RE: 1/5 scale Bell P-39 Aircobra...
Darrell,
I didn't know about the Robart part# P-39 retracts. Those are PERFECT! As for the canopy and, possibly, the exhaust manifolds, those could be made by vacuum forming. I haven't tried that process yet but it seems easy enough once a suitable mold is made.
I bought a set of exhaust manifolds for a Ziroli Stuka from a fellow who made a master from balsa wood and then made the mold from a kit purchased from Micro Mark. The resulting resin manifolds are beautiful to look at with simulated welds cast in. Trust me, this will be the last of your concerns!
Not too long ago there was a guy who was featured in RCM for scratch building a 1/5th scale P-63 with a mid-mount engine. I talked with him over the phone and learned that he used a Moki 2.10 for power. This made real sense as a Moki uses no nitro and alcohol burns relatively cool. He utilized the original dorsal carb intake and the cooling ducts in the wing roots to bring in cool air and vented the heated air via space between the simulated exhaust manifolds and the fuselage walls in addition to the doors (shutters) under the fuselage utilized by the full-scale P-39 and P-63 to control engine temperature. He claimed that he never had an over-heating condition with this configuration.
He said that the Moki made more than enough power and that he flew his plane at half throttle most of the time. Full throttle brought the plane well beyond scale flight speeds due to the streamlining of the Airacobra and he cautioned that the plane did not scrub off speed quickly.
He used a shaft from the Moki's crankshaft to the prop drive washer that was made using tool steel. His weak link was the Woodruff key in this shaft. He said that the Moki was prone to "kicking" during starting and that the hard "kicks" were sufficient to shear the key and damage the shaft. (I'm guessing here but I think he may have used a collar to act as a union between the drive shaft and the Moki's crankshaft.)
I might be willing to give this combination a try. The only area I would deviate in would be in a threaded connection from the drive shaft to the engine's crankshaft. I have a Moki 1.80 sitting around waiting for something to do. Who knows?
Enough of my verbage. I'm excited to see your progress, Darrell. Keep up the great work!
Al