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Aviation History
1962
1962 - 1386.PDF
196 FLIGHT International, 9 August 1%2 Seventeen years after its first flight the Dart turboprop is entering a new and very important phase of its development programme. This Ambas sador is fitted with an RDa.8 to starboard and an RDa.lO to port, both of which are much too powerful to be flown on the trusty Dakota HUCKNALI metered flow through the discrete points determined by the test programme. At Hucknall this work has been undertaken with a Conway. Mechanical Strength Under this heading is carried out investi gations into engine carcases, nacelles and mountings and a wide range of accessories. The establishment includes a compressor test facility delivering at up to 2801b/sq in at 550°C, a typical task for which was testing the compressor outlet casing for the Spey, which has a higher pressure-ratio than any other production engine. Another important programme associated with the Spey is that carried out on the complete Trident nacelle mounting. A rig has been constructed which can be hydraulically loaded to simulatc every design case, including both forward and reverse thrust and landing loads. Six degrees of freedom make the stressing problems unusually complex, and the rig has already completed nine com plete aircraft lives. The natural frequencies of the installation have been fully explored with this rig; there were never expected to be any difficulties in the engine-running range, but the testing has also confirmed that the structure is satisfactory at low frequencies of the order of 8 c/s. Additional mechanical testing is described in the section dealing with the Spey. VTOL aircraft are responsible for research and ad hoc testing on a very broad front. For example, Hucknall has tested pods swivelling at the tips of aircraft wings, and a complete programme involves duplicating thrust, weight, drag and aerodynamic lift for a range of engine angles between vertical and horizontal. No less essential are the hundreds of tests which Hucknall run on air- turbine drives, fuel heaters, bleed valves and anti-icing systems. Noise Hucknall has for 15 years spearheaded the world's efforts to reduce the noise nuisance of modern aircraft. The name of a former manager of the establishment, Mr F. B. Greatrex, is synonymous with jet silencing, and it was under his direction that Rolls-Royce learned how to design a nozzle to give any specified degree of silencing. The original Port of New York Authority investigations by Bolt, Beranek & Newman provided actual PNdb measurements on a number of new transport aircraft, and facilitated the determination of an acceptable scale of values. For the Boeing 707 Rolls-Royce first considered that a reduc tion of at least lOdb was necessary, an attenuation considerably in excess of corrugated nozzle performance at that time. The degree of silencing achieved on many test nozzles was expressed as a function of two principal parameters: the ratio of A new aspectjpf Hucknall operations: helping to develop and prove the Gazelle turboshaft engine for the Wessex helicopter family. Napier Aero Engines are 50 per cent owned by Rolls-Royce diameters of the corrugated nozzle and the corresponding plan, nozzle; and the width of each lobe (and consequently the number of lobes). Making the assumption that all secondary airflow entrained between the lobes was thoroughly mixed with the hoi jet, curves were plotted from which it was possible to derive the area ratio and number of corrugations necessary for the require 1 PNdb. Extensive ground testing was followed by flight research with an Avon-Canberra equipped successively with a seven-lobe nozzle (a design which Douglas fitted to early DC-8s), a nozzle with five deep and five shallow lobes, and an eight-lobe nozzle which was eventually scaled to suit the 707. Bearing in mind that noise increases extremely rapidly with mean jet velocity, it will be appreciated that modern engines of high by - pass ratio attain the same end more efficiently. It is therefore probable that—unless populations acquire a psychological aversion to plain nozzles—such engines should not require jet silencing. The maximum PNdb around a Trident is only about 100, which is most encouraging. On the other hand, modern turbofans tend to create a noise problem on the approach, for high-intensity sound emanating from the initial compressor stages tends to occur in annoying frequency bands. Such noise is strongly directional and is greatly affected by changes in intake geometry; in rear-engined aircraft much of the worst effect may be blanked off by the wing. Thrust Reversal The general design of present Rolls-Royce reversers is well known: the jetpipe is shut off by a pair of curved shutters which, when rotated to the "reverse" position, also expose cascade outlets in the sides of the pipe which direct the jet in a largely forwards direction. Positive control of the direction of the reversed jet can be important, for lack of it has led to trouble in American installations employing simple target-type reverses. Such control has enabled Rolls-Royce to carry out model-test programmes at Hucknall in order to achieve installations which direct the jet most favourably. This direction should, of course, be as nearly as possible directly forwards; but in most installations difficulties arise from blast effects on other parts of the aircraft, the possibility of picking up stones and other debris from the runway and re-ingestion by the engine of its own exhaust. A further complication in some installation1; is that lateral discharge through the reverser cascade means that a reversed landing with one engine out will impart a large unbalanced lateral force. The final configuration is always the result of exten sive experiment; in the Boeing 707-420 the outboard cascade of each inner engine discharges 40 above horizontal. With rear-engined aircraft serious trouble can be caused if reverse-thrust results in a rising column of hot gas along the. plane of symmetry beneath the fuselage. A rectangular-nozzle tunnel has been built in which more or less complete aircraft con figurations can be investigated to a scale of l/20th. Hot gas may be discharged through the reverser cascades, and stones are repre sented by common table salt, which has the correct size/density relationship. To investigate re-ingestion problems charges of salt are fired from the floor of the tunnel at each of a large number of spanwise stations. The salt particles are projected with zero velocity with respect to the free-stream air in the tunnel, while hot gas is dis charged in a representative manner from the reverser. After each test the number of particles collected from each engine intake are counted and a curve plotted. The geometry of the reverser cas cades is progressively adjusted until the catch is zero for each spanwise position. Vibration It is a sine qua non for an engine manufacturer to minimize vibration stemming from the powerplant, but this parameter is a singularly elusive one. It is difficult enough to minimize vibration in a car, and in an airliner the number of potential sources may run into thousands. Rolls-Royce collaborate closely with airframe manufacturers in order to minimize interior noise and vibration arising from fuel pumps, pipelines, air-delivery systems and other sources. These are shown up immediately b> narrow-band frequency analysis of noise and vibration measure ment in flight. This is a comparatively recent technique which has been used effectively on two current civil aircraft. AVON Commercial Flight and ground development of the RA.29 family is still an active programme. Reverser and suppressoi development required special ground rigs, a reverse-thrust Hunter and a Canberra fitted with various test nozzles. Low-temperature starting tests were performed in a cold room at — 30CC, genera! handling and relighting was investigated with the Canberra, and icing tests with a pod mounted under Ashton WE670.
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