Complex sentence: The larger engine required significant changes to the airframe, particularly the engine compartment, the contra-rotating propellers also created instability that required an increase in fin area, all of which cost Browning more than $300,000.
Split into simpler sentences: The larger engine required significant changes to the airframe, particularly the engine compartment. The contra-rotating propellers also created instability that required an increase in fin area, all of which cost Browning more than $300,000.