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THE HIGH SPEED
FRONTIER
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- Chapter 4: The High-Speed
Propeller Program
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- SWEPTBACK PROPELLERS
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- [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).
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- 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....
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- FIGURE 33.-The swept-blade propeller
tested on the 2000-hp dynamometer.
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- [132] ...0.93 on their
800-hp dynamometer, together with appropriate straight blades for
comparison (ref. 155).
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- 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.
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