Flightline: 175 - McDonnell XP-67 Bat/Moonbat
The McDonnell XP-67 was a proposed 1940 interceptor for the USAAC, but the troubled design as well as engine fires cancelled the project.
In 1940 the US Army Air Corps, fearing that the growing war in Europe and Asia would come to the US, issued Proposal R-40C, requesting an interceptor with high-speed. high-altitude, and long range. McDonnell, incorporated as a subcontractor the year previous and eager to begin building their own aircraft, submitted plans and specs for their Model I, powered by an Allison V-3420 which was buried in the fuselage and drove a pair of pusher props via an unusual gearing system. The unproven drivetrain and modest performance projections saw the Model I ranked 21st out of 23 proposals received, but the Air Corps granted McDonnell $3,000 ($58,000 in 2021 dollars) to redesign the aircraft.
The McDonnell Model 1, with four nose guns and pusher props. | Illustration: McDonnell Aircraft Corp.
Try, Try Again
McDonnell returned to the USAAC on 30 June 1940 with their Model II, which maintained the lines of the first model but now featured two Continental XI/IV-1430 engines in underwing nacelles now driving tractor propellers. Wind tunnel tests found that the configuration resulted in too much drag, so McDonnell went back to the drawing board again.
The wind tunnel model of the Model II. Among other changes, the crew was reduced from two to one. | Photo: McDonnell Aircraft
The Third Time's The Charm
After nearly a year of refinements, McDonnell presented their Model IIA on 24 April 1941. The basic lines of the Model II remained, but the engines were moved into nacelles within the wing, with filets blending the wing, nacelles and fuselage in order to maintain an airfoil across the whole of the aircraft. The redesign added turbosuperchargers to the XI/IV-1430-1 inverted V-12 engines, with the exhaust gasses augmenting the thrust. The aircraft's cockpit would be pressurized, which was rather innovative at the time. McDonnell's engineers projected that the final design would reduce drag and produce a top speed of 472mph. Being an interceptor, a number of different gun configurations were suggested, including six .50 cal machine guns, four 20 mm cannon, and a single75 mm cannon, before a final armament of six 37 mm M4 cannon was selected. The Model IIA impressed the Army enough to award McDonnell a contract worth $1.5 million for two prototypes (s/n 42-11677 and 42-11678), a wind tunnel model and the associated engineering data, designating the plane the XP-67.
Orthograph of the XP-67. | Illustration: McDonnell Aircraft
McDonnell teamed with the University of Detroit and NACA for an test program to further refine the aircraft, including extensive wind tunnel tests. As the company had never produced a complete aircraft previously, McDonnell was forced to develop new techniques and tooling for the prototypes, and testing was hampered by competition for NACA's wind tunnels at their Langley laboratory. Issues with the development and production of the Continental engines delayed completion of the XP-67s, and tunnel tests indicated cooling problems which were never really solved. The US's entry in WWII on 7 December 1941 resulted in manpower and supply shortages, further slowing production.
A wing section of the XP-67 with an XI-1430-17 engine and a GE D-23 turbosupercharger used for wind tunnel testing. The ducting and installation of the D-23 were unusual and resulted in numerous delays of the program. | Photo: LMAL Archives
Construction of the prototypes continued through 1942 and '43, though wind tunnel tests showed numerous changes were needed. Tests of a wing section showed that stronger mounts were required, as were changes to the cooling ducts, horizontal stabilizers and the cockpit pressurization system. The ducting in the test wing section was modified, and 51 hours of tunnel tests at Langley and Wright Field validated the solution to the vibration and cooling problems, but the changes were not to be made to the first prototype until after the maiden flight. McDonnell was ordered to stop work on the second XP-67 until flight tests of the first were conducted and the design finalized.
The first XP-67 prototype nears completion an McDonnell's plant in November 1943. At this point in the plane's design the armaments consisted of six 37mm cannon, and the ports for the guns are visible here. | Photo: McDonnell Aircraft
Trials and Tribulations
Serial number 677 was ready for ground trials on 1 December 1943, fitted with Continental XI/IV-1430-17/19 engines and General Electric D-23 turbosuperchargers. The pressurization system was still not sorted out, and a conventional sliding canopy was installed in place of the planned side door; the expected 37mm guns were not fitted either. A malfunction in the exhaust manifold slip rings caused double engine fires during a ground run on 8 December, setting the project back once again. Repairs were completed in January, and the XP-67's maiden flight was conducted on 6 January 1944, nearly two years later than called for in the initial contract. The flight was cut short just six minutes in after both turbosuperchargers began to overheat, causing minor damage. The aircraft was again repaired, and flight tests recommenced on 26 January, followed by the third flight on the 28th. The fourth flight on 1 Feb was again cut short, this time by a bearing failure due to an accidental overspeed of the left engine. By this point it was clear that the engines were not performing as well as was hoped, and the XP-67 was falling short of the promised specs. With the 1430s only producing 1,060hp instead of the promised 1,350, the XP-67 was falling well short of the expected 500mph, topping out at just 400mph, roughly the same speed of a P-38 or P-47. McDonnell requested funding to replace the Continental engine with more powerful Allison V-1710 or Rolls Royce RM 14SM piston engines, supplemented with Westinghouse J39 turbojets installed in the end of the nacelles. The Army rejected these calls, instead demanding more testing of the existing design.
Head-on view of the XP-67, prior to September 1944. | Photo: USAAF
While the XP-67 was grounded awaiting new engines, upgrades informed by wind tunnel tests were added, including revised cooling ducts and the horizontal stabilizer being raised 12". At around this time, work on 678, the second prototype, resumed, with the exception of components potentially affected by an engine change. Flight tests of 677 commenced again on 23 March 1944, with USAAF pilots flying the aircraft starting on 11 May. The Army fliers found the aircraft, which had acquired the unofficial nickname 'Moonbat', or just 'Bat', to be generally satisfactory, with a fair cockpit layout, good ground handling, effective controls at all speeds and an adequate roll rate. The Bat received poor marks in engine power, speed, acceleration and a long takeoff roll, however, as well as a noted tendency to Dutch roll under certain conditions. Most troubling was the aircraft's pre-stall behavior, which began with a buffet well above the actual stall speed, followed by the nose tucking upwards during the stall; the XP-67 also felt tail-heavy in fast turns. Pilots, fearing that a spin would be unrecoverable, refused to test the Bat's spin characteristics. The pilot's report concluded that the Moonbat was generally deficient compared to existing aircraft.
A view from above on the XP-67 in flight shows its unusual planform. | Photo: USAAF
1677 from the side. Pilots reported that the Bat provided poor visibility to the sides and rear. Formation flights would have been challenging, as pilots were unable to see their own wingtips due to the engine nacelles. | Photo: USAAF
By this point the Army Air Force was questioning the need for the XP-67 program, and calls to cancel the contract began. Others remained confident that the plane's shortcomings could be remediated, and that the aircraft could still serve as a test mule for different weapons, and plans were floated to replace the 37mm cannon with 12 .50 cal machine guns or eight 20mm cannon as a possible attack aircraft. Another possibility was to adapt the mixed-propulsion concept to a photo-recon aircraft to compliment or supplant the Lockheed F-5, a modification of the P-38 Lightning. Wind tunnel models were produced to test the configuration, but it's unclear how far along those ideas progressed.
Wind tunnel model of a potential configuration of the mixed-propulsion XP-67E photo-recon variant. | Photo: LMAL Archives
And the axe falls...
During August 1944 the prototype underwent a number of repairs and modifications, including a change in the wing dihedral from 5° to 7° in the hopes of improving stability. On 6 September the tests resumed again, but during the flight the exhaust valve rocker on cylinder #1 on the right engine failed, causing a fire. The pilot was able to land safely, but a brake failure prevented him from angling the plane into the wind, as a result flames spread from the right engine through the aft fuselage into the left nacelle. The aircraft burned for nearly half an hour, and the damage was so severe that the tail collapsed. Aircraft 1677 had a total of 43 flight hours at the time of the accident.
The forward half of the XP-67 after the 6/9 fire, which damaged the wings, nacelles and aft fuselage. | Photo: USAAF
The Army Air Force, seeing no future in the XP-67 program, suspended further work on 13 September, and issued a formal cancellation of the contract on 27 October. Prototype #2, which was 15% complete, was subsequently scrapped. Total costs of the XP-67 project was approximately $4.7 million dollars (roughly $72 million in 2021).