Chapter 4
The High-Speed Propeller Program
[119] The extensive propeller testing at low airspeeds and high rotational speeds in the Propeller Research Tunnel consistently showed a marked loss in efficiency starting at tip Mach numbers of about 0.9. Clearly, this was only a part of the compressibility problem of propellers for 500-mph aircraft, for which high Mach numbers would exist over the entire blade. The fact that the PRT tests showed a considerable delay in the onset of compressibility effects as compared with wind-tunnel section data suggested that estimates of high-speed propeller performance based on strip theory and section data as understood at that time could not be relied upon. Tests of propellers at high forward speeds were needed to provide precise information on the attainable performance.
Unlike many other NACA programs which started with little understanding of the problem, the high-speed propeller program enjoyed a well-established basic understanding; a substantial body of high-speed section data and criteria for design of efficient advanced propellers had been built up over the previous 20 years. In these circumstances, it was obviously unnecessary to explore the problem by testing existing propellers which would clearly prove to be inefficient at high speeds. Instead, a family of advanced high-speed propellers embodying the features known to be needed to favor high-speed performance was defined at the outset. The main purpose of the test program was to determine accurately the attainable high-speed performance as affected by systematic changes in the principal design variables.
The useful but rather uninspiring nature of this test program, together with the tedious aspects of high-speed dynamometer development, made it unattractive to impatient imaginative researchers seeking higher levels [120] of challenge and excitement. But, like many other instances which come to mind, the propeller program seemed to attract the type of talent appropriate to the job-competent, practical, conservative engineers who were willing to devote many years to the exacting tasks of perfecting the large dynamometers and obtaining precision data under difficult test conditions.
The first step taken by NACA toward higher-speed testing was the approval in mid-1936 of plans for the 19-foot, 250-mph Pressure Tunnel, in essence a super-PRT. A new propeller dynamometer powered by large electric motors was a major feature of the plan. By the time this new tunnel was dedicated in May 1939, however, the continued increase in speed of military aircraft plus the growing war threat made it apparent that 250 mph was inadequate for high-speed propeller testing, and a second new Langley tunnel was undertaken-the 500-mph 16-foot High-Speed Tunnel. This facility was intended to concentrate on full-scale propellers and engine cowling and cooling, while the 19-foot tunnel would become involved primarily with scale-model aircraft testing and dynamic-loads research. Accordingly, the new propeller dynamometer project was transferred to the 16-foot tunnel enterprise.
Shortly after Stack had taken up his duties as head of the 8-foot tunnel section in 1939, the outlook for high-speed propeller testing in the 16-foot tunnel was discouraging. Major delays had been encountered in design and procurement of the new electrical equipment for the dynamometer, and it appeared that three or four years, at least, would elapse before testing could be expected. Stack reacted with characteristic impatience. He was quite unhappy at the prospect of a long delay in testing propellers incorporating the new 16-series blade sections. The only apparent solution was to procure a dynamometer for the 8-foot tunnel and run the tests on 4-foot diameter propellers. This would have the advantage of smaller (200-hp) electrical equipment, some of which was already available, and we projected that the desired answers should be forthcoming within about two years. Stack had little difficulty in selling this plan, and it was called the "Emergency Propeller Program" to answer any question of duplication with the 16-foot tunnel plans.
After the repowering of the 8-foot tunnel in 1945, propeller testing was extended to higher speeds (Mach 0.93) with an 800-hp dynamometer, [121] and this program continued until conversion of the repowered tunnel to a slotted throat in 1950.
The 16-foot tunnel program utilizing nominally full-scale (10-foot-diameter) propellers got underway in 1945 with the 2000-hp dynamometer that had been so long in procurement and development. As in the case of 8-foot, this was later replaced by an improved 6000-hp dynamometer when the 16-foot was repowered in 1950. Speeds up to Mach 1.04 were achieved in full-scale propeller testing in the 16-foot slotted throat.
The Ames Laboratory embarked on a limited program of propeller testing in their 12-foot high-speed tunnel in the early fifties when it appeared that the turboprop application required research at high subsonic speeds. Their propeller dynamometer used 4-foot-diameter blades and 1000-hp motors taken from the Langley program, and it incorporated several other features from the Langley installations. Forward speeds up to Mach 0.84 were covered in the one series of high-speed tests made at Ames (reported in NACA TR 1336).
Throughout the period of the high-speed wind-tunnel propeller programs (1938-1958), periodic propeller testing was also done in flight on advanced fighter aircraft. Starting with such piston-engine aircraft as the XP-42 and P-47, the flight work ended in the mid-fifties with testing of three propellers at speeds up to about Mach 1, using a special turboprop engine installation in the nose of an XF-88B jet fighter.