Appendices
Over 35 years after the Hellcat first flew, neither Grumman nor the Navy can definitely confirm the real story behind the early experimental models. Grumman’s historian, H. J. Schonenberg, believes the matter will never be fully explained, and his assessment is undoubtedly correct.
The Navy authorized two XF6F-ls, BuAer numbers 02981 and 02982, which received factory numbers 3188 and 3189, respectively. Grumman records indicate that for an unknown reason the BuAer numbers were exchanged on the XF6F-1s and that 02982, which became Grumman number 3188, was the sole test airframe. Reportedly, it was subsequently modified as the XF6F-3 with the adoption of the Pratt and Whitney R-2800 engine.
Presumably, then, the original prototype became the interim XF6F-4 between the two major variants. According to company records, this aircraft (now BuNo 02981, factory number 3189) was later delivered as a production F6F-3.
Robert L. Hall, the test pilot directly concerned, disagrees. His logbook shows two separate prototypes, an XF6F-1 (02981) and an XF6F-3 (02982). He contends that they could not possibly be the same airplane, since during the week of 25 July 1942 he flew both the -1 and -3. This contradictory evidence mystifies every researcher or historian who addresses the matter, and the puzzle is unlikely to be solved.
Overlooked among the Hellcat prototypes is the sole -2. The XF6F-2 was BuAer 66244, originally powered by a Wright R-2600-15. It was first flown in January 1944 by test pilot Carl Alber. Later modifications involved the Pratt and Whitney R-2800-16 and R-2800-21. Each of these engines utilized the Birmann turbo-supercharger.
Like many such experiments, the Birmann was highy promising but disappointingly erratic. It seemed capable of delivering sea-level performance at 20,000 feet, a potential advantage of extreme value in combat. But in-flight fires were a common occurrence due to the ignition of unburned fuel streaming from the supercharger near the engine’s exhaust stacks. This ever-present danger was unacceptable, and the Birmann was never adopted as standard equipment. The XF6F-2 was eventually modified to -5 standards and delivered as such.
XF6F-2 (BuNo 66244) with Wright R-2600-15 engine equipped with Birmann turbo-supercharger. It gave a sea-level performance at 20,000 feet but was never adopted because of fire danger. The name on the cowl is “Fuzzy Wuzzy,” after the numerous felt tufts which provided aerodynamic information. Photo: Grumman
The last Hellcat model was the XF6F-6. Two examples (BuAer 70188 and 70913) were built, and the first was test-flown by Pat Gallo in July of 1944. The P&W R-2800-18 engine with water injection produced 2,450 horsepower which, combined with a 13 foot 2 inch Hamilton-Standard four-blade propeller, delivered a top speed of 425 mph, or 370 knots. This was 15 mph faster than the F6F-5, and rate of climb was marginally superior.
Further testing of the -6 continued as time permitted, but by mid 1944 Grumman was building nearly 500 Hellcats per month. With its energies devoted to production rather than additional experimentation, the factory did not put the ultimate Hellcat into manufacture before V-J day. Presumably the Navy felt no urgent need for an improved F6F, otherwise the -6 would have progressed further than it did.