The Spit IA should have a Constant Speed Prop

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Originally Posted by 41Sqn_Banks

You are right about the performance, the difference is small. However the handling of Merlin III (Spitfire I) and Merlin XII (Spitfire II) is different.
E.g. Climb boost: Merlin III +6 1/4 vs. Merlin XII +9.

My understanding is that the Spitfire II with Merlin XII had somewhat better cruise power/max rich continuous than the Spitifire I with Merlin III: + 7 lb/sq.in. vrs 4 1/2 lb/sq.in. In other words the Spitfire II could continuous cruise at better than the Spitfire I's 5 minute all out boost limit of 6 1/4 lb/sq.in. (max emergency +12 excluded - which are the same). That's not insignificant in my view.