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Aviation History
1933
1933 - 0801.PDF
FLIGHT, OCTOBKK 19, 1933 \ Old Acrobatic New Acrobahc Proof 1 1 \ New Normal Ultimate Old N ormal 1 S. New Normal Proof FIG.I 5000 I0O0O 15000 20000 25000 50000 WEIGHT OF AIRCRAFT (LBS) a 4 vv^ -^ N X, New Acrobahc Old Aero banc Ulhmare \ New Normal UlNmate ^New Acrobatic Proof __0ld Normal 1 ^ New Normal Proof ! 1 FIG.2 5000 10000 15000 20000 25000 50000 WEIGHT OF AIRCRAFTUBS.) In other cases, it will be necessary to obtain on test factors 20 per cent, greater than those specified. In the case of a test on a complete unit, a convenient procedure, when practicable, is to patch up such members as fail prematurely, and to continue the test up to the 20 per cent, excess load. Corrections to standard com ponent conditions need then be applied only to the members which fail before the full 20 per cent, extra load has been applied. (12) Compliance with the proof factor requirement should be checked, both when approval is based entirely upon calculations, and when recourse is had to mechanical tests. In general, this will not involve any extra calculations. Definition of New Load Factors The new definitions of British new load factors are set out in full in the extract from Leaflet B.l given above. It will be seen that these definitions lay down a much These charts show the load factors for machines in the more exact—and exacting— ' Normal " and " Acrobatic " Categories. Fig. 1 covers the " C.P. Forward " case, standard than would be implied and Fig. 2 the " C.P. Back " case. in the mere statement that a typical aircraft must not break THE NEW LOAD FACTORS apparently weak and the desired standard was only main tained by subsidiary regulations and the accepted practice of the industry. The one of these difficulties which primarily affected the Airworthiness Department was the enforcement of a uniform standard of interpretation and the explanation of this standard to newcomers to the art and practice of aircraft design. The other difficulty primarily affected the industry, and was concerned with convincing possible customers that the low numerical values of the calculated British factors were a pessimistic indication of the real strength of British aircraft. Both these troubles have been swept away by the new defini tion of load factors and the new numerical values set forth in Amendment No. 33 to A.P. 1208. The real increases in strength required are very much less important than the numerical increases in the factors indicate, and consequently the implication that our own past practice was unsound is seen to be without solid foundation. Our British load factors now either equal or surpass the values laid down by the I.C.A.N. (Inter national Commission for Aeronautical Navigation) and have to be determined by methods which differ only from the I.C.A.N, methods in being more strict. These facts should strengthen the position of British aircraft by rendering im possible even theoretical criticism from foreign rivals of that standard of structural safety which has been so well demonstrated in the past by practical achievement and experience. Extract from Design Leaflet B.l of A.P. 1208 (7) Mechanical tests are considered to constitute the final proof of structural strength, but in most cases calculations will be sufficiently accurate and more convenient than mechanical tests. All calculations and mechanical tests are to be carried out in the light of the following requirements :— (a) That any standard structure or component shall not collapse before withstanding on strength test the specified unit external loading multiplied by the specified factor. This factor will be referred to as the ultimate factor. (b) That any standard structure or component shall be capable on strength test of carrying, for a period of one minute, the specified unit external loading multiplied by 62-5 per cent, of the specified ultimate factor, during and after which it shall still be in an airworthy condition. This 62 • 5 per cent, of the ultimate factor will be referred to as the proof factor. (8) The factors quoted in the following Design Leaflet are ultimate factors, unless otherwise stated. (9) A standard structure or component is defined as a structure or com ponent only just complying with the minimum dimensions admitted by the drawings (allowing for all limits and tolerances) constructed throughout of material only just complying with the minimum requirements of the material specification. Standard'compression members, in addition to satisfying the above conditions, have the maximum allowable eccentricity. (10) Any generally recognised method of calculation may be used in estimating the strength of any given structure, and credit may be taken for all redundancies, provided sufficient information is available as to the effect of such redundancies. In doubtful cases, appeal may be made to strength tests either on complete units, or on individual members. Tn general, the structure or component tested will not comply with the above definition of a standard structure or component, and hence corrections, based upon actual measured dimensions and material control tests, must be applied to the •actor at which the specimen broke on test. These corrections may be waived if the factor given by test is at least 20 per cent, greater than the specified factor. (11) Correction of test results to standard component conditions will usually only be possible when the item tested is of simple design and fails in a manner to which the specified material properties are directly applicable. up under the load corresponding to the ultimate factor. Under a regulation to that effect it would be quite permissible for con structors to make their specimens for test—either complete aircraft or components—out of materials as far as practicable in excess of specilication minima in both strength and thickness. It would also be quite per missible—although undesirable from every point of view— for an aircraft so tested to suffer from permanent deflec tions at an early stage in the loading. Both these troubles have been carefully guarded against, the one by the de finition of a " standard " specimen and the 20 per cent, margin on the test of a typical specimen, and the other by the institution of the proof factor. Neither of these ideas is a new feature of British airworthiness methods, but both are now for the first time incorporated in the Civil Airworthiness Handbook—and incidentally it is believed to be the first time that the idea of the " standard specimen has been incorporated in the airworthiness regu lations of any country. The relentless striving of our Airworthiness Depart ment after accuracy of detail in their methods has often caused—and doubtless will often again cause—considerable irritation among designers and constructors. It is all to the good, however, that the full refinements of this meti culous system should be published and expounded in order that users of British aircraft all the world over may appreciate the theoretical as well as the practical worth of a British Airworthiness Certificate. There is, however, one grave danger in the operation of the new svstern which must be avoided if British con structors are" not to be handicapped rather than helped by the high standard of our airworthiness regulations. The factors now laid down are so high that it is essential that they should be treated as genuine ultimate factors. All possible steps must be taken to avoid those " hidden margins " which are no longer necessary for strength and which would therefore merely lead to waste of weight and consequent commercial inefficiency of our aircraft structures. Neither designers nor stress-men have any control over the hidden margin due to materials being over strength and over size, and the 20 per cent, requirement on typical test specimens shows that the Airworthiness Department anticipate a large effect on this account. This is all the more reason why full account should be taken of all re dundancies, and extra allowances of strength customary in the past should no longer be incorporated. Above all, the Airworthiness Department itself should— and undoubtedly will—revise the letter of all its own minor regulations and specifications in the spirit of its new major requirements. Values of New and Old British Factors Apart from the newly instituted " gust " cases which will be the subject of separate consideration, the two prin cipal stressing cases are those known as " C.P. Forward and " C.P. Back." The new ultimate factors and the new proof factors for these cases are plotted in figures (1) and (2), together with the old British factors which they have superseded. The numerical increases for aircraft of any weight may be readily seen from these charts. 1045
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