About the Performance of Type 4 Fighter (Ki-84)

Update Jun 26 2024: some additional information and corrections to this article in Part 2:

• In the engine table within this article, the CR of Ha-45-21 was changed to 7.17, because the performance used in the table reflects the actual engine with its production CR (rather than 8.0, which was planned and used in prototypes).
• The performance of the “Ha-45 Special” was changed from that of Ha-45-12 to that of Ha-45-11 (which it seems to be, see 2nd article).

The performance of the Type 4 Fighter (project name ‘Ki-84’) is a bit of a can of worms, and a subject of frequent debate. This is due to a significant amount of differing data with varying credibility, and often a lack of information to illustrate the background of each record.

When discussing Japanese aircraft performance in World War II, there has been a general concept of separating “actual performance” and “potential performance” — A result of negative factors introduced by the declining state of Japanese industry and the lack of resources, especially towards the end of the war.

“Actual performance” is what was able to be demonstrated by a plane under Japanese fuel, oil, and maintenance conditions. “Potential performance” is said to be what could be demonstrated if the same plane was provided with higher quality conditions, such as those of the United States.

There is a common belief that the Type 4 Fighter exhibited substantially improved performance (speed as high as 687 km/h!) when captured examples were tested by the US, due to the use of high-octane fuel. While it is certainly true that a Japanese fighter could benefit from the US testing medium, the reality is more complex than “higher grade fuel increased the performance”.

In this article, the true performance of the Type 4 Fighter (Ki-84) will be examined from both angles. This is mainly a technical article, with less focus on history.

(This article may be rewritten if more detailed sources come to light.)

About Ki-84’s Engine ー Ha-45

The most important aspect that dictated the performance of Ki-84 was naturally its engine: the Ha-45. One reason is that depending on the time the airframe was made, a different model (or at least differently performing) engine may have been mounted.

Japanese engine naming systems are also quite confusing to those not informed, so this will first be clarified. The two relevant engines mounted to Ki-84 were named the ‘Ha-45-11’, and ‘Ha-45-21’ by unified convention. The Army called these engines ‘Ha-45 Special‘ and ‘Ha-45‘ respectively. Ha-45 was serviced in the Army with the name ‘Type 4 1850HP Engine’. The corresponding Navy service names may also be encountered: ‘Homare Model 11’ and ‘Homare Model 21’.

Because Ki-84 was an Army fighter, I will be using the Army short names ‘Ha-45 Special‘ and ‘Ha-45‘.

Ha-45 was a very technically impressive engine, and was developed by Nakajima Airplane Company on a rapid timeframe from 1940-1942. The 18-cylinder air cooled radial engine initially was designed to produce ~1,800 horsepower, but was increased to a maximum output at sea level of 2,000 horsepower. At the same time, it was of a significantly smaller form factor than its global contemporaries. Only slightly larger than the famous Sakae engine it descended from, Ha-45 had a 1,180 mm overall diameter, 830 kg dry weight, and 35.8 l displacement. For comparison, the American 2,000 horsepower-class R18 engine ‘R-2800’ had a 1,342 mm diameter, ~1,050 kg dry weight, and 46 l displacement.

The Ha-45’s high power for its size of >100 hp per cylinder was achieved with a higher rotational speed of up to 3,000 RPM and higher pressure boost of +500 mmHg compared to prior Japanese engines. Originally, the engine was designed to use 100 octane fuel in order to avoid ‘knock’ at high pressure (premature combustion). However, wartime Japan did not have the means to procure high-grade fuel, and it was ultimately designed to meet its specified power with 92 octane fuel, instead employing a water-methanol injection system within the intake path to reduce the air temperature and avert knock.

+500 mmHg boost was a high manifold pressure for Japanese aircraft engines, and could only be obtained with water injection on the highest quality of fuel available for service (92 octane in the Navy, 91 octane in the Army). In fact, even the rated power of the engine at +350 mmHg required WM injection. It’s important to know that high-power Japanese engines had to be designed within the constraint of using WM injection to reach boost pressures that were obtained and even surpassed by Allied AC engines without it, as the Allies had a distribution of much higher octane fuel. So resultingly, Allied AC engines that did have WM injection could reach extreme boost pressures Japanese engines could never approach.

Power Restriction of the Ha-45

The above specifications describe the extraordinary performance of the Ha-45 engine as it was developed… but a compact, high-power piston engine never had a chance in the actual state of late-war Japan. It’s well established that the mass-produced Ha-45 suffered relentless problems in field service, leading to a terrible operational rate. These problems were caused by a multitude of factors, such as:

• Lack of skilled production. The Ha-45 required precision manufacturing which was not possible on a mass scale by that time.
• Declining fuel quality. It is likely that even 91 octane fuel was not actually up to spec near the end of the war.
• Lack of quality lubricating oils.
• Delicate electrical system.
• Insufficient maintenance capacity. The Army simply did not have the depth of maintenance ability to keep the Ha-45 in good shape during widespread service.

The culmination of these factors caused widespread rises in cylinder temperature, oil temperature, uneven fuel distribution, and various other malfunctions and reliability issues. Resultingly, the Army decided to govern the mass-produced Ha-45 engines down to the about same level as the Ha-45 Special that had been installed in the initial Ki-84 prototypes. The max RPMs were reduced from 3,000 to 2,900, the maximum intake manifold pressure was reduced from +500 mm to +400 mm, and the cylinder compression ratio was reduced from 8.0 to 7.17.

Even with the governed Ha-45 providing as much as 200 less horsepower, the operational rate of the Type 4 Fighter suffered until the end of the war and the power restrictions were rarely relaxed. The operational rate was usually around 40%, and in some units (especially those in the south) it could be as poor as 0-20%. However, there were a few units that managed to tame the suffering Ha-45.

While the readiness of other Type 4 Fighter units continued to decline, the maintenance team of the Flying 47th Squadron managed to reach up to 100% operational rate at a point in time. This was achieved by implementing a command platoon specifically overseeing aircraft maintenance, which was different from the conventional structure within Army squadrons. Led by Captain Masai Kariya, who was nicknamed the “God of Maintenance”, thorough examinations, maintenance, and overhauls were constantly performed on all aircraft. The Army took note of the 47th Squadron’s ingenuity and ordered the maintenance team to instruct other squadrons, but it was too late to make a major difference.

Even under the most exhaustive care of the 47th Squadron, which may have been closer to US standards, the Type 4 was unlikely to demonstrate its potential due to factors out of the team’s control (poor oil, fuel, manufacturing precision).

The purpose of this section was to introduce the multitude of factors that could have caused disparity in the performance of each plane. Now, the performance numbers will be examined.

About ‘US Testing Data’ of Ki-84

Before talking about the historical performance records for Type 4, it is first necessary to look at the (supposed) US testing performance numbers which have appeared in numerous publications. It is often said that a captured Type 4 managed to reach an impressive top speed of 687 km/h (427 mph) with American high-octane fuel and test conditions. Similar claims have also been made for other Japanese aircraft, such as Ki-83 reaching 762 km/h (473 mph), Saiun reaching 694 km/h (431 mph), and Raiden reaching 671 km/h (417 mph).

While the origins of the numbers for some such as Ki-83 and Saiun have not been verified, the Type 4’s numbers are easily located. These numbers are published in a 1946 AAF T-2 report as ‘Factual Data’ and are used to illustrate the claim that the Type 4 compared favorably with the most advanced US service piston fighters ‘P-51H’ and ‘P-47N’.

As it turns out, this data was not the result of an actual flight test by the US. Rather, these exact numbers were originally created in March 1945 and included in a supplement to the Technical Air Intelligence Center (TAIC) manual on Japanese aircraft. At the time, captured Type 4s had not been extensively tested for performance. As per the TAIC manual:

Except where otherwise stated, performance figures represent estimates of the Technical Air Intelligence Center and have been calculated after a careful analysis of information derived from intelligence, captured equipment, drawings, and photographs, using power ratings derived from the same sources. When authoritative evidence is not available, it is the policy of TAIC to give the Japanese Aircraft Performance every benefit of the doubt within reasonable limits.

Japanese Aircraft Performance & Characteristics, TAIC Manual No.1

As there is no indication in the document that Type 4’s performance was derived from real test data, there is no reason to assume so. These numbers, which were calculated in Japanese operating conditions (92 octane fuel), were probably created by deliberately generous estimates so as to not underestimate an enemy fighter. Not dissimilar to the evaluation of Raiden, which was calculated by TAIC to have speed performance significantly higher than what was actually demonstrated. Furthermore, it is overtly stated in the T-2 report that detailed performance was not measured, and mainly flight characteristics were evaluated.

This is not to say that it was impossible for Type 4 to demonstrate speed performance of this magnitude. For example, if the Saiun (which used the same engine as Type 4) was truly able to reach 694 km/h in postwar US testing, it would be reasonable for Type 4 to reach 687 km/h and even more in the same conditions. However, such performance would in all likelihood only be met using higher-than-design manifold pressure by modifying the supercharger ratio and throttle valve control, together with high octane fuel and quality American parts.

As it stands, we seem to have no real detailed data on a US performance test of Type 4 at all.

In Masai Kariya’s book “Story of Japanese Army Prototype Planes“, it is said that Type 4 achieved 689 km/h using 140 octane fuel during US testing after the war. While this is a slightly different number, it seems quite likely to only be a slight distortion of the TAIC calculations.

Japanese Performance Data

As we have no satisfactory US testing data on the Type 4’s performance, we are left with the Japanese performance records.

There were notable differences in the method of establishing aircraft performance between Japan and the USA at the time. For example, the US recorded top speeds using War Emergency Power (WEP), which is the highest possible output the engine may exhibit for a limited time, while the Japanese standard of recording top speed was at ‘Rated Power’, which could be called ‘Military Power’. This is a lower throttle setting that could be maintained for a longer period of time.

Furthermore, as previously touched on, the state of late-war Japan left many possible disparities in the performance of each airframe. Whether a prototype with a Ha-45 Special, a production plane with a governed or ungoverned Ha-45, a prototype suffering from malfunctions at a stage of testing, or a production plane with subpar manufacturing and maintenance. At the very least, we can assume the fuel quality for official performance tests to be ‘adequate’, and the airframe to be clean.

The first record is well known as the “official top speed” of the Type 4. 624 kilometers per hour at an altitude of 6,550 meters was recorded by Major Iwahashi in one of the initial Ki-84 prototypes with a Ha-45 Special engine. This was the highest performance among Army single-engine fighters. In this condition, the plane climbed to 5,000 meters in 6 minutes and 26 seconds. The detailed performance record was published in the ‘Ki-84 Pilot Manual’, and a copy of this test was later captured by the US at Clark Field on Luzon. The speed and climb are as follows.

This speed was quickly usurped by the 4th Ki-84 prototype (1st pre-production) which seems to have been equipped with a fully rated Ha-45 engine. Lieutenant Funabashi flew this test and achieved 631 kilometers per hour at 6,120 meters with a starting loaded weight of 3,794 kilograms. The climbing time to 5,000 meters was improved to 5 minutes and 54 seconds, a difference of 32 seconds. There is no detailed record of this exact test, but there is a record of a Ki-84 with a fully-rated Ha-45 flying with a much lighter weight of ~3,400 kilograms, which is as follows.

From the available numbers, it seems that the increase in rated engine power only caused a speed increase of about 10 kilometers per hour in the prototype stage. This may be due to the plane’s small 3.0-3.1 meter ‘Pe-32’ propeller, which is considerably smaller than even other Japanese 2,000hp class propellers, such as Shiden’s 3.3 meter propeller, or Saiun’s 3.5 meter propeller. The American Hellcat, Thunderbolt, and Corsair all had propellers around 4 meters in diameter. The choice of Type 4’s propeller is said to have been done to reduce the length of the landing gear and overall weight, but may have negatively affected the transfer of power.

The highest speed allegedly reached during Japanese testing was 660 kilometers per hour with the Ki-84 Otsu prototype, which is written in numerous secondary sources. The only major difference between the former Model ‘Kou’ was that the ‘Otsu’ was equipped with Type 2 20 mm Autocannons in place of the two nose-mounted Type 1 12.7 mm Autocannons. While on its own this should actually slightly burden the flight performance, there are several factors that could have benefitted this airframe due to its later construction date.

One possible factor was an improved force of thrust from the engine exhaust. In the initial Ki-84 prototypes and 1st stage of pre-production airframes, the exhaust of each cylinder was routed through a collective exhaust pipe on each side.

From at least the second stage of pre-production planes, the thrust exhaust method was changed to the evidently superior individual exhaust stacks. The increase in thrust improved the top speed to a certain marginal, but unknown extent. Most photos of Type 4, especially deployed airframes, have this exhaust type. At least some of the 1st stage pre-production planes were also remodeled as such.

Another factor is that the Type 4 Fighter was seemingly equipped with a slightly larger 3.1-meter Pe-32 propeller after the initial testing phase. This modest increase helped the engine demonstrate its power and evidently did not require a change of reduction ratio to keep the tip velocity in check. Captured production planes were measured to have this propeller diameter.

Supposing such conditions, it could be possible that the Ki-84 Otsu prototype achieved a top speed 29 km/h higher than the 4th prototype, especially if it was at a lighter load and/or using WEP. But as I do not have the original source for this speed claim, it is only conjecture. Factors such as weight or whether it was achieved during WEP versus military/rated power, or in a dive, are totally unknown.

It’s also possible that the Ki-84 Otsu did not even possess these features (at least as originally completed). According to Nakajima Airplane Company’s data prepared after the war for the US, two prototypes of Ki-84 with 20 mm machine cannons (known internally as ‘Ki-84-Y’) were completed in 1943. During this time, only the first 3 prototypes and 24 pre-production airframes of the Ki-84 had been built. If correct, this information could suggest that the Ki-84 Otsu prototypes were not equipped with the later performance-enhancing features.

Conclusion

From the available data, it seems reasonable to state that Type 4’s maximum speed at rated/military power, gross weight, in Japanese engine settings and fuel grade, was around 631-634 km/h. Perhaps closer or slightly superior to the higher end, assuming both full-rated tests were done with the old exhaust type. But as the engine was largely governed and often suffered from malfunctions for multitudes of reasons, the actual rated speed of a service plane was probably 624 km/h at the most optimistic, even though the ‘Ha-45 Special’-equipped prototype that demonstrated this had the early type of exhaust thrust.

The top speed when using War Emergency Power (+500 mmHg) would naturally be slightly higher than each of these speeds, but likely only by a matter of about ~10-15 km/h using very tentative estimates. My rough guess would be that ~650 km/h was probably the best case top speed of a Type 4 with emergency power, and a fully rated engine in a ‘clean prototype’ airframe at gross weight with all beneficial improvements.

As has been discussed, the supposed 687 km/h obtained in US testing with high-octane fuel was only a wartime calculation for Japanese conditions (92 octane fuel) using optimistic data. While it certainly could be possible that Type 4 would have seen a significant performance boost with US high-octane fuel and modifications to produce a higher boost pressure, no such data seems to exist. Furthermore, it’s probable that the US only tested the Type 4s at Japanese power settings, and it’s unknown how much additional pressure a Japanese mass-produced Ha-45 could even handle.

According to the US report on Type 4 made with a captured plane at Middletown, Ohio in 1946, the aircraft either demonstrated or was estimated to exhibit a climb time of about 6 minutes 40 seconds to 6,100 meters. The starting weight of the plane seems to have been 3,350 kg or 3,500 kg (the two reports list different weights), and this climb time is slightly superior to the 6’50” to 6,000 m time of the Japanese 3,400 kg test, but far from the TAIC calculation of 5’48” to 6,100 m.

There is little doubt that the Type 4 Fighter was “the most powerful Japanese fighter of World War II”, but its ultimate performance was dictated by the harsh conditions it was developed in, and even then it was rarely able to demonstrate its full potential.

Sources

• Ando, Atsuo. Nihon Rikugun-ki no Keikaku Monogatari. 1980.
• Akimoto, Minoru. Nihon Rikugun Seishiki-ki Taikan. Tōkyō: Kantosha. 2002.
• Maema, Takanori. Higeki no Hatsudou-ki ‘Homare’. Tōkyō: ‎Soshisha, 2015.
• Kariya, Masai. Nihon Rikugun Shisaku-ki Monogatari. Tōkyō: Ushio Shobō Mitsuhito Shinsha. 2017.
• Famous Airplanes of the World No. 19: Army Type 4 Fighter ‘Hayate’. Tōkyō: Bunrindo, 2019.
• Rep. Ki-84 Pilot Manual. 1944.
• Rep. Technical Air Intelligence Center Report No. 31, Homare 11 and 21 Engines, Principal Characteristics and Performance. 1945.
• Rep. ADVATIS Translation 92: Specifications and Performance Data of Ki-84. 1945.
• Rep. Kenkyū Shisakubu-nai Chōsa-hyō. 1945.
• Rep. Desc of Experimental Aircraft and Experimental Engines Under Development by the Japanese Army and the Imperial Japanese Navy. 1946.
• Rep. Manual on Japanese Aircraft, TAIC No. 1. 1945.
• Rep. Manual on Japanese Aircraft, TAIC No. 2. 1945.
• Rep. “Ki” 84 Performance Data. 1945.
• Rep. T-2 Report on Frank-1. 1946.
• Rep. [Frank-1 Middletown Report]. 1946.
• If you are interested in Japanese aircraft performance, read the great articles at warbirdperformance.livedoor.blog.