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Aviation History
1955
1955 - 0365.PDF
18 March 1955 365 b A I c1= 2 i n * •fro. ^*—'' oR a Ro"S 5=OIO o / F/g. 2. Ftfect of change of load factor on probability of \: failure. brought to mind an approach to the difficulty that, though economic in form, is human in basis. It is this. When a small aeroplane crashes and its one or two passengers are killed it is regrettable; but the rest of the world treats it as motor car accidents are commonly treated. Even if the accident is shown to be due to structural failure, the public reaction is still small, IO"« IO-5 IO-» IO"7 IO-S PROBABILITY OF FAILURE (log.scole) unless there is a rapid succession of such accidents. When, how- ever, a large civil aeroplane with some 50 passengers crashes, and the failure is clearly a structural one, the reaction is not only national but international. A second such accident brings such pressure to bear that the whole fleet of aeroplanes may be grounded. The overall effect, though not deliberately economic, can be measured in economic terms, and we seem to have here the basis for a dis- cussion of human risks as set by the public sense of human values. Let us consider two extreme cases, one relating to a small non- passenger-carrying aeroplane (A), and one to a large civil aero- plane (B) of the type just mentioned. For both aeroplanes, Q, c and m in equation (6) will be about the same, but if we restrict attention to structural accidents only, C0IA0 will be some 10 or 20 times greater for aeroplane (B) than aeroplane (A), and would be larger than one in both cases. Were structural accidents the only type of failure, equation (6) would therefore suggest that, to satisfy public opinion, the structural accident rate for aeroplane (B) should be about l/10th that for aeroplane (A). In fact, however, there are commonly many more serious accidents to aeroplanes for non-structural than for struc- tural reasons, and this applies perhaps particularly to aeroplane (B). Nevertheless, even when an attempt is made to allow for this, the demand, expressed by our analysis in economic terms, is still for a relative decrease of the structural risk, as between (B) and (A), of at least 3 to 1. Thi& order of change does not, of course, involve a correspondingly large increase of load factor for the large aeroplane; from Fig. 2 and similar evidence it is clear that an in- crease of 10 per cent, or 20 per cent., appropriately applied, might cover the difference. Civil engineers have for long been accustomed, subconsciously or otherwise, to provide somewhat greater margins of safety, how- ever expressed, in cases where human life is peculiarly at stake or when the economic effects of a structural failure would be great. The committee of the Institution of Structural Engineers has reviewed such allowances so far as load factors are concerned, and tried to state them empirically in numerical terms. In other words, it has aimed to bring out into the open and start to ration- alize deliberate provision for the "seriousness" of a failure, whether measured in human or economic terms; and it is interesting that the allowances suggested, though not in the same form, cover a greater range than the "large airliner" reserve factor of 1-1 or 1-2 deduced by the economic-cum-probability argument that I have just outlined. Since we cannot sense a change of aero- nautical structural accident rate until it reaches roughly a 3 to 1 ratio, it may be that the probability of structural failure, as between a small non-passenger-carrying aeroplane and a large airliner, should have a ratio of more nearly 10 to 1. The accident rates from other causes may tend to even out this specialized difference. All this requires further argument; what I hope is that we are seeing the beginnings of ways of linking human and economic risks in our philosophy of structural safety. There is, of course, one aspect of this matter that will cause heart-searchings among us. To adopt different probabilities of failure, however small in themselves, seems to make distinctions between small and large aeroplanes, or between one passenger and fifty, as regards standards of safety. I can only point out that such distinctions would relate only to structural accidents and have, as a matter of history, been made before in other fields, of both structural and transport engineering, and that the general public, both national and international, appear to react in that way. We are here at the ethical basis of our subject; in safety matters, as J. D. North has often wisely pointed out to me, we are all contri- buting to "a social judgment." The conflict, if conflict there is, is between the "purely ethical" view and what may be called the "purely biological" view; aircraft designers are here, as perhaps not infrequently before, facing an engineering version of the modern "doctor's dilemma" with regard to the use of precious drugs. A practical approach at the present time [concluded the lecturer] would perhaps be to say that (assuming the airworthiness of cur- rent small aeroplanes is satisfactory) with large new civil airliners, particularly those with rather experimental features, every effort should be made to ensure that their probability of structural fail- ure is less rather than greater than for the conventional small civil aircraft; and that to ensure this it may be necessary to expend more money, skill and time on structural calculations and proving tests. Sometimes, too, it may be wise to lower general stress levels, whether measured by a reserve factor or not, or to resort to a greater use of redundant structures to provide alternative load paths; and so on. This, rather than a strictly numerical approach, is perhaps the wise way of stating the matter today. MILITARY SKID UNDERCARRIAGE "PARTICULARLY during the past three years, there has beenA a noticeable intensification of effort in France towards over- coming the complete dependence of present military aircraft on 8,000ft concrete runways. Some of the work has been directed towards highly unconventional aircraft, many of which—like the Coleopteurs of the Bureau Technique Zborowski—need no air- field at all. Of all the more conventional machines, the skid- landing Baroudeur and the modified Ouragan with "diabolo" undercarriage with low-pressure tyres are outstanding examples. Something can now be said of the work in this sphere which is being done by the Societe D.O.P. (Dispositifs Oleo^Pneu- matiques). This organization have developed a simple skid-type •gear which can be fitted to the main legs of existing aircraft. The only obvious drawback to its present form is that it cannot readily be made to retract, unless the aircraft is designed to take it from the outset. The D.O.P. skid gear has now been briefly described in France, and it is claimed to be less critical with respect to airfield surface and drift angle than any conventional wheeled undercarriage for the same application. The mechanical design is quite straight- forward. As the drawing shows, a pair of skids are pivoted to the lower end of each main leg, i.e., a total of four per aircraft (the test- bed aircraft being, in this case, a Mistral). Each skid is made of light alloy with steel runners, and measures 43|in by 9|in. The pairs of skids are joined rigidly together on each leg and are stabilized at the rear by a hydraulic jack, in a manner reminiscent of that used on the bogies of the Short Sperrin and Convair B-36. A second hydraulic jack is fitted to actuate a braking claw mounted between the skids. Control of this claw is effected by the normal toe-brake pedals. Associated with the gear are pairs of rollers mounted on the «nds of simple ties. A pair of rollers are positioned in front of «ach main undercarriage and chocked; the aircraft is then taxied The D.O.P. skid under- carriage as fitted to the Mistral, showing the roller units in place. The nose of the machine is to the right. up over the ties join- ing the rollers until the latter slip into attach- ments about half-way along each skid (care has to be taken not to taxi right over and past the rollers). With the rollers fitted, the air- craft is suitably equipped for pro- tracted taxying, or for taxying over hard sur- faces. Once lined up on the runway, the rollers are chocked, the throttle opened, and the aircraft allowed to slide off the rollers on to the skids. Taking off on the skids is stated to present no problem. General ground handling is stated to be significantly better than that normally possible, and very tight turns on the skids have been found to be entirely practicable. The first skid trials with the Mistral (the machine is the second prototype Mistral 53) took place two days before Christmas; the first take-off, hop and landing followed on January 23rd and the first complete flight took place on February 2nd. All the flying has so far been done from grass at Les Mureaux, the pilot being M. Perrin of S.N.C.A.N.
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