In 1946, American test pilots strapped into a captured Japanese army fighter, filled it with good American fuel and went looking for its limits. What they found was uncomfortable. The aircraft could outclimb and outturn the newest Mustangs and Thunderbolts in the US inventory.

The Allies called it “Frank”. To the Imperial Japanese Army it was the Type 4 Fighter, and to its pilots the Nakajima Ki-84 Hayate, the “Gale”. It is generally considered the best Japanese fighter to serve in large numbers in the Second World War. It arrived when Japan no longer had the fuel, the factories or the pilots to make it count.

A Hayabusa that could hit back

The story begins in early 1942, just as the army’s Nakajima Ki-43 Hayabusa was entering service. The Ki-43 was nimble and well liked, but it was lightly armed, unarmoured and caught fire easily. The army wanted a successor that kept its agility and added speed, firepower and protection.

Nakajima’s team under Yasumi Koyama built the Ki-84 around the company’s compact Ha-45 Homare, an 18-cylinder radial that gave close to 2,000 hp at take-off with water-methanol injection. The pilot got 65 mm of armoured glass in the windscreen and 13 mm of armour behind his head and back. Self-sealing tanks protected the fuel. The main production model carried two 12.7 mm machine guns in the nose and two 20 mm cannon in the wings.

The first prototype flew from Ojima airfield in April 1943, and test pilots liked it immediately. It never quite reached the army’s demanded speed: Japanese trials recorded 624 km/h (388 mph). But it was the best-performing Japanese fighter then ready for mass production, and the army ordered it into large-scale manufacture.

Debut over China, ordeal over Leyte

The 22nd Sentai formed with the new fighter in March 1944. According to Luftfahrtgeschichte, it first appeared over Hankow in China in late August 1944, where American P-40 pilots, unaware of the new type, reported meeting Oscars, Tojos and Zeros. Over the following five weeks the regiment ran roughshod over the Allied fighters opposing it.

Greg’s Airplanes and Automobiles examines whether the Ki-84 Hayate really was Japan’s best fighter of the war.

In October 1944 the unit moved to the Philippines for the Battle of Leyte, the Hayate’s first big campaign. There it met P-38 Lightnings and US Navy F6F Hellcats in growing numbers. The fighter was good enough. The numbers were not, and broken landing gear, hydraulic failures and poor maintenance wore the units down as surely as combat did.

Production peaked anyway. In December 1944 alone, Nakajima built 373 Ki-84s, more than any other Japanese aircraft type that month. About 3,500 were built in all, including 94 or 95 assembled in Manchuria.

Home defence and the end

As the Americans closed on Japan, the Hayate fought everywhere the fighting was hardest: over Okinawa, the home islands and against the B-29 Superfortress raids. In mid-April 1945, eleven Hayates raided American airfields on Okinawa and destroyed a number of aircraft on the ground. The odds were growing worse. On 17 April, according to Luftfahrtgeschichte, US Navy pilot Eugene Valencia of VF-9 shot down six Franks in a single fight.

The deeper problem was industrial. B-29 raids hit Nakajima’s Musashi engine plant, and production of the Homare moved to underground workshops. Japanese steel was badly heat-treated, so landing gear legs snapped. Fuel was poor. New pilots arrived with little training, and the Ki-84 was much “hotter” to land than the docile Ki-43. Nakajima even designed versions with wooden and steel parts to save scarce aluminium.

The design kept evolving to the end. The Otsu model carried four 20 mm cannon, and the bomber-destroyer Hei swapped two of them for 30 mm guns. The Ki-84-II replaced parts of the rear fuselage and tail with wood. The all-wood Ki-106 and the steel-framed Ki-113 went further still, and high-altitude versions with turbocharged or bigger engines were still on the drawing board when the war ended.

Even surrender did not end its career. Communist and Nationalist forces in China both took over captured Hayates, and the Chinese communists flew theirs into the 1950s. In the Dutch East Indies, Indonesian independence fighters seized a handful at former Japanese bases, and most of those were destroyed in the fighting with the Netherlands that followed.

The American verdict

After the war, the Allies collected Hayates for evaluation. Two were flown by the Allied Technical Air Intelligence Unit in the Pacific, and two more were tested in the United States as FE-301 and FE-302. One of them was manufacture number 1446 of the 11th Sentai, captured almost intact at Clark Field on Luzon and shipped home aboard the escort carrier USS Long Island.

In American hands, flown with high-grade fuel and properly maintained, the Ki-84 showed what it could have been. The specification Wikipedia takes from the US report gives a top speed of 687 km/h (427 mph) at 7,000 m. The evaluators found weaknesses too, including short range and some poor-quality parts. Their conclusion was still a compliment no Japanese engineer heard during the war.

Archival colour film of Allied Technical Air Intelligence Unit flight tests with a captured Ki-84 and other Japanese aircraft.

The last Hayate

Number 1446 had a long second life. In 1952 it was sold as surplus to Edward Maloney of the Planes of Fame museum in California, who restored it to flying condition as N3385G. In 1973 it went home to Japan. Parts went missing during years at the Arashiyama Museum in Kyoto, and it never flew again.

Today it stands at the Chiran Peace Museum in Kagoshima Prefecture, the only Ki-84 known to survive. It shows what Japan built in 1944: a fighter that could match the best the Allies had, and arrived too late to make a difference.

Sources: Wikipedia (Nakajima Ki-84 Hayate), Aviation History via HistoryNet, Pacific Wrecks, San Diego Air & Space Museum Archives