Clash of the cancelled Round 5: Curtiss XP-55 Ascender versus Kyushu J7W Shinden
Not every fighter flown in World War II looked like a Spitfire*. Some radical new shapes were tried, but reinventing the fighting aeroplane while your nation was fighting in the most destructive war in history was a risky expensive move, but one with potentialy huge rewards. More than sixty years before the Typhoon, Rafale and Gripen made the tail-first ‘canard’ warplane commonplace, two enemy nations looked to master this unordodox configuration. Jim Smith asks why and finds out which was the better machine.
(*or a Spitfire that had swallowed a barrel in the case of the radial-engined aircraft)
Curtiss XP-55 Ascender and Kyushu J7W Shinden
Two fighter designs, the Ascender from the USA, and the Shinden from Japan, featuring swept wings, a canard foreplane and a pusher engine installation. Before looking at the aircraft more closely, it is worth discussing why this layout might have appeared attractive.
Pusher-Canard Design Objectives
There are several possible advantages of using a canard foreplane and a pusher engine installation. A conventional design with a rear-mounted tailplane is normally trimmed and stabilised with a download on the tailplane, which reduces overall lift. A stable canard will have lift on both the canard and the wing, and the pitching moment to rotate for take-off will also be generated by increasing the lift on the foreplane.
If the aircraft is designed with a swept wing, as is both the Ascender and the Shinden, fuselage size, drag and weight can be reduced compared to a conventional design. The swept wing will allow fins and rudders to be positioned aft of the centre of gravity with sufficient moment arm for them to be effective, with the foreplane being used either as an elevator (Ascender), or to carry elevators (Shinden). Reducing the fuselage size will reduce wetted area and drag, and reduce pitch inertia, potentially increasing responsiveness to the controls.
In general, canard designs have benign stalling characteristics, since the foreplane will normally be designed to stall at a lower incidence than the wing, resulting in a pitch down at the stall. At high lift, downwash from the foreplane will reduce the incidence on the inboard wing, helping to ensure this desirable behaviour.
Selection of a pusher engine installation makes sense for a canard and swept wing configuration because the engine can be used to balance out the weight of the pilot and the armament, which can conveniently be located in the nose of the aircraft. Reduction in drag may also be claimed as the fuselage and wings will be free from propeller wash, offering the prospect of cleaner flow over the wings and fuselage. However, a rear-mounted engine may be more difficult to cool, and the efficiency of the propeller may be reduced as it will be operating in the wake of the aircraft. Rudder authority will also be reduced in the absence of the slipstream from the propeller, so directional stability and control may be an issue for some aircraft.
An outstanding example of a relatively current design with this configuration is the Rutan Vari-Eze, which can achieve a max cruising speed of 170kt on only 100hp.
Curtiss XP-55 Ascender
Designed to meet a 1939 specification for a well-armed high-performance fighter, the XP-55 Ascender was one of three unorthodox designs selected for development. The other two were the Vultee XP-54 Swoose Goose, and the Northrop XP-56 Black Bullet. All three aircraft were of pusher configuration; the Ascender having swept wings and a canard; the Swoose Goose having a straight wing and twin tail booms; and the Black Bullet being a tail-less flying wing.
All three aircraft were originally intended to be powered by the 2200 hp Pratt & Whitney X-1800 engine, which was cancelled, leaving an alternative engine to be selected. Curtiss fitted a 1275 hp Allison V1710-95 engine as a substitute in the Ascender, which would have ensured that the aircraft could in no way match the originally estimated maximum speed of 507 mph. Incidentally, the Northrop design had severe stability problems, and was eventually assessed as not airworthy, while the Vultee design substituted the X-1800 engine first with the Lycoming XH-2470 engine, and then proposed to use the Wright R-2160 Tornado, but both of these alternative powerplants were also cancelled.
The Ascender configuration design had been supported by the use of a full-scale, but light weight, demonstrator powered by a 275 hp Menasco engine, the CW-24B, which made no less than 169 test flights, and this helped to overcome initial AF doubts about the configuration. The XP-55 made its first flight on July 19, 1943, but was lost in an accident in November 1943. The aircraft became inverted following a stall, and could not be recovered by the pilot, who bailed out successfully.
The second and third prototypes continued the flight test program, flying in January and April 1944 with a range of modifications to improve directional stability and the effectiveness of the elevators. The third aircraft also featured additional wing and control modifications in response to the accident to the first aircraft. Even with these modifications, handling was poor at low speeds, the engine tended to over-heat, and the maximum speed of 390 mph was well below expectations.
By this time, it was apparent that the performance of in-service aircraft like the P-38 Lightning, P-47 Thunderbolt, and P-51 Mustang far exceeded anything that could be credibly expected from the Ascender. It was also clear to the Air Force that the future lay with jet propulsion, and the XP-55 Ascender joined the XP-54 Swoose Goose and XP-56 Black Bullet in being cancelled.
Kyushu J7W Shinden
The Kyushu J7W Shinden was a small and very neatly-packaged canard fighter concept that was originated by Captain Masaoki Tsurono of the Technical Staff of the Japanese Navy. Although the aircraft was fitted with a 2130 hp Mitsubishi MK9D radial engine, Captain Tsurono had always envisaged that this would be replaceable in the future by a jet engine.
The basic configuration was validated using an unpowered demonstrator, the Yokosuka MXY6, supplemented by some additional flights with a low powered 22 hp engine. With the assurance that the configuration was practical, the Navy instructed Kyushu to proceed with the design and construction of the J7W Shinden, with some technical assistance being provided by a Navy team led by Captain Tsurono.
The design which emerged was centred on a small, moderately swept wing of 36 ft span, carrying the engine on the wing centre section, with a shaft driven 6-blade propeller at the rear of the fuselage. Cooling intakes were fitted to the fuselage ahead of the wing, with the cockpit, armament and forward canard making up the forward fuselage. With its compact packaging, powerful engine, and relatively high power-to-weight ratio, the Shinden might have been expected to offer high speed and good manoeuvrability.
However, timing was against the Shinden. Despite being ordered into production even before its first flight, only two aircraft could be built before the end of WW II, and only 3 flights of the prototype aircraft were made.
What can we learn from those flights? The reported information was that they revealed a strong swing to starboard during take-off, and significant vibration in the propeller and its drive shaft.
The first of these would have been due to the torque of the powerful engine, but would also have been exacerbated by the short moment arm between the wing-mounted fins and the aircraft centre of gravity. A production aircraft might have required a central fin to supplement those on the wings, and perhaps also larger rudder controls.
The vibration in the propeller and its drive shaft has been stated to be due to propeller flutter, and a redesign of the propeller to increase its stiffness would perhaps have resolved this problem. Unexplored issues which would need to be examined in flight test would include handling at low speeds and at the stall, engine cooling, and detailed evaluation of aircraft performance and armament effectiveness.
Despite these concerns, which might well have been resolved in flight and operational testing, the Shinden was a well-executed, forward-looking design, particularly as consideration had been given to a future jet-powered variant, the J7W2.
Curtiss XP-55 Ascender and Kyushu J7W Shinden – Air Combat Comparison
Although design of the XP-55 started from a 1939 requirement, the aircraft did not fly until 1943, with the third and final aircraft not flying until April 1944. The J7W Shinden made its first flight in August 1945. The Shinden only made 3 flights; two of the three XP-55 aircraft were lost in accidents which reflected poorly on the controllability of the aircraft.
Unsurprisingly, there is limited information on which to judge their air combat performance, other than basic dimensional, power and weight data. What information there is suggests that the Shinden would have had substantial advantages over the XP-55 Ascender.
The Shinden had more power, and a greater power to weight ratio, and had a slightly lower wing loading than the Ascender. While the Ascender had a slightly higher aspect ratio, the advantage due to this in sustained turn rate could well have been negated by the higher power to weight ratio of the Shinden. The reported maximum speed data suggests a 70 mph speed advantage for the Shinden, but I would treat this estimate with some caution, as so little flight test of that aircraft was possible.
On sortie rate, there is little to go on. The air-cooled radial engine of the Shinden might have required less maintenance than the liquid-cooled engine of the Ascender, and there is some indication that there were cooling issues for that aircraft. Directional control and propeller flutter issues remained to be resolved for the Shinden.
Curtiss XP-55 Ascender and Kyushu J7W Shinden Assessment
The Curtiss XP-55 Ascender, like the XP-54 and XP-56 with which it competed, turned out to be disappointing, largely because the engine for which it had been designed turned out not to be available. But it also had difficult handling qualities, especially at low speed. Possibly not as dangerous as the XP-56, and perhaps with greater, if still inadequate, performance potential than the XP-54, the XP-55 Ascender could not be considered a success.
The J7W Shinden seems to be a particularly well executed pusher-canard fighter, but with a total of 45 minutes flying, cannot be said to be a proven entity. The limited test flying revealed two early issues, yaw control on take-off and vibration of the propeller and prop-shaft. It is likely that the Shinden could have been developed into a successful fighter, particularly if the planned jet engine installation had gone ahead. But this will never be known with certainty, because the War in the Pacific ended before any meaningful testing or production had been undertaken.
Which aircraft was better? The choice could be as simple as observing that the Curtiss design was a dud, whereas the Kyushu Shinden had not been tested sufficiently to establish whether or not it was also a dud. This approach would, I think, be a cop out, so I am prepared to rate the Kyushu Shinden as the ‘better loser’ on two grounds – its development potential as a jet-powered aircraft, and its aesthetic appearance.