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Aviation History
1953
1953 - 1334.PDF
49© FLIGHT, 2 October 1953 THE ANGLO-AMERICAN CONFERENCE . . . RECENT ADVANCES IN BOUNDARY LAYER AND CIRCULATION CONTROL By C. D. PERKINS and D. C. HAZEN Professor and Ass. Professor, Dept. of Aeronautical Engineering, Princeton University) THE paper was read by Mr. Hazen. He began by discussing the various types of system possible, and illustrated eight of these with excellent flow photographs from a smoke tun nel. (Boundary layer and circulation control can be used to reduce flow separation and thereby increase lift; to control transition of the boundary layer from laminar to turbulent, so reducing drag; or to increase the aerofoil circulation directly, thereby increasing lift at a given angle of attack.) Drag reduction by controlling boundary layer transition had been studied by Mississippi State College, under Dr. Raspet, and a descrip tion of the sailplane used and results obtained was given. The loci of transition between the laminar and turbulent boundary layers was determined with a stethoscope, the former giv ing a hiss and the latter a roar. Eventually, the work showed that even very low suction could reduce total drag—by 54 per cent, compared with a smooth, impervious profile—provided the porosity distribution and suction power just maintained laminar flow without reducing boundary-layer thickness. Artificial thicken ing of the layer near the leading edge might be required at high speed. The use of a trailing-edge suction slot to control circulation had been studied by Regen- scheit, whose work was continued by various American groups. The lecturer gave a theoretical analysis of trailing-edge suction lift effect and included a wide range of results of lift, drag and pitching moment investigations on ten trailing-edge configurations. He con cluded that, taken simply as a flow singularity added to a profile, trailing-edge suction offered possibilities in the field of aerodynamic control for fixed-wing aircraft, or as an alternative to cyclic-pitch control on helicopters. When com bined with a split flap it promised to be an efficient high-lift device. As one of the groups involved in trailing- edge suction experiments, Princeton University had been led into what the paper described as "one of the most interesting and potentially important developments in subsonic aero- CONDENSING the salient points of his written paper into a 30-minute talk, Dr. de Bruyne began by referring to the structure weights of present-day aircraft, quoted pre viously by Mr. Hibbard to be in some cases 22 per cent of the all-up weight. This was a very low figure compared with those of ten years ago, Dr. de Bruyne continued, and yet a further reduction in structure weight was necessary. One attempted method has been to increase the design stresses, but as this was accompanied by a steady fall in fatigue life, other ways of reducing weight were required. Adhesives were one such way. Apart from the saving in weight, the advantages of ad hesives included a smooth external finish, in crease in fatigue life, simplification of design, and reduction of production costs and times. Giving examples of the use of adhesives in existing aircraft, the speaker showed a section of the Britannia leading edge structure, with an inner corrugated skin Redux-bonded to the smooth, thin outer skin which was thickened at the points of attachment. A more ambitious use was found in an all-metal spar produced by the Fokker ComDany; on this, the Redux bonding was done in a flat hydraulic press, and the flanges formed in a rubber die press. In the Comet, there was much Redux bond ing; for example, extruded stringers were Redux-bonded to the fuselage skin. The pres sure bulkheads of the Comet and Britannia, and the trailing-edge reinforcement of the dynamics"—the determination of a new means for creating and stabilizing a vortex at a desired location. Wind-tunnel turbulence was responsible for this, and a diffuser was eventually tested with small expansion angle, leading into a larger chamber via a sudden jump in cross-section, there being large cusps at the tunnel walls. It was found that these cusps housed two trapped vortices, at right angles to each other, the resulting complex flow within the cusps being free from turbulence. Further experi ments with cusps utilizing automatic vortex formation involved aerofoils; the lecturer went on to describe their behaviour under various conditions with increasing suction. The most notable feature was the powerful effect of the vortex on the downwash. In one form of profile /cusp it was possible to obtain adherence at an angle of attack of 120 deg. Princeton was continuing this work, and the lecturer pointed out that—particularly as 1954 marked the 50th anniversary of Prandtl's original concept of flow control—the time had arrived for the construction of a practical, powered aircraft designed around a flow- control system. Opening the discussion, DR. W. F. HILTON (Armstrong Whitworth) dealt at some length with the behaviour of trapped vortices, and pointed out that their presence should reduce the length and cost of a tunnel diffuser sec tion. PROF. SIR MELVILLE JONES favoured perforated suction surfaces, for he felt the former could more readily be kept clean. He urged intensive study of all types of intered or perforated surfaces. PROF. A. D. YOUNG (Col lege of Aeronautics) underlined the importance of smooth surfaces: if the Reynolds number of an ^regularity exceeded 50 the flow would break down : this meant that the profile had to be correct within O.OOlin on a 10ft chord for laminar flow, or within 0.0003in. with suction. MR. G. S. SCHAIRER (Boeing) was concerned, as a practical manufacturer, with the cost of such systems; he also queried the behaviour of the boundary layer near shock waves. Convair B-36, were also indicated by the lec turer as examples of Redux use. Dr. de Bruyne then continued to the second part of his lecture, dealing with what existing adhesives could do, given intelligent design, in future aircraft. The wrinkling experienced when stringers were used could be overcome by a form of continuous reinforcement, for instance, sandwich construction, such as the balsa-cored sandwich used by Chance Vought. Of the synthetically expanded cores, the "honeycomb" type was very successful, al though solid forms were convenient to use. After giving details of tests on the structural efficiency of Redux-bonded honeycomb panels, the lecturer stated "the economic significance of the increase in efficiency is remarkable." He concluded by showing films of British and American peel tests on honeycomb sandwiches. In Dr. de Bruyne's written paper were given details also of methods of surface preparation, application of pressure and heat, inspection procedure, the effect of geometry on the strength of glued joints, fundamentals of adhesion, and detailed test results. The general discussion on structural ad hesives was opened by MR JOHN J. FOSTER (I.A.S.) who reported on his own experience which showed that honeycomb structures of sin cell size gave peel strength test results as high as 100 in lb. High-temperature ad hesives were being developed, he continued, and adhesives and sandwich construction were being used for primary structures. DR. D. WILLIAMS (R.A.E.) traced the progress of work at Farnborough on sandwich construction, and mentioned the difficulty of discovering failures in a structure incorporating sandwich construction. MR. F. TYSON (Handley Page) enquired whether Redux-bonding would be sufficiently strong for use with the stronger cores which were being developed. A form of bonding requiring neither test nor pressure was the ideal, he maintained. Work performed at Imperial College, London, on bonded struc tures was briefly described by DR. J. H. ARGYRIS, and among other subjects discussed were the relative merits of sandwich, integral machined and integral forged construction; the need for ribs in sandwich wing structures, the use of Redux in helicopter blade construc tion; and the use of sandwich construction for prototype aircraft. THE CONTROL OF FLIGHT By PRESTON R. BASSETT (President, Sperry Gyroscope Corp.) MR. BASSETT, in his paper, gave a sum mary of the development and application of instruments to the control of flight over the past 50 years. He divided his lecture into a discussion of what he termed as the "three layers" of instrument development. The first layer covered the introduction of instruments as an aid to the pilot, when the perfo-mance of his aircraft reached such a level that his own senses were no longer able accurately to interpret its performance in relation to the ground or the horizon. During and immediately after the first world war, the inevitable step forward was tackled; the problem of completely blind flvine had to be solved. With the introduction of gyroscopic instruments, regular passenger- carrying operations became a possibility, and parallel development during the years 1929-33 in passenger aircraft design made this a reality. It was in Layer 2, also, that the autopilot was successfully introduced into service as a relief for the human pilot. Layer 3, Mr. Bassett stated, covered the period up to the present day in which the three elements of blind operation, the pilot, his instruments and the autopilot, were co-ordinated into a team. Regarding the essential factor in airline opera tion to be the success of the approach at the destination, he proceeded to enumerate the lines of development which had been and were being followed in solving the problem of instru ment approach. By means of the flight direc tor, or zero reader, the pilot could now make an accurate instrument approach down to the point at which he, better than any machine, could determine the success of the approach. Discussion.—In the ensuing discussion, various suggestions were put forward for easing the pilot's problem in the final transition to visual landing. MR. IVOR BOWEN (M.O.S.) suggested a television picture projected on to the windscreen, but the main suggestions put forward were for a pilot-supervised autopilot approach designed to give the human pilot the ability to concentrate more wholeheartedly on the final visual take-over. MR. C. F. JOY (Handley Page) suggested the possibility of an aircraft designed to land itself without either autopilot or human aid. MR. A. C. CAMPBELL-ORDE (B.O.A.C.) struck nearest to the bone in suggesting that the vital factor in giving the pilot time for the visual take-over lay in reducing aircraft approach speeds. Mr. Bassett, in answering the discussion, stated that, so long as passengers were carried in air craft, the human pilot would have to accom pany them, although future lines of develop ment indicated that he would have a super visory rather than an executive function in aircraft management. However automatic flight became he would never become totally redundant, and he would always require a set of basic instruments with which to check the functioning of his automatic equipment. Future progress in instrumentation, he concluded, must lie in development of flight director-type systems and development of pictorial map position indicators. STRUCTURAL ADHESIVES FOR METAL AIRCRAFT By Dr. N. A. DE BRUYNE (Managing director, Aero Research, Ltd.)
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