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Aviation History
1948
1948 - 0042.PDF
FLIGHT JANUARY 8TH, 1948 NO SWEEPBACK I =—a 1 AH -3 6 AR»5O AR-Et8 07 O8 MACH No. 0 9 1O ! 1 1 1 1 j ASPECT RATIO - 5-8 WNG THICKNESS-12% 4O* SWEEPBACK A , (Left) Fig. S. Effect "A- of aspect ratio and sweepback on aero- dynamic centre. The aerodynamic centres are measured from mean quarter-chord point. (8 CM^ is -— r**1^^ \ \ 1 NO SWEEPBACK\ • 1 20'SWEEPBACK 1 35 ' SWEEPS ACK ( 45» SWEEPBACK REY. N..-I-2 « 1O*. — O-6 O-7 O8 O-9 MACH No: , —TFOR AR-3 0 the \J WING THICKNESS-MX ASPECT" RATIO - 3 O -0 1 ot -ttfcl a AR - 3 0 j f \I — I FOR/ ) WING THICKNESS -12% 50' SWOBBACK A WING THICKNESS- 127. •5'AWING 218 local M value at ( NO SWEEPBACK \ 4O* SWEEPBACK _ / C 0-6 07 08MACH N>. 0-9 10 (Right) Fig. 6. Effect of aspect ratio and sweepback on lift- curve slope, a (lift slope per radian) is the local value atCi,=O.I —i 41 slO SWEEP6ACK W SWEEPBACK ,, 5O* SWEEPBACK ° j@ REY. N« 75«IO* 1 \ 1 0-6 07 OB 09 10MACH No. WING THICKNESS-12% High-speed Research Number it is also necessary to know the absolute pressure inthe working section. For pressures below atmospheric this is measured by means of a special barometer which coversa range down to two inches of mercury and can be read to within o.oiin, while for pressure above atmospheric aBourdon type pressure gauge gives sufficient accuracy. For general work, pressure distribution measurements, orexploration of temperatures, groups of manometer and thermocouple leads have been built into the tunnel, therecording equipment being located in the observation room and permanently connected to distribution boards situatedin the dead space. The next section of the paper dealt in considerable detailwith blockage corrections, strut corrections and choking. Further data are required before a detailed analysis ofresults can be made for the ordinary routine tests on a single aerofoil, but it is obviously desirable that work ofthis nature should continue, since ultimately it will allow the- tunnel to be used well into the regions of mixed suband supersonic flow. , Work of the Tunnel Since its official opening in November, 1342, the tunnel has been in continuous operation, except for a break of about a month in 1945, when it was shut down so that the rolling, yawing and side-force units could be added to the main balance and the mixing vanes fitted to the return circuit. During the first three years the tunnel was run on an average between 90 and 100 hours per month. It has since been necessary to run with a reduced staff, but even so the tunnel has continued to operate at the rate of about 30 hours per month. It has never had to shut down due to failure of any part of the plant. For much of the time it has been used for testing complete models, and every important high-speed aircraft built during this period, and many built before then, has been tested in the tunnel. The paper dealt at considerable length with the prob- lems of research on the effect of sweepback and of com- pressibility on longitudinal stability. The latter has been studied theoretically by Gates and Miss Lyon who, in a generalized theory have set out four important criteria: the static margin, stick-fixed ; the static margin, stick-free ; ttie manoeuvre margin, stick-fixed; and the manoeuvre margin, stick-free. So far only the stick-fixed margins can be measured in the tunnel. The stick-fixed static margin !REY.Ko.-0-5x10« 46 AWING : ASPECT RATIO-3 0 WING THICKNESS-IOSJ 06 0-7 0-8 No. 09 is proportional to the change in stick position required to trim in steady flight to a higher or lower speed than the initially trimmed speed; the stick-fixed manoeuvre margin is proportional to the stick movement per g. At small angles of incidence and on conventionally shaped wings, the aerodynamic centre tends to move forward as the speed increases. At first the movement is slow, but above Mach Numbers of about 0.75 it can become fairly rapid. Ultimately, as the speed is further increased and becomes supersonic, the aerodynamic centre moves back again and tends towards 0.5 of the chord. Designers of high-speed aircraft are now particularly interested in the effects of sweepback. Fig. 5 shows some test results. The effect of sweepback is favourable, the curves of aerodynamic centre having good characteristics up to speeds of about 0.85a. Above speeds of-0.85a, or'* both swept-back wings and delta aerofoils, the aerodynarrfe '^ centre moves rapidly rearwards. In practice, this will result in a large increase in manoeuvre margin. The effect need not embarrass the designer, provided the aircraft can be ke.pt in trim and the controls remain effective and power- ful enough to overcame this general increase in stability. The paper stated that on the subject of controls, further tests had confirmed the results of earlier work, viz., eleven controls on swept-back and delta wing forms ceases to be effective at about 0.93 of the speed of sound, and at still higher speeds there is a reversal in control effectiveness. During recent tests on aerofoils of various planforms and thicknesses the results shown in Fig. 6 were obtained. The fall in lift slope at Mach Numbers of 0.8 in the case of wings without sweepback is apparent, and it is inter- esting to observe that reducing the aspect ratio, while it results in reducing the slope of the lift curve at low speed, also results in a less marked collapse in lift slope at high speed. The lift slope of swept-back wings and delta plan- forms are reasonably maintained, and on the whole improve over the range of speeds it has been possible to test. On the subject of the contribution to stability made by a tail, the paper pointed out that several factors combine which enable the tail to remain effective at high speed.
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