Rad game, love it.
When I run out of airplanes I get a symbol on the main screen to add another one. This doesn't cost me anything. I'm not speeding up the timer for the airplane that is still being built. It's just a free "plus 1 airplane" button, it seems. I didn't buy the airplane generator. All I bought was the advertisment removal, and the star doubler.
Could someone tell me the rules for this? Do I have infinite free airplanes now?