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Aviation History
1962
1962 - 1140.PDF
(,1 FLIGHT International, 12 July 19(: LAMINAR REPORT Handley Page Progress in the Field of Boundary-layer Control By THE TECHNICAL EDITOR FOR 20 years it has been appreciated that the reduction in air craft drag which could be achieved by maintaining laminar (i.e., non-turbulent) flow in the boundary layer enveloping the aircraft surface is sufficiently great to warrant a great deal of research into the subject. One of the leaders in this field has always been Handley Page Ltd, whose director of research is Dr G. V. Lachmann. We discussed their work in our issue for April 13,1956. At this time the laminarized aeroplane appeared just around the corner, and globe-girdling projects were advanced in a spirit of optimism. But it proved much more difficult; only now, after a decade of toil, can the way ahead be seen once more. It was Gray, at the RAE, who first encountered the cause of the delays. He was engaged in work on the AW.52, a tailless jet specially built to close wing-profile tolerance in an endeavour to preserve laminar flow as far aft as possible. Mr Gray found that, while the desired result was obtained across the centre-section, the boundary layer over the slightly swept outer panels became turbu lent right at the leading edge—and remained so even when the skin was polished to a mirror-finish. It was hard to reconcile this with the laminar results obtained in tunnels, until Squire, Owen and Randall demonstrated the theory. Every streamline passing closely across a swept wing is deflected out towards the tip by the transverse pressure gradient over the leading edge. This imparts two components to the boundary layer: the classical vector parallel with the aerodynamic chord and a new transverse vector due to the lateral displacement. The profile of the latter contained a point of inflection, and resulted in the bound ary layer being inherently unstable. Owen calculated the growth of the cross-flow as one worked back from the leading edge and found transition points from laminar flow a few inches back. He found correlation with classic two- dimensional flow and, with others, determined the Reynolds" numbers associated with individual cross-flow profiles. Gray adopted the simple expedient of laying an extra metal sheet above the wing with a gap at its forward edge through which the initial turbulent layer could be sucked. But to keep the prime variable below the critical value for laminar flow needed huge suction. The latest configuration of the H.P.I 17, a laminarized transport con figuration discussed in the text. The span, length and height are respect ively 148ft lOigin, 149ft 6in and 41ft 9in; un-sucked areas are shown shaded. With 100,0001b fuel this aircraft could carry 82,0001b freight or 300 passengers over a 5,000-mile stage at Mach 0.8 with full BOAC reserves This apparent limitation to the laminarization of swept surface was reflected in an almost complete loss of interest on the part of governments and both civil and military operators. The RAE virtually ceased work, while the NACA (now NASA) opposes laminar research on a mistaken belief in critical sensitivity to surface condition. Only Handley Page and Dr Head at Cambridge plugged doggedly on, increasingly supported by a group at Northrop led by Pfenninger. It is chiefly to the credit of the latter that the work now promises to be successful. Pfenninger did not believe in the rigid applicability of the formulae derived for swept wings, and discovered that proper suction no: only reduced the thickness of the boundary layer but, by altering the cross-flow profile, gave it greater stability. Thus the maximum thickness allowable for stability was raised and the critical pars- meter changed by a factor of 4 or 5. This drastically reduced the necessary suction; in fact it has now been found that a fully laminar swept wing will have only slightly greater (5 to 10 per cent) more suction than a straight one, plus suction over the leading edge (which is not required on a straight wing owing to the favourable pressure-gradient, and lack of three-dimensional instability). Handley Page had set an upper limit on economic suction power. and had appreciated that the important factor governing the U environment is not chordal R but unit R (—). The latter has a v value of about 1,500,000 at Mach 0.8 at 40,000ft, equivalent to a critical height of roughness of the order of 0.006in—a perfect!) practical production level. At 50,000ft the critical - is only about 1,000,000. All that was needed now (say, 1958), was money to produce test hardware. Northrop have received a SI3m contract to fly two full- scale aircraft, described in our issue dated December 16, I960 and June 8, 1961. This work will give the US industry a full range of results at high Mach numbers, as well as essential experience in field operations and maintenance. In Britain there has naturally been no parallel full-scale attack or the problem, but traditional compromise on a shoe-string basis ma\ yet yield valuable data. Folland wanted a half Midge wing pressure- plotted, and offered it to the College of Aeronautics on the condition that the college did the work. The MoA offered the other half wing to Handley Page, but it was found preferable to design a new structure from scratch tailored to fit the college's Lancaster attachment. As our drawings show, the test wing is swept at no less than 45 over the critical leading edge. The latter is 13ft long, the root anc tip chord being lOOin and 68in (mean, 84), and the span from the root fence perpendicularly to inboard of the non-laminarized tip is lOOin. An important factor determining cross-flow is the t/c ratio, and this is 12.5 per cent. A symmetrical section has been chosen chiefly in order to compare results from different surface arrange ments on the two sides. Testing will be done over a range of lift coefficients by imparting varying yaw (attack) angles. Flying a: 150 to 200kt at 10,000ft will yield R of 1,400,000 to 2,000,000: equivalent to the same wing at from Mach 0.8 to over Mach 1 a: altitude. A laminarized wing must have suction ducts inside the leading edge and aft of the torsion box, both carrying away the air at some 200ft/sec. From these ducts the suction is applied through some form of spanwise corrugations underlying an outer skin bonded-or with thin slits above the sucking corrugations. If each corruganor were to run along the whole span the velocity would become impossibly high at the root, so it is desirable to insert chordwise collector ducts at intervals not greater than about 14ft. It i' also essential to control both the strength and the distribution of suction. The test wing is divided into a leading edge, a main section extending from 7 per cent to 50 per cent chord, and a rear portion Aft of the leading edge the surfaces are divided into inboarJ.
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