United States of America (1944)
Prototype Ground Attack Aircraft – 1 Built
The Vultee XA-41 was a single-engine aircraft that began life as a dive bomber. Months later, its role was changed to a low-level attack aircraft. The XA-41 performed admirably in flight tests, but the United States Army Air Corps (USAAC) eventually decided that the fighter aircraft then in service were already performing well enough in the attack role. Despite its job being erased, the XA-41 continued development as a testbed, showing off the powerful XR-4360 engine it mounted and how much it could carry. The aircraft itself would have been deadly had it been produced in large numbers, as it boasted four 37mm cannons. As the war went on, the XA-41 was still being tested. Throughout the trials, the aircraft had extremely good performance, even being able to outturn a P-51, but its speed wasn’t quite enough for its role. At one point, it was given to the Navy for testing and eventually it would wind up at Pratt & Whitney (PR). At PR, it served as a testbed through the war and was eventually scrapped in 1950.
The XA-41 began as part of a United States Army Air Corps (USAAC) requirement in October 1941 for a new single-seat attack aircraft. The Douglas A-20 Havoc (and soon to be produced North American A-36 Apache) was performing well at the time, but the USAAC wanted something new. The aircraft requested had to be able to reach at least 300mph (482.8kph) at sea level, have a service ceiling of at least 30,000ft (9,144m), and a range of 1,200 miles (1,932km). For the attack role, the aircraft was to have either 37mm, 20mm, or 50. cal guns mounted in the wings. Given this imposing armament, it is likely the aircraft would have attacked soft targets or even been used for tank-busting.
The USAAC commissioned Vultee Aircraft Corporation, Kaiser Fleetwings, and Curtiss to design a new aircraft for the role. Kaiser Fleetwings developed the XA-39, which would have mounted the R-2800-27 engine. Their aircraft didn’t progress beyond the mockup stage. Curtiss reused their naval XTBC-1 prototype for their part, renaming it the XA-40. This also didn’t go beyond the mockup stage. Vultee’s answer was the V-90, a ground attack aircraft mounting the fairly new and powerful R-4360 engine. Interestingly, the XA-41 started off as a dive bomber, despite it being commissioned as an attack aircraft. It isn’t often stated, but the Army had been interested in dive bombers since 1940, going as far as purchasing several Navy designs. The Army bought several SB2D-1 Helldivers in December of 1940 and renamed them the A-25 Shrike. They also had a troubled history with one of Vultee’s own aircraft, the A-35 Vengeance, which they tried numerous times, but to no avail. The XA-41 was most likely a chance for the Army to have a successful dive bomber or attack aircraft. The Army was satisfied with Vultee’s V-90 design and awarded a contract for two prototypes on November 10, 1942. Shortly after a mockup inspection, the Army interestingly switched the role from a dive bomber to a dedicated attack aircraft. The switch was rather abrupt and caused a delay in the development.
On April 30th, a new contract was signed which included a static mockup. Vultee continued construction on the project until the prototype was halfway completed, at which point the Army decided that the most current aircraft, the Republic P-47 Thunderbolt and North American P-51 Mustang, were already quite capable in the attack role. But this wouldn’t be the end of the XA-41. Maj.Gen Oliver P. Echols, Chief of the Material Division, opted not to cancel the program and instead complete it in order to show the true potential of a new attack aircraft with the new R-4360 engine. This decision went through on November 20th, with the prototype ¾ the way through completion. The 2nd prototype was cut at this point and only one would be built (Serial No. 43-35124/5).
On February 11th, 1944, the XA-41 flew for the first time from Vultee Field, California, with test pilot Frank Davis at the controls, and landed at March Field, California. Several flights after this were conducted at the nearby army base. The aircraft was flown by both Vultee and Army pilots, and both agreed it handled well. There were some problems here and there, which Vultee quickly fixed with some additions to the airframe. On June 25, the Army accepted the XA-41. On July 16th, on its 60th flight, the aircraft was ferried to Eglin Field, Florida. Testing proved the XA-41 was an exceptional aircraft, with many great features. The craft had an excellent turn rate, being able to outturn the P-51. For its size, it carried an impressive arsenal of weapons. But the Army wanted an attack aircraft that could also defend itself if the need would arise, and the 350mph (563.2 km/h) of the XA-41 wasn’t that impressive compared to other aircraft in service. The United States Navy became interested in the XA-41 at some point and the prototype was given to them for testing at Patuxent River, Maryland. The Navy wanted to see if the aircraft could be flown from aircraft carriers. After the Navy briefly tested it, the XA-41 was given to Pratt & Whitney (PR) on August 22, 1944. It was obvious at this point that the XA-41 would never see combat, but would remain in the US as a testbed aircraft. Serving with PR, it was used as a flying testbed for their R-4360 engine, as well as having a supercharger mounted. As testing continued, the aircraft was purchased by PR on October 9 and re-registered as NX6037N. There are few documents that reference the XA-41 post-war. The only thing mentioned is that the sole XA-41 was finally scrapped in 1950, having served for many years at PR.
The XA-41 was a conventional single-engine aircraft. It had a slight gull wing and a tail sitter configuration. The landing gear in the wings would retract inboard and was placed widely to allow better landing performance. During development, the tail wheel had doors installed to completely cover it in-flight. The cockpit was placed far forward and raised to allow the pilot to see over the engine, giving him better visibility when attacking ground targets. The ventral tail had an extension that spanned most of the length of the aircraft. This was added during development. A spinner was also added at some point. The XA-41 mounted the PR XR-4360 Wasp Major engine, which was the main reason the Army and PR were so interested in the project.
For armament, the XA-41 had four M2 Browning .50 cal machine-guns mounted in the wings. For the attack role, it was meant to mount four more 37mm cannons (sources don’t mention what particular kind of gun) in the wings. All armament in the wings was placed outside of the propeller’s range. For bombing, the XA-41 had a bomb bay that could carry four 500Ibs bombs, a torpedo, additional fuel, or two 1,600Ibs weapons. In total, it had up to 6,500Ibs of ordnance. Documents mention that up to 1,100Ibs of additional bombs could be mounted to the wings. The aforementioned competing XA-39 only sported the four Brownings, two 37mms, as well as a predicted carrying capacity of six 500Ibs bombs.
XA-41 – [The sole prototype built, used as a testbed for the XR-4360 engine.]
United States of America – The United States Army Air Corps would have operated it had it entered production. After serving as a testbed for the Army, the Navy and Pratt & Whitney also did tests with the aircraft.
The VL Pyörremyrsky (translates as Hurricane) was a prototype Finnish fighter plane designed to keep up with its contemporaries. It was to be domestically produced, using wood, but using the same engine as the Bf 109 G. Due to limitations brought about due to the war, only one prototype was produced and it wasn’t ready until the end of 1945.
Development and History
As Finland found itself still at war with the Soviet Union in 1942, with no end in sight, it turned to ways to bolster its military force. In order to become as self sufficient as possible, it was engaged in various projects for domestically designed and produced weapons systems. The VL Myrsky project was severely behind schedule and the air force realised that it would be outclassed by the newer Soviet aircraft by the time it reached production. With this in mind, it placed order number 2012/42 on 26th November 1942 for a new aircraft design, under the name Pyörremyrsky.
The State Aircraft Factory (Valtion Lentokonetehdas) was tasked with producing the new fighter and Captain of Engineering Torsti Verkkola was assigned chief designer of the team. The main premise was that the aircraft was to be made out of wood, as much as was possible, and that it was to be comparable with the German Messerschmitt Bf 109G. Verkkola used the Bf 109 as the base for his design, making modifications to allow it to be produced with local skills and materials. However, as the war dragged on, and the Finnish Air Force required more proven aircraft, as well as repairs to the planes already in service, the Pyörremyrsky found itself given a lower priority.
Upon the cessation of hostilities between Finland and the Soviet Union in September 1944, the Pyörremyrsky project had only a partially completed prototype and the Ministry of Defence (puolustusministeriö) cancelled the advance order of 40 aircraft, as well as the second prototype on the 29th September, but they did allow the first prototype to be completed. In Autumn 1945 the prototype, now christened PM-1 (which led to the nickname Puu-Mersu or Wooden Messerschmitt), was ready for pilot tests. On the 21st of November 1945, Luutnantti (Lieutenant) Esko Halme took off in PM-1 from Tampere-Härmälän airfield. The flight only lasted 25 minutes as part of the engines’ exhaust system came loose, forcing an emergency landing as Lt. Halme was unable to see through the exhaust blowing into his cockpit. Despite the incident, Halme reported good flying controls and characteristics. PM-1 would do 2 more test flights at Härmälän airfield before moving to Kuorevesi for Air Force testing. In total 31 test flights were performed, amounting to 27 hours of flight time. All 8 pilots reported the same, smooth and controlled flying characteristics, good speed and turning ability, however it was not quite up to the same performance of the Bf 109 G but close enough. The last flight of PM-1 was on the 22nd of July 1947, when Kapteeni (Captain) Osmo Kauppinen took off for a 20 minute general test flight. After this it was put into storage until it was officially removed from the Air Force’s rolls on the 1st April 1953. This was mainly due to the lack of ability to source new engine parts as part of the armistice Finland signed with the Allies forbade them from receiving military goods from Germany, as well as the decline of the piston aircraft as a fighter.
The Air Force didn’t want such a unique piece of Finnish aviation history to be scrapped however and ordered it to be preserved. It was sent to the State Aircraft Factory’s depot and was refurbished in the early 1970’s and sent to the Finnish Air Force Museum, where it is still on display.
The experiences learnt with the Pyörremyrsky were not totally in vain or wasted as the basic design was used in the development of the VL Vihuri fighter trainer.
Captain Verkkola used the Bf 109’s low-wing cantilever monoplane configuration as his base for the Pyörremyrsky. The Germans were also willing to supply the Daimler-Benz DB 605A-1 liquid cooled V12 engines and VDM 9-12087 three-bladed light-alloy propellers that were used on the Bf 109 series. It was also installed with a German produced Telefunken FuG 7a model of compact airborne receiver/transmitter.
The main body of the aircraft was built using the vast amounts of wood available to the Finns, with metal being used where absolutely necessary, like the cockpit and engine housing. While many believe the design is similar, if not copied from the Bf 109, there are many differences outside of just the materials used. The rear portion of the fuselage is of wooden monocoque design, with the horizontal stabilizers mounted at the near end, as opposed to the Bf 109’s which are mounted on the vertical stabilizer. The wings were of negative transverse V shape and covered in plywood panelling.
Unlike the wing fuel tanks found in the Bf 109, the Pyörremyrsky had a single tank behind the cockpit, protected by a 10mm thick armoured plate. The landing gear was copied from the Bf 109 but the Finns made some changes to eliminate the narrow and problematic system that plagued the Germans. The tailwheel was also retractable, thus helping it with aerodynamics.
Due to wartime shortages, Finland was forced to rely on substandard replacement products. The use of Lukko glue was one of the main reasons for the failings in the VL Myrsky and so it has been suspected that the Pyörremyrsky would have suffered similar issues to its sister aircraft had it been pushed into service or flown for longer periods of time.
Armament was not fitted to the PM-1 but it was designed to be installed with a Motorkanone mounted 20 mm (.78 in) MG 151/20 cannon and two nose mounted synchronized 12.7 mm LKK-42 machine guns. It was also proposed that the wings would have provisions for two 100kg bombs each for fighter bomber duties, but it is not clear if the proposal was ever considered seriously.
Finland – The VL Pyörremyrsky was intended to be used by the Finnish Air Force.
The PZL P.50 Jastrząb (Hawk) entered development in 1937 as a replacement for the outdated PZL P.11 and PZL P.24 fighters. Very little was known about the aircraft until relatively recently, with only a few photographs and documents about this aircraft surviving World War II. Most of what is known at the time of writing is based on accounts and sketches from PZL engineers years after the war and mostly based on memory, which is not the most reliable form of historiographic evidence. Ultimately, the project was cancelled in March of 1939 due to dissatisfaction with its underpowered radial engine, despite an attempt being made to fit an inline vee engine to the aircraft in the form of the PZL P.56 Kania (Kite).
In the latter half of 1936, plans to replace all of Poland’s PZL P.11 single-engine fighters with the twin-engined PZL P.39 heavy fighter were abandoned, and the need for a maneuverable single-engine interceptor was recognized. Briefly, the PZL P.24 was considered in order to fulfill this role, but as it did not offer much of an improvement over the P.11, it was ultimately decided that an entirely new design was needed.
Wsiewlod Jakimiuk, the head of PZL’s fighter team, submitted a proposal in autumn 1936 featuring “an advanced low-wing monoplane which offered improved all-around performance and great scope for future development” (Cynk, p. 259). The aircraft resembled the American Seversky/Republic fighter designs. One of the things that Jakimiuk focused on in the design of this aircraft was to allow upgrading to larger and more powerful engines to be a simple task to accomplish at any later time.
Ultimately, after a heavily-protracted design process plagued by numerous issues, the aircraft never saw combat and only 32 airframes had been built before Poland was invaded by Germany and the USSR in September 1939, but 30 of these were not completed.
In the fall of 1937, the design, now called the PZL P.50 Jastrząb and powered by a British 840-horsepower Bristol Mercury VIII radial engine, was approved and two prototypes were soon built. The first prototype, called the P.50/I, was designed to take engines up to 1,200-horsepower while the second prototype, the P.50/II, was designed for engines of up to 1,600-horsepower. Both versions were to be armed with four 7.7-millimeter machine-guns in the wings although, confusingly, the few images of the P.50/I show two of these guns in the forward fuselage instead. The P.50A production version of the P.50/I was envisioned to have a top speed of 310 miles per hour (500 kilometers per hour) at 14,100 feet (4,300 meters).
The Polish Aviation Command almost immediately ordered 300 aircraft and paid for the first 100 in advance, with the first 50 expected to be delivered by September 1939. In order to speed up development The Dowty Company of Britain was contracted to build the landing gear for the prototype, while PZL and the Czech Avia company would design the production aircraft’s landing gear. Dowty delivered the landing gear over four months late, and the P.50/I Jastrząb prototype, still utilizing the Mercury engine, did not even fly until February 1939.
During the P.50/I’s initial flight trials, it was discovered that the desired performance parameters were far out of reach. The top speed at full load was only 274 mph (442 kph). It also handled low-speed turns very poorly and had a tendency to wobble at top speed. Curiously, it was discovered that the engine was unable to produce full power under any circumstances. It was not until May that it was realized that the carburettor air intake was too small. After enlarging the intake and improving the tail and wing surfaces, the aircraft’s performance was improved. In August, the aircraft finally reached its desired speed but, by the time the war began in September, the aircraft was still a long way from being ready for service. The only known photos of the P.50/I come from a visit in February 1939 by the Italian Foreign Minister, Count Galeazzo Ciano.
Developed alongside the P.50/I was the P.50/II, which had been completed in the spring of 1939 and was still waiting for an engine by the time of the invasion in September. The P.50/II differed greatly from the P.50/I, so much so that it is believed to have received a new designation just before the war began. The canopy was an all-round-vision hood, somewhat similar to the Soviet Yak-1b. The P.50/II also included provision for additional fuel tanks and a 660-pound (300kg) bomb, and two 20mm cannons were added to the wings. The two machine-guns which were in the fuselage of the P.50/I were also moved to the wings. The powerplant was supposed to be the PZL Waran radial engine, capable of up to 1,400hp, with the intended top speed of the P.50/II being 350mph (560kph). However, development of the engine was far behind schedule and it was estimated that it would not be ready before the middle of 1940. In a desperate search for a suitable engine, several different options were considered. These included the 1,375hp Bristol Hercules and the 1,400hp Gnome-Rhone 14N, again both radial engines. In the end, the Hercules appears to have been chosen, and it was around this time that the P.50/II received its new designation.
There was one more derivative of the P.50, beginning in late 1938. Political upheaval was ongoing in the Polish Air Force high command, and General Ludomil Rayski, who favored radial engines, was coming under heavy criticism and was nearing replacement by General Jozef Zajac, who favored vee engines. This replacement would eventually take place in March 1939. Jakimiuk, the designer of the P.50, anticipated this shift and proposed a Jastrząb adapted to take an inline engine. The aircraft was given the designation P.56 Kania (Kite) and was to be powered by the 1,200hp Hispano-Suiza 12Y, and later by the improved 1,600hp Hispano-Suiza 12Z. However, another PZL designer, Jerzy Dabrowski, submitted a competing design bearing the designation P.62, and this design was preferred over the P.56. The P.56 was ultimately cancelled in the summer of 1939.
When General Rayski was ousted from the command of the Polish Air Force in March 1939, the P.50/I was still having severe problems. His replacement, General Zajac, canceled all production of the P.50 almost immediately. 30 P.50A airframes had begun construction at the W.P.1 plant in Okecie and, after the P.50/I began to show improvement, construction was permitted to continue on these aircraft only. Because of the unsatisfactory results of the Mercury engine, plans were made to power the very first of these production aircraft with the 870hp Gnome-Rhone 14Kirs, and this aircraft was very close to being ready in September 1939. This was to be a sort of test aircraft for an improved version of the P.50A, the P.50B Jastrząb B. There were also plans to mount the 1,100hp Gnome-Rhone 14K or the 1,000hp Pratt & Whitney R-1830 Twin Wasp, although these never came to fruition.
“The Hawk Which Would Never Prey”
On September 5th, 1939, the PZL factory in Warsaw was evacuated. Test pilot Jerzy Widawski attempted to escape with the P.50/I prototype, but was accidentally shot down by Polish anti-aircraft guns. Five pre-production P.50A airframes and the P.50/II prototype, including the aircraft intended to be equipped with the 870hp engine mentioned earlier, were moved out of the Okecie plant on September 3rd. These were captured by the Germans and scrapped in 1940, bringing a final end to the P.50 Jastrząb project.
PZL P.50/I Jastrząb – Initial prototype of the PZL P.50 series. Powered by an 840hp Mercury VIII engine, the top speed was intended to be 310 mph, but it only ever reached 275 mph. Armament was four 7.7mm machine-guns, with two in the forward fuselage and two in the wings. One produced, first flew in February 1939.
PZL P.50A Jastrząb A – Planned production version of the PZL P.50/I. 300 were ordered, but only 30 built, all incomplete by the time of Polish capitulation. Fuselage was changed to a razorback rear, akin to the American P-47. It is unclear whether the fuselage machine-guns were moved to the wing, as documents imply they were but sketches of the aircraft still show them in the fuselage. Top speed had been improved to 310 mph (500kph).
PZL P.50B Jastrząb B – Planned development of the P.50A with a more powerful radial engine. One P.50A was planned to fit an 870hp Gnome-Rhone 14Kirs engine as a sort of testbed for the P.50B, but that is all that is known about this variant.
PZL P.50/II Jastrząb II – Second prototype developed alongside the P.50/I prototype as an all-around upgraded version. The canopy was changed. A 660 lbs (300 kg) bomb was added, along with two 20mm cannon to the wings. The engine was upgraded to the 1,400hp PZL Waran engine, giving a projected top speed of 350 mph (560 kph) . Only one was produced, without the engine, and never flew.
PZL P.56 Kania – Planned development of the P.50A Jastrząb A using either a 1,200hp Hispano-Suiza 12Y or 1,600hp Hispano-Suiza 12Z inline Vee engine. None produced.
Poland – 300 copies of the P.50A type were ordered, with only 30 incomplete airframes + 1 P.50/I prototype produced. Did not see service. A single P.50/II prototype also existed, but the type was never ordered.
PZL P.50A Jastrząb A Specifications
31 ft 9.875 in / 9.7 m
25 ft 3.125 in / 7.7 m
8 ft 10.25 in / 2.7 m
208.83 ft² / 19.4 m²
One 840hp (648.8kW) Bristol Mercury VIII nine-cylinder radial engine
3,748 lbs / 1,700 kg
Maximum Takeoff Weight
5,511 lbs / 2,500 kg
310.6 mph / 500 kmh
466 mi / 750 km
Maximum Service Ceiling
14,107 ft / 4,300 m
Four 7.7mm KM Wz 36 machine-guns mounted in the wings OR two in the fuselage and two in the wings
Provision for an unknown weight of bombs, possibly 220.5lbs (100kg), to replace two wing machine-guns
Polish aircraft historian Zabytki Nieva discusses the PZL P.50. Audio is in Polish with English subtitles available.
Video made by the author specifically to accompany this article.
United States of America (1944)
Prototype Escort Fighter – 2 Built
The Consolidated Vultee XP-81 was a prototype mixed power fighter developed in late 1943 by the Consolidated Vultee Aircraft Corporation in order to meet an Army Air Force requirement calling for a high altitude escort fighter. Plagued by slow development and engine problems, the XP-81 would never see active service and development would be terminated in 1947. Despite this, the XP-81 still holds a distinct place in history as America’s first turboprop engine plane to fly and the world’s first plane to fly with a turboprop engine and a jet engine together.
With the formal introduction of the Boeing B-29 Superfortress on May 8th of 1943, it would be clear that a high altitude escort fighter would soon be needed to accompany the Superfortress on its bombing missions over the Pacific. In the summer of 1943, this need was realized and the United States Army Air Force (USAAF) issued a list of design requirements that consists of the following:
1,250 mile (2,012 km) operating radius
Fuel for 20 minutes of combat plus reserve fuel supply for landing
Cruising speed of 250 mph (402 km/h) at 25,000 ft (7,620 m)
Maximum speed over 500 mph (804 km/h)
Combat ceiling of 37,500 ft (11,430 m)
Climb rate of 2500 fpm (feet per minute) / 762 mpm (meters per minute) while at 27000 ft (8230 m)
12 ° angle of vision over the nose
* – The USAAF recommended that the designers use a two engine setup consisting of a propeller engine for long range flights while complemented by a jet engine for high speed combat situations.
Interested in this proposal, the Consolidated Vultee Aircraft Corporation, later known as Convair, began work on an aircraft which would meet the specifications, appointing Charles R. Irving, who was a chief engineer of the Vultee Field Division and Frank W. Davis, the assistant engineer, who was also the chief test pilot, as the leaders of the design team. The project was known as the “Model 102” within Consolidated Vultee. In the early stages of development, the designers faced a dilemma of engine selection. The Pratt & Whitney R-2800 Double Wasp radial engine was considered, as was the General Electric TG-100 turboprop engine. After some evaluating and testing however, the TG-100 was selected as it was deemed to have superior performance for combat and cruising situations. As for the jet engine in the rear, a relatively straightforward choice to mount a General Electric J33-GE-5 (also known as I-40) jet engine was made. After a couple of months of development, Consolidated Vultee submitted a preliminary design proposal to the United States Army Air Force in September of 1943. Relatively interested in this design, the plane was given the greenlight for further development and received the designation “XP-81” by the Air Material Command.
Detailed work on the XP-81 began in January 5th of 1944 and on January 18th, Consolidated Vultee was given the contract (no. W33-038-ac-1887) by the USAAF worth about $4.6 million to construct two flying XP-81 prototypes and one airframe for ground testing under the USAAF project name “MX-480”. Another contract followed on June 20th of 1944 worth $3,744,000 for the two flying examples, the airframe and the testing data. The contract was later modified to include 13 YP-81 under the project name “MX-796”. The construction of the first XP-81 prototype would begin on January 20th at the formerly independent Vultee aircraft factory in Downey, California but problems soon surfaced. Some time in April, the Air Material Command was notified that there would be a delay in the delivery of the TG-100 due to a couple of technical difficulties. As such, construction of the first prototype was delayed as the designers sought out an alternative engine to replace the TG-100 in June.
The Packard V-1650-3 (some sources state V-1650-7), which was the American copy of the British Rolls-Royce Merlin engine, was selected to fill in the gap and the USAAF promptly provided Consolidated Vultee with such an engine taken from a North American P-51D Mustang. Within a week of receiving the engine, Consolidated Vultee engineers were able to install it after making considerable structural modifications to the first prototype’s airframe. A radiator similar to that of the Lockheed P-38J’s “beard” radiator would also be mounted on the XP-81, under the propeller spinner. Unfortunately for the designers however, the change of powerplant would add 950 lb (431 kg) to the plane while taking away 960 hp at takeoff and 1720 hp at top speed. With the relatively slow development, the first XP-81 prototype would finally be completed in January of 1945 bearing the serial number of “44-91000”.
Although the aforementioned issues with weight gain and horsepower loss were present, the Packard engine powered XP-81 was still deemed safe for flight tests, and as such, the first XP-81 prototype was prepared for test flights at Muroc Dry Lake in California and finally took to the skies on February 7th of 1945 with Frank W. Davis in the cockpit. Amazingly enough, 46 test flights were made with the Packard engine and it accumulated a total of 47.75 flight hours. In the testing phase it was noted that with the Packard engine installed, the XP-81 had poor directional stability at low speeds and the occasional splatter of oil on the windscreen by the propellers. Plans to replace the Packard engine were brought up on May 18th of 1945 when the TG-100 turboprop was finally available. The conversion was completed and the first prototype was returned back to Muroc for more tests on June 11th. Due to the new engine installation, extensive ground work had to be accomplished before flight tests were to continue. Throughout June 23rd to December 20th of 1945, numerous ground tests were conducted and a few problems surfaced. For one, the TG-100 was difficult to start and once it did, the pilot would have difficulty controlling the propeller. As this was an early turboprop engine,
reliability was low and the turbine wheels had to be replaced constantly, sometimes only after half an hour of use. The 10 inch (25 cm) oil cooler for the TG-100 was also deemed a problem, and it was thus increased to a 12 inch (30 cm) system instead. Perhaps the biggest problem however, was the throttle lag the XP-81 suffered. Frank W. Davis describes the problem by stating “The pilot had about a 10 second lag when he wanted to go and about 2 seconds lag when he wanted to stop, with both thrust and drag being powerful and non-adjustable when they did occur.” (Consolidated Vultee XP-81, by Steve Ginter). The ground personnel concluded in these ground tests that the current Aeroproduct A542 propeller and drive shafts were incompatible with the TG-100, and that new propellers should be developed. An emergency engine feathering system was also recommended.
The first flight of the XP-81 with the TG-100 engine occured on December 21st of 1945. This was the 47th test flight the first XP-81 underwent. Performance was rather satisfactory, and the flight concluded after a mere 5 minutes. Excessive oil consumption was noted however. Test flights with the TG-100 proved disappointing as the turboprop did not perform as it was advertised, delivering less horsepower than was expected. Out of the estimated 2,300 hp the TG-100 was suppose to achieve, only 1,400 hp was achieved. The I-40 engine was no help either, as it developed nearly 250 lb (113 kg) less thrust than advertised as well. The estimated performance of 478 mph (769 kmh) at sea level was not achieved with only a mere 400 mph (643 kmh) achieved. Due to these factors, the performance achieved was similar to that of the Packard engine installation. Despite these problems, the XP-81 still did well in some aspects. The relatively decent handling and decent climb rate was complemented, as was the light controls. The second prototype (serial no. 44-91001) was produced some time before November of 1946, and was ready for flights by February of 1947. It featured a longer ventral fin than that of the XP-81 and had a four blade Hamilton Hydromatic propeller replacing the Aeroproducts propeller used on the first prototype. Unfortunately, it is unknown what date the second prototype made its maiden flight, but it is speculated that it first flew some time in February of 1947.
In total, 116 flights were made by both of the XP-81 prototypes, 22 of which were done by the second XP-81 prototype. More tests were planned, as on January 14th of 1947, Consolidated Vultee called for the following areas to be studied and tested:
Firearms testing of the Browning AN/M2 and the Hispano T31. Bombs and rockets tests will also be included.
Anti-icing equipment efficiency.
Control characteristics and lateral stability.
Cabin pressurization experiments.
Power plant operations.
However due to the previously mentioned issues of the XP-81 underperforming, the USAAF gradually lost interest in the XP-81 program. Consolidated Vultee was well aware of this, and they had been trying since December of 1946 to improve their design. A proposal was made in December 31st to the Air Material Command to fix the underperforming prototypes. This proposal suggested that an improved TG-110 (the ones that would have been used on the YP-81) should replace the TG-100 and a J33-19 jet engine should replace the J33-GE-5. The Air Material Command however was not impressed by the proposal due to the amount of redesigning and time needed and in early 1947, their engineering department ceased work on the TG-100 turboprop engine. Things would look even more grim for the XP-81 when on January 27th of 1947, the contract for the 13 YP-81 pre-
production fighters were cancelled. Finally on May 9th, the XP-81 program reached its end when the government decided to cancel the contract on its development. The two prototypes were then taken in by the USAAF on June 24th and 25th. Finalization of the cancellation was conducted on June 23rd of 1948 after the USAAF was reorganized into the United States Air Force (USAF) when Consolidated Vultee was reimbursed with $4,578,231 for their work on the program.
Though development stopped for the XP-81 program, the two prototype’s story did not end there. At the time when the USAAF took in the prototypes, the engine and propeller development branches of the Air Material Command was in the middle of developing more advanced propeller control techniques and a suitable machine was needed to perform tests on as wind tunnels and models were not available. The USAAF promptly provided the two XP-81s which were redesignated as “ZXF-81” for this new role. The two planes were then stored in Edwards AFB (previously known as Muroc AAF) for future use. Unfortunately, they were never used and on April 29th of 1949, all useful parts and gadgets were stripped from the two planes by order of the USAF. The two empty airframes were then dragged onto the photography & bombing range of the Edwards AFB.
Despite the XP-81s now sitting in the desert, Consolidated Vultee was still not willing to yield completely. The company tried proposing reviving the XP-81 program using different power plants and repurposing the role. The proposal called for the use of the British Armstrong-Siddeley Double Mamba turboprop producing 4,000 hp and a Rolls-Royce R.B 41 jet engine producing 6,250 lbf (2,835 kgf) of thrust replacing the original engines. The idea behind this was to create a ground attack aircraft which could be exported to other countries. However, this idea was understandably met with skepticism by the Air Force, but an investigation to see the feasibility of this proposal was made. On September 14th of 1950, a report was finalized stating that at least ⅔ of the airframe would need to be modified in order to mount the new engines. New drop tanks, rocket rails, hardpoints and various other parts would also need to be redesigned. Another investigation was done on this proposal by comparing the hypothetical performance to the all-turboprop Douglas A2D Skyshark, a ground attacker aircraft in service with the USAF. It was determined that the Skyshark would outperform the XP-81 with British engines in all aspects, so there was no point in developing an inferior aircraft. Another factor that was noted was the excessive amount of maintenance, training and logistics needed to service the ground attacker. With all these factors in mind, the proposal was discarded by the USAF and Consolidated Vultee finally gave up on the XP-81.
The two XP-81 airframes would remain in the desert exposed to the elements for decades until August of 1994 when Air Force Flight Test Center Museum curator Doug Nelson retrieved them. They were in derelict condition, with the second XP-81 prototype being more damaged than the first. As of 2018, the two airframes remain in the National Museum of the United States Air Force in Dayton, Ohio awaiting future restoration. Although never seeing service, the XP-81 still holds a distinct spot in history as America’s first turboprop engine powered plane to fly and the world’s first plane to fly with a turboprop engine and a jet engine together.
Airframe: The XP-81’s semi-monocoque fuselage was constructed using age hardened 24-SRT aluminum alloy, followed by the exterior surfaces being flush riveted. The entire fuselage is made from metal. The wing design was a NACA laminar flow type, made from aluminum-alloy. The design allowed for a stressed-skin wing which was flush riveted as well, with the rivet heads being milled. Due to the relatively heavy materials used in the wings, the surface was relatively smooth thus allowing for good aerodynamics. The majority of the heavy plating was mounted in the frontal 34.5% of the wings, and thus allowed a decent mount for aerial weapons and permitted ordinance to be mounted. There were spoilers present on each wing which automatically operated in accordance to the ailerons. Another interesting feature was a thermal anti-ice system derived from the hot hair emitted from the TG-100 turboprop and the exhaust. Within the fuselage two fuel tanks were installed directly behind the cockpit, making for a total 811 gallons (3670 L) of fuel. The fuselage also housed the XP-81’s tricycle landing gear which was electrically operated. The main gear was fitted with disc brakes, also doubling as a parking brake.
The canopy on the cockpit was based off of the British bubble design, which allowed for a relatively clean 360° view. This type of canopy was used on many planes in service with the United States and Britain. The canopy would be controlled by the pilot via a hand crank on the left hand side of the cockpit. For fatal combat situations, an emergency canopy jettison system was provided allowing for the pilot to bail out quickly. The pilot’s seat was an ordinary World War II styled seat, but this was eventually replaced with an ejection seat modelled after the one used on the Convair XP-54. As the XP-81 was a long range fighter, an automatic piloting system was also installed. The cockpit would also be pressurized using the air from the TG-100 engine. For pilot comfort, a temperature system was installed allowing for optimal temperatures in all climate and altitudes.
For communication, the XP-81 was fitted with a VHF (Very High Frequency) SCR 522-A radio set. The cockpit also had room for a BC-1206 beacon receiver and an SCR 695 identification friendly-or-foe system, but these were never installed. The pilot would operate the SCR 522-A radio from the right side of the cockpit, where the radio controls were based.
It is also interesting to note that the second YP-81 prototype had a longer ventral fin than the first prototype.
The XP-81’s design called for a General Electric TG-100 (also known as XTG-31-GE-1) turboprop and General Electric/Allison J33-GE-5 (I-40) jet engine as its power plants. The first prototype had a four blade Aeroproducts A542 brand propeller driving the TG-100 while the second prototype had a Hamilton Standard Hydromatic 4260 propeller instead. The TG-100 had a capacity for 8 gallons (30 L) of oil while the I-40 had 3.5 gallons (13 L). In terms of fuel, 811 gallons (3,670 L) was available in the XP-81’s two standard fuel tanks in the fuselage, but could go up to 1,511 gallons (5,720 L) with the installation of drop tanks.
Armament: The standard armament envisioned for the production P-81 would consist of either six 12.7x99mm Browning AN/M2 machine guns with 400 rounds each or six 20x110mm Hispano T31 cannons with 200 rounds each. The loadout of these guns would be in groups of three in each wing. For ordinance, a single hard point was mounted under each wing, allowing the plane to carry two bombs size ranging from 100 lb (45 kg) to 1,600 lb (725 kg), allowing for a maximum of 3,200 lb (1,451 kg). Chemical tanks, drop tanks, depth charges could also be equipped. Alternatively, 14 High velocity Aircraft Rockets (HVAR) could be carried.
XP-81 – Prototype fighter variant powered by a TG-100 turboprop and I-40 jet engine. Two examples were produced and extensively tested up until the cancellation of the project. Both prototypes were redesignated as “ZXF-81” in 1948 and stored in Edwards AFB. They would be stripped of useful parts and towed to the photography/bombing range near Edwards AFB and left there in derelict condition until August of 1994 when they were retrieved by Doug Nelson. The two airframes still survive to this day and are currently awaiting restoration at the National Museum of the United States Air Force in Dayton, Ohio.
YP-81 – Planned batch of 13 pre-production fighter variant powered by a lighter but more powerful TG-110 turboprop engine and an uprated General Electric J33-GE-5 turbojet engine. These planes would have been armed with either the Browning AN/M2 machine guns or the Hispano T-31 cannons. It would have differed from the XP-81 by having the wings moved back 10 inches (2.54 cm). No YP-81s were produced.
ZXF-81 – Post development termination designation for the two XP-81 prototypes. This designation signified that the prototypes were now flying test beds. However, no use of the prototypes after its termination was noted.
XP-81 (British Engines) – Unofficial variant proposed by Consolidated Vultee some time in 1949/1950 calling for the revival of the XP-81 project using British Armstrong-Siddeley Double Mamba turboprop producing 4,000 hp and a Rolls-Royce R.B 41 jet engine producing 6,250 lb (2,835 kg) of thrust replacing original engines. This new variant would be used as a ground attacker that would be solely used for export. This proposal never saw any development and was thus discarded.
United States of America – The XP-81 was intended to be used by the USAAF, but development carried over to the USAF. The project was eventually cancelled.
Consolidated Vultee Aircraft Corporation XP-81 (Taken from “Consolidated Vultee XP-81 by Steve Ginter”)
50 ft 6 in / 15.39 m
44 ft 8 in / 13.61 m
13 ft 10 in / 4.21 m
425 ft² / 39.48 m²
45.9 ft² / 4.26 m²
80 in² / 516.12 cm²
Wings Sweep Back
1x General Electric XTG-31-GE-1 (TG-100) turboprop 1x General Electric / Allison J33-GE-5 (I-40) jet
TG-100: 8 gallons / 30 L I-40: 3.5 gallons / 13 L
12,755 lb / 3,887 kg
19,500 lb / 8,845 kg (Maximum internal fuel with reduced armaments)
Maximum Combat Weight
24,650 lb / 11,181 kg
811 gallons / 3,070 L – Internal Fuel Tanks 1511 gallons / 5,720 L – Internal Fuel Tanks + Drop Tanks 350 gallons / 1,325 L – Individual Drop Tank
Center of Gravity
Max Forward – 17% Max Aft – 27%
Rate of Climb
0 to 5,000 ft / 0 to 1,524 m – 5,200 fpm / 39.31 mps
Time of Climb
30,000 ft / 9,144 m in 9.6 minutes
299 mph / 481 kmh at Sea Level 253 mph / 407 kmh at 15,000 ft / 4,572 m 224 mph / 360 kmh at 30,000 ft / 9,144 m
546 mph / 877 kmh Diving tests were never finalized due to propeller and engine problems. Flight #90 on September 4th of 1946 achieved the highest speed as mentioned above.
Conditions under maximum combat weight
Ferry Range – 2,393 mi / 3,851 km Speed at 247 mph / 397 kmh – 2,002 mi / 3,222 km Speed at 274 mph / 441 kmh – 1,622 mi / 2,610 km
47,000 ft / 14,000 m
SCR 522-A VHF Radio
6x 12.7x99mm Browning AN/M2 (400 rpg, 2,400 total) or 6x 20x119mm Hispano T31 (200 rpg, 1,200 total) Never Fitted on Prototypes, Intended Armament
1x K-14 Gyro Gunsight
2x hardpoints capable of carrying 3,200 lb / 1,452 kg of either bombs, depth charges, chemical tanks or drop tanks or 14x 5 inch / 12.7 cm High Velocity Aircraft Rockets (HVAR)
Empire of Japan (1943) Prototype Fighter Interceptor – 1 Built
The Kawasaki Ki-88 was a fighter interceptor designed in 1942 with the intent of intercepting enemy aircraft heading towards vital military locations. The Ki-88 would never see service, as it was cancelled in 1943 after a mockup and partial prototype were constructed. Although considered by many to be the Japanese copy of the American Bell P-39 Airacobra due to the exterior aesthetic similarities, this is only speculation.
The origins of the Kawasaki Ki-88 began in August of 1942 when Tsuchii Takeo, a designer for Kawasaki, responded to a design specification put forward by the Imperial Japanese Army Air Service (IJAAS). The IJAAS determined that they needed an interceptor aircraft that would defend important military assets like airfields, gun emplacements, and others. The specification also stated that the aircraft had to be heavily armed, provide a stable gun platform and be easily flyable by new pilots.
Takeo began work on the Ki-88 and chose to use a 37mm Ho-203 cannon as the plane’s primary armament, with two 20mm Ho-5 cannons to complement the Ho-203. The placement of the guns prompted Takeo to place the engine behind the cockpit. Many sources state that this was done to copy the American Bell P-39 Airacobra, but that claim is debated. The P-39 Airacobra was in service at the time the Ki-88 was developed, but saw limited service with the United States. It did however, see service during the Battle of Guadalcanal. The Japanese were certainly aware of its existence and possibly captured an example of the P-39. If they did indeed capture an example, Takeo could have simply copied the gun and engine placement. It is important to note that such a “rear-engine” fighter configuration was a rarity in plane design at the time. Another common theory is that Takeo came to the same conclusion as H.M Poyer (designer of the P-39) did during the planning phase and designed the plane without copying the P-39. Other than the engine and gun placement, the two planes are quite dissimilar.
Takeo completed the Ki-88’s design in June of 1943. A full scale mockup and prototype were in the works in mid/late 1943, and estimated that the prototype would be completed in October of 1943. However, after the mockup and plans were inspected by representatives of the IJAAS, it was concluded that the Ki-88 had no real improvements over other designs of the time, and the top speed was only slightly better than the Kawasaki Ki-61 after calculations. The IJAAS immediately lost interest and ordered Kawasaki to cease all work on it.
The Ki-88 was a single seater, single engine fighter powered by a Kawasaki Ha-140 engine producing 1,500hp while driving a propeller using an extension shaft. The radiator was placed under the cockpit at the bottom of the fuselage. There was an air intake placed beneath the fuselage on the left to provide cooling for the supercharger in the Ha-140.
The Ki-88 used a conventional landing gear, in which the main wheels could be retracted into the wings while the tail wheel stayed fixed. There was a fuel tank in each of the wings, beside the landing gear wells.
The size of the Ho-203 canon prevented Takeo from placing the engine into the nose which led him to place it behind the pilot’s cockpit, much like the American P-39 Airacobra. Moving the engine to the back of the cockpit was a smart move, as it theoretically would have made the plane a more stable gun platform. Under the Ho 203, on both sides of the nose, there were two 20mm Ho-5 cannons.
Empire of Japan – The Ki-88 was supposed to have been operated by the Imperial Japanese Army Air Service, but never did so due to the design being deemed as inferior to the Ki-61 and was thus cancelled.
Nazi Germany (1940)
Prototype Wooden Glider – 2 Built
The Ju 322 “Mammut” was a prototype wooden glider developed by Junkers in 1940 in anticipation of the Invasion of Britain. The design was riddled with flaws and eventually scrapped in 1941 after two prototype models were made. Instead, the RLM decided to use the Me 321 as their main heavy glider. No part of the Ju 322 is known to have survived to the present day.
History of the Mammut
Operation Sealion (Invasion of Britain) was to commence in the fall of 1940, and the Germans lacked a means of transporting supplies and troops effectively. In that same year, the Reichsluftfahrtministerium, the German Ministry of Aviation or RLM, issued a demand to Messerschmitt and Junkers to design and develop a glider capable of carrying a very heavy payload. The conditions were that the glider was to be able to carry some of the heaviest equipment in service with the Wehrmacht. Messerschmitt developed the Me 321 as a result, and Junkers with the Ju 322.
The Ju 322 “Mammut” (Mammoth) was designed as a fully wooden heavy transport glider which was originally designed to carry at least 44,000lbs (19,900kg). This weight was around enough for a Panzer III/IV, FlaK 88, or a StuG III/IV or a full load of 100 troops and all necessary support equipment. The Ju 322 was designed so that cargo was to be loaded into the plane from the nose, which could be folded. The cockpit had a single position, and was located on the outside of the cargo hold on the left wing. The glider would be on a carriage which would be dropped right after take off or while airborne. The designers noted that the carriage was extremely heavy, and could not be dropped from a high altitude without it breaking. They also noted that if the carriage were to be dropped from a lower altitude, there was the risk of it bouncing back up and hitting the glider. Many different kinds of gears were experimented on, using from as little as 8 wheels to 32 wheels. As for landing, the glider was fitted with four sprung landing skids. The production variants were suppose to be fitted with three turrets armed with MG 15s. Two turrets would be located on either side of the nose, near the front of the wings and the other turret would be located near the back of the cargo compartment. The Ju 322V1 and V2 were not armed.
After two prototype models were produced, stationary tests began. It was found that the Ju 322V1 had troubles with the materials it was built with. An observation made by engineers were that the wooden structure of the glider were weakened by rot. It was agreed that this was to be blamed on poor manufacturing techniques.
When a Panzer III was loaded onto the plane, the floor broke and the Panzer III fell straight through it. This incident was partly to be blamed on the ramp design and poor wood quality. Due to this flaw, the original design was not able to be met and the maximum cargo weight was reduced twice. The first reduction was to 35,280lbs (16,000kg), the second reduction was to 24,255lbs (11,000kg). The reduced weight of cargo and reinforced floor solved the problem of loading tanks and equipment on, but at the expense of payload. As a result of this along with other changes, the designers had to reduce the plane’s maximum cargo weight to 24,255lbs (11,000kg).
A common misconception is that there was a competition between Messerschmitt and Junkers to develop the best glider and dominate the glider market. However, it was not a competition at all and each company were given specific guidelines. Messerschmitt was allowed to use steel while Junkers was only allowed to use wood. This was because the RLM was anticipating a shortage of steel, in which case the RLM could fall back on the Junkers design. It is also worth noting that the Ju 322V1 used eight tons of steel to strengthen the airframe, despite the RLM’s orders.
As the Ju 322 was in prototype stage, only two models were ordered and constructed. The only two models are known as Ju 322V1 and Ju 322V2. V1 was the only model to see testing, while V2 stood by in case V1 was destroyed. During testing of the V1, construction began on 98 airframes, although none were completed.
The Ju 322V1 made its first and only flight in April of 1941 at Merseburg Airfield. According to the reports, the Ju 90* towplane failed to lift the glider off the ground on full throttle. In a subsequent attempt, the glider was able to get off the ground. However shortly after takeoff, the tow plane pilot noticed two immediate flaws. First, the glider could not maneuver or change direction and it had no pilot during the test. Second, the glider had extremely poor vertical stability such that its wings would sway in small arcs which swung the tow plane dangerously. Because of this, the glider was immediately cut from the tow plane after take off. The glider ended up landing in a field not far from the airfield. It took over two weeks for the glider to be transported back to the airfield by towing. This was the Mammut’s only test flight, and it was deemed a failure.
* – It is interesting to note that the Ju 90 which towed the Ju 332 on it’s maiden flight was one of the two Ju 90s meant to be sent to South Africa before the war, and were therefore fitted with Pratt and Whitney Twin Wasp engines which had 900hp each.
Financially exhausted and convinced that the Ju 322 will not be successful, Junkers finally terminated the projected in May 1941. As a result, the two Ju 332s were cut up and used as firewood, along with all the uncompleted airframes and spare parts still in factories.