HI
you prop an airplane and the best prop for the engine on a given airplane ,so sometimes a 12/4 or 12/6 might be better than a 13/4 for a given planes weight and shape (bi-plane or single wing ETC ) DIFFICULT to say a 13/4 prop on a 55 AX would be best on all planes could be a good place to try and start your testing-vertical performance is always my bench mark for performance BEST REGARDS TONY