Fisher with USAAF P-47D-30-RE "Test PE-219" in postwar testing; note scimitar propeller.
Compressibility effects:
After test flights of a P-47C on November 13, 1942, Republic Aviation issued a press release on December 1, 1942, claiming that Lts. Harold E. Comstock and fellow test pilot Roger Dyar had exceeded the speed of sound. In response, Fisher later observed, "We knew about Mach 1 going clear back to the P-36 and the P-40 ... Nothing could go 600 mph (970 km/h) mph in level flight, but pilots were beginning to dive fighters. We ran into compressibility back in '38."
The desire to develop a propeller that maintained its efficiency at transonic and supersonic speeds led the Curtiss-Wright Propeller Division to design and test several different concepts, including a thin, cuffed four-blade and a three-bladed "scimitar" design. Utilizing a specially modified P-47D-30-RE on loan from the USAAF, Fisher undertook over 100 high Mach number precision dives from 38,000 ft (11,582 m) at speeds from 500 mph (800 km/h) to 590 mph (950 km/h). The typical flight began above 35,000 ft (10,668 m) when Fisher would push over into a steep dive, allowing his airspeed to build beyond 560 mph (900 km/h) (true airspeed). He would then execute a pullout at 18,000 ft (5,486 m), having to maintain an exacting set altitude within plus or minus five ft.
Some of the tests proved hazardous with flexing of the thin blades on the ground run-ups. Test flights also had to be carefully flown as engine power could only be fed in gradually for the same reason. The most serious incident, though one unconnected with compressibility, occurred in August 1948 when a rupture of a high pressure oil line at 590 mph (950 km/h) mph over Allentown, Pennsylvania led to an emergency "blind" landing with the entire aircraft coated in black oil. Several of these dives resulted in speeds of Mach 0.83; one on 27 October 1949 reached the fastest speed a P-47 could attain.
During the test program, Fisher brought his son, Herbert O. Fisher, Jr., along for a Mach 0.80 dive. The youngster was touted as "the world's fastest toddler".
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