In an extended article for Aviation of Japan correspondent Mark Smith and co-author Mike Quan explore the development of the Imperial Japanese Navy Air Service's elegant Nakajima C6N Saiun (彩雲 - vibrant cloud) carrier based reconnaissance aircraft. The article is illustrated with images of Mark's own Saiun prototype model, built from the Hasegawa kit.
From its inception the Imperial Japanese Navy Air Service (大日本帝國海軍航空隊 -
Dai-Nippon Teikoku Kaigun Kōkū-tai) had stringent demands and high expectations of those who designed and built its aircraft. Performance specifications outlined for each new model required the highest level of innovation, financial risk, and imagination – and even then seemed impossible to meet. Successful designs like Mitsubishi’s A6M Zero fighter and Nakajima’s B5N torpedo bomber had proven the wisdom of such a course. But in 1942, Nakajima’s design engineers were asked to deliver against a specification for a new experimental carrier reconnaissance plane that must have come as a jolt.
A top speed of 350 knots at 6,000 meters and a maximum range of 2,500 nautical miles would be required for a machine carrying three crewmen, state-of-the-art radio and camera gear, a rear-seat machine gun, and arresting gear for carrier operations. The vaunted Zero-Sen, a pure fighter design unmatched in performance by any shipboard aircraft of any type then in service, had a top speed under 300 knots.
The need for speed was fundamental to long-range fleet reconnaissance, but had taken on a new urgency. The Japanese knew that the Corsair and Hellcat would soon show up in the Pacific in significant numbers. Japan’s reconnaissance doctrine was heavily reliant upon catapult-launched floatplanes and long-range flying boats, and they had reliable aircraft and seasoned crews for these roles; against US forces in carrier versus carrier battles, however, they would not be enough. While these aircraft had remarkable range, if spotted by Allied fighters, they seldom returned. In the first few months of the Pacific war the Navy employed small numbers of carrier-based attack aircraft for reconnaissance to augment the reporting of “Jake” floatplanes and “Mavis” as well as the “Emily” flying boats just introduced to service.
But this would not prove the answer against the new foe. Grumman F4F Wildcats, guided by Fighter Directors on American CVs who were quickly learning to exploit the USN’s critical edge in radar, kept showing up. The risk of interception was high when enemy forces were known to be within the search radius. If fighters did not score a kill, they could at least divert Japanese aircraft from their assigned search vectors. Other factors pressed also. IJN attack aircraft and their veteran crews were critically needed for their primary role when their own numbers were spread too thin, and the unforgiving nature of ocean navigation over trackless distances regularly claimed its own victims. The hardest part of such a stressful mission came at the end, when weary crews had to find a carrier which might have changed course in their absence, usually under strictly imposed radio silence. The absence of the air superiority the Japanese had known in China had changed the picture.
JNAF carrier units were second to none in equipment, training, and skills. But airborne reconnaissance missions that would give Combined Fleet the needed edge required greater range, speed, and a higher service ceiling than any IJN bird in the inventory. Where were the dedicated eyes of the fleet?
Right around the corner, some said; the scuttlebutt was that something special was on the way from Koizumi that would help Japan reclaim the initiative in the South Pacific. Koizumi was a brand new facility, with Ohta Field and Nakajima’s Ohta plant virtually next door. There the Navy and Nakajima had assembled their brightest lights in experimental flight-testing, as well as a crack team of mechanics and fitters. Specific groups were assigned to investigate persistent problems critical to any new design’s success. According to Rene Francillon, the C6N design employed eighteen prototype and pre-production aircraft before production could commence. But at Koizumi the Saiun improbably evolved into a viable design, as the airplane the Allies would come to know as “Myrt” came into its own.
Yasuo Fukuda was the project’s Airframe Section Chief. Instrumental in design and production of Nakajima’s the B5N (“Kate”) and its successor the B6N Tenzan (“Jill”), he knew what he was up against. Tenzan had held great promise, but was riddled with teething problems that had so far prevented it from attaining production status. The main problem was power. The horsepower necessary to meet assigned requirements was unavailable in any Japanese engine of that time. A three-seat carrier aircraft with a maximum speed of 250 knots, Tenzan was still mired in a long gestation period. And in Saiun, the company was being asked to come up with a three-seat carrier aircraft 100 knots faster!
Though the C6N1 project was originally envisioned to use two yoked 1000 hp engines in order to provide the requisite power, Nakajima’s new air-cooled Homare, then being tested, promised almost as much power without the added weight and maintenance headaches such a complex design was bound to bring. The decision was made for a more conventional single-engine design. Homare (“Honor”) was compact, its diameter only 30mm larger than the Zero’s reliable Sakae. But at the necessary altitude of 6000 meters, it was found that the engine (which would always prove troublesome in service) would only yield 1600 horses. However Nakajima’s design crew did the math, they needed the 400 additional hp originally promised in order to deliver the performance the JNAF demanded. The only hope to make up the difference would require the simultaneous success of several radical design features. While it was a longshot, there was no alternative due to the worsening war situation.
Nakajima would not regret choosing Homare, and concurrent events that supported its wisdom are worth relating here. In 1943 the Kugisho (sometimes referred to as Yokosuka) had begun to test the 18-Shi Field Reconnaissance Aircraft assigned the name Keiun. Without the necessity of arresting gear or a short takeoff run, a land-based recon aircraft should have been able to match or exceed Saiun’s performance. But early in its design stages a decision was made for the in-line installation of two German DB601 liquid-cooled engines, produced domestically in Japan as "Atsuta" (Type 30), in what was termed a "twin-type" mounting, with the two powerplants buried front-and-back in the fuselage. Navy engineers had a privately-purchased Heinkel 119 on hand for study which had the same arrangement. The engines were connected, to each other and to the propeller, via a long extension shaft. It was a bold technique, but one dictated by the lack of a Japanese aero-engine equivalent to the Pratt & Whitney R-2800 or the later Rolls-Royce Merlins; and Keiun’s radical airframe design was not flexible enough to support a more typical powerplant. The gremlins connected with the “twin-type” engine arrangement were never exorcised. The Keiun program consumed precious time, personnel, and priorities for the sake of an airplane that accomplished one ten-minute test flight.
If the efforts expended on Keiun had been applied to existing Kugisho types suitably modified to undertake the land-based reconnaissance role, such as the D4Y Suisei (Judy) and P1Y Ginga (Francis), these aircraft could have been delivered to JNAF units badly needing them. At least in Suisei’s case, this could have easily occurred by the beginning of 1944. The earliest variant, the D4Y1-C, had proven an able purpose-built recon bird, with long range and high speed. The fact that the first two were assigned to Soryu just in time for the Midway battle in mid-1942 indicates how early they might have been ready in numbers. That battle marked the beginning of heavy Aichi D3A (Val) dive-bomber losses, however, and afterward Suisei production would concentrate solely on the dive-bomber version. With later models of the D4Y, the removal of armament, carrier arresting gear, bomb load, and the hefty bomb release mechanism inside the bomb bay would have added significant speed for the recon mission, leaving only the need for radio and camera gear. Just as importantly, the type was easily adapted to a radial engine (D4Y3) when the Atsuta inline models became unavailable, so the decision to continue so long with the Keiun program seems completely insupportable.
The Navy would finally have the land-based reconnaissance aircraft it needed, but for all the wrong reasons: Keiun failed miserably, and by the time Saiun reached production, there were no Japanese carriers left from which to operate. But these two aircraft represent what has often been the critical design decision: to build upon the proven and traditional, or to take the leap of faith for something new or revolutionary.
In a way, Saiun’s designers did both. Airframe Section Chief Fukuda’s prime directive was to keep the airplane as small as possible, much the same way as the Zero was designed with constant emphasis on weight savings. Special care was taken with an economically elegant cowling design to provide an exceptionally clean airflow entry. The same attention was focused on the front fuselage area and the seven sections of the aircraft’s greenhouse canopy.
The wing design, as originally submitted, was a shade less than two-thirds of Tenzan’s wing area, and appeared impossibly small to provide the necessary lift for carrier take-offs and landings while still meeting range and drag numbers. It was agreed that only a laminar-flow wing could provide the necessary lift while reducing drag sufficiently to meet speed requirements. Over 200 NACA airfoils were tested along with Nakajima's own K-Series airfoil, one of the latter being modified by 30-year-old engineer Naito Yasuo. His successful creation resulted from what he called "sharpening" the leading edge profile. As he explained in a two-part article printed in Koku-Fan magazine in the 1970s, this was a “delicate planing” of the leading edge of the airfoil shape that resulted in an increase in the lift coefficient at higher airflow regimes than the normally tested flow regimes then considered as ‘standard,’ as measured by Reynolds number. Encyclopedia Britannica defines Reynolds number as a scientific, mathematical quantity that models/theorizes liquid or gas flow, measuring the aeronautically important factor of when airflow transitions from laminar to turbulent, at which point the drag increases on an airfoil. (Lift coefficient equals drag forces divided by lift forces.) Essentially, Naito was pinning his hopes on a combination of aeronautics and nascent technology which was then unproven in Japan - and in doing so, had stumbled into the same scientific area of research which NACA had conducted in the US that resulted in the laminar airfoil so successfully used most notably on the P-51 Mustang, endowing it with unusual speed and range.
Some business pressures never change. The IJNAF, the ones who had mandated such stringent requirements, balked at approving the new technology Nakajima would require in meeting them. Naito was briskly summoned to explain himself, as the Navy’s own engineers felt that precious time would be wasted in banking on an untested and seemingly impractical design. As a result of these discussions, however, a new wooden model was made, and the wind tunnel tests Naito requested were granted. There his calculations and daring were vindicated; the maximum lift-over-drag coefficient increased, while stall characteristics decreased. This was the key basis for selling the Navy on such a controversially small wing, and Nakajima now set to work to make it as efficient as humanly possible.
Leading edge slats and sophisticated double-slotted Fowler flaps were incorporated into the wing to further increase lift, and the ailerons could be lowered in conjunction with the flaps. The slats were unlike those seen on most aircraft of the period such as the Fieseler Storch or Me109, which deployed automatically at a certain speed above stall; those on Saiun these were operated hydraulically by the pilot through the use of springs.
The inevitable down-side of all these efforts to make the airplane controllable for carrier operations was that the “center of lift” shifted dangerously aft, causing the nose to pitch down perilously at deck landing speed. Nakajima engineers devised an ingenious solution that was to be reflected in many later jet designs: again using oil pressure, the entire horizontal stabilizer was made hydraulically articulated so that its angle could be changed upon final approach and arrest. The tiny square cutout underneath these surfaces is the only external evidence of this critical innovation.
Lift or no lift, though, Saiun would need increased acceleration for that short carrier take-off run, and the largest-diameter propeller feasible (3.5 meters) was chosen for the sake of its additional thrust (3.5 meters). This helped take Saiun from zero to six thousand meters in 6 minutes, 20 seconds. (Nakajima’s Hayate, a single-seat fighter using the same engine, climbed to the same height only ten seconds sooner). In a way that mirrored the Vought Corsair’s needs and development, this in turn mandated a long set of landing gear legs for ground clearance. Due to the selection of a laminar-flow wing, these would have to retract between the spars (instead of forward of the main spar as was the general design practice of the time), so as to least compromise the critical laminar shape of the airfoil. Though this pushed the main gear legs back far enough to give the aircraft an awkward ‘sit’ and made for slightly more difficult ground handling, it was yet another make-or-break point in the airplane’s development. It also mandated another unusual feature. A “smooth” oleo strut was designed by Engineer Sakamoto, an element that Naito Yasuo, writing thirty years after the war, would cite as critical to the aircraft’s success. These struts, when retracted, were raked forward dramatically to compensate for being mounted so far aft.
Unusually large panels of thicker gauge metal were used to skin the Saiun. This would seem an odd choice in an aircraft where speed was essential and any weight savings important. Yet the benefits proved otherwise: Saiun needed less than half of the number of rivets used in Tenzan, and the larger panels cut skin drag and speeded production. An innovative wing spar utilized super-extruded duralumin for the upper and lower spar caps. These features made for an exceptionally strong wing structure (required for the larger panels), and one capable of mounting three fuel tanks in each wing totaling 1300 liters. The under-wing external fuel tank, the largest carried by any Japanese aircraft, added 730 liters. Thus loaded, Saiun could roam for an astonishing 5300 kilometers.
Nakajima had completed the first prototype in March 1943 and flown it on May 15. Flight test results suggested several changes that might refine performance, even as Nakajima was feeling increasing pressure to standardize the aircraft for production as the front pushed further north toward Japan. Significant changes to the design included a new three-blade propeller of different section to replace the original four-blade unit, cowling and oil cooler refinements, and modified horizontal tail surfaces. Originally unable to meet the critical speed requirement for its mission, after these changes the sleek Saiun was finally accepted for production in the spring of 1944, and first deliveries made to operational units in September.
The C6N1 made its combat debut during the battle for the Marianas, an unmitigated disaster for Japan. By then of course it was far too late; though the Saiun was a sterling performer and could usually accomplish its mission well and safely, the photographs and reports it brought back were inevitably disheartening. It was ironic that by war’s end, the only Japanese aircraft in production possessing truly exceptional performance by international standards were not ones bringing bullets or bombs to the fight, but types designed to spy on the enemy: the Army’s late-model Ki-46 Type 100s and the Navy’s C6N1 Saiun. However elegant their design or elusive their performance, they could only bring home bad news. Soon many were flying as pathfinders for Tokko and Kamikaze missions, and a ‘successful’ mission meant coming home alone.
Attempts were made to transform both designs into fighters, utilizing diagonally-mounted cannon neither airframe had been designed to support. The handful of examples of Saiun that were turned into night-fighters to protect Japan’s industrial cities against B-29s saw little if any success. Mounting large-caliber weapons mid-cockpit of such a specialized design was merely a measure of Japan’s desperation.
Counting prototypes and production airframes, 463 examples of Saiun were built. While the Zero was the pride of the Navy for much of the war, Saiun was easily the fastest and most exotic aircraft in its inventory in WWII. It was a remarkable accomplishment of imagination and innovation in the face of Japan’s most crippling aviation shortcoming - horsepower. American testing of at least one aircraft was undertaken after the war upon arrival of several airframes aboard jeep carriers. “Myrt” had earned a deservedly high reputation among USAAF and USN pilots and intelligence officers in its brief time of service; with new plugs and the Homare perfectly tuned, for instance, it could not be caught by Hellcat pilots in a tail chase. While the design elicited admiration stateside for its clean lines and performance, interest was academic; by war’s end American reconnaissance doctrine was firmly established in combat-ready fighter types also equipped with cameras, as had been ably vindicated by the P-38, Hellcat, and P-51. And the new technology of the jet engine was here to stay.
The only surviving example of Saiun is now held in storage by the National Air and Space Museum in Washington D.C. Found at Atsugi Air Base at war’s end, it had been converted to the night fighter role. Some of its cockpit appointments are long lost, and many of those that remain are unique to the armed version, to which only a handful were converted. Roughly disassembled, it is still sadly derelict, and if restored, some areas would need re-fabrication. It was last flown at Middletown, Pennsylvania, after which it sat at its tie-downs for many months through hard Northern weather, its tires soon flat and its once-polished metal surfaces chalking to gray - an ignominious end for a brilliant design.
Engineer Naito Yasuo’s account, as abstracted in English in Koku-Fan magazine, was the central source used for this article. I had only a photocopy of that article obtained long ago, which gave no clue to the particular issues where it appeared. That information would be greatly appreciated so we can provide it here. As this project became more ambitious and I struggled with a translation that had its own problems, and that treated subjects where my knowledge was insufficient, I was out of my depth. In clarifying certain vagaries of that translation as they related to technical matters, in correcting my own mistakes regarding terminology, and particularly in rewriting the section about Reynolds number, I was greatly aided by Mike Quan’s careful reading and editing of the manuscript. His professional knowledge of the subject made such a difference. Errors that remain are entirely mine.
Mark Smith
Image credit: All model photos © 2015 Mark Smith; Interior colour chips © 2015 Aviation of Japan
