Basic features of planes have to be researched after buying an aircraft, so you spend game currency (cash) to make the airplane able to survive and have weapons and ammo to be able to score so you can research more. It is an annoying cycle. I have to admit I paid cash because I was curious whether the "developed" airplane could actually win in battle.
Now I'm super annoyed:
I agree with other reviewers that a great game was rendered a bad game by following the candy crush pay model. The developers played on our need for accomplishment (greed), which makes them greedy themselves. That killed it for me.