The Twelfth British Commonwealth and Empire Lecture “ Aeronautical Development in Australia and its Potential Contributions to the British Commonwealth ” was given by Mr. L. P. Coombes, D.F.C., B.Sc, F.C.G.I., F.I.A.S., F.R.Ae.S., Chief Superintendent, Aeronautical Research Laboratories, Melbourne, before the Society on 22nd November 1956 at the Institution of Mechanical Engineers. Mr. E. T. Jones, C.B., O.B.E., F.R.Ae.S., President of the Society, presided and introduced the Lecturer

MR. E. T. Jones: The British Commonwealth and Empire Lecture was given annually and normally alternated yearly between a resident of the United Kingdom and a resident of a member country of the Commonwealth. This year the lecture was to be given by a resident of Australia and it was his great pleasure to introduce Mr. Coombes. Mr. Coombes had long been a friend and colleague of many of them but as they had a fair number of the younger generation present he would introduce Mr. Coombes properly.

Mr. Coombes had served the profession of aeronautics since 1917 when he started as a pilot in the First World War. For ithese services he was awarded the Distinguished Flying Cross. He was a Fellow of the Society and a Fellow of the Institute of the Aeronautical Sciences. He was also a Fellow of the City and Guilds Institute. Mr. Coombes was elected Chairman of the Melbourne Branch of the Royal Aeronautical Society in 1953 and in 1956 he was elected President of the Australian Division of the Society. He was also a most active and enthusiastic member, or delegate rather, of the Commonwealth Advisory Aeronautical Research Council, a body to which he referred in the paper and which came into existence in 1946.

Mr. Coombes joined the Royal Aircraft Establishment in the Aero Department in 1924 and a year after transferred to the Marine Aircraft Experimental Establishment at Felixstowe. He was recalled to Farnborough in 1930 to take charge there of the seaplane testing tank which had just been erected and many would remember that Mr. Coombes and the late Mr. W. G. A. Perring worked side by side on that tank for many years. He thought it was about 1938 when he and his family left England when there were no facilities at all in Australia for aeronautical research. Indeed there was no organisation in Australia for aeronautical research and he thought that there was no need personally or professionally for Mr. Coombes to go to Australia because he had already assured for himself an eminent aeronautical career in the British Air Ministry. Mr. Coombes therefore was just as much a pioneer, aeronautically speaking, as those men of sail who set out from England two centuries before him. Mr. Coombes was both architect and designer of the Aeronautical Research Laboratories in Melbourne and was now the leader there of a strong team of scientists and engineers who had made quite a reputation in the aeronautical sciences for the high quality of their work.

He would like now to read a message from Australia from Mr. Isbister, the honorary secretary of the Australian Division, which said: “ Please convey to our President, Mr. L. P. Coombes, best wishes for a successful British Commonwealth and Empire Lecture. From Council and members of the Australian Division.”

The increase in jet engine thrust has been accompanied by an increase in the noise generated by the jet stream to such an extent that the associated noise pressures are now capable of exciting vibrations in an aircraft structure which are potentially dangerous from the fatigue aspect. Several examples of fatigue damage of this kind have appeared already on aircraft in this country and in the United States.

The noise generated by a gaseous jet arises principally from disturbances caused by high velocity gradients at the jet boundary. Classical theoretical work on noise generated in this way has been done by Professor Lighthill. He introduced the concept of an “acoustic quadropole” as the elementary sound generator, and showed mathematically that the total acoustical power radiated by a jet varies directly as the eighth power of the jet exit velocity and the second power of the jet exit diameter.

A considerable research effort is now directed at the problem of vibrations excited in aircraft structures by airborne sound from the power plants. Both jet and propeller aircraft are involved, although perhaps for different reasons. With jet aircraft the problem is to make the structure strong enough to withstand the vibrations and the accompanying stress fluctuations; with propeller aircraft the problem is usually to prevent the transmission of sound through the structure into the passenger cabin.

The reduction of the vibration amplitudes of a system subjected to external exciting forces can often be achieved by a suitable alteration in the mass or stiffness of the system. The natural frequencies of the system are thereby changed and are no longer in the neighbourhood of the peak harmonic components of the exciting force. This procedure can be shown to have little effect on the problem of jet efflux excitation (unless a really radical change of stiffness is made, such as could only be obtained by a complete change in the type of construction). This is due to the jet efflux pressure field having a fairly flat pressure spectrum and therefore containing harmonic components of all frequencies (over its range of flatness) which are more or less equal in magnitude. However, despite this flatness the spectrum of the generalised force exciting a particular mode has a large peak at the frequency where the sound wave of that frequency has the same wavelength as the structural mode (the coincidence effect).

The sound pressures, associated with a jet efflux, set up stresses in an aircraft structure which are essentially those occurring in the resonant condition of the structure. These sound pressures contain components in a wide range of frequencies and the resultant stresses in the structure have low magnitude by normal structural standards but, because of the high frequency, they are very important from the fatigue standpoint. A rigorous theoretical approach is being developed at Southampton University which, although extremely difficult to apply to actual aircraft structures, emphasises the factors which must be considered. However, for immediate practical use it is suggested that an approximation can be made to render the problem tractable. It is considered that in an actual aircraft the effects are primarily local and on this assumption an approximation is developed and compared with test results. Reference is made specifically to the de Havilland Comet aircraft.

Reports of noise effects on the rear fuselage structure of large British aircraft indicated that one might expect noise damage in this region on the Vickers Supermarine Type 544 aircraft. It will be seen from Fig. 1 that the jet efflux from each engine passes close to the fuselage side and under the tailplane. The fuselage structure proper is, however, protected from the heat and noise effects at the jet efflux by a fairing which is insulated at all attachments. This fairing, by virtue of its shape, is called the “ pen nib. ”

A study has been made of the effect of inlet conditions on the performance of conical diffusers with 4:1 area ratio and 5 and 10 degrees total angle of expansion. The conditions at entry were varied by using different approach lengths of diffuser inlet diameter, and by means of projecting annular screens of woven wire cloth. With this new technique it was possible to vary the velocity distribution substantially within moderate settling lengths, and to produce velocity profiles with inflections. The suitability of the annular screen method of boundary layer generation for diffuser investigations was confirmed from the comparison with the approach length results.

Energy and pressure coefficients, as well as diffuser energy efficiency and the conversion efficiency, were found to depend on the diffuser angle β and the momentum thickness ratio at inlet θ/D0. The latter emerged as one of the chief parameters controlling diffuser performance. Variation in the inlet shape parameter H of the order of 20 per cent did not significantly affect the pressure recovery or the losses in the diffuser. For moderately thick boundary layers, θ/D0 X β<0·1, the diffuser angle could be eliminated as a parameter by plotting the pressure coefficient against θ/D0 X β. The Reynolds number based on diffuser inlet diameter was, for essentially incompressible flow, 2·5 X 105 in all tests.

An instrument is described which measures the time duration of a specified number of complete cycles of an oscillation over the frequency range OT cycles per second to 500 cycles per second. The accuracy of the reading is dependent on the frequency measured and the number of cycles taken; but, in most cases, an accuracy better than 01 per cent is obtainable. The construction of the instrument is described and circuit diagrams given.

The design of the exhaust ports of a two-stroke cycle engine is mainly based on the area required to reduce the cylinder pressure at the end of the expansion stroke to scavenge air pressure in the minimum practicable time. The flow through the exhaust ports is both sonic and subsonic; in the former case the flow is independent of the exhaust pipe conditions but not in the latter case. In the first stage of the design, however, the exhaust pipe conditions may be assumed to be constant when calculating the port area, subsequent calculations of the pressure drop allowing for the pipe size may then be made by the methods suggested by Wallace, Cole, or Benson. In the following Note formulae are developed for estimating the port area neglecting the exhaust pipe size.

The determination of effective column end fixity by means of vibration testing has been established for the case when the rotational end restraints of the column are equal. It is the purpose of the present work to present solutions for the general case. The method of solution is straightforward. Results are listed in a convenient tabular array.

A method of solution of linear differential equations is derived which requires no special starting procedure, and which is, in principle at least, capable of extension to any order of accuracy. Further advantages of the method are that boundary conditions are easy to satisfy, and that the solution of families of equations differing only in the input function requires little extra computation. The method requires the reciprocation of a matrix of order equal to the number of points for which a solution is to be obtained, but this can be much simplified by suitably partitioning the matrix, and with most automatic high-speed computers the reciprocation of a matrix is in any case an easily programmed operation.