17 October 2011

The Ad Hoc Lashup That Resulted in Japan's Best Fighter of WW2

Japan's finest fighter of World War 2.
Unlike Germany and the Allies during the Second World War, the vast majority of Japanese fighter aircraft were powered by radial engines while other nations had a mix of both radial and inline engines for their fighter units. Only one production-standard Japanese fighter entered service with an inline engine, the Kawasaki Ki-61 "Hien" (Allied code name Tony). Over 2,500 Ki-61s were produced and saw service with the Imperial Japanese Army Air Force (IJAAF) from 1943 to the end of hostilities in 1945. The Hien was designed by Kawasaki's chief designer, Takeo Doi and his deputy, Shin Owada (Doi would later go on to design the postwar YS-11 airliner) in response to an IJAAF requirement for a multirole fighter that would use a license-produced version of the German Daimler-Benz DB601 liquid-cooled inline engine that also powered the Messerschmitt Bf 109. Proving itself to be a promising design, the Ki-61 turned out to have a major Achilles heel in its Kawasaki-built engine which was designated the Ha-140. Though the DB601 would be one of the Second World War's finest engines, by the time the Ki-61 entered service it was an old design that was already being outclassed by more powerful engines of both radial and inline design. Worse yet, the DB601 had very tight manufacturing tolerances and in Japanese license production, getting Ha-140 engines built as well as the DB601 proved elusive given the increasing toll being taken on Japanese industry which found itself constantly short of quality metals and lubricants given the tightening US Navy submarine blockade of the home islands. 

Ki-61 to Ki-100.
Worse would come as the war progressed into 1944 with Kawasaki's Akashi factory constantly being plagued by supplier shortages and properly manufactured crankcases and cylinder blocks. By the spring of 1944, more than half of the Ha-140 engines produced at Akashi that did leave the assembly line failed to pass acceptance testing! As a result, at Kawasaki's facility in Kagamigahara that produced the Ki-61 fighter found itself with increasing numbers of finished fighters sitting in open storage waiting for their Ha-140 engines. Matters worsened on 19 January 1945 when B-29 Superfortresses completely destroyed the Akashi engine plant, leaving 275 Ki-61 fighters without any engines. With the B-29s ranging with near impunity over the home islands, the IJAAF headquarters, the Koko Hombu, suggested that the engineless Ki-61s be adapted to take the most readily available and reliable engine- IJAAF engineers suggested that Kawasaki find a way to fit the engineless fighters with the Mitsubishi Ha-112 radial engine, a 14-cylinder twin row radial engine comparable in power to the Pratt & Whitney R-1830 radial used on the Consolidated B-24 Liberator and the Douglas C-47 Skytrain/Dakota. 

Takeo Doi was initially not pleased with the Koku Hombu's directive to fit the Ha-112 to the Ki-61. The radial engine was 48 inches in diameter and the fuselage of the Ki-61 was only 36 inches in diameter. But seeing as to how there was little alternatives and the need for fighters great, Doi had his engineering staff work around the clock to make the Ha-112 work with the Ki-61 Hien. Fortunately Kawasaki was in possession of a Focke Wulf Fw 190 which had been delivered to the Japanese several years earlier for engineering study. Using the cowl and engine mount of the Fw 190 as a starting point, Doi and his team even replicated the horizontal alignment and collection of the exhaust pipes of the Fw 190 which in the Kawasaki design actually added between 6 to 9 mph of thrust augmentation to the aircraft's speed. Designated the Ki-100, the prototype aircraft made its first flight a mere three months after start of design work- a figure all the more incredible in light of the tremendous toll the B-29 attacks were taking on Japanese industry. By the time the third Ki-100 prototype took flight, the Koko Hombu was sufficiently impressed that all the engineless Ki-61s were ordered to be converted to Ki-100 standard. 

On many occasions, Ki-100s bested Hellcats and Corsairs.
"Sufficiently impressed" would be an understatement- the Ha-112 radial engine actually made the Ki-100 superior in every performance parameter to the Ki-61, much to the surprise of Takeo Doi. Although just slightly slower in cruise than the Hien, the Ki-100 was more maneuverable and had a faster climb rate. The IJAAF even took the Ki-100 prototypes into mock air combat against a captured P-51C Mustang. With the flight test program only lasting three weeks, the first Ki-100s went into combat with the 18th Sentai in Chiba on the same night in March that B-29s had laid waste to sixteen square miles of Tokyo in a firebombing attack. Less than six weeks elapsed since first flight to first combat! Production of the Ki-61 was terminated immediately and in less than three months all 275 engineless Ki-61s were rebuilt as Ki-100s AND the Kagamigahara factory also managed to build nearly 75 new-build Ki-100s that featured a cut back rear fuselage to improve aft vision for the pilot. As fast as Ki-100s could be built they were funneled straight to home defense units who found the Ki-100 a formidable fighter. Numerous large dogfights against Grumman F6F Hellcats and Vought F4U Corsairs found the Ki-100 on the winning side with more American fighters shot down than Ki-100s. By May 1945 the Ki-100 finally had met its match as long range P-51D Mustangs from Iwo Jima were now escorting the B-29s all the way to Japan and back. Dogfights with P-51Ds ended up swinging in favor of pilot skill and numerical advantage rather than any deficiency on the part of either aircraft. 

The one weakness of the Ki-100 was its high altitude performance as it lacked a turbocharger and as a result, few B-29s fell to the Ki-100. Work was underway for a supercharged high altitude Ki-100 when the war came to an abrupt end with the atomic bombing of Hiroshima and Nagasaki. Unfortunately for Japan, only about 500 Ki-100s were built before the war ended, but compared to other Japanese fighter designs, the Ki-100 proved to be Japan's finest fighter aircraft as a swansong for the once-feared IJAAF. From inception to the end of the war, the lifespan of the Ki-100 was barely ten months with an ad hoc marriage of engine and airframe out of necessity! The only surviving Ki-100 can be seen today at the RAF Museum. 

Source: Air International, October 1976, Volume 11, Number 4. "The Last Swallow of Summer: The Extraordinary Story of the Ki-100", p185-191.

01 October 2011

Rivet Haste: Rebirth of the USAF at the end of Vietnam

Heritage Flight F-4E Phantom painted in Vietnam-era colors.
In several prior postings I had talked about how the United States set about improving the performance of its fighter pilots over the skies of Vietnam. The first one dealt with the testing and analysis of MiG fighters that had been obtained secretly by the United States and the second posting covered the origins of Red Flag, the exercises in Nevada that gave pilots valuable experience under simulated combat conditions. The Navy also instituted measures to improve its fighter pilots' performance after the Ault Report was commissioned by the Chief of Naval Operations to review naval air-to-air tactics. One thing to keep in mind is that there were numerous parallel measures being undertaken in the late 1960s and early 1970s to improve the dogfighting skills of American pilots. The main American fighter of the day that was in service with both the US Navy and USAF was the McDonnell Douglas F-4 Phantom II. The Air Force in particular, also looked at ways of improving the performance of the Phantom, a design that was originally intended for fleet air defense but found itself in swirling low speed dogfights over Vietnam for which it was never designed. 

The Phantom had two idiosyncrasies that proved to be a liability in a dogfight. Being a big hulking fighter with two J79 engines that left a nice smoke trail for the enemy to spot were one of just many faults of the aircraft. As the engines of the Phantom were optimized for high speed, so to were the aircraft's aerodynamics. In the low-speed regime during a dogfight, the F-4 was prone to what was called adverse yaw- normally when the pilot wanted to turn one direction, he only had to move the stick in that direction. At low speeds and high angles of attack that were commonplace in a dogfight, however, if a pilot pushed the stick to the left, the downward-deflected aileron on the right wing would produce more drag than lift, causing the Phantom to yaw back to the right even though the pilot wanted to turn left. As the yaw increases, the effective sweep on the left wing decreases and it starts to produce more left and the F-4 snaps to the right and then into a spin. All this happened nearly instantly and pilots had to compensate for the adverse yaw when rolling left or right by using the rudder aggressively during close air combat- instead of moving the stick into the direction of the turn, the rudder was deflected. So a left turn meant keeping the stick centered and pushing the left rudder pedal down. This causes the Phantom to yaw to the left and this decreases the effective sweep on the right wing- it therefore creates more lift and the plane now snap rolls into the direction of the turn. This took a lot of practice and it was suspected that a significant number of Phantom combat losses were due to adverse yaw conditions. 

The ergonomics of the Phantom cockpit weren't exactly great, either, with a mass of switches and dials that most Phantom drivers felt were scattered randomly over the instrument panel and side consoles. Most derided by the pilots were the switches on the panel by the pilot's left knee that controlled the Sidewinder and Sparrow missiles- one of the switches was a three-position switch that cycled through the Sidewinders and Sparrows. It felt identical to the other control switches on the missile panel and the last thing pilots wanted to do was look at the switch to make sure they flipped the right one in the middle of a dogfight. Resourceful pilots in Vietnam got hold a piece of plastic tubing that was used to sample oil from the engines. Cutting it into a two-inch length, it was slipped over the end of the switch so it stuck out from other switches. All a pilot had to do during combat was swat the tubing by his left knee to cycle through his missiles. 

South Korean F-4E shows off its leading edge slats.
When the F-4E entered service, its most obvious advantage was that it had an internal cannon in the nose, rectifying a situation often encountered in Vietnam when the MiGs were inside of the minimum launch parameters for either the Sidewinder or the Sparrow missiles. But the F-4Es had another advantage and that was what was called a "soft wing"- the wing now had leading edge slats that were controlled automatically by the flight control system whenever the angle of attack would reach a certain level. When the AoA hit this preset level, the slats would automatically pop out and it increased the lift across the wing- this had a dual effect on the Phantom- it eliminated the adverse yaw condition, even at high angles of attack. And secondly, the Phantom became practically spin proof as adverse yaw got eliminated- accidents from spins dropped dramatically and pilots could now haul around the beast in the sky without worrying about loss of control. In fact, some pilots felt that the soft wing F-4E flew just like the Northrop T-38 Talons used in advanced flight training. 

When the USAF brass got word of the improvised plastic tubing modification used in Vietnam, it became apparent after some analysis that many MiG kills were missed due to the missile switches being configured incorrectly. And truth be told, it was pretty embarrassing to know that to make a multimillion dollar fighter more effective in combat needed a cheap piece of tubing. The Air Force came out with a modification to the F-4E that was numbered 556. Pilots called it the "five-five-six mod" and what it did was add a switch on the outside of the left throttle. Operated by the pilot's left pinkie finger (hence it was called the "pinkie switch"), it if was pushed forward, the Sparrow missile was selected. If it was in the middle setting, a Sidewinder was selected. And if it was pulled all the way back, the F-4E's gun was selected. Since the pilot's hand was always on the throttles, the switch was readily accessible. The second part of the 556 mod was another switch added to the front of the left throttle that allowed the pilot to instantly reconfigure the weapons systems from air-to-ground to air-to-air. No longer was there a switch sequence to follow. Push one button, that was it. Other changes consolidated the bomb dropping switches into a single small panel. 

TISEO unit on the F-4E.
While the soft wing and the pinkie switch did much to ease the pilot's workload in air combat, there was still the issue of the rules of engagement requiring visual identification of a MiG before shooting. This restriction pretty much rendered the Sparrow useless as a beyond-visual range (BVR) weapon that could swat MiGs down from unseen distances. The biggest problem with the Sparrow with the RoE in use over Vietnam was that most engagements began head-on. At that distance, the Sparrow could be fired at a maximum of ten miles and a minimum of three miles. But head-on, the MiGs were much smaller and harder to see even if with a radar lock. With closure rates approaching one mile every three seconds, by the time the Phantom crew had a positive visual on the MiG, it was already too late to fire a Sparrow. The solution was called TISEO- Target Identification System Electro-Optical. Some F-4Es had an electro-optical telescope mounted on the leading edge of the left wing above the inboard pylon. The telescope had 4x and 10x magnification and displayed the image on the WSO's radar display. Now when the backseater had a radar lock, he could command the TISEO to track the target and then switch to 10x magnification for visual identification while the MiG was still in the firing envelope of the Sparrow missile.

With the soft wing, the pinkie switch, and the TISEO, the USAF realized it had the tools to make its fighter pilots more lethal. In the late summer of 1972, the Rivet Haste program was established. Rivet Haste would be the Air Force's acknowledgement that years of substandard training and poor doctrine had to be reversed. Rivet Haste combined the slatted wing, TISEO, and 556 mod of the F-4E and teamed them up with handpicked crews with combat experience over Vietnam. They were assembled at Nellis AFB and here the key of Rivet Haste would take place- intensive training with the new F-4Es against the secret MiG force that Air Force flew out of Tonopah north of Las Vegas. Each Rivet Haste crew were paired up to stay together through training to deployment for combat- this allowed the pilot and his WSO to develop their own system of coordination in the cockpit and carry that to combat. Six pilots and six WSOs were in each Rivet Haste group and each pilot/WSO would fly at least three missions against the MiGs at Tonopah. Each month, a new group of six pilots and six WSOs were assigned to Rivet Haste. Crews assigned to Rivet Haste saw it as a quantum leap in air combat training that the USAF had never had before, but the war in Vietnam ended less than two weeks after the first Rivet Haste crews arrived in Southeast Asia. 

It wasn't all for naught, though. The 556 mod spawned a cockpit design philosophy called HOTAS- Hands On Throttle And Stick- this meant that every function that a pilot needed in air combat (and even in air to ground combat) would be instantly accessible either on the throttle or stick. Many of the crews that were part of Rivet Haste would go on into important leadership positions in the post-Vietnam USAF that would result in a greater emphasis on realistic training. The realistic training provided to Rivet Haste crews would be one of several foundations that would result in the Red Flag exercises. 

As the saying today goes "Fight like you train, train like you fight". 

Source: Sierra Hotel: Flying Air Force Fighters in the Decade After Vietnam by C.R. Anderegg. Air Force History and Museums Program, 2001, p11-13, 32-35.