ch4-7

THE HIGH SPEED FRONTIER


Chapter 4: The High-Speed Propeller Program

TRANSONIC AND SUPERSONIC PROPELLERS



[132] The 800-hp dynamometer placed in operation in the repowered 8-foot tunnel in 1946 (fig. 34) was used to extend the testing of the related NACA blade families to higher solidities and higher power loading, including dual rotation. It was now possible to explore propeller performance at airspeeds up to Mach 0.93 where obviously the entire blade was operating supercritically. With large spinners supersonic helical Mach numbers could be obtained over the entire blade. Still deeper penetration into supersonic operation was achieved with the 6000-hp dynamometer used in the repowered and slotted 16-foot tunnel at airspeeds up to Mach 1.04 (fig. 35). Both dynamometers incorporated important improvements over their earlier counterparts (refs. 156, 157). The strut-choking effect for the 800-hp installation in 8-foot was largely avoided by locating the plane of the propellers in the subsonic throat region of the Mach 1.2 plaster nozzle, following the scheme sketched in fig. 14. The propeller program in 8-foot was completed before the slotted throat was installed in 1950, all propeller research at Langley thereafter being conducted in 16-foot or in actual flight.

After the war, with the growing reality of the turbo-propeller, the prospect of using propellers at speeds well beyond 500 mph, upward to....




[133] FIGURE 34.-The 800-hp Propeller Dynamometer used in the repowered 8-Foot High-Speed Tunnel.

....transonic and even low supersonic flight speeds, seemed likely. High efficiencies had been maintained up to 500 mph by the devices of improved (primarily thinner) blade sections and reduced rotational speeds (high blade angles, or high advance ratio, V/nD). For normal subcritical operation best efficiency is obtained at reference blade angles of about 45° corresponding to a V/nD of about 2. At 500 mph the best blade angle is typically about 60°. Obviously, this device cannot be continued indefinitely as the speed increases because the blade angle ultimately becomes so high that efficiency starts to fall drastically due to the unfavorable inclination of the force vectors. One of the important contributions of the 8-foot tunnel program was to delineate the V/nD limits for best efficiency at speeds up to Mach 0.93. It was found that the increasing-blade-angle approach remained effective up to about Mach 0.85 but by Mach 0.9 and beyond a reversion to lower blade angles resulted in best efficiency.

A major additional asset in the use of high rotational speeds at....



[134] FIGURE 35.-The 6000-hp Propeller Dynamometer installed in the repowered slotted 16-Foot High-Speed Tunnel.

[135] ....transonic speeds is a large reduction in the physical size of the propeller, from 26 feet in diameter for V/nD = 6 to 12 feet for V/nD = 2, in an example given in ref. 155. A small propeller of this kind has supersonic conditions over virtually the entire exposed blade and is thus referred to as a "supersonic propeller" even though the design forward speed may be still subsonic.

The aerodynamic criteria for design of transonic or "supersonic" propellers with low profile losses were clear: use the thinnest possible blade sections, sharp or very small-radius leading edges, and little if any camber. The first two of these criteria had in fact been obvious for some 25 or 30 years-since the thin propeller tests of Reed, and the section tests of Briggs and Dryden. Propellers tapering from about 5-percent thickness ratio at the base to 2-percent at the tip with either zero or very small cambers yielded efficiencies of 75 to 80 percent at a forward Mach number of 0.9. At Mach 1, peak efficiencies as high as 0.75 were obtained (refs. 155, 158). The recovery in lift and 1/d observed in airfoil section tests at high supercritical speeds where the separated flow disappears (fig. 4) is also seen in the propeller tests; curves of peak efficiency against flight speed level out and may rise slightly at speeds beyond about Mach 0.9 (ref. 155).