johnbr
2nd Lieutenant
from this pdf.
Fisher P-75A Eagle
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Does anyone have a PDF copy of the 51 page brochure on the Fisher p-75 Eagle that was once on the website Retro Mechanix?
Oh, here's the translation of the article
Monograph
by Alain Pelletier
The Fisher XP-75 Eagle
The idea seemed brilliant.
In times of war, belligerents often pull out all the stops, especially when they find themselves in a critical situation. In this regard, Germany's last-ditch programs are well known. Less well-known are some American programs, such as the Fisher Body P-75 "Eagle" fighter.
This program devoured nearly $10 million in the space of a few months. All this to give birth to... an iron!
by Alain Pelletier
The Fisher XP-75 Eagle
The idea seemed brilliant.
In times of war, belligerents often pull out all the stops, especially when they find themselves in a critical situation. In this regard, Germany's last-ditch programs are well known. Less well-known are some American programs, such as the Fisher Body P-75 "Eagle" fighter.
This program devoured nearly $10 million in the space of a few months. All this to give birth to... an iron!
From the early months of World War II, the Wright Field Technical Center in Ohio became the cradle of equipment for the U.S. Army Air Force. It was here that aircraft programs were developed and undeveloped during numerous meetings. It was here that manufacturers presented their projects. Above all, it was here that major decisions affecting the nation's aircraft industry were made.
On Thursday, September 10, 1942 (1), during one of these meetings, the managers of a little-known –– not to say unknown –– company presented a project (2) that was certainly attractive... at least on paper. This company was Fisher Body, a subsidiary of General Motors, based in Cleveland, and which until then had been responsible for producing sub-assemblies for the B-25 "Mitchell" assembled in its Kansas City factory by North American. But, if this company was not known, it was not a beginner. His name was Donovan Reese Berlin. He was responsible for the design of such famous aircraft as the Northrop A-17, the Curtiss P-36 and the Curtiss P-40 (3).
In the Pacific at that time, the American air force was far from having acquired mastery of the air and one of the main problems it faced was to intercept its attackers in time.
This is why, as early as February 1942, the USAAF had drawn up the specifications (4) for a fighter capable of climbing at more than 5,500 feet per minute, flying at more than 700 km/h at 6,000 m, and covering at least 4,000 km in a single flight. The preliminary project presented by Fisher Body could only interest the military, who were pressed for time and ardent supporters of standardization. In fact, it was a question of designing a fighter with an exceptional climb speed in a very short time. To do this, it was planned to use a certain number of sub-assemblies from aircraft already in production (wings, empennage, landing gear, etc.)
The very next day, this preliminary project was presented to General Oliver P. Echols, Deputy Chief of Staff for Materiel, Maintenance, and Delivery (MM&D). Given the favorable reception the said project received, on September 24, Fisher officially sent a report to the USAAF which took into account a certain number of remarks (5), and sent on October 7 a first estimate of the project. Three days later, without further ado, a pre-contract of $407,877.60 was signed with Fisher Body for the construction of two prototypes, designated XP-75 (6). This was more than General Motors had hoped for, but, to put events in context, it is important to know that Donovan Berlin enjoyed a considerable reputation in aeronautical circles at the time and that this reputation surely weighed heavily in the decision.
In October 1942, Fisher Body submitted the final specification document, numbered X-92, and the following month, the Engineering Department received instructions to supply the appropriate engines and propellers. The X-92 document was amended again on December 19, and the Authority of Purchase (AP) was prepared on this basis. The AP, for $428,278.48, covered the acquisition of two aircraft, a wind tunnel model, and the calculation file. Delivery of the first aircraft was to take place within six months of signing.
As originally conceived, the project included a Douglas A-24 tail (land-based version of the SBD Dauntless), a Chance Vought F4U Corsair landing gear, and North American P-51 Mustang outer wing sections mounted with negative dihedral (thus creating a gull-wing). This latter solution was not adopted, and a straight wing composed of Curtiss P-40 elements was ultimately chosen.
For the engine, the engineers had little choice: the most powerful liquid-cooled engine of the time was the Allison V-3420 (7). The result of an experimental program launched by the Air Corps in 1937, this powerplant was in fact a hybrid engine consisting of two Allison V-1710 engines mounted at 60° on the same crankcase, the two crankshafts driving a single propeller shaft. Installed, as on the Bell P-39 "Airacobra", close to the center of gravity of the aircraft, that is to say behind the pilot, this engine would make its debut on the XP-75 (8).
Change of Direction
During the winter of 1942 and spring of 1943, design work on the new aircraft progressed well. In November and December 1942, a model was installed in the 1.50 m diameter wind tunnel at Wright Field; the results obtained were excellent except for the maximum speed, which turned out to be significantly lower than the calculated speed. A full-scale layout model was also built. It was inspected twice, on March 8 and May 31, 1943. The first of these two inspections was unfortunately cut short because the Wright Field engineers judged that the model was not sufficiently detailed, not by a long shot. The installation of the engine, accessories and radio equipment was barely sketched. This prompted a second inspection on May 13. Although no major modifications were required, a large number of detail corrections had to be made. In addition, Air Force engineers asked Fisher Body to conduct the necessary studies to determine whether it was possible to use wing panels from the North American P-51 "Mustang" armed with four cannons.
But all this came at a high cost; on June 22, 1943, Fisher Body requested additional funding to continue the program, the modest sum of $934,501.52! This funding was granted on July 15, although by that date the final contract had still not been signed. A conference had taken place on July 6, 1943, in General Echols's office that had fundamentally changed the direction of the program.
Indeed, Wright Field had decided to change its tune. The Air Force urgently required a long-range escort fighter. It must be said that for several weeks, the bombers of the 8th Air Force had been taking a rather rough ride over Germany. Tuesday, August 17, 1943, was etched in the memory as one of the darkest days experienced by the USAAF. 36 of the 230 bombers sent to Schweinfurt, and 24 of the 146 bombers sent to Regensburg had not returned...
Faced with this alarming situation, the Wright Field engineers had agreed that, with certain modifications, the XP-75 could very well do the job. The requirements now covered six additional experimental aircraft transformed into long-range escort fighters and no fewer than 2,500 production aircraft called P-75A "Eagle". This request was confirmed by letter on July 10 and resulted, on October 1, 1943, in the signing of the final contract for an amount of $2,911,434.70, covering the supply by Fisher Body to the Air Force of eight prototypes (two XP-75s and six XP-75As), a static test cell, a layout model, spare parts and the calculation file. However, a clause stipulated that if the first production aircraft did not meet the specifications, the entire contract could be called into question. The schedule was as follows: first prototype ready to fly on September 30, 1943, first of six additional experimental aircraft delivered in December, and last on February 2, 1944. On November 5, the eight prototypes concerned received their serial numbers: 43-46950 and 46951, 44-32161 to 32166; the 2,500 P-75A-1-GC were numbered from 44-44549 to 44-47048. With this "juicy" contract, Fisher Body doubled its efforts.
In June 1943, a conference was held with NACA representatives (11) to decide what steps could be taken to improve the aircraft's poor roll rate. It was decided to provide P-40 wings to NACA to see how the ailerons could be improved. Further tests were conducted in the NACA wind tunnel during July, and a 1:6.5 scale model was built for more precise experiments in the Massachusetts Institute of Technology (MIT) wind tunnel. Furthermore, the tests conducted in September 1943 in the NACA vertical wind tunnel led to the installation of a ventral keel on production aircraft to improve the aircraft's spin behavior. Vibration and flutter tests were conducted in early November 1943.
The Ugly Fat Duckling!
The first flight took place on Wednesday, November 17, 1943, but history has not preserved the name of the test pilot. As new aircraft joined the flight test program, it quickly became apparent that the XP-75's design was flawed in many ways.
Chief among the difficulties was instability due to poor estimation of the center of gravity position, unexplained power losses, insufficient engine cooling, poor roll rate, poor spin behavior, fuel sloshing in the fuselage tank, inefficient brakes, etc. It was a lot for one airplane!
Therefore, throughout 1944, flight tests had only one objective: to eradicate all these defects, and many others of lesser importance. The causes of these problems were multiple, but we can mention a few for certain. For example, the use of an experimental engine (the V-3420 was in fact making its debut on the XP-75) by a manufacturer who was cruelly inexperienced and whose organization was geographically dispersed. The design office was located in Detroit, Michigan, and the production center in Cleveland, Ohio, nearly 300 km away. Added to this was the terrible pressure exerted by the USAAF to start production as quickly as possible, which left Fisher Body no time to plan the program properly. Despite these working conditions, most of the problems were corrected and the aircraft had reached a generally satisfactory level by the time the first production "Eagles" were ready.
However, these flew at some 40 km/h less than the speed guaranteed by the manufacturer. The maximum speed recorded with the third production aircraft was 672 km/h at 6580 m and at a weight of 6,160 kg, the engine delivering 2,210 hp at 3,000 rpm. The rate of climb in the same conditions was 2,900 feet/min; this was far from the 3,500 in the specifications! It was to correct this inadequacy that this same aircraft was equipped with an experimental exchanger which allowed the engine power to be increased. This modification would have been generalized if large-scale production had been launched.
Mark E. Bradley, Wright Field's aircraft project manager, was soon flying the XP-75. "It was heavy, unwieldy, and unstable. No pilot would want it, I was sure of it. It was hopeless," he recalled (12). He was quickly convinced that the solution to the long-range escort fighter problem was to install an extra fuel tank in the P-51. He managed to convince General Orval Cook, head of the Production Division, but he had more difficulty with General Echols. When he told him about the aircraft's lack of maneuverability, he was told, "We don't want it circling around the bombers. All we want it to do is stay with them and protect them!"
Bradley was appalled by this response. There was reason to be. Nevertheless, after much argument, he obtained Echols' agreement to convert a P-51. An additional 320-liter fuel tank was installed in the fuselage with the success we know, and the first P-51Bs thus modified were delivered to the 8th Air Force starting in December 1943.
$1,579,772.40
In February 1944, given the incessant number of modifications, Fisher Body found itself faced with a worrying drift in the program's costs. This is why, once incurred, the manufacturer requested an additional $1,579,772.40, which was granted on March 28, 1944. But the program was seriously beginning to... fail, especially since on March 4, for the first time, P-51 "Mustangs" had escorted the bombers over Berlin (13).
Further vibration and flutter tests were conducted in February 1944. And the first official performance evaluation tests were initiated in June 1944, but due to engine malfunction, the results were questionable. No further official performance evaluation was attempted until a production aircraft was available. In September and October 1944, production aircraft Nos. 1 and 2 were sent to Eglin Field, Florida, for tactical evaluation tests. Shortly thereafter, No. 1 was destroyed in an accident. In any case, these tests were never completed. The termination of the program was decided at a meeting at Wright Field on October 23, 1944, and officially confirmed on November 8.
For the balance of all accounts...
In fact, this abandonment was not quite a one. It had been specified that total production should not exceed 20 aircraft. A visit to Fisher Body in Cleveland allowed an update on the progress of the work. There were then five production aircraft in flying condition or about to be: aircraft Nos. 2 to 6, since No. 1 had been destroyed. After numerous meetings, it was decided to transfer these aircraft to the Engineering Division to continue the development of the Allison V-3420 engine. As a result, contract No. W535-AC-33962 was terminated and a new contract (No. W33-038-AC-6620) was negotiated with Fisher Body to complete the development or construction of aircraft Nos. 2, 3, 4 and 5, and to store No. 6 to serve as a source of spare parts.
In any case, work on the aircraft had to be stopped on January 31, 1945.
This decision resulted in the allocation of the necessary funds, namely $4.7 million (14). Aircraft No. 2 (No. 44-44550) was flown to Moffett Field, California, where stability and propeller tests were continued in the large wind tunnel. After these tests, the aircraft was simply scrapped. The installation of the experimental heat exchanger on No. 3 (44-44551) was completed and the aircraft was handed over to Allison on June 28, 1945, in accordance with contract No. W33-038-AC-11440; this $95,600 contract covered 100 hours of flight tests. It ended with the victory over Japan, but the aircraft was retained by Allison free of charge. Aircraft numbers 4 and 5 (numbers 44-44552 and 44553) were flown to Patterson Field, Ohio, and stored in case they were needed to further develop the V-3420. Other modifications to the aircraft included modifications to the landing gear to improve taxiing conditions, relocation of the radiators, installation of a cockpit ventilation system, replacement of the propeller blades, etc.
In total, the Fisher Body P-75 "Eagle" program had cost a whopping $9,373,629.37, the equivalent of 180 production P-51 "Mustangs" (15). A real waste for an aircraft that, according to many, should never have existed.
In the months following the program's termination, most of the aircraft built were simply scrapped. Only one of those shipped to Patterson Field (now Wright Patterson AFB), the last of the series, was given to the US Air Force Museum at the same location.
Footnotes
(1) The day before, September 9, General Arnold had submitted to the Chief of Staff of the USAAF his AWPD-42 plan specifying the number of aircraft needed to achieve air superiority. This plan provided, among other things, for the expansion of the 8th Air Force in Great Britain and contained a first draft of the Combined Bomber Offensive (CBO).
(2) This article is primarily based on the final report on the acquisition, inspection, testing, and acceptance of the General Motors / Fisher Body Divsion XP-75 aircraft, dated June 26, 1946 (AAF Technical Report No. 5505).
(3) Aviation Fana No. 317 & 318. Berlin had left Curtiss in April 1942.
(4) In the United States, the initial specifications are called Request For Proposal, or RFP.
(5) Report No,. A-89, revised September 18, 1942.
(6) Draft Contract No. W535-AC-33962 of October 10, 1942.
(7) In this designation, the letter V indicates the arrangement of the cylinders, here in V, and the number 3420 indicates the cylinder capacity expressed in cubic inches, or 56044 cm³ or 56 liters.
(8) Few aircraft were equipped with the V-3420: the XP-58, the XP-75, the XB-19 and the XB-39.
(11) National Advisory Committee for Aeronautics, now NASA.
(12) Excerpt from the memoirs of 72 Wright Field test pilots, published as "Test Flying," Westchester House Publishers, Omaha, Nebraska, 1993.
(13) In fact, the first escort over Berlin had been carried out the day before by Lockheed P-38 "Lightnings" of the 55th Fighter Group. On March 4, it was P-51s of the 354th and 4th FG that escorted the "Big Brothers" over the capital of the Reich.
(14) Exactly $4,733,312.28 authorized on February 27, 1945
The Eagle in Detail
The Fisher Body P-75A's wing was cantilevered. It consisted of three parts: a rectangular central section and two outer panels borrowed from the P-40 but with modified ends. The central section was located at the base of the fuselage and served as a support for the engine.
With a wingspan of 15.03m and a surface area of 32.24m², the wing had an aspect ratio of 7. It was set at an incidence of 1˚30' and the external parts had a dihedral angle of 6˚. The profile used was a NACA asymmetrical biconvex profile of the 2200 series (NACA 2215 at the root, and NACA 2209 at the wingtip (16). The ailerons had a total surface area of 1.95m² (larger than those of the P-40); the two flaps, with a total surface area of 4.41m², could be lowered by 42˚30'.
The fuselage was of semi-monococque construction (made up of frames held together by stiffeners, the whole being covered with sheets which contributed to the strength of the whole), made of Aclad 24ST aluminum alloy covered with a layer of aluminum to protect it against corrosion. It was bolted to the wing.
The cockpit was placed very high and very far forward for an aircraft of that era, equipped with a full-visibility canopy (known as a "teardrop" canopy) and an armored windshield.
The entire tailplane was borrowed from the Douglas A-24 dive bomber, but a number of modifications were introduced throughout the program. As a result, while its shape was clearly recognizable on the first prototype, this was no longer the case on later aircraft.
The landing gear, that of the Chance Vought F4U Corsair carrier-based fighter, retracted flat into the wing using a hydraulic system. Its wheelbase was 7.90m and its track width was 6.08m. It used 81x22 cm wheels and Goodyear hydraulic brakes.
The engine was mounted in the center of the fuselage, behind the cockpit (as on the Bell P-39 and P-63 fighters). It drove two 3.83m diameter, counter-rotating, hydraulically driven, three-bladed Aero-Products propellers. It was an Allison V-3420-23 with 24 cylinders arranged in a double V. Its compression ratio was 6.65. With a two-stage turbocharger, its performance was as follows:
The fuselage housed two tanks (1,215 liters) and the central section of the wing four (810 liters). If necessary, droppable auxiliary tanks of 284 or 416 liters could be added.
The aircraft's armament consisted of six 12.7 mm Colt-Browning machine guns in the wings, each firing 235 rounds, and four synchronized 300 rounds in the nose. In addition, two hardpoints under the center of the wing allowed the aircraft to carry two 227 kg bombs, two fuel tanks, or two M-10 smoke pods. The sighting device was an N-9 (the P-51D's); an N-6 machine gun was embedded in the leading edge of the starboard wing.
Radio navigation equipment included an MN-26 (or MN-26Y) radio compass for convoys), a BC-1206-A heading indicator, an SCR-522-A or SCR-274-N radio, and an SCR-695-A IFF (transponder).
Footnotes
(1) The Curtiss P-36 and P-40 used strictly the same profiles
On Thursday, September 10, 1942 (1), during one of these meetings, the managers of a little-known –– not to say unknown –– company presented a project (2) that was certainly attractive... at least on paper. This company was Fisher Body, a subsidiary of General Motors, based in Cleveland, and which until then had been responsible for producing sub-assemblies for the B-25 "Mitchell" assembled in its Kansas City factory by North American. But, if this company was not known, it was not a beginner. His name was Donovan Reese Berlin. He was responsible for the design of such famous aircraft as the Northrop A-17, the Curtiss P-36 and the Curtiss P-40 (3).
In the Pacific at that time, the American air force was far from having acquired mastery of the air and one of the main problems it faced was to intercept its attackers in time.
This is why, as early as February 1942, the USAAF had drawn up the specifications (4) for a fighter capable of climbing at more than 5,500 feet per minute, flying at more than 700 km/h at 6,000 m, and covering at least 4,000 km in a single flight. The preliminary project presented by Fisher Body could only interest the military, who were pressed for time and ardent supporters of standardization. In fact, it was a question of designing a fighter with an exceptional climb speed in a very short time. To do this, it was planned to use a certain number of sub-assemblies from aircraft already in production (wings, empennage, landing gear, etc.)
The very next day, this preliminary project was presented to General Oliver P. Echols, Deputy Chief of Staff for Materiel, Maintenance, and Delivery (MM&D). Given the favorable reception the said project received, on September 24, Fisher officially sent a report to the USAAF which took into account a certain number of remarks (5), and sent on October 7 a first estimate of the project. Three days later, without further ado, a pre-contract of $407,877.60 was signed with Fisher Body for the construction of two prototypes, designated XP-75 (6). This was more than General Motors had hoped for, but, to put events in context, it is important to know that Donovan Berlin enjoyed a considerable reputation in aeronautical circles at the time and that this reputation surely weighed heavily in the decision.
In October 1942, Fisher Body submitted the final specification document, numbered X-92, and the following month, the Engineering Department received instructions to supply the appropriate engines and propellers. The X-92 document was amended again on December 19, and the Authority of Purchase (AP) was prepared on this basis. The AP, for $428,278.48, covered the acquisition of two aircraft, a wind tunnel model, and the calculation file. Delivery of the first aircraft was to take place within six months of signing.
As originally conceived, the project included a Douglas A-24 tail (land-based version of the SBD Dauntless), a Chance Vought F4U Corsair landing gear, and North American P-51 Mustang outer wing sections mounted with negative dihedral (thus creating a gull-wing). This latter solution was not adopted, and a straight wing composed of Curtiss P-40 elements was ultimately chosen.
For the engine, the engineers had little choice: the most powerful liquid-cooled engine of the time was the Allison V-3420 (7). The result of an experimental program launched by the Air Corps in 1937, this powerplant was in fact a hybrid engine consisting of two Allison V-1710 engines mounted at 60° on the same crankcase, the two crankshafts driving a single propeller shaft. Installed, as on the Bell P-39 "Airacobra", close to the center of gravity of the aircraft, that is to say behind the pilot, this engine would make its debut on the XP-75 (8).
Change of Direction
During the winter of 1942 and spring of 1943, design work on the new aircraft progressed well. In November and December 1942, a model was installed in the 1.50 m diameter wind tunnel at Wright Field; the results obtained were excellent except for the maximum speed, which turned out to be significantly lower than the calculated speed. A full-scale layout model was also built. It was inspected twice, on March 8 and May 31, 1943. The first of these two inspections was unfortunately cut short because the Wright Field engineers judged that the model was not sufficiently detailed, not by a long shot. The installation of the engine, accessories and radio equipment was barely sketched. This prompted a second inspection on May 13. Although no major modifications were required, a large number of detail corrections had to be made. In addition, Air Force engineers asked Fisher Body to conduct the necessary studies to determine whether it was possible to use wing panels from the North American P-51 "Mustang" armed with four cannons.
But all this came at a high cost; on June 22, 1943, Fisher Body requested additional funding to continue the program, the modest sum of $934,501.52! This funding was granted on July 15, although by that date the final contract had still not been signed. A conference had taken place on July 6, 1943, in General Echols's office that had fundamentally changed the direction of the program.
Indeed, Wright Field had decided to change its tune. The Air Force urgently required a long-range escort fighter. It must be said that for several weeks, the bombers of the 8th Air Force had been taking a rather rough ride over Germany. Tuesday, August 17, 1943, was etched in the memory as one of the darkest days experienced by the USAAF. 36 of the 230 bombers sent to Schweinfurt, and 24 of the 146 bombers sent to Regensburg had not returned...
Faced with this alarming situation, the Wright Field engineers had agreed that, with certain modifications, the XP-75 could very well do the job. The requirements now covered six additional experimental aircraft transformed into long-range escort fighters and no fewer than 2,500 production aircraft called P-75A "Eagle". This request was confirmed by letter on July 10 and resulted, on October 1, 1943, in the signing of the final contract for an amount of $2,911,434.70, covering the supply by Fisher Body to the Air Force of eight prototypes (two XP-75s and six XP-75As), a static test cell, a layout model, spare parts and the calculation file. However, a clause stipulated that if the first production aircraft did not meet the specifications, the entire contract could be called into question. The schedule was as follows: first prototype ready to fly on September 30, 1943, first of six additional experimental aircraft delivered in December, and last on February 2, 1944. On November 5, the eight prototypes concerned received their serial numbers: 43-46950 and 46951, 44-32161 to 32166; the 2,500 P-75A-1-GC were numbered from 44-44549 to 44-47048. With this "juicy" contract, Fisher Body doubled its efforts.
In June 1943, a conference was held with NACA representatives (11) to decide what steps could be taken to improve the aircraft's poor roll rate. It was decided to provide P-40 wings to NACA to see how the ailerons could be improved. Further tests were conducted in the NACA wind tunnel during July, and a 1:6.5 scale model was built for more precise experiments in the Massachusetts Institute of Technology (MIT) wind tunnel. Furthermore, the tests conducted in September 1943 in the NACA vertical wind tunnel led to the installation of a ventral keel on production aircraft to improve the aircraft's spin behavior. Vibration and flutter tests were conducted in early November 1943.
The Ugly Fat Duckling!
The first flight took place on Wednesday, November 17, 1943, but history has not preserved the name of the test pilot. As new aircraft joined the flight test program, it quickly became apparent that the XP-75's design was flawed in many ways.
Chief among the difficulties was instability due to poor estimation of the center of gravity position, unexplained power losses, insufficient engine cooling, poor roll rate, poor spin behavior, fuel sloshing in the fuselage tank, inefficient brakes, etc. It was a lot for one airplane!
Therefore, throughout 1944, flight tests had only one objective: to eradicate all these defects, and many others of lesser importance. The causes of these problems were multiple, but we can mention a few for certain. For example, the use of an experimental engine (the V-3420 was in fact making its debut on the XP-75) by a manufacturer who was cruelly inexperienced and whose organization was geographically dispersed. The design office was located in Detroit, Michigan, and the production center in Cleveland, Ohio, nearly 300 km away. Added to this was the terrible pressure exerted by the USAAF to start production as quickly as possible, which left Fisher Body no time to plan the program properly. Despite these working conditions, most of the problems were corrected and the aircraft had reached a generally satisfactory level by the time the first production "Eagles" were ready.
However, these flew at some 40 km/h less than the speed guaranteed by the manufacturer. The maximum speed recorded with the third production aircraft was 672 km/h at 6580 m and at a weight of 6,160 kg, the engine delivering 2,210 hp at 3,000 rpm. The rate of climb in the same conditions was 2,900 feet/min; this was far from the 3,500 in the specifications! It was to correct this inadequacy that this same aircraft was equipped with an experimental exchanger which allowed the engine power to be increased. This modification would have been generalized if large-scale production had been launched.
Mark E. Bradley, Wright Field's aircraft project manager, was soon flying the XP-75. "It was heavy, unwieldy, and unstable. No pilot would want it, I was sure of it. It was hopeless," he recalled (12). He was quickly convinced that the solution to the long-range escort fighter problem was to install an extra fuel tank in the P-51. He managed to convince General Orval Cook, head of the Production Division, but he had more difficulty with General Echols. When he told him about the aircraft's lack of maneuverability, he was told, "We don't want it circling around the bombers. All we want it to do is stay with them and protect them!"
Bradley was appalled by this response. There was reason to be. Nevertheless, after much argument, he obtained Echols' agreement to convert a P-51. An additional 320-liter fuel tank was installed in the fuselage with the success we know, and the first P-51Bs thus modified were delivered to the 8th Air Force starting in December 1943.
$1,579,772.40
In February 1944, given the incessant number of modifications, Fisher Body found itself faced with a worrying drift in the program's costs. This is why, once incurred, the manufacturer requested an additional $1,579,772.40, which was granted on March 28, 1944. But the program was seriously beginning to... fail, especially since on March 4, for the first time, P-51 "Mustangs" had escorted the bombers over Berlin (13).
Further vibration and flutter tests were conducted in February 1944. And the first official performance evaluation tests were initiated in June 1944, but due to engine malfunction, the results were questionable. No further official performance evaluation was attempted until a production aircraft was available. In September and October 1944, production aircraft Nos. 1 and 2 were sent to Eglin Field, Florida, for tactical evaluation tests. Shortly thereafter, No. 1 was destroyed in an accident. In any case, these tests were never completed. The termination of the program was decided at a meeting at Wright Field on October 23, 1944, and officially confirmed on November 8.
For the balance of all accounts...
In fact, this abandonment was not quite a one. It had been specified that total production should not exceed 20 aircraft. A visit to Fisher Body in Cleveland allowed an update on the progress of the work. There were then five production aircraft in flying condition or about to be: aircraft Nos. 2 to 6, since No. 1 had been destroyed. After numerous meetings, it was decided to transfer these aircraft to the Engineering Division to continue the development of the Allison V-3420 engine. As a result, contract No. W535-AC-33962 was terminated and a new contract (No. W33-038-AC-6620) was negotiated with Fisher Body to complete the development or construction of aircraft Nos. 2, 3, 4 and 5, and to store No. 6 to serve as a source of spare parts.
In any case, work on the aircraft had to be stopped on January 31, 1945.
This decision resulted in the allocation of the necessary funds, namely $4.7 million (14). Aircraft No. 2 (No. 44-44550) was flown to Moffett Field, California, where stability and propeller tests were continued in the large wind tunnel. After these tests, the aircraft was simply scrapped. The installation of the experimental heat exchanger on No. 3 (44-44551) was completed and the aircraft was handed over to Allison on June 28, 1945, in accordance with contract No. W33-038-AC-11440; this $95,600 contract covered 100 hours of flight tests. It ended with the victory over Japan, but the aircraft was retained by Allison free of charge. Aircraft numbers 4 and 5 (numbers 44-44552 and 44553) were flown to Patterson Field, Ohio, and stored in case they were needed to further develop the V-3420. Other modifications to the aircraft included modifications to the landing gear to improve taxiing conditions, relocation of the radiators, installation of a cockpit ventilation system, replacement of the propeller blades, etc.
In total, the Fisher Body P-75 "Eagle" program had cost a whopping $9,373,629.37, the equivalent of 180 production P-51 "Mustangs" (15). A real waste for an aircraft that, according to many, should never have existed.
In the months following the program's termination, most of the aircraft built were simply scrapped. Only one of those shipped to Patterson Field (now Wright Patterson AFB), the last of the series, was given to the US Air Force Museum at the same location.
Footnotes
(1) The day before, September 9, General Arnold had submitted to the Chief of Staff of the USAAF his AWPD-42 plan specifying the number of aircraft needed to achieve air superiority. This plan provided, among other things, for the expansion of the 8th Air Force in Great Britain and contained a first draft of the Combined Bomber Offensive (CBO).
(2) This article is primarily based on the final report on the acquisition, inspection, testing, and acceptance of the General Motors / Fisher Body Divsion XP-75 aircraft, dated June 26, 1946 (AAF Technical Report No. 5505).
(3) Aviation Fana No. 317 & 318. Berlin had left Curtiss in April 1942.
(4) In the United States, the initial specifications are called Request For Proposal, or RFP.
(5) Report No,. A-89, revised September 18, 1942.
(6) Draft Contract No. W535-AC-33962 of October 10, 1942.
(7) In this designation, the letter V indicates the arrangement of the cylinders, here in V, and the number 3420 indicates the cylinder capacity expressed in cubic inches, or 56044 cm³ or 56 liters.
(8) Few aircraft were equipped with the V-3420: the XP-58, the XP-75, the XB-19 and the XB-39.
(11) National Advisory Committee for Aeronautics, now NASA.
(12) Excerpt from the memoirs of 72 Wright Field test pilots, published as "Test Flying," Westchester House Publishers, Omaha, Nebraska, 1993.
(13) In fact, the first escort over Berlin had been carried out the day before by Lockheed P-38 "Lightnings" of the 55th Fighter Group. On March 4, it was P-51s of the 354th and 4th FG that escorted the "Big Brothers" over the capital of the Reich.
(14) Exactly $4,733,312.28 authorized on February 27, 1945
The Eagle in Detail
The Fisher Body P-75A's wing was cantilevered. It consisted of three parts: a rectangular central section and two outer panels borrowed from the P-40 but with modified ends. The central section was located at the base of the fuselage and served as a support for the engine.
With a wingspan of 15.03m and a surface area of 32.24m², the wing had an aspect ratio of 7. It was set at an incidence of 1˚30' and the external parts had a dihedral angle of 6˚. The profile used was a NACA asymmetrical biconvex profile of the 2200 series (NACA 2215 at the root, and NACA 2209 at the wingtip (16). The ailerons had a total surface area of 1.95m² (larger than those of the P-40); the two flaps, with a total surface area of 4.41m², could be lowered by 42˚30'.
The fuselage was of semi-monococque construction (made up of frames held together by stiffeners, the whole being covered with sheets which contributed to the strength of the whole), made of Aclad 24ST aluminum alloy covered with a layer of aluminum to protect it against corrosion. It was bolted to the wing.
The cockpit was placed very high and very far forward for an aircraft of that era, equipped with a full-visibility canopy (known as a "teardrop" canopy) and an armored windshield.
The entire tailplane was borrowed from the Douglas A-24 dive bomber, but a number of modifications were introduced throughout the program. As a result, while its shape was clearly recognizable on the first prototype, this was no longer the case on later aircraft.
The landing gear, that of the Chance Vought F4U Corsair carrier-based fighter, retracted flat into the wing using a hydraulic system. Its wheelbase was 7.90m and its track width was 6.08m. It used 81x22 cm wheels and Goodyear hydraulic brakes.
The engine was mounted in the center of the fuselage, behind the cockpit (as on the Bell P-39 and P-63 fighters). It drove two 3.83m diameter, counter-rotating, hydraulically driven, three-bladed Aero-Products propellers. It was an Allison V-3420-23 with 24 cylinders arranged in a double V. Its compression ratio was 6.65. With a two-stage turbocharger, its performance was as follows:
| Regime | Altitude (in meters) | Power (hp/RPM) | Manifold Pressure (inches of mercury) | Duration (minutes) |
| Takeoff | 0 | 2600/3000 | 50.5 | 5 |
| Normal | 0 | 2100/2600 | 41.5 | Continuous |
| Normal | 5180 | 2100/2600 | 41.0 | Continuous |
| Emergency | 0 | 2885/3000 | 57.5 | 5 |
| Combat | 6100 | 2300/3000 | 48.5 | 15 |
The fuselage housed two tanks (1,215 liters) and the central section of the wing four (810 liters). If necessary, droppable auxiliary tanks of 284 or 416 liters could be added.
The aircraft's armament consisted of six 12.7 mm Colt-Browning machine guns in the wings, each firing 235 rounds, and four synchronized 300 rounds in the nose. In addition, two hardpoints under the center of the wing allowed the aircraft to carry two 227 kg bombs, two fuel tanks, or two M-10 smoke pods. The sighting device was an N-9 (the P-51D's); an N-6 machine gun was embedded in the leading edge of the starboard wing.
Radio navigation equipment included an MN-26 (or MN-26Y) radio compass for convoys), a BC-1206-A heading indicator, an SCR-522-A or SCR-274-N radio, and an SCR-695-A IFF (transponder).
Footnotes
(1) The Curtiss P-36 and P-40 used strictly the same profiles
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