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Aviation History
1957
1957 - 0010.PDF
10 FLIGHT, 4 January 1957 TRANSONIC TUNNELS ... First of the afternoon papers was Some Aspects of TransonicTunnel Operation in Industry, by Mr. F. E. Roe, B.Sc.(Eng.), D.I.C., of English Electric. Early in 1950, Mr. Roe said, theoriginal jet-driven tunnel of his company was converted to transonic operation. The working section was reduced to onesquare foot, surrounded by a 3ft x 2ft plenum chamber, and various slotted and perforated walls were tested. More successwas achieved with slotted walls, and these were used for a year for model testing before further experiments with detailed modi-fications resulted in a maximum Mach Number of 1.22 and an improved Mach-number distribution. Interference effects were first noted in tests on a scale pitot-static head, and a limited series of tests were then undertaken on a number of simple bodies of revolution. These tests revealedthat the flow pattern around the bodies could be modified by the slotted walls, and that the interference effects increased withmodel size and bluffness. The interference effects were pro- nounced at both subsonic and supersonic Mach Numbers.Further data showed that model blockage played an important part in the validity of force data measured on a typical aircraftmodel. Nevertheless, in spite of the uncertainty of interference effects, sufficient correlation had been obtained between wind-tunnel and flight-test results on the P.I aircraft to encourage the use of slotted-wall tunnels in the transonic range. The final paper, presented by Mr. F. OTiara, MA., of R.A.E.,Bedford, was entitled Notes on the Assessment of Results Obtained in a Transonic Wind-tunnel. In it, the speaker statedthe need for a comparative assessment of the possible overall errors in force and pressure measurements in relation to desirablestandards of accuracy. Problems of flow uniformity, accuracy of model manufacture,and interference effects were discussed in turn, and details were given of the transonic working section of the 3ft tunnel atR.A.E., Bedford. After comparing results obtained in the tunnel with thosefrom free-flight model tests, Mr. CHara went on to discuss illus- trative examples of model results with a number of full-scaleflight comparisons. He concluded by commenting on the prob- lem associated with maintaining representative flow conditionson sting-mounted models. Final Discussion Period General discussion on the dav's topics was opened by Prof.A. R. Collar, chairman of the wind-tunnel design committee of the Aeronautical Research Council. Among his questions werethe following: How were contraction sections for transonic tunnels designed? Was the shape of diffusers based on experi-mental work or largely arbitrary in design? What proportion of the total available power was needed for the auxiliary suctionplant? Assuming two ventilated walls and two solid, could block- age interference be eliminated, leaving only Shockwave reflection?Was not direct measurement of tunnel speed possible, by spark observation or measurement of sound? Following Prof. Collar's remarks, Mr. Hills said the A.R.A.contraction shape was a modification of an existing contraction, and mat it had been model-tested. The shape near the nozzle was the most important consideration. A shape equivalent to a five-degree cone, later modified to 5i deg, had been the basis of the diffuser design; it was not thought that model tests could helphere because of Reynolds Number effects. Auxiliary power amounted to rather less than half the main-motor power; as thepressure ratio was 3 :1, the actual power required was high. He did not think more-elaborate methods of speed measurement wereneeded. Mr. Vessey thought that two ventilated walls would be sufficientto avoid blockage. For ordinary model tests, plenum-chamber pressure would be used for speed measurement for some timeyet. If one had a normal contraction and not a supersonic nozzle, about half the fan power would be needed in auxiliary suction.Mr. Kirk agreed that more design work could be carried out on contraction sections. Mr. Roe, speaking of speed measurement,said that his company had tried pressure measurements on the tunnel wall (at the point where the flow stabilized) and in theplenum chamber. If the model affected the plenum-chamber pressure, he claimed, the plenum-chamber pressure affected themodel, and results were therefore questionable. Another speaker suggested that the length of working sectionsshould be at least four times their height. From experimental evidence, the pressure of two plain walls in a ventilated workingsection seemed to affect the pressures on the model very little. Was the testing of half-models a valuable technique? The mainspeakers said they did intend to continue half-model testing; Mr. O'Hara called attention to the effect of leaks around the wall-root, and the effect of low aspect ratios. The power requirement expressed in relation to working-section area, the subject of another question, was stated by Mr. Hills to be about 430 h.p. per sq ft at M = 1.4, for the A.R.A.tunnel, and (by Mr. Vessey) about 400 h.p. per sq ft at M= 1.2 to 1.3 for the R.A.E. 8ft x 6ft tunnel. Dr. Hilton's estimate for theArmstrong Whitworth tunnel was about 2,000 h.p. per sq ft, however. "There is quite a discrepancy here," he commented.But pressure ratio, not power, was the important factor. Work at the College of Aeronautics on the use of vortex stripsahead of half-models to reduce boundary-layer effects was men- tioned by another speaker, who submitted that the tricky calcula-tion was to determine the appropriate model size for a given working section. What Reynolds Number had been aimed at inthe design of the A.R.A. tunnel? Mr. Hills replied that a Reynolds Number of about 45 x 10*,based on wing chord, had been the objective, but this figure did depend on the type of model used. Mr. Vessey said that theCranfield method of "sweeping away" the boundary layer had been followed up at R.A.E., Bedford, with interesting results. Itappeared a useful device for low-speed tests. Other questions referred to the possibility of employing strutsfor model-mounting in addition to stings; the problem of the re-entry of the air from the plenum chamber into the tunnel; andthe permissible blockage percentage if a limited range of tests were acceptable. The day's discussions had provided a useful insightinto transonic testing problems—limited only by the "security" considerations mentioned so often during the day by the chairmanof the meeting. The full publication of the main papers in the Royal Aeronautical Society Journal will record many aspects ofthese problems in this country. What is immediately obvious is that there is a large amount still to be learned. REMAINING RARITIES (continued from page 8) them could be resurrected to flying standard in the unfortunateevent of LF363 coming to untimely grief. The Spitfire is ixi a slightly happier position, and a few con-tinue to render their daily services—though for how much longer one prefers not to think.* Everyone knows of the specimenstationed at Duxford and used each year to share the lead of the Battle of Britain fly-past with the lone Hurricane; but how manypeople realize that four or five Spitfire L.F.16s purr round the London area on Army co-operation work from Hornchurch, orthat a similar number of P.R. 19s is based at Woodvale, in Lan- cashire, and that each morning one climbs to about 40,000ft toobtain high-level met. information? These last few remaining 19s were drawn from the ranks of No. 541 Squadron, the lastregular unit in this country to operate Spitfires and which gave way to the modernity of Meteor P.R. 10s only in 1951-2, whenit left its base at Benson and moved east to form a pan of B.A.F.O. Apart from those Spitfires flying in the Service, a Griffon-powered Mk 14, G-ALGT, still operates as an engine test-bed for Rolls-Royce at HucknaLl, while a Mk 5b, once on the civilregister as G-AISU and formerly flown by G/C. (now A. Cdrc.) A. H. Wheeler, is preserved by its manufacturers and flies today *No longer; since this paragraph was written the Air Ministry hasannounced the •withdrawal of the remaining regularly active Spitfires (see page 30).—Ed. (albeit occasionally) in the military markings of its time and withthe serial AB510. On the other hand, G-AIDN, the prototype Spitfire T.8, made its last flight last July, when V. H. Bellamy col-lected it on a ferry permit to place it in its permanent resting-place with the Hampshire Aeroplane Club at Eastleigh. Lastly, we have the Tiger Moth. Although dozens are flyingwith clubs and a few are privately owned, the type was officially declared obsolete by the R.A.F. just over two years ago. TheRoyal Navy, however, are not always in such haste to dispose of their better aeroplanes; in addition to preserving a Swordfishand Seafire 14 they have just purchased four Tiger Moths from civil sources to use for glider towing and for giving air experienceto cadets and midshipmen. Two of these Tiger Moths, with the ultra-modern serials XL714 and XL715, were delivered recentlyafter overhaul by Hants and Sussex Aviation at Portsmouth. Several other military-type aeroplanes are maintained by theirmakers, among them Hawker's veteran Hart G-ABMR, Fairey's Fulmar G-AIBE (the original prototype), Gloster's GladiatorG-AMRK, and a few others. While it is to be hoped that these may last for many more years one cannot have title same con-fidence in the future of those machines still in Service hands; but presumably we are wasting our time in reminding the Air Ministrythat time marches on and that only action now can give us a Lan- caster, Mosquito, Beaufighter, Hurricane and Spitfire for per-manent preservation.
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