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Aviation History
1933
1933 - 1328.PDF
,90 SUPPLEMENT TO FLIGHT THE AIRCRAFT ENGINEER DECEMBER 28, 1933 •25 24 •23 •24 •23 *>^® 0 ^» (A) (B) © ~* SYMMErRICAL SECTIONS 00 SERIES dKmc SKT 2434- 53 f 43-63 SERIES O « =-04 + 8 = -06 dKm dK, •2507- -34 t 2 •02 •03 •04 •05 FIG.I7 44-64 SERIES o X > -04 8 = -06 -2504- -416 t2 4S-65 SERIES © « = 04 t 8 - 06 ^=•250- -426 tl •02 (-2 03 Slope of Moment Curves this empirical correction is applied to the remaining series of aerofoils. Considering that (a0 — aa) is obtained as differences between relatively large quantities, the results show remarkable consistency. The majority of the cases show definite increases in (a0 — ax) with camber, and to the degree of accuracy of the tests we may write «o •= + 57-3 Jt/4 Km0 F - 10y. (11) where F2 = y0fz(x)dx F, = 1 -4-9«2 y = Maximum camber ratio. This relationship shows that to obtain an aero- dynamically untwisted wing of tapering thickness but constant centreline proportions, the incidence along the • , 57-3 F2 wing must be varied as -| JT- =- 4- 10y. The angle given is that of the datum line of the section with respect to some arbitrary plane. To the limits of experimental error it now seems pos sible to forecast the no-lift angle of any section of the series. The check is given (as for the moment coefficient) on sections N.60 and N.60.R. Taking first N.60, the value of F2 = 0.007. .-. a„ = + ~ (- 0-0396 - ^Z ) _ io x 0-041 TT/4 \ P, / 57-3 / I - 0 0396 0007 0-41 0-7854 \ 0-923 .-. Estimated a0 = — 4-0° to N.A.C.A. datum Actually a0 = — 5-43 to flat under surface. = -5-43 + 57-3 X 0-029 = - 3-77° to N.A.C.A. datum. Taking now N.60.R. where F3 = 0 0141, 57-3 / 0-0141X 00003 0-7854 \ 0-923 -1-48° to N.A.C.A. datum I 10 X 0 032 Actual oco = — 1 = -1 48°+ 0-001 X 57-3 42° to N.A.C.A. datum. This agreement is within the accuracy to be expected, and shows that the empirical correction may be applied with confidence to practical sections. We will now consider the slope of the moment curve with lift. The theoretical value of 0.25 is not realised for any of the sections, but the extrapolated value for t = o is approximately 0.25 where there is any camber of the centreline. For the symmetrical sections the value for t = o is only 0.243 (see Fig. 17). 45 •4 b •35 •3 dKm dKL •25 - b t z FK3.I8 3c -35 c -4c 45c 5c POINT OF MAX. CAMBER Slope of Moment Curves for Cambered Sections The fall off with thickness is a function of t2, the tests falling into two groups. The first group is the symmetrical sections and those with the maximum d¥Lm camber at 0.3 c. The rate of fall off of ~ny— for this dKx group is approximately £ f. •12 15 18 « MAX THICKNESS RATIO 1310 /
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