THE 456th FIGHTER INTERCEPTOR SQUADRON
THE PROTECTORS OF S. A. C.
The Fisher P-75 "Eagle"
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The Fisher P-75 Eagle
The Fisher P-75 Eagle was intended to fill the Army Air Forces' (AAF) 1942 need for an interceptor. Its unique design featured two coaxial contrarotating propellers connected by dual drive shafts running under the cockpit to a 24-cylinder liquid cooled engine located amidships. The original concept called for use of proven airframe components such as P-40 wing panels, A-24 tail, and F4U landing gear--to reduce the design and testing period.
The first of two XP-75s using component parts made its initial flight on November 17, 1943. Flight tests revealed unsatisfactory performance. This, combined with a mission change from interceptor to long-range escort, caused major changes in the original design. Ultimately, the idea of using proven airframe components had to be abandoned. The AAF ordered six XP-75s of the revised configuration along with 2,500 P-75As. The improved version was still unsatisfactory and after three Eagles had crashed, the entire program was cancelled on November 8, 1944. Only eight XP-75s and six P-75As were built.
Long-range Escort; 2,494 canc.
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Span: 49 ft. 4 in.
Length: 40 ft. 5 in.
Height: 15 ft. 6 in.
Weight: 19,420 lbs. loaded
Armament: Ten .50 cal. machine guns and two 500 lb. bombs
Engine: Allison V-3420 of 2,885 hp.
Maximum speed: 404 mph.
Cruising speed: 250 mph.
Range: 2,600 miles
Service Ceiling: 36,400 ft.
The P-75 Eagle
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The Fisher P-75 Eagle
In February of 1942, the USAAF issued a Request For Proposals (RFP) which called for aircraft companies to submit designs for a fighter/interceptor having an exceptional performance. Maximum speed was to be 440 mph at 2000 feet, operational ceiling was to be 38,000 feet, and range was to be 2500 miles. A special requirement was added for the initial climb rate, which was to be no less than 5600 feet per minute. High speed, high climb rate, high ceiling, and long range, all in one package--a tall order for any aircraft company.
In April of 1942, the famous designer Donovan Berlin (who had been responsible for such successful designs as the P-36 and the P-40) left the Curtiss company after many years of service to take over the directorship of the Aircraft Development Division of the Fisher Body Division of the General Motors Corporation. As one of his first assignments, he took it upon himself to work on a design in response to the USAAF RFP. In September of 1942, Fisher submitted their proposal to the USAAF. The Fisher proposal used the most powerful liquid-cooled engine then available, the twenty-four cylinder Allison V-3420. This engine was basically a pair of coupled V-1710 engines, mounted side-by-side in a W-type configuration. Significant savings in cost and time were to be gained by employing major assemblies from existing aircraft already in production in the manufacture of the new interceptor.
On October 10, 1942, a contract for two prototypes was awarded to Fisher under the designation XP-75. In assigning the XP-75 designation to the Fisher design, the designations XP-73 and XP-74 were skipped, for reasons which are not altogether clear even today. The aviation historian James Fahey claims that the P-73 and P-74 designations were deliberately omitted as a result of political pressure applied to the Army by Fisher. According to the story, Fisher wanted the Army to assign to its new escort fighter a "nice symbolic number", a number that would sound nice in advertising copy and would make for memorable slogans--something like "The French 75 in World War 1, the Fisher P-75 in World War 2" was envisaged. The Army agreed, and skipped the designations P-73 and P-74 and gave Fisher the P-75 designation.
Serial numbers 43-46950 and 43-46951 were assigned to the two XP-75 prototypes. The Allison engine was to be located behind the cockpit a la P-39 Airacobra, and was to drive a set of contrarotating propellers via an extension shaft and a reduction gearbox. The engine was cooled by means of a large duct in the ventral fuselage. Initially, it was planned that the outer wing panels of the P-51 Mustang would be used in an inverted-gull configuration, and that the tail assembly of the Douglas A-24 (Army version of the SBD Dauntless) and the undercarriage of the Vought F4U Corsair would be used. However, at an early stage it was decided to drop the inverted gull-wing configuration and go with a straight wing design utilizing outer wing panels from a P-40.
By the summer of 1943, the USAAF had a more urgent need for long-range escort fighters than it did for fast-climbing interceptors. On July 6, 1943, the USAAF ordered six more prototypes that would be adapted to fulfill the long-range escort role. They were assigned the designation XP-75A, and the serial numbers were 44-32161/32166. They were to be powered by an Allison V-3420-23 engine, and were to be armed with six 0.50-inch machine guns in the wings and four 0.50-inch guns in the fuselage nose. At the same time, the USAAF decided to order no less than 2500 production P-75As, although they did stipulate in the contract that if the production aircraft did not meet specifications the order might be cancelled. Maximum speed was to be 434 mph at 20,000 feet and 389 mph at sea level. These production P-75As were to be built at the Fisher plant in Cleveland, Ohio.
The first XP-75 flew on November 17, 1943. It was powered by an Allison V-3420-19 engine rated at 2600 hp for takeoff and driving a pair of contrarotating propellers. As planned, the wing was of a straight center section with P-40 outer panels with modified tips. The tail assembly was from an A-24, and the main undercarriage members were taken from an F4U Corsair.
All six XP-75A long-range escort versions were in the test program by the spring of 1944. Some problems were encountered with instability, since errors had been made in the initial estimate of the aircraft's center of mass. The coupled Allison engine failed to give its full rated power. The engine cooling was inadequate, aileron forces were excessively high, and the spinning characteristics were poor.
The P-75A production aircraft featured a modified tail assembly and had a bubble-type canopy replacing the framed and braced cockpit hood of the earlier versions. It featured the V-3420-23 engine of the XP-75A. The first P-75A flew on September 15, 1944. By that time, most of the bugs had been ironed out of the design. However, at that stage in the war, the Republic P-47D Thunderbolt and the North American P-51D Mustang were more than adequately fulfilling the long-range escort role, and the USAAF decided that there was no longer any need for a new escort fighter. Consequently, the USAAF decided to terminate the P-75 development program, and the production contract for the P-75A was cancelled on October 27, 1944 after only six examples had been built. The serial numbers of the six P-75As built were 44-44549/44553.
By the time of contract termination, the first and second P-75A had been delivered to Elgin Field, Florida for tactical suitability trials, the third machine was in the shop being fitted with an experimental intercooler, and the fourth and fifth machines were almost complete. Although the USAAF no longer had any need for the P-75, it was decided to go ahead and finish these machines and use them for development work. The sixth machine was to be placed in storage and scavenged for spare parts to keep the rest flying.
The five production P-75As never completed official performance trials, but enough testing was performed to confirm the fact that the maximum speed was at least 30 mph below that guaranteed by the manufacturer. The third machine received an experimental intercooler installation which permitted substantial increases in engine power.
The following specification for the long-range XP-75 are those quoted by the manufacturer; One 2885 hp Allison V-3420-23 twenty-four cylinder liquid-cooled engine. 433 mph at 20,000 feet, initial climb rate 4200 feet per minute st 10,000 feet, 3900 feet per minute at 20,000 feet. Service ceiling was 36,400 feet and absolute ceiling was 29,500 feet. Range with maximum external fuel was 3500 miles. Weights were 11,495 pounds empty, 13,807 pounds normal loaded, and 18,210 pounds maximum. Dimensions were wingspan 49 feet 4 inches, length 40 feet 5 inches, height 15 feet 6 inches, and wing area 347 square feet. Armament consisted of six 0.50-inch machine guns in the wings (235 rpg) located outboard of the propeller arc and four 0.50-in machine guns (300 rpg) in the fuselage nose synchronized to fire through the propeller arc. In addition, a pair of 500-pound bombs could be carried.
The last production P-75A (serial number 44-44553) is on display in the Annex at the Wright-Patterson Air Force Base Museum in Dayton, Ohio. I saw it there in June of 1992. It was in fairly good shape, but needs some restoration work. This was apparently the aircraft that had been scavenged to keep the other P-75s flying, so more than a few things may be missing.
War Planes of the Second World War, Fighters, Volume Four, William Green, Doubleday, 1964.
American Combat Planes, Third Enlarged Edition, Ray Wagner, Doubleday, 1982.
Fighters of the United States Air Force, Robert F. Dorr and David Donald, Temple Press Aerospace, 1980.
The American Fighter, Enzo Angelucci and Peter Bowers, Orion, 1987.
By Joe Baugher
The P-75 Eagle
|Manufacturer||General Motors Corporation|
|First flight||17 November 1943|
|Status||Cancelled 6 October 1944|
The General Motors P-75 Eagle was a fighter aircraft for which the Fisher Body Division of General Motors Corporation in September 1942 submitted a proposal to meet a United States Army Air Forces requirement for a fighter possessing an extremely high rate of climb. The proposal was for an aircraft that used the most powerful liquid-cooled engine then available, the Allison V-3420 (essentially a pair of 12 cylinder Allison V-1710 engines mated to a common crankcase), and components from existing aircraft. Only eight XP-75s and six P-75As were built before the program was cancelled.
Design & Development
In October 1942, the contract for two prototypes, designated XP-75, was signed with the Fisher Body Division of GM. The design concept was to use the outer wing panels from the P-51 Mustang, the tail assembly from the Douglas A-24 (SBD), and the undercarriage from the F4U Corsair in a general layout much as in the P-39 Airacobra with the engine located amidships with the propeller driven through an extension shaft. At an early design stage, however, P-40 Warhawk outer wing panels were substituted for the P-51 panels.
In mid-1943, the need for long-range escort fighters became more urgent than fast climbing interceptors so a decision was made to order six more XP-75 airplanes modified for the long-range role. At this time an order for 2,500 production aircraft was also let, but with the stipulation that if the first P-75A was not satisfactory the complete order might be canceled.
At the time, General Motors was busy in several projects towards the war effort, including the mass production of several different aircraft types, including the Grumman TBF Avenger. Some sources claim that the P-75 was the result of a scheme to get General Motors out of being forced to build Boeing B-29 Superfortresses; the P-75 project being a "high-priority" project to help GM avoid the added strain of Superfortress production.
Powered by a V-3420-19 twenty-four cylinder engine rated at 2,600 hp driving co-axial contra-rotating propellers, the XP-75 flew for the first time on 17 November 1943. The second XP-75 flew shortly thereafter, with all six long-range XP-75s entering the test program by the spring 1944. The test program brought up a number of deficiencies, including miscalculation of the fighter’s center of mass, failure of the engine to produce its expected power, inadequate engine cooling, high aileron forces at high speed, and poor spin characteristics. Redesigns were introduced into the long-range XP-75s including a modified tail assembly, new cockpit canopy, and a V-3420-23 engine that corrected most of the problems by the time the first P-75A Eagles entered flight test in September 1944.
By this time, the Army Air Force decided to limit the number of combat aircraft types in production and not enter into large-scale production of new types that might not be available before the war ended. As the P-47N Thunderbolt and P-51D Mustang demonstrated excellent long-range capabilities, the production run of the P-75A Eagle was substantially terminated on 6 October 1944. It was decided to use the five completed production aircraft for experimental work and development of the V-3420 engine. As a result of those events, the P-75A did not complete formal performance trials due to termination of the production contract.
Length: 40 ft 5 in (12.32 m)
Wingspan: 49 ft 4 in (15.04 m)
Height: 15 ft 6 in (4.72 m)
Wing area: 347 ft² (32.24 m²)
Empty weight: 11,495 lb (5,214 kg)
Loaded weight: 13,807 lb (6,263 kg)
Max takeoff weight: 18,210 lb (8,260 kg)
Powerplant: 1× Allison V-3420-23 liquid-cooled 24-cylinder double-Vee, 2,885 hp (2,150 kW)
Maximum speed: 433 mph (697 km/h) at 20,000 (6,100 m)
Range: 2,050 mi (3,300 km)
Service ceiling: 36,400 ft (11,100 m)
Rate of climb: 4,200 ft/min (21.3 m/s)
Wing loading: 39.8 lb/ft² (194.3 kg/m²)
Power/mass: 0.21 hp/lb (0.34 kW/kg)
6x 0.5 in (12.7 mm) wing mounted machine guns
4x 0.5 in (12.7 mm) fuselage mounted machine guns
2x 500 lb (227 kg) bombs
Pictures USAF Museum Dayton, OH
Green 1961, p. 87.
Green & Swanborough 1978, p. 7.
Green, William. War Planes of the Second World War - Fighters (Vol 4). London: Macdonald & Co. (Publishers), 1961 (Sixth impression December 1969). ISBN 0-356-01448-7.
Green, William and Swanborough, Gordon. WW2 Aircraft Fact Files: US Army Air Force Fighters, Part 2. London: Macdonald and Jane's Publishers Ltd., 1978. ISBN 0-354-01072-7.
Winchester, Jim. The World's Worst Aircraft: From Pioneering Failures to Multimillion Dollar Disasters. London: Amber Books Ltd., 2005. ISBN 1-904687-34-2.
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