USE OF THE BERTIN AEROTFtAIN FOR THE INvSTIGATION OF FLIGHT EFFECTS ON AIRCRAFT ENGINE EXltCllST NOISE R.G. Hoch SoeiCtG Nationale d'2tude et de Construction de Moteurs d'Aviation Paris, France and M. Berthelot Soci6tB Bertin & Cie. Plaisir, France

1ati.ve Velocity effect to static data. If, in the rear arc, the relative velocity law applies fairly well, in the forward arc, however, a remarkable insensitivity to flight or soretimes a noise increase is observed. Long disputed, these observations are now almost universally atcepted. but explanations given for this unexpected fact are contradictory inasmuch as simulated flight experiments ir. vind-tunnels, for instance, show effects which fit better the existing theoretical mdels.

Abstract A prototype of the ACrotrain has been modified by S.N.E.C.M.A. and SociStC Bertin to investigate flight effects an jet noise and jet suppressor performance. To this end, special atcention was given to the reduction of parasitic noise frcm the vehicle and internal noise from the G E - J R 5 turbojet engine which pawcrs the h6rJtrain. The vehicle, its performance, the operational techniques, the measurement and analysis procedures used are fully descrihed, togcther with the results of the calibration tests of this unique and very flexible facility which presents many advantages compared to aircraft in flight. Typical results, consistent with the trends shown by clean aircraft noise data, are presented.

Considering chis situation, engine mnufacturers started to design test facilities allowing the experimental investigation of flight effects on jet noise in particular, and, accessorily, on the other engine noise sources. These test facilitiss can be divided into three major categories, as schematically illustrated in Fig. I :

I. Introduction

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Progressively increased stringency of noise cerrification regplation hay forced aircraft ar.d engine wnufacturers into a continuous effort to reduce engine noise and to optimize the aircraftiengine configurations. Margins left between noise limits specified by those regulations and noise levels achievable by current noise reduction technology, whilc remaining ccmpatibls with product viability in terms of performnce and economics, have become quite narrow. With this evolution, manufacturers have progressively been facad with difficult problems related to the assessment of flight effects on the various noise sources in an engine. The prediction of these effects in terns of level, directivity and spectra is still fairly unreliable today, because fundammtul phenomena affecting the radiation from a noise source in f1igh.t are not well understood and a relevant theory is badly needad, in spite of the fact that considerable w o r k has been devoted to these problems over t!ie last years.

Figure I . Possible Experimental Techniques for the Evaluation of Flight Effects.

(i) Aircraft in flight, including either aircraft in current service, or flying test beds for in-flight development of engines.

Jct noise remains one of the m s t critical of these problems, not only for supersonic aircraft powerplant but also for high bypass turbofans, mainly at take-off, owing to the successful results obtained in suppressing turbomachinery noise. Therefare, the initial design of a turbofan intended to meet given noise level objectives, and :he plans of its future developed versions, will unsvoidingly be faced with the problem of jet noise and its prediction in flighc.

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(ii) Land tracked vehicles carrying the source to be investigated, this source being used or not for vehicle propulsion. In this category, taxying of aircraft along a runway, ground-based vehicles. sled systems, air cushion vehicles like the ,"AGrotrain". rotating amis, etc., . should be mentioned. (iii) Static simulation facilities including open-section wind tunnels and the free jet technique, where measurements are made outside of the flow surrounding the jet'to be investigated, and open-section and closed-section wind tunnels, where measurements are made inside the flow.

A model of in-flight jet noise based on a r e l a tive velocity law has been long relied upon and, on the o t h e r hand, it was thmght that attenuations given by jet noise suppressors in static conditions would be fully maintained in flight. However, the accumulation of good test data, obtained with progressively improved measurement and analysis tech-' niques, has progressively shown chat, far engine jet noise in fli.ght, actual facts were f a r from what. could be expected from the mere application of a reCopyrighi American lnrfiiuteof AeiOnaulicIand A b i r o i i s ~ t iInc,, ~ ~ , 1916. All right- reicrvrd.

Without gcirig into 2 detailed discussion of advantages and disadvantages of each technique, it is worth recalling that real flight testing is expensive and does not allow a sufficient degree of fle-

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