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By the way, back in the mid-80's I decided to build a 1/48 P-82 using two Monogram P-51D's. It won't work!
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I guess the XP was the actual one where they had to swap the engines from side to side to get it to fly.
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They had more in common with the H than the D - but purely in appearance. The guys restoring the XP-82 said there was about one part in common between the H and the 82 aircraft, and it was nothing major. And the canopy is that of the P-51F, or something like that
The P-82 fuselage was stretched as compared to a P-51D, by a lot. If you try to insert a plug in the aft fuselage of a P-51D model to reach the P-82 length you find out you can't do it that way. The fuselage tapers such that it radically changes shape over that added length. Talk about needing putty!
Fine Scale Modeler had an article on how to convert a 1/32 P-51D to a P-51H using a combination of Monogram and Hasegawa (I think) parts and I used that as a guide. The author joked about doing a 1/32 P-82 next and that inspired me to give it a try.
The P-82 differed from the F-82 primarily in keeping the Merlin engine in the usual P-51 installation. It worked great.
The F-82 changed to the V-1710 with the auxiliary supercharger such as used in the P-63. They switched because:
1. The Merlin required payment of $1500 each to RR for licensing.
2. And probably because the first SECAF was a former GM executive, and Allison was part of GM then (ironically, it is now part of RR).
The V-1710 gave them so many problems that virtually the entire production run of F-82's was shipped from Inglewood to the former Vultee plant at Downey, where they sat for years waiting for reliable engines. NAA tried fixing the V-1710, added antibackfire screens, and that seemed to help. The Air Force told them they were not an engine company, did not know jack about Allison's products and should stop insulting the engine company by fiddling with their products. Of course, by that time Allison was focused on jets and did not give a rat's rump about the V-1710.