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Aviation History
1947
1947 - 1590.PDF
-312 FLIGHT SEPTEMBER I5TH, The Anglo-American Conference Three Further Lectures Summarized : Sweepback : Airscrew -Turbines : Helicopters Practical Design and Problems Arising from Sweepback By K. J. Richards, M.A., B.Sc, A.F.R.Ae.S.A FTER stating that there are manysweepback problems, particularly k on liigh-altitude transport aircraftwhich have not really been touched upon yet, the lecturer, who is ChiefAerodynamicist of Vickers-Armstrongs, Ltd., went on to give a broad review ofthe many problems involved in the use of sweepback. Mr. Richards agreed thatwhilst sweepback has a beneficial effect on critical Mach number, the quantita-tive gain to be expected is difficult to assess owing to the paucity of testresults available and thus the impossi- bility of satisfactory rules being pro-pounded. Mr. Richards then went on to dealwith the effect of fuselage and nacelles on sweepback characteristics and in thisconnection suggested that, as shown in the accompanying illustration which in-dicates the serious adverse effect that improperly designed wing nacelles couldhave on the critical Mach number, it would appear that nacelles will need tobe underslung or to be so well faired over as to be considered as part of thewing itself. There is thus urgent need for small power units which can beburied completely in the wings. On the score of wing weight, thelecturer observed that, apart from the increased structural span of wings withsweepback, consideration of aeroelastic distortion at the increased speeds willundoubtedly affect wing weight consider- be increased by about four or five de-grees of the all-up weight. The advent of sweepback, suggestedMr. Richards, is not going to postpone the necessity for the complete solution tothe change of trim problem in the presence of shock waves but, on the con-trary, is going to aggravate it by the addition of two more variables, sweep-back and speed : since also the speed will be higher, the onset of the changes oftrim can be expected to be more severe. Aircraft will, increasingly, be designedof necessity with a very limited scope of speed development so that it is impera-tive that engine designers should state boldly the anticipated development inthrust and fuel economy during the next ten years so that airframe designers canbe equally bold in allowing for them. In terms of manoeuvrability the lec- WE give hereunder and on thefollowing pages summaries of a further three lectures given before the joint convention of the Royal Aero- nautical Society and the American Institute of the Aeronautical Sciences which, opened by Sir Frederick Handley Page on September 3rd, ended with a banquet on September 9th, at the Guildhall, London. Additional lectures will be included in subsequent issues. turer stated that model tests on completeaircraft demonstrated that for the normal types of wing section, the maximum lifthas fallen to such an extent at the Mach numbers made possible by sweepbackthat it is impossible to fly straight and level without stalling. Tunnel tests atthe R.A.E. demonstrate this quite clearly and also show that there is an apparentadvantage from the use of lamina flow aerofoil sections on the wings. With sweepback, wing bending, aswell as wing torsion, gives rise to V^changes of incidence and unless consider-^"able c MACH N° Effects of body and nacelles on critical Mach number plotted against drag. ably, and it is thus difficult to estimatewith any great accuracy what weight in- crease arises directly from sweepback,but probably the best indication is given by an empirical lormula developed at theR.A.E. where, if the basic aspect ratio. t/c ratio and wing loading are re-tained, an increase of wing weight with angle of sweepback which is roughly pro-portional to the secant of the sweepback angle will be incurred. Thus, for fortydegrees sweepback the wing weight will gcare is taken at the design stage, losses of longitudinal stability of ten percent of the mean chord, can easily be obtained. This can quite easily resultin high speed instability which will dis- appear as speed is reduced and give wayto excessive stability at low speeds. Probably the best way to reduce this sta-bility loss, suggested Mr. Richards, is by designing the tailplane to distort in apredetermined manner. .With large sweepbacks, it may be worth while todesign the tail as an all-movement trim, elastically mounted without an elevator,longitudinal control being obtained by means of elevons on the wings. On the subject of design problems atsuper-critical speeds, Mr. Richards ob- served that the remarkable advancementof jet engine efficiency had produced the condition of our having to design forthese super-critical speeds on high speed fighters and bombers at higher Machnumbers than ever before with the added variable of sweepback. Wind tunnelsare of little use in this connection and, although the dropped model and the guided rocket had offered some allevia-tion, it cannot be gainsaid that de- signers will have to find out their prob-lems the hard way by constructing their aircraft and putting them right afterthey show their flying characteristics, and, whilst this is most unsatisfactory,it is, in the circumstances, the only way of proceeding. After considering themain suggestions for counteracting tip stall qualities, Mr. Richards went on toreview the other class of sweepback, that is, twenty to thirty degrees, in which tipstall is not apparent, citing the exampk that it is possible with a thirty degreesweepback to avoid tip stall if the aspect ratio is below five. On the subject of flaps, the lecturerstated that whilst a normal gain in maxi- mum lift is obtained with small andmoderate sweepback, the overall gain is zero or even negative with angles offorty and fifty, the flaps acting purely as an incidence reduction device. We have,therefore, to develop entirely from scratch new methods of lift increasewhich will still operate for these large angles of sweepback. In connection with gust alleviation,Mr. Richards agreed that whilst this problem is not in any way confined toswept-back wings, the advent of sweep- back has resulted in . flight at muchhigher speeds and lower wing loadings, both of which aggravate gust accelera-tions very severely, and there is an un- doubted need for some form of alleviationdevice which »will respond about twice as quickly as those at present envisaged.With swept-back wings it may be pos- sible to make more use of the inherentalleviation resulting from aeroelastic dis- tortion, but it is impossible to saywhether this can be accomplished with- out losing stability or increasing thedanger of flutter. The Propeller-Turbine Aero Engine By F. M. Owner, M.Sc, M.S.A.E., F.R.Ae.S.M R. OWNER who, as Chief Engin-eer of the Bristol Aeroplane Com pany, has been chiefly responsiblefor the design of the Bristol Theseus and Proteus airscrew turbines, after brieflyreviewing the specialist claims of th^ piston-engine / airscrew, gas-turbine / air-screw and pure jet forms of propulsion, in conjunction with the operating condi-tions favoured by each, suggested that, for the next fifteen years, cruising speedsof large transport aircraft will not exceed 500 m.p.h., and thus they will not flyat much above 40,000 feet. This is the acknowledged reserve of the airscrev.-turbine. In considering airscrew-turbine design,the lecturer stated that the prime require- ment in attaining thermal efficiency isto remove heat from the exhaust jet, since, although this reduces the jftthrust, the energy thus abstracted can be more efficiently utilized by being fed to(lie airscrew. Of the mear.3 whereby he?? energy may be thus converted the choic
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