XP-67 “Moonbat” – The Plane That Loved to Catch Fire

The XP-67 Moonbat was an experimental fighter and McDonnell Corporation’s first serious opportunity to impress the US military establishment. Featuring a pioneering morphed fuselage that gave it the appearance of a bat, the XP-67 was trailblazing in both a metaphorical and literal sense.

Contents

Origins

In July 1939 McDonnell Aircraft Corporation was officially established and looking to make a splash by bagging its first ever aeronautical contract. Keen to get the ball rolling, later that year James McDonnell proactively approached the Army Air Corps to see if they were interested in some of the company’s novel fighter designs. Although McDonnell’s proposals were rejected for not meeting current Air Corp requirements, his tenacity was rewarded when the Army invited his company to submit blueprints for the Request for Data R-40C competition issued in March 1940 for a fighter aircraft.

The result was the McDonnell Model I proposal, which outlined a fighter powered by either an Allison V-3420-B2 or a Pratt & Whitney H-3130 that was to be mounted in the fuselage aft of the pilot. The engines were to drive two pusher propellors via extension shafts and an angled gear drive – a rather unconventional layout for its time.

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Despite not winning, the Air Corps were sufficiently impressed by the inventiveness of the submission to award the young company its first ever sale in the form of a Purchase Order worth 3000 dollars on June 6th 1940. Having piqued the interest of Army officials, McDonnell submitted another proposal in the same month for a two-passenger Model II fighter that was to be propelled by a pair of Continental I-1430s – a more conventional twin engined arrangement.

McDonnell Model I blueprints from 1939.
McDonnell Model I blueprints from 1939.

McDonnell’s efforts were again unsuccessful, but following feedback from the Air Corps it was decided to present a revised version on May 5th 1941 that was renamed Model S-23-A. McDonnell envisioned that this radical fighter would principally be used to engage and destroy bomber formations. He also reasoned that the overall efficiency of the craft could be optimized if the fuselage contours were blended into an airfoil shape, thus complimenting the wing’s lifting abilities and significantly decreasing drag. 

This time McDonnell’s perseverance and creativeness paid off and they were granted Contract W535-AC-21218 on August 2nd 1941 by the Chief of the AAF Experimental Engineering Section, who required them to create two prototypes and to transfer any associated data. The new project would be given the moniker XP-67 by the Army and by April 15th 1942, after a relatively smooth development period, a full-scale mock-up was available for inspection. 

The XP-67

The XP-67 was 13.65 meters in length, 4.80 meters in height, had an empty weight of 8,049 kilograms and a maximum weight of 11,521 kilograms. By merging the centre fuselage and the rear segments of the engine nacelles in a concerted attempt to maintain true aerofoil sections, the XP-67 resembled a bat in appearance and was replete with chiropteran wings with a span of 16.76 meters and an area of 38.46 meters squared. 

The XP-67 was installed with dual Continental XI-1430 turbosupercharged twelve-cylinder inverted-vee liquid-cooled engines which were placed inside lengthy engine nacelles, which in turn drove a pair of four-bladed propellors rotating in opposite directions. Interestingly, the exhaust at the back also doubled-up as an extra source of thrust.

Further adding to the mix were thrust augmenters installed in the engines, which were able to squeeze out some extra velocity from the engine gases after they had passed through the turbosuperchargers.

The XP-67 prototype.
The XP-67 prototype.

Another innovation that was planned but never implemented involved a novel use of the XP-67’s turbosuperchargers. Via these mechanisms, on take-off the engines were to emit 1300 horsepower of thrust, which would change to 1600 horsepower once the plane had reached an altitude of 7620 meters.

Performance-wise the XP-67, which could soar to a maximum altitude of 11,400 meters, had a top speed of 652 kilometers per hour, a cruising speed of 435 kilometers per hour, a rate of climb of 13 meters per second and a combat range of 2,384 miles.

Read More: XP-55 Ascender: the Back-to-Front Fighter

The production version of the XP-67 was anticipated to have a significant arsenal of weapons. The first proposed layout comprised six 0.50 inch machine guns and four 20 mm cannons. A later specification revised this, and instead opted for six 37 mm cannons with 45 rounds each in the wing sections. Towards the end, a massive 75 mm gun installation was even considered, but like the production edition of the XP-67, this would never come to fruition.

Continental XI-1430 V12 engine.
The Continental XI-1430 inverted V12. This engine is often said to be the source of the XP-67’s problems, but it was actually the airframe around it that was at fault.

Furthermore a handful of planned features, including a pressurized cabin, an oxygen system, and a set of drooping ailerons, were also never added to the single XP-67 prototype. Neither were a ream of additional components and layouts that were to be potentially installed on the second prototype and production model.

Here engineers toyed with substituting the contra-rotating propellors for handed propellors, and also considered using a mixed powerplant arrangement in which the front of each nacelle would house either a Packard V-1650 or Allison V-1710 inline engine with a two speed supercharger. Both were to be augmented by an I-20 turbojet.

Yet, like all the other unheeded suggestions offered up, they would never see the light of day.

Flight Tests

Rolled out by early December 1943 and registered with serial number 42-11677, the XP-67 was painted with a standard Olive Drab and Neutral Gray finish, which was complimented by a smidgeon of green dappling around the leading and trailing edges of the flying surfaces.

The XP-67 first undertook a battery of high speed taxi tests at Lambert Field, where it was slightly damaged after one of the turbosupercharger units cracked and became engulfed in fire, burning the interior of the engine nacelle. It was an incident that would set the tone for upcoming trials, foreshadowing many later disasters.

Moonbat from above.
This top-down view gives a good impression of the XP-67’s unique shape.

What made this all the more dangerous was that since they were buried deep within the nacelle the blaze would not be noticed by the pilot until the exterior paint began to smoke and blister. 

Nevertheless, repaired on December 8th 1942, it was next transferred to Scott Field in Illinois where it was prepared for its first flight, set to take place on January 6th 1944. That day with test pilot E.E. Elliot at the controls, the XP-67 performed a 6 minute flight, starting with a brief circuit of the test field but ending with an emergency landing due to faulty turbo compartments.

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In response, they were encased in stainless steel firewalls to prevent fire spreading. Precautions having now put in place, a second and third trial were completed without a hitch, but trouble was brewing on the horizon once again.

The XP-67's blended engine nacelle.
The XP-67’s blended engine nacelle during wind-tunnel testing.

On February 1st 1942 a routine test flight suddenly went awry after the XP-67’s engine bearings were completely burnt away due to powerplant complications. Sent back to the McDonnell plant in St Louis, the cooling ducts were completely remodeled while the horizontal tail surfaces were raised 30.5 centimeters in order to improve longitudinal stability characteristics.

Moreover with the engines completely wrecked Continental were forced to supply the XP-67 with replacements while the original propulsion system was being fixed. But when testing was resumed in March the new powerplants were found to be substandard, immediately putting a halt to proceedings.

Flight evaluations, now conducted by Army Air Corps pilots, were finally restarted on May 11th 1944, after which a further spate of problems, such as engine roughness, improper aileron balance, and faulty main undercarriage door closure were identified and corrected over several months. It was during this period also that the XP-67’s difficult handling characteristics were fixed with the addition of small dorsal fin and an increase of the angle of tailplane dihedral from 5 degrees to 7 degrees.

Moonbat front on during work.
This aircraft has an incredible head-on appearance, thanks to its blended fuselage and wings.

On the other hand, issues with the engines unfortunately persisted, reaching an unexpected and ruinous climax on September 6th 1944 after the XP-67 had clocked up a total of 43 hours flight time. It was on this date that XP-67 prototype was damaged beyond repair after a fire engulfed the right nacelle mid-flight, forcing Elliot to make an emergency landing. Upon touchdown gusts of wind started to blow the fire over the fuselage, causing even further destruction.

It must be noted however that the experimental engines were not actually the source of the problems – it was due to the airframe improperly catering to them (ducts, mounts, feeds, etc).

Cancellation

With flight trials only 15% completed, on September 13th 1944 at a meeting of Air Corps and McDonnell representatives it was advised that the entire project should be cancelled, an opinion also motivated in part by the fact that by 1944 the need for a bomber destroyer was virtually non-existent. Six weeks later a formal termination notice was issued, meaning the near complete second prototype was also scrapped.

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However, although McDonnell’s earliest foray into fighter-planes had been a failure, success would soon come with the development of the FD-1 Phantom, the US Navy’s first jet fighter, a craft that would fully cement the company’s reputation as a titan of industry.

Specifications

Capacity: 1
Length: 44 ft 9 in (13.6 m)
Wingspan: 55 ft (16.8 m)
Height: 15 ft 9 in (4.80 m)
Empty weight: 17,740 lb (8,000 kg)
Gross weight: 22,100 lb (10,000 kg)
Powerplant: Two x Continental XI-1430, 1600 hp
Maximum speed: 405 mph (650 km/h)
Range: 2,400 miles (3,860 km)
Service ceiling: 37,000 ft (11,300 m)