I have often pulled my MPM Kingcobra kit out of the stash to look things over and with good intentions to build. Alas, it always ended up back in the closet ...... more or less due to the lack of interesting schemes for it.  Thoughts up front were to do a whiffer bubbletop P-63D version, but this just never took off.  While looking thru my "Allied Aircraft Piston Engines of WWII" book I reread the chapter on Allison’s turbo-compounded V1710 and its mention of its intended mating to the P-63H. Okay so what is this aircraft?  A couple of quick searches and I was soon reading an article: “Supercharging the Allison.”  A very interesting article. I was further intrigued by two things: a 3-view drawing of the P-63H and a book cited in the references.  Looking at the drawing, I realized that was this was what I was looking for and I'm pretty sure I heard the kit calling from the stash! I found and ordered the book, “V's for Victory” and soon read this and another, "Cobra", a history of Bells aircraft. These are all very good reads with gobs of reference data and great pictures and drawings.

          Out again came the kit.  Looking at the kit and the drawings, I realized this could be fairly easy to do.  A basic airframe with a few more holes drilled for exhaust piping.  And I might as well do some cutting and fit a bubble canopy to it .... the kits canopy were uselessly yellowed anyway. 

            First off, lets start with the basic Kingcobra.  It was an evolutionary design of the Airacobra, though it shared no parts. A longer fuselage allowed a two-stage Allison driving a four-blade propeller for better critical altitude, while overall aerodynamic refinements (with extensive  studies and recommendations by NACA) and a laminar flow wing improved overall performance.   It used Bells trademark nose-wheel, and put the engine behind the cockpit, the propeller being driven by an extension shaft.  This left the nose area free for a large 37mm cannon and two .50cal machine guns.

            Performance-wise, it was definitely a step up from the Airacobra.  Though slightly slower than it's contemporaries, it had a phenomenal roll rate and could turn tighter than any of them (it was known to out-turn T-6 Texan trainers). 

            So, good speed, great maneuverability, heavy firepower.........sounds like a winner.  So why didn't the USAAC want anything to do with it?  Well, it didn't offer any marked increase in performance, certainly not enough to re-equip existing squadrons with established types and supply lines.  But the biggest factor was the range.  It just didn't have enough.  Now, the Airacobra had sufficient internal fuel capacity for the requirements it was built around, namely a 1 hour endurance.  And remember, thinking at this time sported US fighters defending the shorelines against invading bombers. So the one hour endurance seemed adequate.  However, that requirement translated into actual combat conditions rather poorly; the Airacobra was always short-legged.  Okay, “fine”, you say.  They should have learned from their "mistakes".   But no, the Kingcobra shares this excuse.  One must remember that the Kingcobra was being designed when the Japanese attacked Pearl Harbor, and first flew exactly one year later.  As it was, the Kingcobra went thru life with the smallest internal fuel load of any production US fighter. 

            Okay then, why not add more fuel?  Simple.  But where?  Remember, this is a tiny fighter.  Not an abundance of space anywhere.  Behind the cockpit?  Nope .... engine there.  Nose is filled with landing gear and weapons.  Wings already have tanks.  Other wing areas area occupied with landing gear and ammo belt racks for the ,50 machine guns (in under-wing pods because there was no space for them in the wings).  Wait!  What about the wing/fuselage center section?  Perfect place for fuel tanks. Nope.  This was occupied by Bells very compact and efficient cooling radiators and duct-work.  Bell should have opted for a normal chin or belly scoop instead of the internal radiators.  A less elegant solution for sure, but it may have solved the range issues.

            Alright then, lets throw some drop tanks under the wings.  Even with these as standard equipment, the Kingcobras total fuel load was less than the internal fuel load of the Mustang.  So the King was in effect a very potent point defense fighter when we had no need for one.

            Now lets talk about that power-plant.  The Allison V1710.  It's gotten a lot of bad press, especially compared to the Rolls-Royce Merlin.  Odd when you think about it, the engines were developed at the same time and were of similar layout, displacement, and power. Yet the Merlin has attained almost legendary status while the V1710 is almost an also-ran.  Most people will tell you about the Mustang and how the Merlin transformed the dog into a world beater.  (truth be told, below 15000ft the Allison Mustangs could run rings around the later versions, which were heavy trucks in comparison .... But I digress). Well, the big difference wasn't the engines themselves .... like I mentioned, they were surprisingly similar.  The difference was in what supplies the air to the engines, the supercharger.  Rolls Royce was somewhat lucky.  They had a two stage supercharger developed for the ill-fated Vulture engine  that was grafted to their Merlins which allowed the engine to supply its rated power to a much higher altitude than the single stage supercharged Allison that it replaced in the P-51.  (Interestingly, if one compares single stage Merlins to single stage Allisons, or dual -stage to dual-stage,  it's a dead heat ....but nobody compares apples to apples).

            So why wasn't the V1710 designed with a second, or auxiliary stage supercharger?  Well, it was ....in a way.  The Army Air Corps stipulated that the single stage supercharger always be supplemented with the then new turbo-supercharger.  In the end, this did not occur on most aircraft.  And those aircraft were altitude limited as a result.  Once the realization of turbo production and installation difficulties became apparent, impetus was put behind auxiliary stage development (Allison had recommended this years prior, but interest and development money was nonexistent).  What emerged from Allison was an auxiliary supercharger package that was essentially an add-on (a result of a great modular design philosophy taught by GM and seen thru-out the Allison design.  Case in point: while a typical Merlin had around 11000 parts and 4500 piece or different parts, the Allison had around 7100 with only 700 piece parts). Two-stage Allisons performed on par with two-stage Merlins but ..... and there is always a but .... they were longer and would need redesigns to fit.  Since it wasn’t prudent to interrupt wartime production for no real performance gain, the Merlins stayed.

            The Kingcobra was designed with the two-stage Allison and its performance showed.  It was the equal of any of the contemporary US fighters (except in range of course).   But equal isn’t good enough ...... so lets go one better shall we?  Lets see, supercharger is good, dual stage is better (for altitude performance anyway). What about turbochargers?  Well, turbos are essentially a different style of second stage supercharging using exhaust heat energy instead of energy from the engine.  But everything has its downsides.  Driving a supercharger, or two, requires power from the engine.... a LOT of power.  Turbocharger power is essentially free, coming from otherwise wasted exhaust heat energy.  But there is a lot of extra plumbing involved, increasing weight and complexity.  (One thing turbos do give is better fuel efficiency, IE: range.  Fun fact: a Merlin powered Lightning would have given comparable performance but reduced range compared to the turbo'd Allisons).  Lets try this.  Lets mate an engine to a dual stage supercharger.  But lets run the exhaust to a turbo, one connected directly back to the engine crankshaft. The turbo would supply the power to run the supercharger and any excess would add to the engine total, or more to the point, the propeller.  Design the final exhaust right and maybe get a few pounds of additional thrust there.

            This is just what Allison proposed with the V-1710-127 , the worlds first turbo-compounded engine.  Bell designed this into the P-63H, shoe-horning the power plant into the tiny airframe.  It was a tight fit, and left no room for exhaust ducting internally.  The exhaust exited the engine, ran rearward outside the airframe.  They re-entered and turned downward to enter the GE CT-1 turbo.  While this my seem crude at first, it had some benefits.  Namely it helped cool the exhaust to keep it below the critical temp of 1725F.  What all this did was give  the V1710, an engine that started around 750hp, almost 3100hp to play with.  And this wasn't some napkin-scribbled pipe dream.  This engine was built, tested, and installed into the airframe.  Unfortunately it was never flown.  As promising as it was, both Bell and Allison asked that the project be stopped.  They understood that the new jet turbine offered even greater potential.  With that ended the prospect of the ultimate V1710.