Staff Writer (Updated: 5/5/2016):
P-39 Quick Glance
Generally a very pleasing aircraft to look at, the P-39 design came about at a time when streamlining aircraft shapes were just coming into their own. The P-39 was a vast departure from most aircraft being conceived of at the time and featured several design elements that distinguished the type from her contemporaries. Chief among these was in the internal layout, the Allison series engine mounted in the middle of the fuselage just aft of the cockpit. Engineers ran an extended shaft from the engine through a center bearing underneath the pilots feet to the front fuselage section where the three-blade propeller and reduction gear were mounted. As a result of this engine placement, the engine had to be fed through intakes mounted along the fuselage as opposed to a conventional placement in the nose. In early P-39 forms, this meant intakes were added to the sides of the fuselage just aft of the cockpit. Other forms mounted these intakes along the wing roots and the most identifiable form saw the intake affixed to the top of the fuselage. Wings were low-mounted monoplane assemblies and the empennage featured a traditional "T-style" arrangement with a single vertical fin. The P-39 also featured a powered tricycle landing gear system, a relatively new concept in the art of aircraft design for the time.
In what turned out to be effectively an early form of the "bubble" canopy, the Airacobra featured a complex canopy design which offered up unparalleled vision over the entire design. The pilot sat in a very ergonomically-minded cockpit that featured two automotive-style "swing" doors to either side of his seat. The windows in these doors were fully retractable and done so through a car-like crank handle. Upon having to exit his aircraft in the event of damage or power loss, the pilot simply jettisoned the doors via lever and rolled out one side or the other, eventually slipping off the edges of the respective wing. If enough time was allotted, he could even make his way to the wings edge and make a controlled jump.
Cockpit design was simple and similar in scope to the P-38 Lightning and P-40 Warhawk. Control levers were dominated by the throttle lever, easily the largest of the group. Most of the main control gauges were held in a center column running from knee-to-face height and offering up easy access. The throttle and other controls were held off to the lower left in a separate assembly that included the gun controls. Space-wise, the cockpit fit the build of a standard 5'8" pilot (standard for the time at least) and offered up limited comforts for sorties that could very well last several hours. Beyond that, however, many-a-veteran would curse the system's limited conveniences on longer trips. Internally, nothing was spared in constructing a cockpit worthy of any pilots own life. The cockpit featured air-tight sections to keep deadly fumes from the nose-mounted armament out and deadly fumes from the engine from creeping in from the rear. All vital systems were held in this area of the fuselage, which itself featured rugged and sturdy construction practices that would become synonymous with Bell Aircraft for a time.
A Changing World - For the Worse
The Bell P-39 design came about at a time when the world was turning increasing hostile on both sides of the United States. Troubles in Europe were readily apparent with the political and military movements occurring in Germany and Italy. Expansionism by the Empire of Japan in Asia were another area of real concern and forced an internal evaluation of American military power and might as it then stood. Systems then in service were found to be wholly inadequate for the new methods of war and attempts were now in motion to rectify the critical situation. Enter Larry Bell - president of the Bell Aircraft Corporation.
If change was what the United States military was after, then change is what it found in this visionary. Bell had already made a splash on the aircraft design scene with their failed - yet incredibly interesting - first attempt with the Bell FM-1 "Airacuda". This "bomber-destroyer" was to have been the military answer for an ultimate bomber interceptor with the most distinct design feature being the 37mm cannons mounted in engine nacelles on each wing (including their respective gunners as well). Engines were of a "pusher" type (Allison brand) but the aircraft proved to be simply too heavy for the intended role. The aircraft did go on, however, to field an entire squadron before production ended.
In any case, development of the Airacuda (beginning the company tradition of using a naming convention featuring the word "air" in official designations) forced Bell's team to tackle the seemingly impossible. The experience garnered in the creation of the XFM-1 Airacuda was priceless and convinced the promising team that they could develop an aircraft for the US Army's new requirement - an interceptor. Besides the Curtiss P-36s, P-40 Warhawks and Seversky P-35s then in service, the US Army had very little to go on in the way of tangling with Japan's best fighters and bombers. She fielded no such aircraft to effectively seek out attacking bomber formations at night, no aircraft to threaten the new crop making up Japanese fighter command and no aircraft to efficiently bomb or cannon ground targets in any way with repeated success. The canvas was generally a blank one and Bell and his team set to work.
Birth of a Cobra
Bell's engineers took some new information to heart. First and foremost, armament of contemporary American fighters at the time was generally inadequate for use against the new generation of bombers. Aircraft were still being fielded with a World War 1-style gun arrangement featuring just two rifle-caliber (7.62mm) machine guns. Bell engineers took a radical approach to arming their prime fighter design and made the centerpiece of the aircraft a powerful - though slow-firing - Oldsmobile-brand 37mm cannon. This alone brought the new design leaps ahead of anything then service. The 37mm cannon would be supplemented with heavy caliber machine guns for additional punch in the form of 12.7mm (.50 caliber) types mounted in the wings. The design crew's optimism was not in question as such an aircraft would surely be reckoned with in any skies around the world but it did present several speed bumps in the thought process. A large caliber weapon as requested by the design team meant that there would have to be proper and adequate placement for the system somewhere in the fuselage. As a result, the P-39 was basically a design set around the position of this cannon - effectively being that the Airacobra was an aircraft designed around its own primary armament.
From one point of view, the addition of the 37mm cannon forced a design that was highly unorthodox while on the other it provided the aircraft a solid center of gravity for which to fire such a powerful weapon. Being centered in the fuselage and firing through the front, the aircraft could be of such high containment that eventual weight changes within the structure would generally have very little effect in the way of the aircraft's overall performance specifications. The weapon was hence mounted in the upper forward portion of the fuselage nose with the barrel protruding and firing out through the propeller hub. Two 12.7mm machine guns were also added to the upper-forward portion of the fuselage to compliment the cannon arrangement with more concentrated direct fire. These systems were synchronized to fire through the spinning propeller blades and had their rate-of-fire affected to a point but not wholly detrimental. Additional 12.7mm machine guns were added to either wing - one to a wing - for an impressive overall armament capability. The resulting aircraft, with the powerful and turbosupercharged (the turbosupercharger was mounted on the port side of the fuselage)Allison V-1710-17 liquid-cooled engine of 1,150 horsepower, was now designated officially as the XP-39.
The XP-39 was, at least outwardly, very similar to the final P-39 models soon to reach production. However, several elements soon became apparent with the program that would forever change the destiny of the aircraft - and effectively tarnish its rather impressive image. Despite production already beginning on several YP-39s, the US Army involved itself evermore in the Airacobra's design. With several old hands still holding power for this branch of service, the US Army decided to modify the Airacobra design to make it more ideal for use as a close-air support platform capable of operating under the control of ground forces instead of being its own independent aerial entity. As such, the P-39 was now being asked to provide better performance under a 10,000 foot ceiling limit. This meant the deletion of the ultra-important turbosupercharger - a requisite component in the aircraft's design that was all necessity when performing against the specifications found in the new German and Japanese fighters. It was thought that the deletion of the component would assist in making the P-39 a more stable and better-performing low-level system. What it did, however, was rob the aircraft of its performance gains that so wowed interested purchasing parties. The air intake was now moved to the upper portion of the fuselage (the more identifiable "look" to the P-39 that we recognize today) and another Allison engine was selected for production that would provide better low-level performance akin to what the US Army was looking for.
The initial production P-39 systems became the P-39C models and were quickly followed by the P-39D series. The aircraft almost took on the designation of P-45 due to the radical list of changes from the original proposal through it was eventually settled that the P-39 designation was sufficient in detailing the significant changes and additions. P-39C models had a pair of 7.62mm (.30 caliber machine guns) mounted in between the existing 12.7mm heavy machine guns. Sixty examples of the ordered P-39C models effectively removed the 12.7mm heavy caliber wing-mounted machine guns and replaced them with 4 x 7.62mm types (two machine guns to a wing with 1,000 rounds each) and became P-39D designations while still retaining their nose-mounted 12.7mm heavy machine guns (200 rounds each). The 37mm cannon (30 rounds) was still in place. The completed P-39C and D models initially went to war with the 31st Pursuit Group (later becoming "Fighter" Group). It was also found later that the P-39 made for a stable bombing platform, particularly in the dive-bombing role, and had a single 500lb bomb affixed to the centerline fuselage underside.