History – So we all know the story of the airplane designed with the engine placed behind the pilot to make room in the nose for a big cannon. And a bad decision made to drop the proposed turbo-supercharger. Not having a supercharger would make the aircraft a poor performer. And the mid-engine made it a death trap in a spin. While there is an element of truth to some of that, the Airacobra story is much more complex.

Bell Aircraft responded to a 1937 Air Corps request for a single-engine high-altitude "interceptor." Despite being called an interceptor, the proposed aircraft's role was simply a refinement of the traditional pursuit (fighter) role to intercept aircraft at higher altitudes. Specifications called for a heavy armament including a cannon, a liquid-cooled Allison engine with a General Electric turbo-supercharger, tricycle landing gear, a level airspeed of at least 360 mph (580 km/h) at altitude, and a climb to 20,000 ft (6,100 m) within 6 minutes. This was the most demanding set of fighter specifications USAAC had presented to that date. It also came at just the wrong time. The P-39 just missed the advances in technology that would benefit the later types that would replace it. Turbo-supercharging was still cutting edge and presented many problems for a single engine fighter with size and weight being the most prominent.

Much is made of Bell’s idea to place the engine behind the pilot to accommodate a 37mm cannon. However, Bell’s Chief Designer, Robert Woods, never mentioned this as a priority of the design. In the March 1940 edition of Aviation Magazine, Woods writes an article promoting the P-39. He describes the purpose of the mid-engine design allowed for better aerodynamics by enabling a more streamlined nose profile. He describes the P-39 nose as” being equivalent to the shape of a high velocity bullet.” He also makes much of the center-of-gravity being where most of the weight is concentrated. This decreases moment inertia which should increase the aircraft’s ability to turn quicker, aiding maneuverability. He then talks about the advantages of the tri-cycle landing gear. Landing incidents were the number one cause of USAAC aircraft accidents at the time and landing with a nose wheel was known to be easier than with a tail dragger. He mentions that the design makes room for heavy armament, but it doesn’t seem to be high on his list of reasons for the design.

The original XP-39 had radiators and air-intakes in fairings on each side, and the turbo impeller and exhaust outlet protruding from the bottom of the fuselage, quite like the installation on the P-38. Unfortunately, the drag produced by this layout soaked up any power increases and it just wasn’t performing to expectation. It was found that the weight savings of deleting the turbo and moving the radiator inlets to the wing allowed better performance with a simplified design. It is a myth the P-39 had no supercharger; it was equipped with the then contemporary single-stage gear driven supercharger as in the P-40. So, what happened? Turbo-supercharging did make more horsepower at higher altitudes than the gear driven superchargers of the time. This was proven in high altitude bombers of the period. But these larger aircraft could absorb the size and weight penalty of the system. The P-38 successfully implemented the turbo, but it too was a larger multi-engine airplane. And in truth the P-38 suffered some extensive teething issues.

Technology advances narrowly missed by the 1937 requirement would benefit the next generation of designs. The P-47 would successfully integrate the turbo in a single engine fighter. But notice how much larger and heavier this design is, the largest single engine fighter to be produced. But a great performer due to an advanced engine producing 2,000 hp. Then there is the legend of the P-51 being fitted with a British engine. Key among advantages of the Rolls-Royce engine compared to the Allison was its gear driven two-stage supercharger that performed well at higher altitudes. Also notice the US Navy stayed away from turbos as all their designs used mechanical supercharging.

So, the XP-39 was re-designed and actually achieved the USAAC performance requirements. However, the XP-39 was some 2,000 lbs. lighter than the production fighter fitted with guns, armor, and self-sealing fuel tanks. But, like its stablemate the P-40, it was operational in strength when we were thrust into war. These mid-thirties designs flown by pre-war trained pilots had to hold the line until the vast numbers of advanced aircraft, flown by pilots produced by an amazingly expanded training program, could be fielded. As such they hold an honored place in American history and their sacrifices must never be forgotten.

First produced in 1969 as one of Monogram’s heralded quarter scale fighter lineup. This kit is a 1983 vintage re-issue. The Monogram line which began in the fifties adapted over the years to a changing hobby clientele. They began with products that appealed to the average modeler who in the fifties were younger boys. So those models were less about detail accuracy and more about working features and gimmicks. Youngsters don’t put their toys on a shelf to look at, they play with them. As those boys grew older and more informed, they began to take the hobby more seriously. Monogram responded with more accurate and detailed kits. The operating features such as retractable landing gear went away in favor of interior details with removable panels to display the builder’s handiwork. This is the point in time this kit was produced. There is a basic engine molded with a fuselage half with removable covers. There is full gun bay, and a cockpit door can be posed open to allow one to look around in there. Top of the line in 69, it has been superseded by more modern offerings. Still, it is a rather pleasant build and certainly looks the part if one doesn’t look too closely. Decals are for one option, a P-39F of the 488^th Fighter Squadron 59^th Fighter Group.

These old kits provide a pleasant build experience with few hassles, save some gaps here and there to be filled. One has to decide first whether to build up the armament bay for display or use that space to add weight. Since I was modeling the unarmed XP-39 I glued a .45 slug to the gun compartment chassis. It fits well and provides the right amount of weight to keep the model off its butt. I assembled the fuselage and wings per instructions then began the conversion process to the XP-39. There are some excellent photos of the aircraft available. Basically, it is filling gun ports in the nose and wings (including shell ejection chutes) and making a pointed spinner. Then you sand away all the gun and ammo access panels on the wings. Next is to re-scribe sanded off panel lines. So, panel line detail is of the raised variety on this kit. But it works out ok as one would have to examine the area carefully to see the difference in the raised and scribed lines under paint. And this model isn’t going to any shows. To make the pointed spinner I cut off some of the cannon barrel molded to a fuselage half that also serves as a prop shaft. I glued this piece in place in the spinner. Then I dripped thick super glue down the shaft. I did this repeatedly a little at a time allowing each application to dry. This built up nicely into the desired cone shape. Then cut off the protruding barrel and with just a bit of sanding you achieve a nice point. The only major surgery required was cutting off the centerline bomb rack. This left a hole that had to be filled. I used Testor’s contour putty for this. Lastly, this kit provides two sets of exhaust stacks, the early 12 port and later 6 port exhausts. The early 12 port is correct for the XP-39.

The XP-39 was in unpainted aluminum. Another departure from the production P-39 was the landing gear also left unpainted. Though the photos are b&w, they suggest the landing gear doors were painted green zinc chromate (or that unique Bell interior green). I just used regular interior green on mine including the cockpit. The airframe was airbrushed with Model Air metallic Aluminum 71.062. Engine compartment covers and landing gear were painted Mission Models Duralumin for some contrast. The spinner was painted Testor’s Chrome Silver. The anti-glare panel was masked and painted Tamiya XF-1 Flat Black as were the tires. The rudder was painted Model Color White. I used some highly thinned flat black enamel as a wash to pop engraved hinge and panel lines.

I scrounged some suitable national insignia stars, US ARMY, and Curtiss Electric prop logo decals from some P-40 sheet in the big box of stuff. I found some rudder stripe decals that belonged to some biplane kit I think. It sorta fit with some trimming. The blue had to be augmented by piecing some decal of matching color to make the wider blue stripe on the top half of the rudder. I finished by touching up a few short red stripes with red paint. There are no other markings present on this aircraft.

So building the XP-39 allows one to display a shiny silver Airacobra that you don’t see every day. I enjoy building these old Monogram kits. They are like trips down memory lane. The 83 re-pop shows only slight wear of the molds in the form of some molding seams on gear struts and such, easily scraped away. The raised panel line detail is still crisp, and nothing is warped as in today’s Revell re-issue of some these old classics. More modern offerings offer better detail accuracy, for s price. But for an enjoyable problem free build to stick on your shelf I can recommend this old kit. You can still find these on e-bay sometimes at a reasonable price.

Aviation Magazine, March 1940. Wikipedia, the internet, and U-Tube.

David Cummings

9 October 2023

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