McDonnell XP-67 Moonbat by Anigrand

1/72 scale
Kit no.  AA-2056
Cost: $45.00
Decals: One version – USAAF
Comments: Cast resin with single vacuform canopy; engraved panel lines, detailed wheel wells, individually mounted propeller blades, some experience with resin kits recommended

History

The XP-67 Moonbat has the distinction of being the McDonnell Aircraft Corporation’s first fighter design, no small honor given McDonnell’s subsequent spectacular success with the F-4 Phantom II.  If McDonnell’s FH-1 Phantom of the 1940’s, the U.S. Navy’s first production jet fighter, could be considered the F-4’s father, then the XP-67 Moonbat was certainly its grandfather, for the Moonbat influenced the design of the FH-1.

The XP-67 was a twin-engine prototype fighter of World War II with a futuristic, blended airframe design reminiscent of today’s stealth aircraft, having virtually no right angles or sharp edges from nose to tail.  Designed as a heavy fighter to be armed with either four or six 37mm cannon, part of the XP-67’s mission would have been to intercept the long-range Amerika Bomber that Nazi Germany was suspected of developing in the early 1940’s.  No American fighter save for the Bell P-39 Airacobra, with its single 37mm cannon, had anything like the Moonbat’s contemplated firepower.  Initially planned to have lighter armament in the form of four .50 caliber machine guns and two 20mm cannon, the Moonbat’s weaponry was improved in response to the German bomber offensive against England, the “Blitz” of 1940-41, and the potential threat of German development of a long range, trans-Atlantic bomber force.

From the start, the XP-67 suffered from powerplant problems: its engines were not only underpowered, but prone to catch fire.  While conceived as an advanced fighter, it was hampered by what were essentially pre-war performance specifications.  McDonnell developed the Moonbat in response to Request for Proposal R-40C issued by the U.S. Army in 1940.  R-40C called for radical new aircraft designs, and was part of an effort to expand and re-equip the Army Air Force in anticipation of what by 1940 seemed an increasingly likely war with Nazi Germany.  The result was McDonnell’s Model I, a conventional and generally unimpressive aircraft save for one feature: an Allison V-3420 engine buried deep in twin engine nacelles, with a complex and as yet undeveloped drive gear.

The Army rejected the Model I, but wanted to encourage innovation, so it awarded McDonnell a $3,000 contract for preliminary engineering data. On June 30, 1941, McDonnell submitted a revised design, the Model II.  It was yet more conventional than the Model I, and the Army rejected it also, but showed continuing interest in the work of McDonnell’s design team.  Told by Uncle Sam to keep at it, on April 14, 1942, McDonnell submitted the Model IIa, which would ultimately become the XP-67.  The Model IIa was replete with aerodynamic curves due to its blended design, and left an overall impression of sleek deadliness.  This time, the Army reacted with enthusiasm.

But the radical nature of the XP-67’s design nearly killed it on the drawing board.  A faction within the Army Air Force wanted the project shut down, arguing with some justification that McDonnell’s company lacked the robust engineering staff required to develop what was — conceptually speaking — a completely new type of airplane, although they excelled as subcontractors building components for other manufacturers.  With an eye to the post-war future, McDonnell lobbied hard both in Washington and at the Wright Field offices, ultimately getting the go-ahead from none other than General Henry “Hap” Arnold himself.

Wind tunnel testing revealed that manufacturing tolerances would have to be rigidly controlled in order to maintain an extremely smooth finish to the XP-67’s exterior skin and benefit from the laminar flow airfoil design, achieving the promised top speed of 472 mph.  But the results from Langley’s wind tunnel facility pointed up an even more critical problem: the XP-67 would have extensive engine cooling problems as a result of its leading edge duct design.  The culprit was also the revised powerplant — the pair of Continental X1-1430-1 engines and their tendency to overheat.  McDonnell wanted  to immediately re-design the duct system, but Colonel C.F. Green of the National Advisory Committee on Aeronautics advised that a relatively minor design change in the ducts and adjacent leading edge areas would cut drag by 25 percent and increase cooling “potential” by some 200 percent.  Unfortunately, these changes would not be incorporated into the design until the second prototype, construction of which was later placed on hold in October 1943, when it was only 15 percent complete.  The failure to redesign the duct system immediately would play a critical role in the Moonbat’s demise.

Taxiing tests began in December 1943 as the first prototype neared completion, missing only its armament, pressure cabin, oxygen system and aileron drooping controls.  On December 8, 1943, fires started in both engine nacelles during an engine run-up.  The cause was traced to exhaust manifold slip rings which had failed, igniting the self sealing oil tanks.  The fires were quickly extinguished, but the problem was a
warning of things to come.

During its first test flight on January 6, 1944, the XP-67 handled well but quickly overheated, forcing test pilot E.E. Eliot to make a hasty landing 15 minutes after takeoff.  Soon afterwards, McDonnell learned that the troubled Continental engines would not be available for production models of the XP-67.  Recognizing this as a blessing in disguise, McDonnell seized the opportunity to lobby the Army Air Force for Allison or even Rolls Royce Merlin engines.  The Merlin was especially desirable, for it had transformed the P-51 Mustang from a good low level fighter and reconnaissance platform into a superb high-altitude fighter escort.  The AAF made noise about providing the best available engines, but shied away from a hard commitment.

The next test flights on January 26 and 28, 1944 revealed that the XP-67, while it handled beautifully, was seriously underpowered.  Its Continental X-1430 engines generated 1350 hp take off power and 1150 hp from sea level to 25.000 feet at normal power.  This was deemed sufficient in 1940-41, when the Continental was first selected as the powerplant.  But this was 1944, and in comparison the XP-67’s Allied contemporaries already in service exhibited superior performance — the Lockheed P-38L Lightning’s twin  Allison V-1710’s generated 1425 hp each; the Republic P-47D Thunderbolt’s Pratt & Whitney R-2800, 2535 hp; the North American P-51 Mustang’s Packard-built Merlin, 1590 hp; and the British Supermarine Spitfire Mk. Va with it’s V-12 Merlin, 1478 hp.  Keenly aware of these performance statistics, James McDonnell intensified his lobbying for Merlin engines for the Moonbat.  Ultimately these efforts failed; when pressed on the matter, the Army made it clear that 100% of the production of the American license-built Merlins was spoken for — they were earmarked for the Mustang, period.

Meanwhile, other aerodynamic problems surfaced during additional flight tests.  In accelerated turns the Moonbat grew tail heavy and began to buffet at a speed well over its expected stall speed.  In comparative flights with the North American P-51B Mustang, the latter had a superior rate of turn and better maneuverability generally.  And, saddled with inferior Continental engines for which no replacements were forthcoming, the XP-67’s top speed was a disappointing 405 mph, over 65 mph slower than promised, and over 30 mph slower than the Mustang.

Final Fire
September 6, 1944 was a grim day for the Moonbat.  On that day, E.E. Eliot took off for another test flight and noticed a loss of power in  the right engine at 10,000 feet.  Cutting the throttle, he quickly brought the plane down toward the field, watchful for signs of fire.  He saw flames at 3,000 feet and, in an effort to save the aircraft, landed on a runway with a crosswind to ensure the flames would be blown away from the XP-67.   The landing went smoothly until the brake for the right landing gear failed, turning the plane sharply to the left, a position in which the wind fanned the mounting flames back toward the airplane.  Before the fire could be put out, the fuselage was burned so badly it was nearly in two pieces, and the left wing and nacelle were severely damaged with the left wheel well completely burned out.  The XP-67 program, with an expense of $4.7 million and total flying time of 43 hours with its lone prototype, was soon cancelled. What might have been a leading fighter of the late war period was dead.

Cancellation of the XP-67 was partly driven by the fact that the war was drawing to a close — by September 1944, Allied victory seemed inevitable, with speculation in some quarters that the war, in Europe at least, would be over by Christmas.  The need for innovative propeller-driven fighter designs was waning at a time when jet fighters had begun to appear — Germany’s Me 262 had a top speed over 120 mph faster than the P-51 Mustang’s, and Lockheed’s P-80 Shooting Star soon to become America’s front-line jet fighter, took its maiden flight just two days after the XP-67’s.  Still, one wonders what might have been.  Had there been adequate time for McDonnell to resolve the Moonbat’s cooling and aerodynamic problems, if it had been fitted with the Merlin engines that company president James McDonnell lobbied so hard for, the XP-67 may well have taken its place as one of the great fighters of World War II.

As it was, the Moonbat’s design strongly influenced that of McDonnell’s FH-1 Phantom, the U.S. Navy’s first jet fighter, for which the U.S. government executed a contract for 100 aircraft on March 7, 1945 — launching one of the finest companies in world military aviation.  Viewed in this context, the demise of the XP-67 Moonbat could be seen as a glorious failure.

The Kit

The Moonbat is cast in pale yellow resin and consists of 35 parts and one vacuform canopy.  My example was unfortunately riddled with tiny sinkholes which had to be filled with putty and sanded smooth.  They were present in such numbers that it is safe to say I eliminated only about 80 percent of them.  There are very nice engraved panel lines throughout the airframe, and are prominent enough that the modeler need not worry about obscuring them with sanding.  There is a small decal sheet with national insignia and a pair of serial numbers — nothing special, but then there was only one prototype before the program was cancelled.  There are parts for two spinners with individually mounted propeller blades.  These latter parts (a couple of extra blades are provided) will require some sanding and clean-up, and may have one or two sinkholes as well (mine did).  The landing gear are each a single part with separately mounted wheels.  There is an impressive amount of exterior detail, not merely the panel lines but the cooling ducts on the engine nacelles.  The interior of the wheels wells are thoroughly detailed and well-molded.

 Construction

The major components of the fuselage consist of 16 resin parts: two for the upper and lower halves of the fuselage; two for the upper and lower halves of each engine nacelle; one for each wing tip; one for the tail, one for each elevator, two engine exhausts to be cemented on the rear of each nacelle, and one for each of the three landing gear doors.  Cyanoacrylcate cement (super glue gel) is highly recommended to ensure secure bonds for each step of the assembly — accordingly, the XP-67 was assembled with Zap-a-Gap cement.  The first step is to cement the top and bottom halves of the fuselage – this must be done carefully as the Moonbat had a blended, laminar flow design throughout its airframe, with as few sharp edges as possible, the likes of which was not seen again until decades later with the SR-71 (1960’s), the F-16 (1970’s) and the B-2 Bomber (1980’s and 90’s).  Photos of the XP-67 appear to indicate a first attempt at stealth design, but the Moonbat’s laminar flow airframe was more about speed.  Designed in 1941-42, the goal of the XP-67’s designers was for it to reach a top speed of over 470 mph, which no fighter of the day had yet achieved.

Once the cement is dry, the fuselage must be sanded smooth until the seam lines between the top and bottom halves are invisible — this is relatively easily since Zap-a-Gap lives up to its name and fills any gaps nicely.  Since it hardens quite effectively and can require more sanding than other glues, a medium to light grade sanding paper is recommended, so as not to scratch the resin too deeply and make more work for yourself.  The final smooth look and feel is worth the extra time and effort.  A great deal of effort went into eliminating tiny sinkholes throughout the wings and tail surfaces, using Squadron putty, Mr. Surfacer 500, and extensive sanding.

Next come the engine nacelles, which must be cemented and finished in an identical fashion as the fuselage .  Each nacelle consists of two resin pieces (top and bottom halves) including not only the nacelle but the wing section between the fuselage and the nacelle, and a wing section outboard of the nacelle.  The nacelles therefore include wing sections which must be cemented to the fuselage.  There are locator pins to assist with this, but they are crudely done, insufficiently deep, and do not align properly with the holes in the fuselage.  I sanded the locator pins off and relied on Zap-a-Gap, but even this left a significant gap at the underside seam where the wing/nacelle joins the fuselage.  I filled these gaps (top and bottom) with the strongest modeling putty I know of, Milliput.  Milliput hardens to a stone-like density, and this ensured not only a solid join, but increased structural strength once it was sanded down.

Each outboard wing section (wingtip really) must then be cemented onto each nacelle/wing section.  The vertical tail section and elevators, each separate resin parts, are then cemented to the aft section of the fuselage.  Sanding and putty are necessary for each new part added to the airframe.  Some sanding of the pilot’s seat will be necessary to ensure a good fit in the cockpit, which is bare save for the seat, the bomber-style control wheel, and the plain instrument panel hood, which also forms the bare instrument panel.  Since this latter part lacks any detail, and I added an instrument panel decal from the spares box.  The pilot’s seat is also bare save for a headrest, and I added seatbelts from an Eduard photo-etch set for American WWII aircraft.  I decided to depict the XP-67 in flight, so I did not have to bother with the landing gear.  However, the landing gear doors are ever so slightly undersized, and cannot simply be cemented over the detailed wheel wells without falling in.  I solved this problem by packing the wheel well areas with just enough Milliput to fill the empty space and provide support for the landing gear doors as they were pressed into place.  No cement was necessary; once the Milliput hardened, it held the doors fast.

Finally came the vacuform canopy.  Great care and repeated dry fitting must be taken when trimming this part, by increments, to ensure a good fit.  Feeling super-cautious as the kit provides only one canopy, I took my time trimming it with decal scissors, and for the last few millimeters resorted to sand paper.  When I had it to the right dimensions, it was time to paint it.  I am not a fan of vacuform canopies, for despite their realistic thinness, they are a pain to paint.  I generally mask my own canopies, but my method, using Tamiya tape and an Xacto blade, is fine for injection molded canopies, but would risk irreparable damage to a vacuform example due to the pressure required to cut through the tape.

Most kits with vacuform canopies are not mass-produced and do not justify the investment Eduard would have to make to produce a set of canopy masks, so the modeler’s two alternatives are to use a liquid masking agent or paint it freehand.  I opted to paint it freehand since the Moonbat’s greenhouse style canopy did not easily lend itself to a liquid masking agent.  Now, Anigrand’s vacuform canopy for the Moonbat bears visible but singularly faint lines for the canopy frame, so that it could only be painted in extremely good natural light at mid-day.  I made sure to work only within this window of time when the sunlight was at its maximum, using Humbrol olive drab enamel paint and a very fine paintbrush.

Painting

The Moonbat is airbrushed in Gunze Sangyo acrylics for WWII U.S. aircraft, H52 Olive Drab semi-gloss (which airbrushes to look quite flat, but when brush-painted looks glossy), over H53 Neutral Gray, also semi-gloss.  The cockpit is painted Humbrol Interior Green, an enamel.

Decals

The kit decals are excellent, being strong, relatively thin and having a semi-gloss sheen, and are equivalent to Aeromaster in quality.  They appeared to have a residue on them that looked like it would compromise the finish, but the residue vanished once the markings were treated with Micro Sol to help them sink into the panel lines.  A final coating of Future sealed them with no problem.

Conclusion

This is a simple yet challenging kit of a fascinating World War II subject.  Being all resin, it requires patience and more careful attention than plastic kits, including possibly considerable puttying and sanding to ensure a smooth airframe surface.  Highly recommended.

References

• A Handbook of Fighter Aircraft by Francis Crosby; Copyright 2002 Anness Publishing Limited, London.
• “McDonnell’s First Fighter: The XP-67 Moonbat” – Wings Magazine, December 1973