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Aviation History
1927
1927 - 0365.PDF
MAY 26, 1927 37 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT angles, and is accompanied by a rapid decrease in lift coeffi- cient and increase in drag coefficient. " At V/c = 1 -08, standing compressional waves (bow waves) were observed at a distance of about 0-5 in. in front of the leading edges of the airfoils." In R. & M. 1040 Mr. H. Glauert, in empirical corrections on Ki tip speed, states :— " The corresponding correction to the drag coefficient is not known with any certainty, but it is relatively unimportant." R. & M. 884, however, shows that a tip speed of 0-85 times the speed of a sound may reduce the efficiency of a low-pitch airscrew (that is, an airscrew suitable for a heavy bomber with an ungeared engine) from 75 to about GO per cent. Both these reports, however, suggest increase in Jift coefficient with increasing speed. N.A.C.A. Technical Note No. 244 shows that propeller sections tested at 1 and 20 the values computed from the pressure integration at Edgewood are very much higher than those obtained at Lynn at angles near 0°. We should expect the drag coefficient for the whole section to be somewhat higher than the Lynn values owing to the smaller aspect ratio, but the difference is too great to be attributed entirely to an aspect ratio effect. It seems highly probable that there is a com- paratively large effect of Reynolds number on drag coefficient even at these high speeds. We hope to obtain more definite information on this point in later tests. " The drag coefficient curves given in Report 207, Figs. 16 to 21, show a rapid rise in coefficient for angles near 0°, as, for example, Figs. 21 for airfoil 6. The maximum speeds reached in most cases were between 0-8 and 0-95, the speed of sound. The Edgewood tests show that this rapid increase is followed by a region of nearly constant coefficient, and in fact there is some indication of this in 120 •16 a; xa tu o 1-12 •08 1-04. •00 © De ——« --Aver notes —— -age o aver ——— F moc age Fc ——-' els iselac a,/ c PN-9 / MO-I / / © Q PN-7 300 4-00 500 600 700 800 TIP SPEED IN FT./SEC. = irnD 900 1000 Fig. 25. atmospheres have, if anything, a lower lift coefficient at higher values of —- . but in N.A.C.A. Report Xo. 220 the •writer of the report suggests that the high full scale Kq may be due to scale effect on k,. We have, therefore, both evidence and suggestions in favour of (a) higher and lower \7hit coefficients with increasing ~~~ , and increasing V Vc, and^ also suggestions that increase of drag coefficient with >,ve is negligible and evidence that it is very marked. The position is still further complicated by the fact that an integration of the drag components in N!A.C.A. 255 give a higher value to drag than that measured in N.A.C.A. 207. !t a.u^ora °* the former report comment as follows :— When we come to drag coefficients there is a somewhat different state of affairs. We should expect the drag j'oefhcient computed from the pressure integration to be -pVcr *han the true drag coefficient because of skin friction. >>e might also expect the coefficient for a section near the "(••ntre to be somewhat lower than the average for the whole for I*11' Hence> it.is Probable that the total drag coefficient 1 +V. °^e section is greater by some unknown amount wan that computed from the pressure distribution. However, a few observations in Report 207. In other words, the drag curves are probably somewhat similar to the well-known (javre curve for projectiles." N.A.C.A. Technical Note No. 225 gives indirectly the increase of torque coefficient experimental over calculated for models and a number of actual aeroplanes plotted as- the ratio of drag horse-power over calculated horse-power against tip speed. There is a notable departure from the normal curve in the case of the P.N. 9 multi-engine machine, and a not inconsiderable departure in the case of the P.N. 7 and the M.O.I. These are explained by the writer of that report as being due to abnormal body conditions, M.O. 1 having an exceptionally large fuselage and thick wings immediately behind the propeller. P.N. 7 and P.N. 9 having outboard engines with small nacelles, and the P.N. 9 large geared propellers. So far as the results up to 800 ft./sec. are concerned it would appear that the increase of Kg might be due as much to the relative sizes of the fuselages and the propellers as either scale effect or high value of V/Vc. It is unfortunate that comparisons between the calculated and actual thrust coefficient are not available for the same propellers. The reader may well appreciate the difficulty of dissociating 330c
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