Curtiss XP-55

The first prototype (S/N 42-78845) in flight.

The 1930s were an interesting time for military aircraft, with the breakneck pace of technological development pushing the envelope farther with every year. Nations around the world were experimenting with all sorts of unconventional designs, some of which were far more successful than others.

On November 27th, 1939, the United States Army Air Force issued Circular Proposal R-40C. R-40C was for a new fighter that would be superior to all existing types while having a low cost and easy maintenance. Specifically mentioned in R-40C was the willingness to see unconventional configurations. More than 50 designs were submitted, though only a handful made it to the end. The winners of the competition all had created advanced, and highly unorthodox, pusher fighters.

The 'pusher' configuration for combat aircraft had been around since 1915 and offered several advantages over what is now seen as the conventional 'tractor' configuration, the most notable being improved visibility for the pilot and the ability to cluster weapons in the nose. Even so, this wasn't really explored in the United States until the 1930s with Bell aviation leading the way with aircraft like the deeply flawed Bell Aircuda, and other projects like their Model 16, the XP-52, and the related XP-59 (no real relation to the jet, but that is a story for another time).

The three companies that built prototypes were Northrop with the XP-56, Curtiss with the XP-55, and Vultee with their XP-54. Preliminary contracts were issued on June 22nd, 1940, and all called for single-seat aircraft, powered by the mighty liquid-cooled Pratt & Whitney X-1800-A3G, which would produce 2,200 horsepower from its 24 cylinders. But the engine's potential was never realized, as it was canceled in October 1940, forcing the use of other power plants for the new fighters.

The first prototype on the ground.

Unlike the Vultee or Northrop designs, the XP-55 had a canard configuration, with no actual tail. The elevator surfaces were located on the nose of the aircraft, and the vertical rudder surfaces were located near the tips of the wings, which were sharply swept back. The ventral and dorsal fins behind the cockpit contained the air intakes for the radiator (a requirement for the liquid-cooled inline engine) and the supercharger.

Curtiss approached the issue of bailing out by patenting a system to eject the three-bladed propeller entirely. Like the other competition aircraft, the XP-55 used a fully retractable tricycle style of landing gear, a first for any Curtiss design.

A page of the propellor ejector patent.

Power was ultimately provided by the Allison V-1710-95 inline engine, the same engine that drove Curtiss's P-40, among many other aircraft. This engine, also known as the V-1710-F, could produce about 1,150-1,425 horsepower at 3000 RPM. It was also the only V-12 liquid cooled engine used by the United States during the Second World War. The Allison engine was lighter, but less powerful than the planned X-1800 (1,400 lb v. 4,200 lb dry weight, and 1,425 hp v. 2,200 hp maximum power). A supercharger was added to provide better altitude performance.

A cluster of four Browning M2 .50 caliber machine guns was mounted in the nose, with a total of 800 rounds available (200 rounds per gun).

One of the advantages of the pusher configuration was the ability to mount all of the guns in the nose. On conventional aircraft, wing mounted guns were angled slightly in, so that their paths of fire would converge at a point out in front of the aircraft. But with all of the guns in the nose, that was no longer needed.

A British Hawker Hurricane, showing the principle of gun convergence. In the XP-55 and other pusher aircraft, all of the guns can be concentrated in the nose, making this process mostly unneeded.

The in-company name for what would become the XP-55 was the CW-24, and was ambitious even among the aircraft designed for the competition. Pusher aircraft were unusual on their own, but the heavy sweep of the wings, the position of the control surfaces, and the design of the undercarriage were all firsts for the company. The projected top speed was more than 500 miles per hour, driven by the 2,200 horsepower Pratt & Whitney X-1800-A3G engine.

On June 22nd, 1940, Curtiss received a contract for a powered wind tunnel model, engineering data, and an optional prototype. A 1/4 scale model with two different wing designs was ready by November 2nd. The first wing design was a conventional airfoil, while the other was a new laminar design that would reduce drag. Both of them had the severe sweep that was retained to the final stage.

The full-size wind tunnel model at Langley. Note the man standing by the supports.

During testing from November 1940 to January 1941, the vertical rudders were added to the wingtips to control yaw. The USAAF was unimpressed with the model's performance but gave the go-ahead to continue work with the laminar wing. It was during this time that the Pratt & Whitney X-1800-A3G was canceled, forcing a change in the planned engine to the venerable Allision V-1710-95.

One of the small-scale wind tunnel models that would become the XP-55.

Curtiss went even farther to prove the design's viability, constructing a full-size, manned version with its own money. This testbed, designated the Model-24B (powered by a 275 horsepower Menasco C-68 engine), flew for the first time in November 1941 at Muroc Lake in California. This aircraft was built of welded steel tubing, covered in fabric, with a fixed undercarriage and a wooden wing. 169 flights were made until May 1942, which indicated that the design might have potential.

The Model-24B manned testbed.

The Model-24B in flight.

On July 10th 1942, the Army ordered three prototypes from Curtiss. It took around a year for the Curtiss factory in St. Louis to build the first prototype (S/N 42-78845) which flew on July 13th, 1943 with Curtiss's test pilot Harvey Gray in the pilot's seat.

Assembly of the first prototype.

An engine run test of the first prototype.

The first prototype on a taxi test on July 11th, 1943. With Harvey Gray in the pilot's seat.

Modifications had to be made though, as in the first test flight it was found that the takeoff distance was far too long. The elevators on the nose were enlarged, and the flaps and aileron trim were connected to operate together. Beyond that, the flight testing was proceeding well until November 15th when it pitched forward in an outside loop until it stopped inverted during a stall test. The engine quit due to fuel starvation, and the aircraft plummeted 16,000 feet before Gray was able to bail out. The aircraft was a total loss.

The wreck of the first prototype, still inverted. The burned area around the wreck can be clearly seen.

The second prototype (S/N 42-78846) was almost identical to the first except for the changes to the elevators, and the tab systems on the ailerons and elevators. It first flew on January 9th, 1944, with a very restricted flight profile until testing with the third prototype gave solutions to the problems to prevent a similar loss.

The cockpit of the first prototype.

Prototype three (S/N 42-78847) was armed with the planned four .50 caliber machine guns in the nose and was further modified based on what had been learned from the loss of the first XP-55. The travel limits of the nose elevators were extended, and larger and longer (four feet) wingtip extensions improved the stall characteristics. This aircraft flew on April 25th, 1944, and some issues were found immediately. With the larger range of travel on the elevators, it was possible to adopt too steep an angle of attack on takeoff, which would cause the elevators to stall. More modifications were made, and it wasn't long before stall tests were passed, though there were still problems. There was effectively no warning before stalling, and while recoverable, they required far too much time and altitude to escape from.

The second prototype on the ground.

A stall warning device was added, and the second prototype was modified to the same level as the third before official trials began on September 16th. While a decent aircraft while level and climbing, at low speeds and during landings there was a tendency for pilots to overcontrol and stall the elevators. Even with the stall warning system, they were dangerous, and still required a long time and lots of altitude to recover from. Cooling of the engine was also lacking, and it was prone to overheat. By October 2nd, the XP-55's shortcomings were clear, as was the future of aircraft design in the form of jets. So further work on the XP-55 was abandoned.

The nickname for the XP-55, 'Ascender', apparently started as a joke from one of the design team that ended up sticking.

Work being done on the third prototype.

Both the second and third prototypes survived the testing, but only one would make it past 1945.

Prototype three was part of the show at an open house event at Wright Field on May 27th, 1945 that was to get people to buy war bonds. The aircraft was flown by Captain William C. Glasgow of Niagara Falls, New York. Glasgow was a 28-year-old combat veteran, having been shot down over Germany, taken prisoner and escaped. He had been awarded the Silver Star, the Distinguished Flying Cross, the Purple Heart, and an Air Medal with six oak leaf clusters.

The third prototype on the ground, showing the landing gear, canairds, and gunports.

At around 1600 hours, Glasgow in the XP-55 was leading a flyby of fighter aircraft. One of them was a P-38 Lighting being flown by Major Richard 'Dick' Bong, with a Captain John Ducas in the back. During the flyby, Glasgow made one roll but was unable to make the second. Ducas saw the XP-55 crash near airport road and burst into flames.

During the crash, the aircraft sideswiped a car on Airport Road, covering it with burning aviation fuel. A local man named Wesley Roehm and a family friend, Kathleen Eyre were both killed. Roehm's wife Susan and their two children suffered critical burns, and their fate is unknown. Glasgow did not survive the crash.

A portrait of Captain William Glasgow.

That same month, the second prototype was flown to Warner Robins Field in Georgia. Later it was moved to Freeman Field before a planned transfer to the National Air and Space Museum.

Specifications:

Allision V-1710-95

12-Cylinder Vee air cooled engine

1,275 horsepower

Dimensions:

Wingspan: 44' .5''

Length: 29' 7''

Height: 10' 3/4''

Wing Area: 235 ft^2

Weight:

Empty: 6354 lb

Gross: 7330 lb

Maximum: 7939 lb

gallons of fuel

Performance:

Top Speed: 390 mph @ 19,300'

377.5 mph @ 16,400'

At Sea Level: mph

Cruising Speed: 269 mph

Service Ceiling: 34,600 Feet

Climb Rate: 20,000' in 7.1 minutes

Normal Range: 635 Miles @ 296 mph

Maximum Range: 1440 Miles

There is some footage of the XP-55 in flight. The second and third prototypes are seen in this footage.

What is Left?

Astonishingly, the second prototype has survived to the present day. After an extended period on display at the Paul Gerber facility in Suitland, Maryland, the last XP-55 was sent to the Kalamazoo Aviation History Museum for restoration in December 2001. Over the next six years, the Ascender was restored and now resides at the Air Zoo in Kalamazoo, Michigan.