 1.8 is for conventional airplanes with mechanical flaps. Note that these values are approach lift cocfiicients which are considerably lower than maximum lift coefficients because of the various angle-of-attack an3 speed margins required for safety of cpt,ration. The hatched area represents typical powered-lift conditions in the higl:,rr wing loading range and extends from field lengths of about 609.6 m (2000 ft) tt~ bout 1371.6 m (4500 ft).
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