Great, but needs little tweaks
I have to agree with the guy below me about the Axe..
The idea is awesome, the game play is almost perfect.. but many.. if not most of the planes are pretty useless. If you got rid of all of them and used an upgrade system with a handful of different chassis this would be a really ridiculous game.
Suggestion for Steambirds 3:
A few different chassis varying in speed and armor as to leave upgrades to the player.
The ability to earn and add different guns varying in range, speed and power.. the ability to earn and add different armor packages or engines to adjust speed and strength.. the ability to earn and add aerodynamics packages to increase maneuverability.
The ability to add skills based on a points system tailored to the chassis. (ie. Heavy planes get more skill points)
You could still "sell" guns and armor to generate your return.
Otherwise.. what you have here is very good.