History
Background
On November 27, 1939, the US Army Air Corps issued a Request for Proposal R-40C that sought a high-speed, long-range, high-altitude interceptor with improved performance, armament, and pilot visibility over existing fighters. The interceptor was to be specialized to exterminate enemy bombers. The specifications already outlined that the interceptor must be capable of reaching 20,000 feet in 7 minutes and attaining 425 mph at 15,000 to 20,000 feet. The armament would be either at least four guns or cannons. The specifications were very ambitious, and they intended to encourage manufacturers to produce innovative interceptors that would outperform any existing fighter in the era. In addition, they allowed the manufacturers to come up with unconventional, radical aircraft designs.
Just 22 other manufacturers had issued proposals to meet the Army’s request; the XP-54, XP-55, and XP-56 designs were among the accepted proposals.
Design and Development
Northrop submitted a proposal for the XP-56 design in response to the US Army Air Corps’ R-40C. Its design was very radical in 1939-1940 since it featured no horizontal tail, only a small vertical tail. It would be powered by an experimental pusher engine driving contra-rotating propellers. The aircraft was a flying wing with a small central fuselage added to hold the engine and a pilot, and it would be covered in magnesium. The wing was swept back and of gull form with downward-sloping tips, as previously seen on the Northrop N-1M. The pilot would have an excellent view forward and downward since the pilot would be located near the nose and ahead of the wing’s leading edge. The aircraft was to have a tricycle undercarriage, which can be fully retractable. Overall, with this configuration, it would be forecasted to have less aerodynamic drag than a conventional airplane.
The US Army ordered Northrop to develop the design work on June 22, 1940. The design was intended to be powered by a Pratt & Whitney X-1800 (or XH-2600) liquid-cooled engine with a turbo-supercharger, which would drive a pusher contra-rotating propeller at the end of the short fuselage and behind a ventral fin. After reviewing the design work, the US Army awarded Northrop a contract to work on the prototype on September 26, 1940, and have it ready for delivery within 12 months.
However, Pratt & Whitney stopped development of the X-1800, which impacted all developments of XP-54, XP-55, and XP-56. The Pratt & Whitney R-2800 engine with a two-stage/two-speed supercharger was selected as a replacement for the XP-56 on March 12, 1941, and approved on July 21, 1941. It would become unsuitable for the aircraft, as it had a large diameter and required a larger fuselage to house it. It was expected that the new engine would increase the weight by 2,000 pounds. The thorough redesign had delayed the XP-56 program by five months to accommodate a different engine.
The XP-56 was to be built with R-2800-29 to accommodate the 9.7-foot Curtiss Electric six-blade contra-rotating propeller with extended concentric shafts and a two-speed remote gearbox. The gearbox and cooling fan were integrated into the engine. In addition, an explosive charge was armed and encircled the gearbox to blow off the propeller to allow the safe pilot bailout. The XP-56 may have been the first aircraft with a dual-rotation propeller as a pusher. Weapons were to be two 20-mm cannons with 100 rpg and four .50-cal. machine guns with 400 rpg, presumably all mounted in the nose or wings or spread across both.
While work was still proceeding on the delayed prototype, a second prototype was ordered in February 1942.
Prototype #1
Delivery of the XP-56’s new model R-2800-29 engine was late, and once the engine was fitted to the prototype’s airframe, ground vibration tests were performed to monitor and avoid destructive resonances. The all-metal first XP-56, constructed with arc-welded magnesium, was finally completed in March 1943. The engine runs immediately revealed the problem, with an excessive propeller shaft flex that caused the engine to fail. It was remedied a month later. On April 12, 1943, the XP-56 made its initial taxi runs but had a serious yaw problem. Another effort was made to rectify the uneven and grabby brakes. Manual hydraulic brakes were substituted and installed. The aircraft was not ready for the initial flight since its powerplant issues necessitated changes in engine and gearbox mounting. More months were wasted awaiting the replacement from Pratt & Whitney.
On September 3, 1943, the aircraft was moved to Rogers Dry Lake. When it resumed with taxi testing there, the elevated engine temperatures became a persistent problem. The aircraft made its first flight on September 6, 1943, but it was only flying five feet above the ground and 130 mph for 30 seconds before landing. The nose heaviness was noted again. On a second flight the same day, it flew 25 to 50 feet and 170 mph, yet it exhibited uncontrolled yaw, roll, and nose-down pitch movements. The test pilot got it back on the ground safely, but it landed so heavily that the tail bumper was torn away.
After reviewing flight and wind tunnel data, the directional instability with the landing gear down was identified, so the recommendation was to increase the height of the shallow upper fin. The testing resumed on October 8, 1943, with the aircraft performing several high-speed taxi runs and low hops over the lakebed. During a taxi run, the left tire blew out, and the aircraft swerved sharply off the runway before going out of control. The aircraft fell backward across the lakebed and was destroyed. The test pilot survived but suffered moderate injuries. The prototype was out of action and never flew again.
Prototype #2
Further testing was halted until the arrival of the second completed prototype in March 1944, with the construction beginning in January 1944. The camouflaged aircraft was already redesigned and fitted with a bigger dorsal fin in an attempt to rectify the yaw problem. Its weight was increased by 1,180, and the center of gravity was shifted forward with ballast. It included reworked rudder control linkages and wingtip aero boost ducts to provide ram air pressure power and operate split control surfaces used for lateral controls. A small wheel was installed at the tip of the ventral fin, replacing the bumper.
The aircraft flew on March 23, 1944, for the first time, with the nose gear doors removed. The nose heaviness was still observed and reported. The aircraft was able to rotate and take off at speeds above 160 mph. The aircraft got off and circled the desert site, and it climbed to 2,500 feet and accelerated to 180 mph. However, the pilot was fighting hard against the very high stick forces and rudder sensitivity. The aircraft landed and ended the flight after just 7.5 minutes.
The center of gravity was reset to its original location for the next flight on March 31, 1943. During the second flight, the pitch response during takeoff was improved. The undercarriage was retracted after takeoff, but the aircraft approached a stall and quickly extended the gear. The XP-56 flew to about 7,800 feet, but it was reported to not handle well and suffered a severe vibration above 250 mph. It landed and ended the flight after 35 minutes. Four flights were later conducted, some with the gear doors reinstalled. Speeds up to 320 mph were achieved, but overall control and trim authority remained serious flaws that needed to be ironed out. The increase in power may be necessary since the aircraft was underpowered in its weight and did not meet the expectations of the flight performance.
On the seventh flight, May 12, 1944, the test pilot reported wing heaviness and stability issues that appeared to be worse than those in recent flights. The engineers were puzzled by the degradation and could not isolate the cause. The XP-56 project was beyond salvage since the aircraft appeared to be a lost cause due to unsolved stability and control problems.
There was some talk that the National Advisory Committee for Aeronautics would plan to use their wind tunnel at Moffett Field, California, to investigate the shortcomings of the XP-56s that led to their unairworthiness. However, the higher priority of other projects led to the postponement of the XP-56 wind tunnel tests and cancellation in late October 1944.
On the ninth flight, the ventral fin was damaged during the landing, requiring the aircraft to be shipped back to Hawthorne, California, for repairs. The aircraft was then returned to Muroc. The aircraft’s tenth and last flight was made on August 11, 1944, and extreme tail heaviness on the ground, low power, and excessive fuel consumption were reported. After landing, the fuel mixture was found to be too rich, and a pitch linkage between the two propellers had failed, allowing one set to go to full low pitch. After consideration, it was concluded that the second XP-56 was unairworthy and became too dangerous to continue the testing, abandoning the whole project after a year of inactivity.
By January 1946, the US Army Air Forces were developing new jet-powered fighters that put an end to the XP-56 project, as there was no need for a new propeller-driven fighter aircraft. The aircraft had 5.9 hours of flight logged, and it remained in storage at the Smithsonian Institution’s National Air and Space Museum in Washington, D.C., for further restoration work.
Epilogue
The XP-56 was another victim of an underpowered engine that had doomed the project from the beginning. Nevertheless, the aircraft served as an important pioneer of innovative aircraft designs during the pre-war era. Although the XP-56 project was a doomed failure, it was not a serious loss for Northrop since it had learned a lot about flying wing designs. They gained data during the XP-56 project and put it into good use in their further Northrop designs, such as the XB-35 piston-engined bomber, the YB-49 jet-powered bomber, and the B-2 stealth bomber.
