Zero Model 21: unraveling the performance data (part 1)
by Richard L. Dunn c 2004
INTRODUCTIONThe Mitsubishi Type Zero Carrier Fighter Model 21 was the fighter that opened the war in the Pacific over Hawaii and the Philippines. It was the primary fighter used by the Japanese navy from the beginning of the war until early 1943 and remained in front line service until well into 1944. As important as this fighter was in the Pacific air war there is little agreement in published sources about some aspects of its performance. In particular, the aircraft's maximum speed is given by different post-war publications in a range from 316 mph to 345 mph. This disparity of nearly 30 m.p.h. is sufficiently broad that at the lower end the aircraft might be deemed relatively slow by 1942 fighter standards and at the upper end it might be considered relatively fast and definitely competitive. This article attempts to unravel conflicting data and provide a likely, if not definitive, answer to the question of the Zero's maximum speed.
CONFLICTING DATAA review of a selection of popular aviation literature reveals the following range of maximum speed data for the Zero 21: (1) 316 m.p.h. at 16,570 ft. (Taylor, p.253); (2) 316 m.p.h. at 16,400 ft. or 509 k.p.h. at 5,000m (General View, appended chart); (3) 321 m.p.h. or 279 knots at 20,000 ft. (Mikesh, p. 123); (4) 326 m.p.h. at 16,000 ft. (Reardon, p.113); (5) 331.5 m.p.h. at 14,930 ft. (Francillion, p. 16); (6) 332 m.p.h. at 16,570 ft. (Caiden, p. 158); (7) 336 m.p.h. at 19,685 m.p.h. (Green, p.46); and (8) 345 m.p.h. (Sakai, p.48). Earlier figures from wartime sources would reveal an even greater range of estimates of the Zero's speed but the figures here are all given in publications written well after the war, several by well respected aviation authors and historians. Only two of these publications list the source of their data. Both Reardon and Mikesh cite Informational Intelligence Summary No. 85, December 1942 as their source. Citing an actual test report tends to give their figures an air of authenticity. The difference between the two apparently arises because Mikesh has left speed at 15,000 ft. blank rather than adjust his table to indicate 16,000 ft. as shown in Reardon.
Intelligence Summary No. 85 contains performance data on the Zero captured at Akutan Island in July 1942. The Zero was restored to flying condition and test flown at San Diego in September-October 1942. Interestingly Reardon mentions the Navy's test report (Performance and Characteristics Trials, Japanese Fighter", Technical Aviation Intelligence Brief #3, 4 Nov. 1942), however, he cites performance figures not from the Navy test report but from the latter Army summary. A reading of Reardon's book hints he never had access to nor read Intelligence Brief #3. The only cogent comment in the book on the condition of the captured aircraft at the time of the tests is Reardon's question, decades after the fact, to one of the navy pilots (E. Saunders) that flew the captured Zero. Reardon asked was "the repaired airplane 100 percent?" The reply was "about 98 percent." There is no indication Reardon probed further nor that he was aware of the details contained in Intelligence Brief #3 concerning the airplane's aerodynamic or mechanical condition.
The source of information on the low end of the Zero's speed range is not given by either Taylor or in "General View"; however, it is fairly easy to track down. It closely reflects the "official" maximum speed given in a Japanese manual on the Zero's flight characteristics (JICPOA Item No. 5981, dated October 1943, captured on Kwajalein February 1944). The maximum speed given in the manual is 275 knots (a fraction above 316 m.p.h.) at 4,400m (14,430 ft.). Erroneously the manual gives the rated altitude of the Zero's Sakae 12 engine rather than the critical altitude of the aircraft as given in other sources. This same speed of 275 knots is mentioned by Sakai as the Zero's speed "under normal full power conditions."
Of the various "maximum speed" figures cited above it seems evident that the low-end figures given by Taylor and "General View" do not reflect a true maximum speed. These figures are lower than tests results achieved with a captured Zero in less than 100% condition and also are equated by Sakai with the aircraft's speed under "normal full power" (military or 30 minute rated power) not over boost (roughly "war emergency power" in U.S. terminology).
Above the low-end data that do not give a true maximum speed (sources 1-2 cited above) and the U.S. test data cited in Mikesh and Reardon (sources 3-4) are figures ranging from 331.5 m.p.h. to 345 m.p.h. No specific source material is cited for these figures though Sakai's high-end figure of 345 m.p.h. presumably is based on his recollection and years of personal experience flying the Zero 21. The problem is to try to validate which, if any, of these figures reflect the actual performance of the Zero as operated by the Japanese.
A LOOK AT THE U.S. TESTSIt might be worth noting briefly that the Akutan Zero tested at San Diego was not the only captured Zero tested by the U.S. A Zero 21 captured and rebuilt in China was also briefly tested (Holloway). The maximum speed obtained in those tests was only 289 m.p.h. at 15,000 feet. However, this performance is remarkable as the aircraft was operated at 2050 r.p.m. (it was found capable of only 2075 r.p.m.) vice the Zero's rated 2500 r.p.m. and maximum take-off rating of 2550 r.p.m. (even so the Zero out climbed a P-40K!). Needless to say test figures obtained at such low revolutions bear no relationship to the aircraft's true performance.
The San Diego tests on the Akutan Zero were conducted from September 26 to October 15, 1942. Intelligence Summary No. 85 gives no indication of tests conditions and provides only one set of data on the Zero's speed. No doubt the information was disseminated because it was deemed both the best available and reasonably reliable. Intelligence Brief #3, the original source, was apparently not widely disseminated because it contained technical information of little interest to combat crews and also because it showed multiple tests had revealed differing results including figures superior to the results the authorities apparently decided were most acceptable.
The most important thing to note about the results published in Intelligence Summary No. 85, at least for this study, is that they likely understated the Zero's performance. Intelligence Brief #3 (from which the Summary No. 85 data was taken) states: "It is probable that the airplane in original condition was somewhat faster than is indicated here, due to lack of flush fit at wheel well fairings and cabin enclosure in the overhauled plane, and the addition of non-specular paint." These defects may relate to the 98 percent condition of the airplane mentioned by Admiral Saunders in Reardon's book. However, the test report also reveals a more profound defect in the tests. The Zero was tested at a maximum boost of 35 inches of mercury. The aircraft was operated at 38 inches of mercury for only brief periods because the engine ran rough and there was fear of losing the test aircraft. Thus the aircraft was not tested at its overboost rating (N.B. "overboost" is something of a misnomer, the 38 in. Hg boost was obtained by operating a boost shift lever that allowed and regulated this level of boost). Possibly also significant was the fact that the automatic mixture control was inoperative and the carburetor had to be adjusted manually during the tests. (Additional note: the U.S. notations 35 and 38 in. Hg actually reflect Japanese ratings of +150mm/35.4 in. Hg and +250mm/37.8 in. Hg).
Prior to the release of Intelligence Brief #3 the Navy's Bureau of Aeronautics sent a memorandum dated Oct. 19, 1942 to the War Department giving preliminary Zero performance data. This report gave maximum speeds for the Zero 7-10 m.p.h. faster than the data that was later widely disseminated. Maximum speed at 16,000 was given as 335 m.p.h. Intelligence Brief #3 stated that the earlier data was revised due to tests with improved instruments, reduction to standard conditions and corrections for compressibility.
It is interesting to note that despite the wide dissemination given to Intelligence Summary No. 85, later Technical Air Intelligence Center summaries often attributed to the Zero 21 [ZEKE Mk. 1] a slightly higher maximum speed (328 m.p.h.) and indicated this figure came from flight tests. Whether this figure was derived from later flights tests or was based on additional adjustments to the San Diego test figures is unclear. The Akutan Zero was subsequently flown in an aircraft identification training film (starring Ronald Reagan) and did under go National Advisory Committee for Aeronautics testing (a preliminary study of stability and flying qualities took place with NACA instruments installed). Whether additional tests that might modify earlier performance findings took place has not been determined by this author.
In addition to quantitative data the San Diego tests also involved comparative trials with American fighters. These comparisons allow us some ability to gauge the results obtained in San Diego against reports of actual performance in combat. The two most telling data points for this purpose are the Brief's statement that the Grumman F4F-4 Wildcat was faster than the Zero at low level and the report that the Bell P-39D Airacobra could catch and pull away from the Zero at 20,000 feet and had a decided superiority in speed at lower altitudes ("At 10,000 feet, from a cruising speed of 220 mph indicated, the P-39 still accelerated rapidly away from the Zero").
It was demonstrated above that the 316 m.p.h. figure clearly is not an accurate representation of the Zero's maximum speed as that term is commonly used in western literature. This section has demonstrated that brief U.S. flight tests in China resulted in a set of completely bogus data and that much more extensive tests of the Akutan Zero in San Diego have deficiencies that render them less than fully accurate and understate the Zero's performance.
The conclusions in the paragraph above do not aid in deciding which of the higher figures (332-345 m.p.h.) is more likely correct. Both to attempt that and to verify whether the San Diego tests understated the Zero's performance, we undertake an examination of combat data.