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THE HIGH SPEED FRONTIER


Chapter 4: The High-Speed Propeller Program

HIGH-SPEED FLIGHT TESTS OF PROPELLERS



[135] Throughout the forties the Flight Research Division at Langley measured propeller performance on several piston-engine fighter aircraft at speeds in excess of 400 mph. The test propellers were generally typical of advanced service practice and they provided useful data on compressibility effects as they were encountered in actual operating conditions (refs. 160, 161). The results were invariably consistent with expectations based on the wind tunnel programs, but were rarely directly cornparable because of differences in the test propellers. Principal figures in the flight work were T. Voglewede, A. Vogeley, and J. Hammack.

The successes of the high-speed research airplanes in the late forties had led to thinking at Langley about a possible "propeller research-airplane," and the flight division eventually succeeded in promoting such a project. Aimed primarily at potential long-range military applications, [136] it was developed as a joint effort with the services; the Air Force provided the XF-88B airplane and the test propellers and associated equipment, and the Navy provided the turbojets and the T-38 turboprop engine which was installed in the nose of the XF-88B to power the test propellers. Unfortunately, this program did not start to produce results until the mid-fifties when interest in high-speed propellers had almost disappeared. Three propellers were eventually tested at flight speeds up to slightly above Mach 1 on the XF-88B (fig. 36). By the time the results were analyzed in 1957, the Subcommittee on Propellers for Aircraft had been disbanded, eliminating a main heading on this subject in the NACA Annual Report. Thus, we find in the 1958 (Final) NACA Annual Report only an obscure reference to these interesting data, the crowning achievement of a difficult and costly project, under the heading "Low-Speed Aerodynamics." Peak efficiency of 80 percent had been measured at Mach 0.95 on a thin "supersonic" propeller, generally confirming the levels indicated in the Langley high-speed wind tunnel programs (ref. 162).

COMMENTARY

Taken as a whole, the high-speed propeller program is clearly one of the more substantial NACA contributions. The magnitude of the undertaking was well beyond anything that might realistically have been expected from private industry, and this was another example of NACA fulfilling its proper governmental function.

There were occasional noteworthy flashes of inspiration. The impressive blade pressure distribution surveys afford perhaps the best example. Several innovative developments had to be brought together to make these measurements possible. These unique data still have been only partially analyzed and remain available to enterprising future researchers.

Like several other NACA programs, high-speed propellers has its mythology. Evidence of this can be found, for example, in the writings of G. W. Gray whose book (ref. 163) states, "Almost everything in the way of improving propeller efficiency for high-speed flight rests on the utilization of the 16-series airfoils. "The principal source of his education in high-speed propellers and the NACA reviewer of this material was.....




[137] FIGURE 36.-Supersomic propeller (feathered) driven by T-38 turboprop engine mounted on XF-88 propeller research airplane.

[138] .... Stack, a man who had outstanding talent for technical salesmanship. Any qualifications or words of caution which Stack probably included were undoubtedly lost in his effusive account of how NACA had created efficient 500-mph propellers. He often used the term "16-series" to encompass all the improvements embodied in the NACA propellers, including the all-important reduced thickness ratios, a usage which Gray evidently misunderstood.

The progress in high-speed propeller technology made in the NACA program took place in an environment of dwindling user interest. In 1949 T. B. Rhines of the Hamilton Standard (Propeller) Division of United Aircraft complained poignantly that ". . . . various representatives of the aircraft industry imply that even if the [high-speed] propeller is good it is not wanted" (ref. 155). There was still hope that 500-600-mph transports might need transonic propellers, especially for long range, but with the advent of the Comet and the 707 this application also faded and the NACA high-speed propeller program ended with the transition to NASA.

The oil crisis is now forcing new considerations of high-speed propellers, and we may see a renaissance for both military and commercial applications.