Lateral stability was lost as power increased. It became an issue when the switch from the Allison V-1710 to the Merlin V-1650 increased the power due to the Merlin's very much superior supercharger design and increased power at altitude, so from the P-51B forward. The dorsal fin extension or fillet was developed to counter this because it did not interfere with production and could be retrofitted, whereas a larger vertical stabiliser would have been an impractical retro-fit.
Modifications introduced during a production run were a major cause of delay and delivery shortfalls. If you look at the Spitfire, it got larger and larger fins rather than strakes/fillets as it got more power, but they were design modifications introduced for specific Marks or batches.
Modifications introduced during a production run were a major cause of delay and delivery shortfalls. If you look at the Spitfire, it got larger and larger fins rather than strakes/fillets as it got more power, but they were design modifications introduced for specific Marks or batches.






