There had been earlier indications that at high supersonic speeds the trailing edge of a minimum-drag airfoil should be blunt rather than sharp. This situation resulted from the fact that the suction losses behind a blunt trailing edge were more than offset by the lower pressure forces on the fore part of the airfoil made possible by the blunting.
At hypersonic speeds the pressure forces on the fore part were predominant and, to minimize these forces, it was necessary to divert the oncoming air as little as possible.
If it was assumed that some thickness of the airfoil was required for strength, minimum flow diversion would be accomplished if the airfoil was in the form of a thin wedge with an absolutely blunt, or bluff, trailing edge. At lower supersonic speeds, where the fore drag was somewhat less important, the trailing edge was rounded down (boat-tailed) a certain amount to reduce base suction drag.
But how much should the airfoil be boat-tailed for any particular speed? This was the nature of the problem which Chapman attacked. His approach, of classic form, began with theory and ended with experiment
. The theory is contained in TR 1063 (ref. B-24) . The confirming experimentation, in which he was aided by William Wimbrow and Robert Kester, is reported in TR 1109 (ref. B-25) .
The study of wing planforms begun so enthusiastically in the early postwar years carried over into the current period; and the results of a large part of this work, performed in several wind tunnels at Ames, are summarized in RM A53A30 entitled "Lift, Drag and Pitching Moment of Low Aspect Ratio Wings," by Charles F. Hall.
This report gives particular attention to airplane configurations having delta, or triangular, wing planforms and likewise considers the matter of twisting and cambering delta wings to reduce the increment of wing drag caused by lift.
As the angle of attack of a wing is increased to produce lift, the resultant force vector tilts backward, thus increasing its component in the drag [204] direction. This additional increment of drag, due to lift, is reduced by any leading-edge suction that may be generated by the wing.
Although theory indicates that a plane wing with leading edges swept within the Mach cone will develop leading-edge suction, in actuality the predicted suction does not appear. It was realized that the higher drag-due-to-lift resulting from this unfortunate circumstance could significantly reduce the performance of supersonic airplanes in climb and cruising and could curtail their range.
In his investigation of this matter, Charlie Hall found that a substantial reduction of drag-due-to-lift could be achieved with a delta-wing airplane if the wing was twisted and suitably cambered throughout its span.
For practical reasons it was desirable, he found, to incorporate the camber only in the forward portion of the wing. In the selected arrangement the cambered part of the wing appears as a segment of a cone varying linearly in extent from zero at the root to a maximum at the tip.
Ames engineers felt that conical camber had the potential for significantly improving the performance of future delta-wing airplanes.
Area Rule. One of the most significant developments of this period was the discovery, by Richard Whitcomb of Langley, of a method, called the Transonic Area Rule, for reducing the transonic drag of aircraft.1 Actually the theoretical basis for the Area Rule had been established a little earlier, but it was not until Whitcomb, using the new transonic tunnel at Langley, made his independent discovery that the significance of the method was fully appreciated.
The Transonic Area Rule expresses the concept that the transonic drag of an airplane is strongly dependent on the distribution of the cross-sectional area of the airplane, including the wing and all other components, and that as far as pressure drag is concerned the airplane could be represented by a body of revolution having the same longitudinal distribution of cross-sectional area as the airplane.
The optimum distribution of area was not precisely determined by Whitcomb, but it was concluded that the area distribution curve should be nicely rounded and free of sharp peaks or humps.
The fuselage of a transonic airplane usually had an area distribution of this kind, but the addition of a wing, an engine nacelle, or a wing-tip fuel tank produced a hump in the area curve which caused the pressure drag of the airplane in the transonic range to rise to a high peak.
Airplanes that designers hoped would be supersonic were thus sometimes limited to sonic speed as a result of unexpectedly high transonic drag.
Whitcomb showed that the hump in the area curve' and thus the high transonic drag, could be eliminated if the added area contributed by the wing was balanced by a deliberate reduction in the cross-sectional area of the fuselage at the wing juncture. The resulting fuselage shape had a constriction like a Coca-Cola bottle, and the terms "coke bottle" or "Marilyn Monroe" fuselage were often heard.
