Key Points

Introduction

The DC-3 Emerges

The C-53 Skytrooper

The Definitive C-47 Skytrain

The RD4

After the War

Vietnam EC-47s and AC-47s Spooky

Our C-47

Construction

Service in World War II

Transfer to Civilian Use in Australia

Back to the United States

Characteristics of the C-47A

General Characteristics

Powerplant

Performance

Bibliography

The Author

For aircraft, the “top speed” is measured at the aircraft’s “critical altitude”—the altitude at which level speed is maximum at full military power. (Military power is the highest power an engine can produce on a sustained basis.) At lower altitudes, air denseness creates more air resistance, slowing the aircraft. At higher altitude, the thinner air keeps the aircraft engine from performing at full power. Sometimes, manufactures also provide a military power speed at Sea Level and occasionally at several altitudes.

Of course, if an attacker is higher than the target, the attacker can gain speed in a dive. In turn, if an attacker has to climb to reach its target, it will be flying slower. Nevertheless, an aircraft’s maximum speed at its critical altitude is the most common single metric for an aircraft’s “speed.”

The Zero was officially the Type 0 Aircraft Carrier Fighter.[1] This base designation was followed by a model number. The first model was Model 11, meaning that it was the first major version of the fuselage and engine. It had fixed wing tips. After 66 aircraft were built, folding wing tips were added to permit more Zeros to fit on aircraft carriers.[2] This second model was the Model 21, to indicate a different fuselage. The Model 21 was used almost exclusively in the first year of the war.

The Imperial Japanese Navy (IJN) also had a short designation for its aircraft. The Zero was the A6M, meaning that it was a carrier fighter (A) designed by Mitsubishi (M). The six meant that it was the sixth carrier fighter from Mitsubishi. The prototype version was the A6M1. The first production models (the Models 11 and 21) were designated as A6M2s.

The Zero’s designer, Jiro Horikoshi, reported speeds for several versions of the Zero—but not for the A6M2 Model 21 or 22. For example, he noted that the prototype had a requirement to fly at 500 km/h (311 mph) at 4,000 meters.[3] He also noted that the A6M1 prototype, using the Mitsubishi Zuisei Type 13 of 850 hp, reached a maximum speed for 480 km/h. However, after atmospheric conditions were taken into account, it had a corrected speed of 508 km/h (316 mph).[4]

Hirokoshi then noted that the more powerful Sakae 12 was placed in the third test aircraft, which became the A6M2.[5] However, he did not mention the speed of this improved version. Presumably, it was faster, but by how much?

To figure that out, we need to do some calculations. Horikoshi did note that the later A6M5, which had a rated speed of 565 km/h (351 mph), was 20 km/h faster than the A6M3.[6] So the A6M3 would have a speed of 545 km/h or 339 mph. He also noted that the A6M3 was a disappointment, being only 10 km/h faster than the A6M2. This would mean that the A6M2’s speed was about 535 km/h or 332 mph.

Fortunately, these tortured calculations are not the only basis for judging the Model 21’s speed. In a 1941 contest, Japan flew the Zero against various other Japanese fighters. The Zero’s speed was recorded as 530 km/h (329 mph or 286 kts). It is not known how heavily the Zero was loaded for this contest.

Another flight test source is the U.S. Intelligence Service[7] report on a crashed and restored Model 21 in late 1942. This aircraft had been flown by Flight Petty Officer Tadayoshi Koga. Koga crashed in Alaska on June 4, 1942, although tests on the aircraft did not begin until late 1942.

Figure 2: Koga's Zero (Source: USAAF photographs)

Flight tests on Koga’s Zero gave the results shown in Figure 3. The maximum speed was 326 mph at 16,000 feet.

Figure 3: Tests of a Captured A6M2 Model 21

Dunn[8] has taken exception to these test numbers. He noted that the Zero was not in optimum condition. He also noted that the Navy’s full Technical Aviation Brief #3, on which the information in the briefer Intelligence Service Report was based, listed different speeds from different tests, with some speeds 7 to 10 mph faster than those shown in Figure 3. (Presumably, some speeds were lower.) He also noted that Technical Air Intelligence Service summaries after Technical Aviation Brief #3 often attributed a higher maximum speed of 328 mph, indicating that this speed came from flight tests. The source of this flight test data was not indicated in these subsequent summaries, but the difference between 326 mph and 328 mph seems to be too small raise concerns.

A number of websites list (without citation) a speed of about 331 or 332 mph for the Model 21. Often this speed is listed at the critical altitude of around 4500 m or 15,000 feet. Three major books on the Zero also list this speed without citation. Mikesh[9] lists the speed of the Zeke 21 as 287 kts (331 mph) on page 97. However, in his detailed description of Zero flight data, he lists and cites the U.S. Intelligence Service data. Jackson[10] also lists the speed of the A6M2 as 534 km/h (332 mph) at 4500 m (14,930 ft) without citation. Gunston’s[11] book provides the only specific information on the source of this speed. It says that the 332 mph speed (534 km/hr) is for the third example of the A6M2 (the one in which the larger engine was introduced). But the book also fails to indicate the source of this statement. It may be that whoever created the uncited 331 mph speed estimate went through the same calculations we did earlier in this paper based on Horikoshi’s book and got the same results.

So far, everything has fallen into the narrow range between 326 mph and 332 mph. However, the last source we have is Saburo Sakai, who was Japan’s top surviving ace. Sakai described his battle with a Hellcat in 1943. [12] This would have taken place in an A6M2 or a slightly faster A6M3. In his book, he reported a normal top speed of only 317 mph (275 kts). This is not surprising, because combat aircraft normally are not as tweaked as test aircraft.

Sakai also noted that the Zero’s engine had an emergency overboost, which got it to 345 mph.[13] In fact, most World War II fighters had emergency overboost that could be used for a few minutes in combat. Normal maximum power was usually called military power, while overboost was often called war emergency power. For example, the P-40E had a rated military power of 1,150 hp at 44.6 inches of supercharger pressure; its war emergency power was 1,490 hp at 56 inches of pressure. Resorting to overboost was an extreme measure because using it for more than a few minutes would destroy the engine, so top speeds are always quoted based on military power.[14]

Overall, then, the top documented speed of the A6M2 Model 21 seems to be about 317 mph to 332 mph at about 16,000 feet, with an overboost speed of about 345 mph. The latter speed, by the way, was the maximum speed on the Zero’s air speed indicator.[15]^,[16]

As the war progressed, Zeros did not become much faster. According to Horikoshi, the A6M5, the last major variant, was only about 20 mph faster than the original production Zeros. In fact, when the U.S. Navy tested a captured A6M5 Model 52, the latter’s top speed was only 335 mph at 18,000 feet.[17] This was only nine miles per hour faster than Koga’s Zero.

Hirokoshi noted that “Even the best fighters become obsolete within two years during times of war.”[18] He began to design a replacement, the A7M Reppu, in 1940.[19] However, he did not succeed until it was too late to put the new aircraft into production.[20] As he noted, Japan had a shortage of engineering manpower that took Japan twice as much time to design a new airplane or redesign an old airplane.[21]

So the Zero’s maximum speed at the start of the war was about 317 mph to 331 mph, with tests being on the higher side. How fast was this compared to U.S., British, and German aircraft? Figure 4 shows some comparative data for aircraft that would have been in combat in 1942. An attempt was made to use good sources, but the numbers shown are not always reliable. The goal is to give a rough but probably accurate picture of relative speeds. Note that this was two years after the Battle of Britain, so we are not concerned with the earliest models of most European aircraft. This table has several judgment calls. Nevertheless, the pattern is clear.

Figure 4: Aircraft Speeds in 1942

In terms of U.S. aircraft in the Pacific, the Zero was markedly slower than the P-40. However, the P-40 was heavy and only had a single-stage supercharger. This limited to combat below about 16,000 feet. The Zero could fly higher and used this height advantage to dive and gain superior speed. The F4F-3 used at Coral Sea was about the same speed as the Zero, while the F4F-4 used at Midway and later substantially slower.

The table unfortunately only shows speeds at a single altitude or a few altitudes for each plane. When discussing relative speed, it is desirable to do so at many different altitudes for the various aircraft used in the comparisons.

In addition to straight-line speed, pilots want to be able to climb and dive rapidly if there is an attacker moving into position to shoot. Tests of the captured Zero showed that Zero was not impressive in a dive. It could dive about as fast as an F4F Wildcat, but its dive was much slower than other aircraft. [27] Although technical problems prevented the P-40 Warhawk from being tested against the Zero, pilots quickly discovered in early combat that the P-40 could dive much faster than the Zero. In fact, we know that early Zero models had wings that could rip off in overly-rapid dives.[28] Testing also showed that when a Zero dove quickly, its engine would cut out briefly, giving the target a little time to accelerate downward before the Zero began its dive.[29] Also, once into a fast dive, the Zero’s controls became extremely heavy, and there was strong tail buffeting.[30]

Tests of the captured Zero also showed that while the Zero climbed better than U.S. aircraft, its sustained climb rate was not remarkably faster than U.S. aircraft.[31] Although it was much lighter, its engine was also less powerful.

However, the Zero was far better at U.S. aircraft at “zoom,” which is initial climb if the pilot pulls back on the control stick. The Zero could practically stand on its tail in a zoom, and in many cases, it could do a full loop and get on the tail of an attacker. Heavier U.S. aircraft had little if any zoom capability.

Although the A6M2 Model 21 Zero could fly at 317 mph to 331 mph, it was not very maneuverable at its highest speeds. The Zero’s great maneuverability was due mainly to its low weight and extremely large ailerons. Ailerons roll an aircraft from left to right or right to left. Once the roll is completed, the wings pull the aircraft around in a turn. If an aircraft cannot roll rapidly, its ability to survive in air combat is limited.

The Zero’s ailerons gave great maneuverability at low speeds, but as speed increased, these big ailerons became extremely heavy. The Intelligence Services’ evaluation of the Zero led to the advice that American aircraft should only engage the Zero at speeds above 300 mph. At such high speeds, the Zero’s ability to roll quickly into a turn was greatly degraded.

In fact, the Zero’s problems with maneuverability did not begin at 300 mph. In October 1944, the U.S. Navy conducted tests of a captured A6M5 Model 52.[32] It found that the ailerons started to become heavy at only 200 kts (180 mph). Yoshimura also reported that the ailerons started to become heavy at 186 mph (300 km/h or 162 kts).[33] By 230 mph (370 km/h or 200 kts), he reported, it was difficult to make a slow roll.[34] Mitsubishi added balance tabs to increase roll rate, but these were later deleted due to buffeting problems.

Acknowledgment