Some Drag Problems of the Moment
Authors: Relf E.F. 
Published: November 2014

THE great advance in aerodynamic efficiency of aircraft achieved during the last few years has largely been due to the continued efforts that have been made to reduce drag to a minimum. It is of interest, at the present stage of development, to review broadly the present state of knowledge on these matters, and to see where vital information is still lacking. It is not so long ago that we were greatly puzzled by the large differences in drag that were sometimes observed in different wind tunnels and in comparisons between results from tunnels of the compressed air type and those obtained in flight. It can now be said that the reason for these differences is well understood, but that it is not yet possible to account for them, or to predict them at all accurately, from the theory of boundary layer flow. Considering only wings with smooth surfaces the drag depends on the shape of the section, which defines the pressure distribution over its surface, and on the point at which transition from laminar to turbulent flow occurs in the boundary layer. If the pressure distribution and the transition point are known it is now possible to calculate quite closely what the profile drag will be. But theory has not yet provided a means of determining where this transition point will be on surfaces of different shapes and in air streams whose turbulence is characteristic of the wind tunnel on the one hand, and the free atmosphere on the other. The experimental determination of both profile drag and transition point in flight has done much to clarify ideas on the whole question of wing drag. It is now definitely known that the transition point may occur much further from the leading edge in the non‐turbulent atmosphere than it does in a wind tunnel at the same Reynolds number, so that the drag is lower in the former case. It is also known that this far‐back transition can occur up to Reynolds numbers as high as 17 × 106 in flight, and that its occurrence is related to the nature of the presssure gradient over the surface, that is, to the shape of the wing contour. Two important questions at once arise. Will this far‐back transition still occur on suitable sections no matter how far the Reynolds number is increased, and is it possible to encourage a still farther back transition by any alteration of section that is practically possible? The answer to these questions may well have a profound effect on design. To answer the first we must have an aeroplane capable of attaining the desired high Reynolds number and having a sufficient spanwise length of wing, undisturbed by airscrew slipstreams or discontinuities such as ailerons, to enable the measurements to be made. Such a machine docs not exist at the moment. There is some indication that the second question may admit of a favourable answer in the range of Reynolds number now available. The transition point would almost certainly be well forward on a flat plate or very thin wing section, unless, possibly, the turbulence in the air was incredibly small. In a stream such as that of the Compressed Air Tunnel it occurs practically at the leading edge at Reynolds numbers above 5 × 106. On the other hand, tests on a 25 per cent thick wing indicate a mean transition point for the two surfaces at about 20 per cent of the chord in this tunnel and, on full scale, transition as far back as 40 per cent of the chord has been observed. It seems likely that the farthest the transition could possibly move back is to the point at which a laminar boundary layer would separate from the surface. This point is calculable to a fair degree of accuracy, though the calculation has not yet been made for an aerofoil. It has been made, and the laminar separation observed experimentally, for an elliptic cylinder with axes in the ratio 3 to 1, and here it occurs at 63 per cent of the chord. The inference is that there is some hope that in non‐turbulent air it may be possible to find wing sections for which transition is delayed beyond the points hitherto observed, though how near it may prove possible to get towards the laminar separation point it is not possible even to guess.