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Aviation History
1957
1957 - 0051.PDF
FLIGHT, 11 January 1957 51 Research, Development and Technical Issues The 1956 Wright Brothers Lecture by Sir Arnold Hall—Part 3 IN the two preceding issues we have published the first and secondparts of the Wright Brothers Lecture delivered recently by Sir Arnold Hall, F.R.S., M.A., F.R.Ae.S., before the Institute of the AeronauticalSciences in Washington; here we print a third and final instalment. Ihe illustrations have been re-drawn from those accompanying the paper,which is reprinted almost in full. Sir Arnold Hall is technical director of the Hawker Siddeley Group.I NOW return to the question I left earlier, which was whetherthe high-subsonic aeroplane merited long term attention to reduction in cost. The answer is "yes," since the otherregimes now opening up do not offer the high-subsonic air- craft such competition that a large part of the market can be ex-pected to turn to them, anyway for a long time. The jet-propelled, high-subsonic aeroplane is an ideal tool of medium-to-long-rangecivil aviation—or will become so when the ancillary services, such as traffic control, match it, as they undoubtedly can. I wouldlike to discuss for a moment the two ways mentioned previously by which greater economy could be sought in this class of aircraft.The influence of size is not apparent from the range equation because it comes from a greater economy of weight in the design;but there is little doubt that an aeroplane flying at Mach number 0.87 and weighing 450,000 lb, as compared with the next genera-tion's size of nearly 300,000 1b, would show an effective reduction in the cost of the seat-mile. But there is a clash between ultimateeconomy and the service the passenger expects. It is roughly the case that settled transport systems provide a frequency of servicecomparable with the travelling time—a not unnatural situation, since it would be odd to have to wait an hour to make a five minutejourney, or equally to have daily departures for a sea journey of four weeks. In Fig. 14 is a diagram, due to Sir Arthur Gouge(and reproduced with permission from the Journal of the Royal Aeronautical Society), showing the way in which transport systemsof different types fall into the pattern of "time between departures roughly equal to journey time." The subsonic aeroplane, bygreatly decreasing the journey time, will generate a demand for higher frequency, and this, on a given size of market, must delaythe need for greater size in aircraft with the economy that it brings. Nevertheless, because the market for civil aviation will increase somuch in coming years, still bigger jet-propelled aircraft are by no means unlikely to be developed, but I think this will not happenuntil frequency has first been increased over present figures. There is another factor which may influence adversely the possibilityof gaining further economy by increase of size, and that is the noise generated by the aircraft. Noise is, of course, related tothe power installed in the machine and a bigger machine will also be noisier; this point is discussed in a later section.The high-by-pass-ratio ducted-fan seems, in many ways, the Fig. 14. General transport problem: journey time related to frequency of service. (Though originally drawn to illustrate a much earlier lecture, this graph still holds good for comparative purposes.) 8 / 6 3 2 Idoj 10 R 6 4 3 2 Ihr 50 40 50 20 lOmin / TYPICAL SHORT RANGE (AHO- LOCAL BUS SERVICES^ / . • SEA Tfl SdTON- DtRECT LONDON -J0'BURG-= LOT / CXw- j NEW III YORK (AIR) V - 9-10 hr 80 MILE RAIL JOURNEY LONDON-PORTSMOUTH UJSPORT NEW YORK SERVICE RAIL J HIP >--• ideal power plant for these high-subsonic aircraft, if its promisecan be achieved in practice. Fig. 15 shows the relationship between all-up weight and payload for a 500-knot trans-Atlantic aeroplanepowered with simple-jet engines and with ducted-fans. (This takes account of the higher specific weight of the fan.) I thinkthe reason that a ducted-fan is not available is not because these arguments are invalid but because the main sweep of jet enginedevelopment has hitherto been pressed forward by the needs of fighters, where cruising fuel economy is by no means the solecriterion for success. But in addition to this, as mentioned in the discussion of specific fuel consumption, the ducted-fan is an enginethat has a narrow field of dominance; below Mach number 0.8 DUCTED FAN-. JET ENGINE 100000 200000 GROSS WEIGHT(lbj 3OQO00 lOmin 20 50 40 Ihr 2 54568 10 JOURNEY TIME 2 5 4 5678 Fig. 75. Ducted tan and turbojet propulsion for high-subsonic airliners. The aircraft all have a still-air range of 5,500 n.m. at M =0.83 and are designed to a uniform severe runway requirement. The latter accounts tor differences between the curves and data published for existing or projected designs. it is defeated by the propeller-turbine, and above Mach number 1it is defeated by the simple-jet engine. It happens that the region of the fan's dominance is that at which long-range civil aviationfinds its best economy, and at which it is likely to play a large pan for many years in providing travellers with service. The argu-ments of the past which condemned the ducted-fan on grounds of lack of flexibility for application over a wide speed range were validas they were put, but perhaps did not take account of the fact that an important section of aviation will operate in conditions in whichthe fan has much to offer. Life of Aircraft Structures. The only structural topic I wantto discuss, apart from the generalizations contained in previous sections, is the question of the strength of aircraft, such as thosethat must have a very long working life for civil use, or military aircraft which are especially subjected to fluctuating loads whenused, for example, for ground attack. I think two matters have become particularly controversial: first, should structural designbe based on the philosophy of "guaranteed life within which structural failure will not occur" or on the philosophy of "if it fails,it does not cause immediate catastrophe"? (these are the so-called "safe life" and "fail safe" policies); second, is the use of a fatiguetest on the complete aircraft justified? Since I and my then- colleagues at the Royal Aircraft Establishment used a completeaircraft fatigue test for a particular accident investigation, such a test has become mandatory in British civil airworthiness require-ments. This has been a subject of much discussion, as has also the question of whether the particular form of test we used (the"tank test") is the best way to do the work—assuming it has to be done. I am grateful to my late colleagues, Dr. P. B. Walkerand Mr. Atkinson, of the R.A.E., for discussions which have helped me to clear my mind on these points. The "safe life" approach may be denned as the determinationof the length of service that an aircraft structure—or part of it— can endure without failure in fatigue. When the safe life of anypart is reached, the part concerned is to be replaced by a new one. If this is impossible the aircraft has reached the end of itsuseful life and must be withdrawn from service. The "fail safe" approach permits failures in the primary struc-ture provided the failure is certain to be found by inspection before the strength is reduced below specified standards. Theimplicit assumptions in this approach are that the "fail safe" characteristic can be established, and that reduced static strengthof the structure can be accepted. The suggestion that a reduced standard of strength can be accepted for the period between theoccurrence of a failure and its detection and rectification raises the question of how the reduced standards are to be arrived at.This is a matter which has been under serious official considera- tion recently in U.S.A. and in England, and I will not intrude onthe discussion in detail. But whatever arguments on probability
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