The term "specification creep" may have been invented to describe the Vultee XP-54 program.
Request for Data R40-C
In late 1939 the United States Army Air Corps issued Request for Data R40-C to several manufacturers, including Bell, Consolidated, Curtiss-Wright, Grumman, Lockheed, Republic, Vought-Sikorsky and Vultee. Hughes, Boeing, McDonnell and Northrop were added to the list.
R40-C set out ambitious targets for aircraft performance and production schedules.
There were two categories:
- Improved versions of existing designs that would be capable of 425mph @ ~15,000ft and would be ready for production in fiscal year 1941 (spans late 1940/early 1941)
- Completely new designs that would be capable of 525mph @ ~15,000ft, to be ready for production in fiscal year 1942.
The performance requirements for the second category were set out in Air Corps Type Specification XC-622, issued in November 1939.
These specifications set out performance parameters far in excess of existing types and types then under development by any aircraft manufacturing nation.
The general requirement was for a single-seat, single-engine pursuit type with at least four guns. The gun choices were 0.30" and 0.50" machine guns and 20mm and 37mm cannon.
Vultee Model 70
Vultee submitted three variants of the model 70 for the competition, all using the two-speed single-stage supercharged Pratt & Whitney X-1800 (XH-2600) of 1,800hp+. The main difference between the variants was the propeller configurations.
The basic configuration was a twin boom pusher aircraft, single seat with a wing span of 40ft and length 37ft 6in.
The radiator was mounted in the nose with the proposed armament of one 0.30" mg, two 0.50" mg and two 20mm cannon.
Vultee's three Model 70 variants were ranked 1st, 2nd and 6th in the competition, the winner being fitted with 3 blade contra-props.
Pratt & Whitney X-1800
In 1937 Pratt & Whitney's George Mead visited the UK and was able to have a look around the UK aero engine manufacturers. He was impressed by the sleeve-valve concept and particularly by the Napier Sabre, then under development.
Upon his return, Mead instigated a H-block sleeve valve engine, conceptually based on the Sabre and sharing its capacity of 2,240 cubic inches.
This was to be the X-1800, which stood for eXperimental, 1,800hp, the expected power output.
The X-1800 was quite slim and when mounted with cylinders vertical it was though ideal for pursuit type aircraft, its width allowing for a slim fuselage, With cylinders horizontal it could be buried inside a bomber's wing.
Mead became ill and ended up running the project from his home. The engine would be enlarged to 2,600 cubic inches, with further versions of 3,130 cubic inches and 3,730 cubic inches.
Other Pratt & Whitney engineers, such as Luke Hobbs, weren't very keen on Pratt & Whitney working on liquid cooled engines. In 1940 Hobbs was able to convince Hap Arnold, head of the USAAC, to drop the X-1800 from production plans so that Pratt & Whitney could concentrate on the development of the R-2800 and R-4360.
Vultee was awarded a contract for engineering data and a wind tunnel model in June 1940. This contract also included options for a full sized mock-up and one prototype aircraft.
The options were exercised in August 1940, the prototype to be known as the XP-54.
The performance estimate at this stage was 510mph @ 20,000ft, 425mph @ 5,000ft and time to climb to 20,000ft of 6.3 minutes. The armament was to be 6 x 0.50" mgs.
In September 1940 the USAAC requested that the XP-54 incorporate a pressure cabin. This change was due to combat reports for Europe and anticipating combat altitudes of 35,000ft.
Acknowledging the impacts on delivery of the project, the USAAC suggested that the first prototype could have provision to be modified for a pressurised cockpit at a later date. They also requested that Vultee provide a quotation for the development of a pressure cabin as a separate project.
The Army informed Vultee of its intention to purchase a second prototype, to be designated XP-54A, this was to contain the pressure cabin but only provision for tactical equipment. It was expected that this aircraft would be ready for flight testing before the first.
When the Pratt & Whitney X-1800 program was discontinued it left the XP-54 without an engine (also the case for the XP-52 and XP-56). The Army Materiel Command informed Vultee of the cancellation in October 1940 and recommended the Lycoming XH-2470 as a suitable replacement.
The XH-2470 was a 24 cylinder engine with an H layout. It was developed from Lycoming's opposed 12 cylinder engine, the O-1230, which had been based on the Army's "hyper" cylinder. Lycoming had realised the O-1230 was too small and lacked the power required for modern military aircraft, and some simply doubled the engine to increase capacity and power.
It was similar in weight to the X-1800, at 2,430lb/1,102kg, but its rating of 2,300hp was several hundred more horsepower. The XH-2470 was to use contra-rotating propellers in the XP-54.
The change from the X-1800 to the XH-2470 was relatively straightforward, performance was expected to improve due to the extra power and Vultee's revised cost estimate for the prototype was lower than it was for the X-1800.
The revised performance estimates were 520mph @ 17,500ft, 450mph @ 5,000ft, time to climb to 20,000ft of 6.0 minutes.
The change order for the revised specification was issued in December 1940 and approved in January 1941. Serial number 41-1210 was assigned to the first prototype.
The Xh-2470 with contra-rotating propellers was slow in development, so the decision was made to replace this with the XH-2470-1 with single rotation 4 blade propeller.
The XH-2470-1 had reduced power, the maximum being 2,200hp @ 12,500ft (XH-2470 had the same power at 14,000ft).
The performance for the XP-54 was revised to 485mph @ 17,500ft and 430mph @ 5,000ft.
The change to the XH-2470 had reduced the weight of the aircraft to 8,544lb/3,875kg, this increasing to 9,045lb/4,103kg with the -1 engine.
In order to speed up delivery the XH-2470-1 was derated, resulting in 13% less power between 5,000ft and 12,000ft and 7% less power above 15,000ft.
At this time Vultee was also asked to consider alternate engine installations.
We're the Army, We Love Turbochargers
In late May 1941 Vultee was informed that Brigadier General Echols had directed that the XP-54 be fitted with turbochargers. Vultee prepared an estimate for the additional cost and delay the change would make to the program.
It was also at this time that the Army requested that Vultee provide a quote for the fitment of a Wright R-2160 Tornado equipped with either one or two General Electric turbochargers, or two Wright turbochargers, and a pressure cabin. At this point in time the R-2160 was a long way from a flight cleared engine, let alone one with contra-props.
The addition of two Wright Type TSBB turbochargers was expected to delay the XP-54 schedule by 6 weeks. Vultee noted that the addition of the turbochargers made it impossible to fit the required SCR-274 radio, but that the British TR-1143 (SCR-522) radio would fit. With the turbos fitted the estimated weight of the XP-54 was up to 11,500lb/5216kg.
Some 600lb/272kg of extra structure was required to allow the provision of the fitment of the R-2160 Tornado with two turbochargers. Discussion between Vultee and the Army revolved around whether the first prototype should be revised to fit the Tornado as well as the second prototype, as Vultee felt that it was at this stage developing two aircraft. And consideration was made as to whether complete redesign would be required for the production version.
All the changes and requests led to a series of design conferences being held in September 1941. At the first conference Vultee presented two specifications - one for the XP-54 fitted with a turbocharged XH-2470-1 with single rotation propeller, with provision for dual rotation propellers at a later date, and the second identical except for the fitment of a turbocharged R-2160. Design weight was now 18,000lb/8,165kg and armament consisted of 4 x 0.50" mg and 2 or 4 x 20mm cannon, or 6 x 20mm cannon or 4 x 0.50" mg and 2 x 37mm cannon.
The XH-2470-1 fitted to the XP-54 was to swing a 12ft 2in/3.7m four blade prop, the reduction gear ratio of 2.6:1 provided a prop speed of 1,269rpm from it engine speed of 3,300rpm.
The proposed R-2160 with single rotation propeller had a reduction ratio of 4:1, meaning a prop speed of 1,037.5rpm from the engine speed of 4,150rpm.
The slower rotational speed and increased power for the Tornado installation would, therefore, require a larger diameter propeller. The Army suggested that Vultee use the larger propeller and that the armament should be 4 x 0.50" mgs plus 2 x 37mm cannon.
Vultee told the Army that it would be impossible to use the larger propeller as it was too big to fit between the XP-54's booms. Their counter-proposal was to use the H-2470-4 instead, this being fitted with a two speed reduction gear, which would enable improved take-off performance and maintain the altitude performance. In the tradition of the experimental H-2470 series of engine, the -4 program was running late.
With no guarantee that the H-2470-4 would be available, the decision was to continue with the -1, while the Tornado version would be fitted with contra-props.
During the conferences in late 1941 the design was assessed and the following parameters agreed for the two prototypes:
- To be powered by the H-2470-1 with single rotation propeller and turbochargers
- Provision for the H-2470-4 with 2 speed reduction drive (expected to be available for the second prototype) and turbochargers
- Allowance for the fitment of the R-2160 with two speed reduction drive and contra-rotating propellers
- Cooling provisions for a 2,500hp engine
- Critical altitude of approximately 30,000ft and service ceiling of approximately 40,000ft
- Fuel to allow 1 hour operation at maximum power at critical altitude
- Pressure cabin to be fitted to both aircraft, a 10,000ft pressure altitude to be maintained to 35,000ft, and the pressure differential to be maintained above that altitude
- Armament consisting of 4 x 0.50" mgs with 500 rounds per gun and 2 x 37mm cannon with 60 rounds per gun
Design work on the XP-54 (Vultee Model 70-A) was stopped with about 50% completed. Vultee noted that all the changes specified by the Army during the program required a redesign of the aircraft.
Authorisation for the redesign came in November 1941, Due to the extent of changes the company designation was change to Vultee Model X84E, or simply Model 84.
Another change was that the second XP-54 was to be fitted with the General Electric XCM turbocharger in place of the Wright Type TSBB turbochargers.
Guaranteed performance for the prototypes was as follows:
XP-54 #1 XP-54 #2 Speed @ Critical Altitude (30,000ft) 476.5mph 476.5mph Speed @ 27,000ft 479.0mph 478.5mph Speed @ 20,000ft 444.0mph 443.5mph Speed @ 3,000ft 384.0mph 384.0mph Combat Ceiling 32,100ft 30,900ft Service Ceiling 37,000ft 34,500ft Normal Design Gross Weight 17,133lb 17,373lb 7,771kg 7,880kg
Vultee XP-54 (Model 84) Descriiption
The Vultee XP-54 was a single seat pursuit type with inverted gull wings, twin booms to support the tail, single XH-2470-1 H-24 engine with turbochargers and single rotation pusher propellers.
The fuselage was a simple cylindrical shape, the twin booms were angled inwards towards the rear.
The wing profile was a NACA laminar flow profile, the inner wing section containing ducts for radiators, oil coolers, intercoolers and induction air.
The inner wings (between booms) used split flaps, the inner section of the outer wing used splotted flaps.
The main dimensions were:
Length 54ft 9in 16.7m Fuselage Length 37ft 2in 11.3m Wing Span 53ft 10in 16.4m Wing Area 455.5sq.ft 42.3m² Tail Span 187in 4.75m Tail Area 75sq.ft 7.0m² Normal Weight 18,735lb 8,498kg (fuel 223 USG/844l) Overload Weight 19,735lb 8,952kg (fuel 378 USG/1,431l)
Type Lycoming XH-2470-1 Power - Take-off 2,300hp @ 3,300rpm Power - Normal 2,000hp @ 3,100rpm Turbocharger XP-54 #1 2 x Wright Turbo TSBB XP-54 #2 1 x General Electric XCM Usable Fuel 363 USG 1,374l Maximum Fuel 378 USG 1,431l
The XP-54 is notable for its unusual entry/exit to the cockpit.
Due to the height of the fuselage from the ground, getting into the aircraft using a conventional canopy would have been difficult. This is compounded by the need for the cabin to be pressurised and for the effect of the pusher propeller on safe emergency exit of the aircraft.
Vultee, therefore, came up with an elevator seat concept. The main canopy was fixed, the pilot entering from below on an electrically powered arm which contained the seat and swung the pilot up into the cockpit. The arm doubled as emergency exit, as it swung down and released the pilot low enough to ensure that he would not hit the propeller.
Vultee : XP-54 : Swoose Goose by SDASM Archives, on Flickr
The gun installation was another unusual system. The 37mm cannon had a substantially different trajectory than the 0.50" mgs also fitted. To overcome this difficulty a new type computing gunsight had to be designed and fitted, while the 37mm cannons were fixed to the nose section which could be pivoted hydraulically up 3° and down 6°. Meanwhile, the 0.50" mgs were fitted to an elevating mechanism mounted to teh nose, so that they could be elevated to a different angle than the 37mm cannon.
All of this complexity, of course, added weight.
XP-54 Flight Characteristics
For its first flight on January 13, 1941, the XP-54 was fitted with Curtiss propeller. The take-off run was excessively long, and the propeller was not considered capable of taking advantage of the power of the engine.
For the second test flight the propeller was changed for a Hamilton Standard 4 blade propeller. Take-off distance was improved by approximately 10%, but it was still over 3,000ft/914m.
Ground handling was considered excellent, owing to the tricycle landing gear.
The overall handling characteristics were considered normal, stability was good about all axes, though some longitudinal instability was found during normal power climbs at 180mph and CoG aft of 28% MAC. Considering that in combat with ammunition expended the CoG could move as far aft as 34% MAC, ballast in the nose would most likely have been required.
Rudder and elevator control was light, and require little trimming fo speed and CoG changes. Aileron forces remained steady at increased speeds, and were relatively light.
Flutter was tested in dives to 460mph TAs in dives and was deemed satisfactory.
Climb tests showed the XP-54 had a sea level rate of climb of 1,620ft/min (8.2m/s), which was not very good for a pursuit type aircraft in 1943.
Staling speed at 18,000lb (8,165kg) was 110mph with the flaps down (10° inner flaps, 30° outer flaps) and 120mph with the flaps up. Lowering the flaps and gear induced a nose down trim change, which was easily controllable.
Approach speed for landing was 140mph IAS, touch down at approximately 115mph IAS. Control during landing was good, and ground run varied between 1,100ft and 1,500ft (335m and 457m).
Unsurprisingly, the XP-54's performance did not live up to the projections.
Speed measurements were taken with three different surface finishes:
- Factory Finish - Camouflage
- Superfinish - Camouflage
- Superfinish - waxed and polished
Altitude Power Factory Superfinish Superfinish Setting Finish Camouflage Waxed Sea Level Military 287 296 304 Sea Level Normal 273 281 289 10,000ft Military 318 327 337 10,000ft Normal 304 311 321 20,000ft Military 352 361 372 20,000ft Normal 335 344 356 28,000ft* Military 380 389 404 32,000ft* Normal 373 382 397 * Critical Altitude All speeds mph TASSpecifications (XP-54)
Data from Green and Swanborough 1978, p. 84.
- Crew: one, pilot
- Length: 54 ft 9 in (16.69 m)
- Wingspan: 53 ft 10 in (16.41 m)
- Height: 14 ft 6 in (4.42 m)
- Wing area: 456 ft² (42.4 m²)
- Empty weight: 15,262 lb (6,923 kg)
- Loaded weight: 18,233 lb (8,270 kg)
- Max. takeoff weight: 19,337 lb (8,771 kg)
- Powerplant: 1 × Lycoming XH-2470-1 liquid-cooled piston engine, 2,300 shp (1,715 kW each)
- Maximum speed: 381 mph at 28,500 ft (613 km/h at 8,700 m)
- Range: 500 mi (805 km)
- Service ceiling: 37,000 ft (11,300 m)
- Rate of climb: 2,300 ft/min (11,7 m/s)
- Wing loading: 40 lb/ft² (196 kg/m²)
- Power/mass: 0.13 hp/lb (0.20 kW/kg)
The XP-54 program was beset with delays and changing requirements. It increased in size from roughly that of a P-47 to be bigger and heavier than a P-38, but with less power. Its weight more than doubled over the life of the program, which had implications for its performance.
It went from a low/medium altitude moderately armed fighter to a heavily armed high altitude interceptor (bomber destroyer?).
It changed from one experimental engine (the X-1800) to another (the XH-2470) and was expected to adopt another (the R-2160) at a later stage. Of these only the XH-2470 made it to flight status, and none made it into volume production.
The Army's insistence on turbochargers, pressure cabin added a lot of weight and complexity. The Army's infatuation with the 37mm cannon caused even more design compromises and weight added.
Against the XP-55 and XP-56 it looks reasonably successful, aided by its far more conventional design. Against the performance requirements and compared to contemporary and later piston fighters, whose performance it was supposed to eclipse, the XP-54 was a total failure.
Gerald H. Balzer, American Secret Pusher Fighters of World War II
Graham White, Pratt & Whitney's Dependable Masterpiece