Summary The drag at cruise CL of a number of recent aircraft—mostly large subsonic transports—has been analysed in terms of figures-of-merit derived from Melvill Jones’ original concept of a streamline aeroplane. The values have been compared with those for some representative aircraft from the 1930-1940 period. The paper then proceeds to highlight some of the main sources of excess drag; for the recent aircraft, these are grouped under broad headings such as “excrescences”, “Mach-number effects” and “nacelle installation or interference”. The results of the analysis are somewhat disturbing. Perhaps the most striking feature, bearing in mind that only a restricted class of aircraft have been considered, is the very wide spread between the figures for the poorest and the best of the recent aircraft. Also, the detailed drag breakdown varies widely from one aircraft to another. The evidence in the paper suggests that, in many cases, present drag standards can and should be improved.

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It is well known that the performance at high subsonic and transonic speeds of a swept-back wing-body combination in which the wing is untwisted and has the same section at all stations along the span and in which the body is not specially shaped to allow for the presence of the wing, falls far short of what would be predicted for the corresponding infinite sheared wing. For example, with a sweep of 45° and a thickness/chord ratio of 6 per cent it has been found experimentally that a rapid shock-induced increase in drag occurs above a Mach number of about 0·95 and a peak value of CD is obtained at Mach numbers slightly in excess of 1·0, whereas it can be estimated that for the corresponding infinite sheared wing, sonic speed in a direction perpendicular to the isobars (the lines joining points where the pressure is equal) would not be obtained until a Mach number of 1·18 was reached. The poorer performance of the finite swept-back wing results principally from the fact that the pressure distributions for sections near the root and tip are distorted in shape from what would be obtained on an infinite sheared wing and, as a result, the isobars tend to lose some or all of their sweep. With a moderate aspect ratio such as 3, such effects extend over most of the span at high subsonic speeds.

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