Barit1,and all, apologies ,you are correct that the P38 had counter -rotating
props,but outwards,,as different from the prototype...must check book before scribing,but it`s been a long week !! Interestingly the prototype Whirlwind had counter-rotation props as well,but went to same rotation in production; pity we didn`t put a couple of Merlins in it,or even Allisons..
I guess we should say the Gannet had co-axial counter-rotating props,in a similar manner to Kamov helicopter rotors are co-axial,but coupled by the gearbox at constant rpm.....confused ?
Dar, back to the original Q,I would suggest if not already done that some form of `tunnel` testing has been done to give an indication of whether inward,or outward rotation is more beneficial with respect to increased /decreased slipstream effects ,resonance near fuselage/prop tips,buffeting of tailplane etc.As you can see ,history reflects things that sometimes `seem like a good idea` don`t always work as expected... However, hope it does work...