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Aviation History
1943
1943 - 2420.PDF
394 FLIGHT OCTOBER 7TH, 1943 Power, Speed and Weight Qross Weight Accurately Determined from the Power/Speed Ratio By W. NICHOLS, A.R.Ae.S. TABLE 1. CHARACTERISTICS AND CALCULATED EXAMPLES OF REPRESENTATIVE BRITISH LIGHT AIRCRAFT. Aircraft Moth Minor ... Mosscraft Tiger Moth ... Magjster WJcko Cygnet Owlet Percival Gull VI Parnall 382 ... Horse power 90 90 130 130 130 140 140 200 200 • -HIM. 9-49 9-49 11-4 11-4 11-4 11-85 11-85 14-15 1415 Max. Speed m.p.h. 118130 109145 140135 127178 * 155 Wma, 10-8611-4 10-44.12-04 11-8511-62 11-2713-35 12-47 DisposableLoad Ib. 580520 655613 745645 703970 795 A 0150-1 ' 0-05015 0-140-2* 0-25*0-0 0-05 -• 1-020-93 0-961-09 1-101-22 1-31-07 1-18 Gross Weight Calculated Ib. 1548 1402 1749 1855 1999 2090 2243 2484 2464 Gross Weight Actual Ib. 1550 1400 1770 1863 2000 2100 2300 2450 2450 Error Ib. 2 2 21 8 1 10 57 S4 14 Per cent Gross Wt -013+0-14 -1-2 — 0-43 -0-05 — 0-5 -2-5+ 1-4 +0-57 *Stressed-skin Metal Construction. THE methods by which the gross weight may bedetermined at the beginning of a new design are onthe whole empirical, and every piece of information that may be obtained from the specification for the design in question is carefully examined, for if the design is to be successful the original estimate should be accurate to a high degree. The designer in these days has a wealth of tabulated data at his disposal for almost every phase of .aircraft design, but so far there does not appear to be any reliable method for the determination of the gross weight, and due to the complexity of the problem and the inconsistency of average data this is not surprising. Methods in use at present include past experience of similar designs, average data tabulated from similar existing aircraft or calculations based upon the power loading to be expected. Another method is to calculate the detailed weights for the- power unit and the total removable load, and to assume that this weight will constitute a certain percentage of the gross weight, again based upon average data. In these days of scientific progress such methods are, to say the least, haphazard and involve a considerable amount of time ; and more often than not the estimate arrived at will require frequent readjustment as the design proceeds. In the issues of Flight of July 30th and August 20th, 1942, methods were presented for estimating the gross weight of the- fighter and the bomber respectively, which gave very accurate results. Since then the writer has had the opportunity to in- vestigate the problem from a different angle. From this investigation some logical conclusions have been estab- lished. To simplify this discussion only the case for light aircraft and trainer aircraft will be considered here for the present. With aircraft in this category the investi- gations were originally begun, though it should be em- phasised that the method to be described applies to all orthodox, aircraft, but with the more powerful and heavier class modifications to the simple formula must be intro- duced. As a beginning, characteristics for a number of light aircraft and trainers of both British and American design were tabulated, some of which are given in Tables 1 and 2. From these data all attempts to estimate accurately gross weight from average figures proved inconsistent and in many cases hopelessly inaccurate. This led to the discovery that for this class of aircraft the empty weight agreed fairly closely with VH.P.maxX 100, but the variation in many cases was too great for an accurate estimate to be generally made, and some modifica- tion to allow for the variation became necessary. The formula then became : Empty Weight = VH.P.max X 100 X 0 - - (1) where jS is a variable factor. The next step was to solve for ft, from which the con- clusion was arrived at, that this value was related to ^Vmax in miles per hour. can be correctly written : This proved that the formula . It will be noticed that a further value, A, has been intro- , duced. The reason is the following : w Algebraical formulae cannot be evolved to cater for the 7 skill or otherwsie of the structural designers ; also the pur- pose of the aircraft will have some bearing upon this value. For example, a trainer aircraft will need to be rather more robustly constructed than an ordinary light aircraft used for normal flying purposes ; also in this country the fully aerobatic C. of A. will need to be considered when specified. Consequently the value of A makes «.ll the difference between an accurate estimate within i per cent, of the gross weight, and one. of, say, 5 per cent, to 7 per cent. Suitable values of A have been tabulated in Table 3 which apply to aircraft in this category only up to say, TABLE 2—CHARACTERISTICS AND Aircraft A.eronca l.uscorabe Taylorcraft ... Piper Cub Coupe Culver Monocoupe ... Porterfield ... Rearwin Speedster RyanSX. ... Hone power 65 65 65 75 80 90 90 125 125 145 v'H.P.,,,,*. 8-06 8-06 8-06 8-66 8-94 9-49 0-411 11-18 11-18 12-04 Max. Speed m.p.h. 105 115 105 100 140 130 135 ISO 120 150 CALCULATED EXAMPLES OF REPRESENTATIVE AMERICAN VV max. 10'25 10-73 10-25 10-0 11-83 11-4 11-62 12-25 10-95 12-25 Disposable Load IB. 480 650 518 535 585 837 503 630 625 800 A 005 0-05* 0-02 015 0-05 017 0-05 0-05 0-0 015* P 0-83 0-8 0-8 1-010-8 1-0 0-860-96 1-02113 Gross Weight Calculated Ib. 1149 1195 1163 1409 1300 1586 1319 1703 1765 2160 LIGHT AIRCRAFT. Gross Weight Actual m. 1155 1200 1150 1400 1305 1610 1326 1700 1775 2150 Error 16. 6 5 13 9 5 24 7 3 10 10 Percent Gross Wt — 0-5 -0-42 + 1-14+ 0-65 -0-4 -1-5 -0-54 +0-18-0-6 + 0-5 • Stressed-skin Metal Construction.
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