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Aviation History
1958
1958 - 0718.PDF
14O STANDARD NOZZLE STANDARD NOZZLE WITH 57in TUBE (3in) STANDARD NOZZLE 75 15O 3OO 6OO 12OO 24OO 15O 3OO 600 I2OO 24OO 48OO FREQUENCY - OCTAVE BANDS (c/s) 48OO 96OO 734 FLIGHT, 30 May 1958 (Left) Rolls-Royce tests on ejector- type nozzles. Measurements were taken 50ft from the end of the silencer tube at 30 deg to the jet axis, with an efflux velocity of l,850ft/sec. (Right) A suggested ejector and thrust-reverser combined in a rear- fuselage engine installation. PLAN VIEW RETRACTABLE EJECTORJ THRUST REVERSER NACELLESIDE VIEW COMBUSTION AND PROPULSION . . . suppress the high-pitched sounds. An ejector shroud is anothermeans of sound reduction, but to be effective must have the almost prohibitive length of four or five nozzle diameters. Prof.Richards suggested a ducted fan as the most effective extractor of momentum from the exhaust. [The scheme illustrated by thelecturer resembled that of the original Power Jets/Whittle design of circa 1946.] Changing the nozzle to a slit—either an annulus or a jet flaptrailing-edge—reduces the width of the mixing region and shifts the major noise output to a higher frequency. As an example ofthe advantageous atmospheric damping thus obtainable, whereas the English climate has a negligible attenuating effect on soundsbelow 500 c/s, at 4,000 c/s in 40 per cent relative humidity reductions of 4 db per 100 yd are obtainable—i.e., air viscosityalone can reduce the major noise output of a slit-jet by more than 15 db in a quarter of a mile. Aspect ratio appears to be importantin the frequency-shift method—again favouring the Jet Flap. Apart from rockets, the power radiated in jet noise is negligiblefrom the performance aspect; internal and external flow losses (drag) in effective suppressor nozzles is not. Prof. Richards classi-fied power losses thus: Internal losses arising from poor flow inside the jet-pipe where the shape is abruptly changed; thrustloss from any "cosine effect" (transverse deflection) when all the gas is not blown along the stream direction; increased base dragwhere the airflow cannot pass freely between the various sections and/or tubes of the suppressor nozzle; increased nacelle drag dueto abrupt changes of shape, causing flow separation or change in Merit for the nacelle; interference effect of the nozzle uponMerit of the adjacent surfaces, pod mounting, wing, fuselage, etc. Dividing the flow within the jetpipe to direct it through thesuppressor nozzle almost inevitably results in some flow separa- tion and thrust losses, of one to two per cent. An exception is theRolls-Royce Avon nozzle ( a "mild" one) which gives an overall reduction of 5 db. The convergent-divergent nozzle today neededfor optimum cruising efficiencies—giving 1 to 2 per cent more thrust than do convergent nozzles—put the designing of sup-pressors beyond mathematical treatment and into the empirical field if over-expansion and consequent separation are to beavoided. External drag arises as base drag due to separation and reducedflow over blanked areas, as additional cooling drag, or as an alteration in the wing or pod Merit. In the Boeing multi-pipe andAvon fluted nozzles there are large flow deviations which require greater transverse pressure gradients as the airspeed increases,which makes them inherently less effective as suppressors at high speed. In general, the drag of the Boeing nozzles is probablylimited to the effect of the additional wetted area plus some com- pressibility effects at high Mach numbers. If a nozzle design issuch as to cause uneven flow in the jetpipe cooling tunnel, then it will increase the cooling drag—normally of the order of 0.8 percent. In the case of the Boeing podded installation any extension of the suppressor aft of the wing trailing edge is unacceptable,since it seriously reduces the effective sweep of the strut shock- front and with it the aircraft's Merit. Here, and in some fuselagepod installations, Prof. Richards thought it would be advantageous to fish-tail the nozzle layout. [Prof. Richards was employed byBoeing as a consultant last summer.] The experience of seven years could be summed up as: modestacoustical reductions of 7 db, possibly 10 db, by nozzle shape, the noise-reduction mechanism being by a quick reduction of velocitywith a rather lower noise output if frequency raising occurs; a minimum of 1 per cent, possibly 2 per cent, effective thrust loss;marginally acceptable noise reductions on current aircraft, but inadequate suppression as thrusts become greater with larger,heavier and faster aeroplanes. Further progress to keep pace with the rise in power will entailthe drastic re-design of airframe or engines. "The first lies in the adoption of the Jet Flap principle . . . which gives not only alower noise level, but also a greatly enhanced areodynamic per- formance of the aeroplane. Since the engineering and aerodynamicproblems of the Jet Flap are likely to make this solution a long- term one, the second and most likely approach to the noise sup-pression is that which has been advocated by the noise-suppression researchers from the very commencement of their researches andis indeed the one clear-cut recommendation immediately deduc- able from Lighthill's theory, viz, the reduction of jet velocity. Itis only quite recently that engine designers (other than Rolls- Royce on the Conway) have accepted the need to reduce jetvelocity at take-off and have realized that such engines of the ducted fan or high by-pass ratio type can be designed with equal,if not better, performances than the orthodox jet engine." Even with improved engines, noise-suppressing nozzles will berequired and Prof. Richards offered some controversial suggestions based on evidence that quicker mixing is the answer, with limita-tion of thrust-losses the problem, i.e., an airflow problem requir- ing engine/airflow integration. Engine designers should beprepared to forgo the single jetpipe and accept compromise immediately aft of the turbine disc. The rear-fuselage pod sug-gests a high aspect ratio multi-nozzle system. The promising ejector suppressor would be of manageable two-dimensional pro-portions with a narrow nozzle. It should be retractable, could be arranged as a thrust-reverser and would be an invaluable shieldagainst acoustical fatigue of the rear fuselage structure. Many jets are already angled away from the fuselage to avoidstructural fatigue, and the radial spread of multi-nozzle jets could still further increase the mixing rate. Photographs of a two-dimensional choked jet showed mixing angles as large as 60 deg aft of the displaced shock cells. At Southampton, experimentswere being made to induce such spread angles in circular jets, mechanically, by shaking the jetpipe and by superimposing a highintensity, very high frequency sound field asymmetrically about the jet axis. If a jet splays on to a curved surface and spreads intoa thin sheet noise is reduced. It might be possible to use retract- able curved surfaces at take-off, or even direct the jets on to theunder-surfaces of wing and flaps; stresses are already so high that steel skins may be used on these surfaces. CANADA'S GOLDEN ANNIVERSARY TAETAILED planning for next year's celebrations of the fiftieth•*-* anniversary of flight in Canada is now well under way. A national co-ordinating council is working with local committees inorganizing a number of air shows and other special events to commemoratethe flight of J. A. D. McCurdy at Bad- deck, Nova Scotia, on February 23,1909. The council includes representa-tives of the Services, government departments, the aircraft industry andoperators, and also major aviation associations such as the CanadianAeronautical Institute and the Royal Canadian Flying Clubs Association.Service participation will include the construction of a full-scale replica ofMcCurdy's Silver Dart aircraft, which will be flown at Baddeck and thendisplayed at various points across Canada. Officers of the council include Gordon Stringer of Canadair,president; Victor Koby of Spartan Air Services, secretary; and W/C. Harold Pearce, M.B.E., CD., F.R.P.S., recently appointedas national co-ordinator. W/C. Pearce has recently retired from the R.C.A.F. following 31 years' service. His appointments haveincluded command of the R.C.A.F. Photographic Establishment; Director of Photographic Survey Operations; exchange officer atR.A.E. Farnborough; Senior Technical Staff Officer of Maritime Air Command; and Chief Technical Services Officer at the All-Weather and Maritime Station at Comox, B.C. W/C. Pearce, co-ordinator of the anniversary plans.
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