The Curtiss XP-55 Ascender was another response to Circular Proposal R-40C, which was issued on November 27, 1939. It called for a fighter that would be much more effective than any extant--with a top speed, rate of climb, maneuverability, armament, and pilot visibility, all of which would be far superior to those of any existing fighter. In addition, the fighter was required to have a low initial cost and had to be easy and inexpensive to maintain. The Army specifically mentioned in R-40C that they would consider aircraft with unconventional configurations.
No less than 50 responses came in. Many of them were quickly ruled out, but by the end of 1940, four designs were considered sufficiently worthy of further study. These were designs submitted by Bell, by Curtiss, by Northrop, and by Vultee.
The Curtiss entry, designated CW-24 by the company, was perhaps the most unconventional of the four finalists. It was to be one of the last projects supervised by Donovan Berlin before he left the Curtiss company to go over to Fisher to work on the P-75. The CW-24 was a swept-wing pusher aircraft with canard (tail-first) elevators. The low-mounted sweptback wings were equipped with ailerons and flaps on the trailing edge as well as directional fins and rudders mounted near the wing tips both above and below the airfoil. The elevators were located near the front of the nose in a horizontal surface. A completely-retractable tricycle undercarriage was to be used, the first time such an undercarriage was to be employed in a Curtiss fighter. Curtiss proposed to use the new and untried Pratt & Whitney X-1800-A3G (H-2600) liquid-cooled engine, mounted behind the pilot's cockpit and driving a pusher propeller. Project maximum speed was no less than 507 mph!
On June 22, 1940, the Curtiss-Wright company received an Army contract for preliminary engineering data and a powered wind tunnel model. The designation P-55 was reserved for the project.
Since the USAAC was not completely satisfied with the results of the wind tunnel tests, Curtiss-Wright took it upon itself to build a flying full-scale model. Designated CW-24B by the company, the flying testbed was powered by a 275 hp Menasco C68-5 engine. It had a fabric-covered, welded steel tube fuselage and a wooden wing. The undercarriage was fixed.
After completion, the CW-24B was shipped out to the Army flight test center at Muroc Dry Lake (later Edwards AFB) in California. It made its first flight there on December 2, 1941. Although the maximum speed was only 180 mph because of the low engine power, the CW-24B proved out the basic feasibility of the concept. However, early flights indicated that there was a certain amount of directional instability. The original auxiliary wingtip fins were increased in area and moved four feet farther outboard on the wings, which enhanced the directional stability. The wingtips were made longer, and further improvements were obtained by adding vertical fins to both the top and the bottom of the engine cowling. 169 flights with the CW-24B were made at Muroc between December 1941 and May 1942. After that, the airplane (having been assigned the USAAC serial number 42-39347) was transferred to Langley Field, Virginia, for further testing by NACA.
During the flight testing of the CW-24B, work on the CW-24 fighter project continued. On July 10, 1942, a USAAF contract was issued for three prototypes under the designation XP-55. Serial numbers were 42-78845/78847. Since the Pratt & Whitney X-1800 engine was experiencing serious program delays (it eventually was cancelled outright before attaining production status) Curtiss decided to switch to the Allison V-1710 (F16) liquid-cooled inline engine for the sake of reliability and availability. Armament was to be two 20-mm cannon and two 0.50-inch machine guns. During the mockup phase, it was decided to switch to the 1275 hp Allison V-1710-95 engine, and the 20-mm cannon were replaced by 0.50-inch machine guns.
The first XP-55 (42-78845) was completed on July 13, 1943. It had essentially the same aerodynamic configuration as did the final CW-24B. It made its first test flight on July 19, 1943 from the Army's Scott Field near the Curtiss-Wright St Louis plant. The pilot was J. Harvey Gray, Curtiss's test pilot. Initial flight testing revealed that the takeoff run was excessively long. In order to solve this problem, the nose elevator was increased in area and the aileron up trim was interconnected with the flaps so that it operated when the flaps were lowered.
On November 15, 1943, test pilot Harvey Gray was flying the first XP-55 (42-78845) through a series of stall tests when the aircraft suddenly flipped over on its back and fell into an uncontrolled, inverted descent. Recovery proved impossible, and the plane fell out of control for 16,000 feet before Gray was able to parachute to safety. The aircraft was destroyed in the ensuing crash.
At the time of the crash, the second XP-55 (42-78846) was too far advanced in construction for its configuration to be conveniently modified to incorporate any changes resulting from an analysis of the cause of the loss. The second XP-55 was essentially similar to the first one, apart from a slightly larger nose elevator, a modified elevator tab system, and a change from balance tabs to spring tabs on the ailerons. It flew for the first time on January 9, 1944, but all flight tests were restricted so that the stall zone was carefully avoided until the third XP-55 had been satisfactorily tested.
The third XP-55 (42-78847) flew for the first time on April 25, 1944. It was fitted with the designed complement of four machine guns. It incorporated some of the ideas learned from the investigation into the cause of the loss of the first XP-55. It was found that stall characteristics could be improved by adding four-foot wingtip extensions of greater area and by increasing the limits of nose elevator travel. However, the first flight revealed that the increased elevator limits resulted in the pilot being able to hold such a high elevator angle during takeoff that the elevator could actually stall. After modifications, stall tests were performed satisfactorily, although the complete lack of any warning prior to the stall and the excessive loss of altitude necessary to return to level flight after the stall were undesirable characteristics.
An artificial stall warning device was introduced to try and correct some of these problems, and between September 16 and October 2, 1944, the second XP-55 (42-78846), which had been modified to the same standards as that of the third aircraft, underwent official USAAF trials. The trials indicated that the XP-55 had satisfactory handling characteristics during level and climbing flight, but at low speeds and during landings there was a tendency on the part of the pilot to overcontrol on the elevators because of a lack of any useful "feel". Stall warning was still insufficient, and stall recovery still involved an excessive loss of altitude. Engine cooling was also a problem.
The performance of the XP-55 was not very impressive and was in fact inferior to that of the more conventional fighters already in service. In addition, by 1944, jet-powered fighter aircraft were clearly the wave of the future. Consequently, no production was undertaken, and further development was abandoned.
The name Ascender had originated as a joke on the part of a Curtiss engineer. The name stuck, and eventually became official.
The third prototype (42-78847) survived the testing program, but was destroyed in a crash during an airshow at Wright Field, Ohio on May 27, 1945, killing the pilot.
The sole surviving XP-55 (42-78846) was flow to Warner Robins Field in Georgia in May of 1945. It was later taken to Freeman Field to await transfer to the National Air Museum at the Smithsonian Institution in Washington. For a long time, its fuselage was on display at the Paul Garber facility in Suitland, Maryland. In December of 2001, the aircraft was sent to the Kalamazoo Aviation History Museum for restoration. By 2007 the work was complete, and the plane is now on display at the Main Campus building of the Kalamazoo Airzoo.
Specs of the XP-55:
One 1275 hp Allison V-1710-95 (F23R) twelve-cylinder liquid-cooled Vee engine. Four 0.50-inch Colt-Browning M2 machine guns with 200 rpg. Maximum speed 390 mph at 19,300 feet, 377.5 mph at 16,900 feet. Normal range was 635 miles at 296 mph. Maximum range was 1440 miles. An altitude of 20,000 feet could be attained in 7.1 minutes. Service ceiling was 34,600 feet. Weights were 6354 pounds empty, 7330 pounds normal loaded, and 7939 pounds maximum. Dimensions were wingspan 44 feet 0 1/2 inches, length 29 feet 7 inches, height 10 feet 0 3/4 inches, wing area 235 square feet.