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Aviation History
1928
1928 - 0037.PDF
JANUARY 19, 1928 marked by crosses), and the average error for all the machinesis but 6 • 2 per cent. By way of comparison between the results given by the twoformulae, Prof. Warner gives Fig. 189 (the right-hand chart) from which it is seen that, as was to be expected, the use of an.exponent of -39 leads to higher estimated speeds for machines with high ratio of P/S (i.e., "wing power"), and a lowerspeed for low-power machines. As his reason for this change in exponent (i.e., from 1/3 to -39) he states that, as a matterof observation, the high-speed aeroplanes are generally those on which parasite drag has been most carefully reduced. Thisdoes not quite seem to tally with British machines, and last week we suggested that the coefficient K in the newsuggested B.H.P./Span2 formula might be progressively reduced so as to prevent ruling out high-power machines. Thesame object could be achieved by altering the exponent as there is one suggested by Walter Diehl in American N.A.C.A.Report No. 173, and quoted by Prof. Warner in his book referred to above. This reads as follows : Vmai. = yVmiu. W/P where Kj has an average value of 20, while \/^ ranges from 0-88 to 0-94. Warner states that even if the propeller efficiency be lumped in with K and the product of Kt and ^/TJ be taken constant at 18, the probable error, as judged from 15 cases, is less than 3 per cent. A few tests on British machines seem to indicate that we should have to increase the product of K,^ and ^/»j to 20 or even more, to get good agreement. 7.0 1.8 1.6 1.4 1.2 1.0 .9 +"/ / / , /.v 1 11 1 I i f / / // • / / / / 11 i 11 1 I 1 y / 4 J 1 1 • f (4 j r i 1 1 / I .i J r f • 1 j 1 1 II ! u t1 1 1 1 I 2.0] 1.8 16 14 I * n / / i1 1I 1 1 ili 11 iIj I1 j I)ll (SI 1 11i -¥— 60 80 100 120 140 160 180 ZOOMaximum Speed 60 80 100 120 W0 160 180 200Maximum Spefcd SIMPLE FORMULA FOR ESTIMATING TOP SPEED: The left-hand chart shows Fig. 188 from/P\0 39 Warner's book, the full line corresponding to the formula V = 127(— . The dotted curves correspond to the same formula, but that on the left has K = 114, and that on the right has K = 140. The chart on the right shows (full line) an exponent of 0-39, while the dotted curve represents an exponent of 0 333. The latter chart is Fig. 189 from Warner's book. done by Professor Warner, but in the opposite direction,i.e., choosing -3 for example. A comparison with British machines suggests that Prof.Warner's formula for speed based on " wing power " would need to use K = 140 to bring the estimated speeds up tothose actually attained by most British machines. In his original article Mr. Walker objected to the use of" wing power " as a basis because it might tend towards encouraging large chord and poor aspect ratio. It wouldseem possible, however, to combine the span2 and the " wing power" formulae by using B.H.P./aspect ratio, denning thelatter as Span2 /area. This, however, would discourage area, and it might be better to include area by using as a basisB.H.P./area x Span2. The plain cube root would not apply in either of these two cases, but some exponent smaller than•333 would have to be used. To us it seems essential that area should be included in such a way as to encourage botharea and span, but the latter more than the former. While it is, obviously, impossible in the space which wecan devote to speed formulas to discuss all possible forms, The use of Diehl's formula for handicapping, even the simplified form, would, furthermore, have the disadvantage that it would necessitate weighing the machines to get the power loading and, what is an even greater objection, to determine accurately the low speed. Provided this were practicable, however, probably very good agreement might be obtained. It would, at any rate, seem logical to take into consideration the power loading and the minimum speed. Doubtless many of our readers will have their own views on the subject of formulae suitable for handicapping purposes, and we shall be glad to hear of new suggestions and to receive criticisms of those already made. Only in that way will it be possible fully to discuss the subject. It is important that the formula to be used this year should be chosen as soon as possible, and, as pointed out by Mr. Walker in his article, once it has been decided upon, it should not be subject to sudden and frequent changes. At least one clear year should be given before an important change in formula is made, so that constructors who wished to produce machines specially for the formula might have time for doing so. 37
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