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Zero Model 21 Performance:
Unraveling Conflicting Data (part 2)

continued from part 1

FIELD DATA

This part of the study will review comparative performance data gathered in the field during combat in 1942. The author is fully aware that data derived from operational reports cannot be controlled in the same way that careful engineering tests can be. On the other hand engineering tests conducted without key information regarding the item under test may be grossly misleading (the China Zero for example). Despite the possible existence of individual anomalies, operational data may be considered highly pertinent when a large body of relatively consistent data can be assembled.

The Bell P-39 and Grumman F4F-4 are key points of comparison and merit brief comment. The maximum speed of the P-39D is generally cited as 360 m.p.h. at 15,000 feet (Dial, p. 272). That for the F4F-4 is given as 318 m.p.h. at 19,400 feet (Taylor, p. 501) or as more pertinent here 274 m.p.h. at sea level (Baugher). The F4F-4 engaged the Zero in both carrier battles and in numerous combats over Guadalcanal operating from land bases. The P-39D (and its P-400 export version) operated against the Zero both over Guadalcanal and over New Guinea. It should be kept in mind that during the period under review Zeros flying from land bases over Guadalcanal almost always entered combat with their external fuel tanks attached. Zeros in combat over New Guinea generally flew without such tanks or dropped them before combat. Without the tank the Zero would have been somewhat faster than flying with the tank attached.

A report summarizing the combat performance of the P-400 and F4F-4 against the Zero over Guadalcanal in late September 1942 stated: "At all altitudes under 10,000 feet the P-400's can pull away from the Zero (P-400 speed about 360 m.p.h. F4F-4 about 40 m.p.h. slower). Zeros are faster than the F4F-4's at all altitudes and more maneuverable_" (Performance).

In a report based on questioning forty fighter pilots of VMF-121, 212 and 251 and VF-71 concerning combats in October 1942 the discussion of comparative performance was brief: "A Zero is faster, more maneuverable, and has a higher rate of climb than our F4F-4s" (Observations).

In an after action interview given in November 1942 Major John Smith, commander of VMF-223 at Guadalcanal, said little about the Zero's performance until asked a direct question and then replied: "They had much more performance than we had. I think they did because we just couldn't stay with them at all, and dog fight at any altitude."

The F4F-4s of VF-5 commanded by Lt. Commander LeRoy Simpler flew against Zeros from a carrier in August 1942 and were land based on Guadalcanal during September and October 1942. Upon returning to the U.S. Simpler was apprised of the test report that said an F4F-4 was equal in speed to a Zero at low level. His comment was that the report was "flat wrong."

The reports above are all measured pronouncements by command authorities after careful study or related by experienced combat leaders. In none of the comments in the reports cited above is there any hint that the F4F-4 could equal the Zero in speed even at low level. In fact the contrary is expressly noted. This is despite the fact that the Zeros were handicapped by an external fuel tank. The P-400's speed advantage below 10,000 feet was also enhanced for the same reason.

Airacobras clashed with the Zero on April 30, 1942 in a low level action near Lae, New Guinea. From May to August 1942 combats between Airacobras and Zeros took place on a regular basis over New Guinea. After the first few combats Lt. Col. Boyd D. Wagner wrote a report on the early actions. After commenting that the Zero outperformed the P-39 markedly in maneuverability and climb, Wagner commented on the relative speeds of the aircraft at low altitudes. According to Wagner: "_the Zero was able to keep up with the P-39 to an indicated 290 mph. At 325 indicated just above the water, the P-39 pulled slowly out of range." Wagner also commented that the P-39's performance above 18,000 feet was very poor.

In later actions combat reports sometimes offer helpful insights into the relative performance of the two aircraft. Lt. Paul G. Brown chased a Zero at 12,000 feet. "He nearly stayed away from me at 350 mph" (Brown). In a low level action: "I indicated 320 mph straight and level at 1,000 feet. Zero kept me in range" (Royal). In another action on the same day Zeros encountered P-39s and P-400s at 21,000 feet. "Zeros stayed with the Airacobras. I dived 12,000 feet indicating 450 miles per hour and Zero stayed with me and followed me to ground level firing. Lt. Martin pulled him off me" (Price). "4 Zeros were over Kokoda and attacked us on the way home. We were barely able to out speed them at 10,000 feet. We were indicating about 350 mph in a very slight dive. Their probable speed 340 mph" (Egenes).

From the Japanese side also comes confirmation that the Zero could hold its own with a P-39 in low-level speed. Sakai relates that on July 22, 1942 he chased a P-39 low over the sea and the P-39 was unable to pull away from him (Sakai, p. 137). The Airacobra was eventually forced to turn in order to take up a course to its base. In the ensuing dogfight Sakai shot the aircraft down. It was probably a P-400 of the 35th Fighter Group.

This compilation of reports indicates the Zero was either equal to or close to the P-39 in speed at the altitudes of the various encounters. The P-39 was in turn up to 40 m.p.h. faster than the F4F-4 according to reports from the South Pacific Theater. There the Zero was found to be consistently faster than the F4F-4. There is a disconnect between the San Diego test results and multiple reports from the combat zone.

Lest there be any doubt, crash intelligence reports show that the Zero 21s in use in the Southwest Pacific were close contemporaries of the Akutan Zero (No. 4593, completed 19 Feb. 1942). Many crash reports identify production dates for Zero 21s lost in the SWPA as February 1942 or earlier.With the exception of a single appearance by A6M3s (30 August 1942) all the Zeros in combat over Guadalcanal during the period under review were also Zero 21s.

CONCLUSION

The field data reviewed by this study indicate that Zeros operated by the Japanese performed relatively better against the Wildcat and Airacobra than did the Zero tested at San Diego. If the comparative performance of the San Diego Zero understated the performance of a typical Japanese operated Zero, this strongly indicates the quantitative performance was also understated. This tends to verify the conclusions reached in the section reviewing U.S. test results. The reasons for this seem obvious. The San Diego Zero was in less than perfect aerodynamic condition and was not operated at its optimum engine capacity or with automatic mixture control engaged. The figures cited in Summary No. 85 and repeated by Mikesh and Reardon are inaccurate and too low to represent the true performance of the Zero in Japanese operations.

The author has been unable to establish the basis for the performance figures higher than the San Diego test results (332-336 m.p.h.) but lower than Sakai's (sources 5-7 in the section Conflicting Data). They are close to the first test results obtained at San Diego (335 m.p.h.) but those results were not deemed reliable. Absent the basis for these figures nor knowing the conditions that yielded them they are difficult to assess.

Sakai distinguished between normal full power speed (316 m.p.h.) and over boost (345 m.p.h.). His normal full speed is exactly the same as the Zero's maximum speed given in the captured Japanese manual. The San Diego test report, while revealing that the San Diego Zero was not tested at over boost, does confirm Sakai's assertion that such a rating was available. Sakai has credibility that is primarily based on his personal familiarity with the Zero 21 aircraft. These additional factors only bolster his credibility.

The evidence assembled in this report strongly indicates that Sakai's version of the Zero's maximum speed (345 m.p.h.) is highly credible and probably the correct one. Additional support for this conclusion is found in an intelligence document issued in 1944: "Performance data given for the ZEKE Mk. 1 [Allied code name for the Zero 21] was obtained in actual flight tests. Although emergency speed obtained in tests was 328 m.p.h., calculations indicate a maximum speed of about 345 m.p.h. as possible for a short period of time" (Intelligence Summary No. 44-11).

SOURCES

Wartime reports:

"Performance and Characteristics Trials, Japanese Fighter" Technical Aviation Brief #3, Aviation Intelligence Branch, Navy Department (4 Nov. 42) (extract)

"Flight Characteristics of the Japanese Zero Fighter Zeke" Informational Intelligence Summary No. 85, Intelligence Service, U.S.A.A.F., Dec. 42 (rev. Mar. 43)

Memorandum of Oct. 19, 1942, Bureau of Aeronautics, Navy Department to War Department, "Preliminary Zero Data, 10 Oct. 42 (revised)"

Captured Document (Zero Flight Manual), Joint Intelligence Center-Pacific Ocean Area Item No. 5981, Kwajalein, received 19 Feb. 1944

"Zero Test - Mitsubishi Type O Evaluation, Feb. 1943" HQ, 23rd Fighter Group, 6 Feb. 1943 ("Holloway")

"Observations of Marine Fighter Pilots at Guadalcanal October 16 to October 31, 1942" (Bauer), United States Pacific Fleet, South Pacific Force, Naval Air Combat Intelligence (extract) ("Observations")

"Performance of P-400 and F4F-4 in Guadalcanal Area" (Commander, Aircraft, South Pacific Force, 28 Sep 42) ("Performance")

Interview, Major John Smith (Navy BuAero Nov 42)

Interview, Lt. Cdr. LeRoy Simpler (Navy BuAero Feb 43)

"Report on first action against Japanese by P-39 type airplane" (B.D. Wagner May 42)

Combat Reports (RAAF Form A.108A) for Lt. P.G. Brown, 36 FS (27 May 42); Lt. F. Royal, 39 FS (4 Jul 42); Lt. J.C. Price, 39 FS (4 Jul 42); and, Capt. E. L. Egenes, 40 FS (6 Jul 42) (cited by pilot's last name)

Informational Intelligence Summary No. 44-11, Mar 1944, Assistant Chief of Air Staff (Intelligence)

Books and other sources (generally cited by author's name):

Baugher "Grumman F4F Wildcat" online

Caiden, Zero Fighter, Ballentine (NY 1969)

Dial, "The Bell P-39 Airacobra" Aircraft In Profile vol. 7, Doubleday (NY 1970)

Francillion, "The Mitsubishi A6M2 Zero-Sen" Aircraft in Profile vol. 6, Doubleday (NY 1969)

General View of Japanese Military Aircraft in the Pacific War, compiled by the staff of "Airview" Kanto-sha (Tokyo 1956)

Green, Warplanes of the Second World War, Fighters vol. 3, Doubleday (NY 1961)

Mikesh, Zero, Motorbooks International (Osceola, WI 1994)

Reardon, Cracking the Zero Mystery, Stackpole (Harrisburg PA 1990)

Sakai et al, Samurai, Ballentine (NY 1958)

Taylor, Combat Aircraft of the World, Putnam (NY 1969)