Chapter 4: The High-Speed Propeller Program
[130] Propeller researchers were in the forefront of the rush to apply the. sweep principle in the mid-forties. The Alsos mission had reported that the Germans had tested a full-scale propeller with sweptback tips but no data on the results were available (ref. 153). A couple of years later, we obtained a translation of Quick's 1943 paper on the early German tests which indicated, rather inconclusively, that there was some advantage of sweep at high tip speeds (ref. 154).
It was clear from the outset that incorporating sweep in a propeller blade was a very complex matter, structurally as well as aerodynamically. [131] Any appreciable sweepback in the outer region of the blade had to be accompanied by sweepforward in the inner portion, the two portions being joined at an unswept "knee" (fig. 33). The first attempts to explore swept propellers, a brief flight program by the Curtiss-Wright Corporation and two propeller tests by the 16-foot tunnel group (ref. 154), involved only small amounts of sweep and showed small or negligible gains. They set the stage, however, for a better-planned effort involving more highly-swept blades and comparable unswept blades to provide meaningful evaluations of the sweep effects. The full-scale swept propeller tested in the 16-foot tunnel was designed for moderately high speed and power. A second propeller designed by Whitcomb to have the largest amount of sweep (45°) that could reasonably be incorporated within structural limitations was tested in the 8-foot tunnel at speeds up to Mach....

photo of
FIGURE 33.-The swept-blade propeller tested on the 2000-hp dynamometer.
[132] ...0.93 on their 800-hp dynamometer, together with appropriate straight blades for comparison (ref. 155).
Both of the swept propellers showed a delay in the onset of compressibility losses to higher tip speeds than those of the straight blades of equal thickness. However, the delay was only about a quarter of what might be expected from the simple sweep theory. Offsetting the beneficial high-speed effect were generally lower levels of efficiency and other aerodynamic problems for the swept propellers. But the major conclusion brought out in the analysis stated that an unswept blade of slightly reduced thickness could always be found which would have equally good high-speed performance, better overall performance, significantly lower blade stresses, and freedom from the other structural complications of the swept propellers. This emphatic and disillusioning result put an end to any further attempts to exploit swept propellers.