NOTE: The below drawing were an old version and have been replaced with ones more in line with my original vision with more Convair and less Lockheed influence. The new drawing can be found
At This Post.
=================================================
11 July 1944
Consolidated Vultee Aircraft Corporation
Research & Development Laboratory
San Diego, California, USA
It has been more than year since AVCO merged their Consolidated Aircraft and Vultee Aircraft divisions. In that time, all the senior Vultee management left the company and Consolidated became the primary headquarters and R&D facilities for the new Consolidated Vultee Aircraft Corporation— now being informally called “Conviar.”
Luckily, many of the Vultee engineers were still with the company and were able to continue their various projects and undertake new projects with which the Consolidated group had little experience. Primary among these projects were the continued work with Lockheed on the P-38 and the in-house developed Very Long Range fighter derived from the P-38, the XP-81.
Most of their efforts in the second half of 1943 were on the P-38, specifically a new two-seat version commissioned by the Army Air Forces Fighter Branch to improve pilot introduction and familiarization with the type in an effort to reduce new pilot accident rates. The first Vultee TP-38, built to P-38H-10 specification, the TP-38H-VN, was delivered to the Air Corps for tested and acceptance in November, and they had completed 18 full airframes by the end of the year. In the time since, the old Vultee plant in Nashville had increased production to just over 20 aircraft per month.
After the dissolution of P-38 RTU Groups of the U.S. based 4th Air Force, and their re-formation as combat groups, the Army no longer needed as heavy a concentration of TP-38s so they distributed them to the airbase training groups—for new pilot instruction—and among all active P-38 Groups, including overseas Combat Groups. Almost as soon as the forward Groups received the two-seater P-38s they started to modify them for use in Combat.
In Europe, the 20th Fighter Group had previously converted several single-seat P-38s to be bomb leaders by removed the armament compartment, replacing it with a bombardier station with a glazed nose and Norden Bombsight. Once they received their Vultee TP-38s, VIII Bomber Command soon realized that the aircraft could be converted into carry an H2X “Mickey” (AN/APS-15) RADAR for target mapping, with the back seat becoming the Radar Operator’s station—there were even rumors of VIII Bomber Command planning on experiments using H2X equipped TP-38s in place of Mickey equipped heavy bombers to act as guide bombers when bombing through overcast in an effort to save reserve the heavies for ordnance delivery, although no word as to whether or not any of these missions have been carried out has yet come back to Vultee.
In the Pacific, another use for the two-seat lightings was found. The 5th Air Force had successfully installed the standard nose armament into a TP-38 and slung a Navy sourced AN/APS-6 intercept radar pod under the nose so that the aircraft could be used as night interceptor while waiting for greater numbers of the purpose-built P-61 to arrive in theater.
This last adaptation, coupled with delays in P-61 delivery, had prompted Fighter Branch to reach out directly to Vultee over to find a more permanent solution. The request was for Vultee to adapt the long-range search and intercept radar developed for the P-61, the SCR-720, to the TP-38 and still allow installation of “armaments of sufficient firepower and capacity to successfully engage the majority of Enemy Aircraft, not less than four (4) AN-M2 .50 HMGs or four (4) A/N-M2C 20mm cannons, or any combination thereof.”
By drawing on experience from Europe on modifying the nose of the P-38 to fit the H2X radar, which was of similar size to the SCR-720, the Vultee engineers were able to design an installation for the latter radar which also included the required minimum of four .50 caliber Browning machine guns in what they were calling a “chin tray” with up to 500 rounds per gun.
The Army had accepted the design and ordered several TP-38Hs to be converted by the factory as XP-38M (with the field modified P-38 Night fighters using AN/APS-6 radar now officially designated as P-38L) for testing and acceptance. If successful, they were expecting much of their TP-38 production to be shifted to P-38M Night fighters by the end of the year.
The work on developing the radar equipped P-38, in turn, informed the much larger Convair fighter project, the XP-81.
From the initial conception, Convair had approached it as a Two-Place VLR Escort—with their design focused on the future of extremely long flights in the company of the truly massive XB-36. Convair’s idea, not dissimilar from North American Aviation’s proposal for the competing XP-82, was that the two pilots could share the flying load during these long missions, reducing overall pilot fatigue. The Army, however, was still somewhat resistant to the idea of a two-seat day-fighter, so the initial order for two prototypes had been revised to include a single place fighter, XP-81A, and Convair’s original two-seater, now called XP-81B.
The altitude of the planned escorts, in support of the XB-36, required that the aircraft have at least partial pressurization. Convair accomplished this into the design by drawing on Lockheed’s work with the early P-38A and the more recent pressurized example of the XP-38J.
The XP-81 gondola was designed as a semi-monocoque fuselage sharing the 38” outer diameter with the P-38 but, being designed for pressurization from the outset, the interior design of this core section was of a near-perfect cylinder, tapering only slightly aft of the pilot station, and capped at either end by pressure bulkheads. This would produce an inner pressure vessel discrete from the unpressurized equipment spaces.
The addition of a pressure bulkhead forced them to revise the gondola design slightly, including moving the nose gear slightly farther forward, but even though this slightly enlarged certain dimensions of the gondola it ended up allowing a more aerodynamic cylinder and improvements to overall equipment fitment.
Most importantly, the re-designed gondola and use of a “chin-tray” permitted an increase in the nose armament to YP-49 levels of four .50 caliber machine guns and two 20mm cannon. In addition, the arrangement will also allow the optional installation of an intercept radar, such as the AN/APS-6 with its 17” dish, directly in the centerline of the nose, permitting the aircraft to act as both a day fighter and single seat night fighter without modification.
Even more so, the application of the “Chin-Tray” gun installation was applied to the revised XP-81B two-seater design so that the example planned to be delivered to the Army Air Force would already include an SCR-720 and four .50’s plus show how additional guns could be carried by installing two 20mms in extended lower wing fillet mounts with up to 75 rounds per gun in magazines in the wings just inboard of the flaps.
Convair hoped that by delivering these two examples with the XP-81 aircraft it would illustrate to the Army that the airplane could be armed in several different ways by applying varying gun packages in combination. Once example would be the option of removing the radar, moving the four top-mounted machineguns from the XP-81A nose farther forward and include all four of the chin-tray machineguns found on the XP-81B for a full eight-gun nose. This eight-gun nose could also be used on the XP-81B by getting rid of the SCR-720, and could then also be supplemented with the two 20mm cannons under the wing-roots. The overall goal was more to show the Army the potential for the aircraft to be adapted for specific missions or needs, in the field, without the need of providing custom variants for each.
The gondola was only a small part of the design, of course. Every part of the airplane, although outwardly resembling the P-38, was redesigned to improve performance, increase range, and simplify manufacture.
As with any high-performance military aircraft, the power plant was of utmost importance. They had initially considered designing the airplane around General Electric’s experiment axial-flow Turbo-Prop, the TG-100, but it was still in the early stages of development and the even the “on-paper” design specifications for it showed it would produce about 2300 shaft horsepower—about the same power available from Allison’s new V-1710-F29 turbo-charged, water-injected, engines already in use on the new P-38J. The questionable availability of the new turbo-prop without much promise of better total output and the estimation of greater fuel use meant that Convair, instead, focused the design around improved V-1710s.
The problem came when the estimated Gross Weight of the new aircraft showed that—even when considering the improved aerodynamics—if fitted with the same engines, it would offer only slightly increased performance and range over the existing P-38J.
The struggled with this over the first few months of development, even while pressing ahead with the majority of the design. The question of engine installation held back some of the most critical design work. It was a junior Allison engineer, Tomislav Kaczmarczyk—the son of Polish immigrants—who was finally able to provide a way forward.
Tom Kaczmarczyk had been up at Wright Airfield in December and January to help the Army work out any problems with the newly developed water-injected engines. While he was there, he heard about some experiments NACA was running at their Cleveland Laboratory using a “Blowdown” turbine to recover power directly from the engine exhaust. He mentioned this experiment to his superiors in the hopes they would be able to get access to more information and that the solution, if successful, may solve the power plant problems for the XP-81.
Allison was able to gain access to the preliminary data from the NACA experiments and immediately set out to apply the methods of exhaust power recovery to the V-1710. Their initial design, based directly on the NACA test installation, used a single 12” diameter blowdown turbine drawing power through two nozzle boxes (one per engine bank) and connected through a reduction gear to a crankshaft extension extending from the engine driven supercharger. This design was enough for them to test the concept and prove that the theory could be applied to the Allison engine but its ultimate performance was limited by the nozzle arrangement and the limitations imposed on the system by the heat of the exhaust.
Still, the estimated 9%-11% improvement in power over the range of operational engine speeds was enough for them to send to Convair to propose its use in the XP-81, which Convair happily accepted.
Once Convair sent their revised engine specification and requirement to the Air Corps, Allison was provided additional budget to accelerate development of the new engine. By the middle of June, Allison had sent the expected dimensions, specifications, and weights of the new engine to Convair, allowing Convair to finalize the initial aircraft design.
During all this, Convair had continued their work on the XP-81. The wing was where most of their time had gone.
To improve performance, reduce parasitic drag, and increase range, Convair worked up a NACA 6-Series laminar flow wing. The first challenge was to provide the space required for fuel. With the goal of 500 U.S. gallons of internal fuel space, they wanted to provide MAIN and RESERVE fuel tanks in the center wing section large enough to accommodate the bulk of this fuel requirement. This meant a broad chord and relatively think wing, which was made even larger by use of the laminar flow 6-series wing. They settled on a 63-113—essentially an improved version of the current P-38’s 23016 with its 0.2c leading edge extension—as it permitted the space needed for a large MAIN tanks and adequate RESERVE tanks, even after providing sufficient space for the leading-edge mounted radiators.
The engine nacelles were lengthened forward of the main spar to accommodate the extra space and weight of the power-recovery turbines, a setup which was now being Turbo-Compound. This shift in the Center of Gravity forced Convair to extend the booms aft by 12 inches and re-design their original outer wing sections. Originally conceived with a trapezoidal planform design similar to the P-38, with equal sweep on both leading and trailing edges, the shift in weight and the more rear-ward center of lift of their selected NACA 65-113 airfoil forced them to alter this to increase the aft weep and dramatically reduce the forward sweep, to 14° 53’ 42” and 2° 41’ 39”, respectively. An effect of this change in planform was that the main spar of the outer wing would need to be swept
forward at 5° 56’ 24” to keep it at the appropriate 44%-42% chord.
To provide additional strength to this forward swept loading, a leading shear beam was added in addition to the trailing shear beam of the outer wing at about 21% Chord. Using the chord-wise internal corrugations like those in the P-38’s leading edge to anchor this forward beam with the main spar caused the entire structure to act as a torsion box to resist undesired movement in the wing.
Since this forward sheer beam was not directly part of the leading edge this allowed for the design to use a replaceable leading edge sub-assembly, similar to the P-38, but unlike the P-38 this leading edge would not need to carry the weight of fuel, since the space within the torsion box, between the Main Spar and forward beam, could accommodate a 65 U.S. Gallon fuel cell.
The expected stall speeds of the laminar flow wing on a heavier airframe, however, caused the flight dynamics engineers some concern. To alleviate the expected high-stall speeds and improve overall low-speed handling, they added Handley-Page leading edge slats to the outboard 96.5 inches of the otherwise empty lead edge sub-assembly.
The wing design went through several iterations, each being adjusted and the theoretical performances calculated. The initial design, even farther forward swept with a straight leading edge, led to nicely lower wing-loading but also a greatly reduced aspect ratio which would have limited the airplane’s high-altitude performance. They settled on the final design as a compromise which offered good wing loading and an aspect ratio similar to that of the P-38, ensuring good High-Altitude performance in support of the projected high-flying B-36. Convair went one step farther by designing the wingtips to be removable and replaceable and by designed two different types, intended for different missions: a “long span” semi-elliptical tip for long-range, high-altitude flights; and a “clipped” wing tip for improved low-to-mid-altitude maneuverability.
With the trailing edge of the wing extended back at almost 15 degrees in the final design, they were able to finish designing the fuel cells and internal spaces of the center section, finally settling on a 115 U.S. Main tank between the Main Spar and Rear Shear Beam; and a 70 U.S. Gallon Reserve tank forward of the Main Spar and aft of the Radiators. With the 65 U.S. Gallon Outer Wing tanks, this would give the XP-81 their target internal fuel capacity of 500 U.S. Gallons—a 25% increase over the P-38J.
Between the extra fuel, improved aerodynamics, and projected 10% +/- efficiency of the engines—even when considering the additional weight of the larger airframe, larger engines (due to the additional turbines), pressurization bulkheads and equipment, and water (for the Water Injection system, stored in tanks aft of the nose gear, under the cockpit)—the XP-81 was expected to have a range increase better than 30% over the P-38J, for a total range on internal fuel of over 2000 miles.
To farther improve this (minimum) expected ranged, Convair was designed the wing from the onset with 2000 Pound wet store on Station 49, the mid-point of the center wing section, similar to the P-38, and supplementing it by including 1000 pound wet store at each wing tip. Total possible fuel capacity then—with 500 gallons internal, two 310 gallon tanks on the main racks, and two 165 gallon tanks at the wingtips—would be as much as 1,450 Gallons and a theoretical range of at least 5000 miles. In additional to these wet-racks, they were also providing provision to carry up to 650 pounds on the outer-wing hard points at Station 200, just out board of the flaps and inboard of the ailerons.
The final design from which the first mockups were currently being built was something of which everyone involved in the project was proud.
{If you're not getting the joke, Google it. If that doesn't give it to you, I'll explain.})
