If you die and lose your rings, just idle overnight in training zone 1 with a dagger equipped, and the mouse positioned on the tip of it. (You can turn the monitor off) The next day you should have all your rings back.
How to beat the Behemoth: It has a slightly smaller firing range than your Bristols. Once the balloons (Or Turtles) are down slowly approach the Behemoth until your planes start firing. Then turn away from the Behemoth while moving the slowest distance possible. If done right your planes will shoot it around 15 times then turn away before getting shot. Use 180 turn and repeat.
1949 is easy, just lure the Turtle bomber away from the balloons and gas it, then pick off the balloons one by one, they have a shorter firing range than you, so you can fly up to them, shoot, and quickly turn away.
Great game, pity there's only one plane you can control though. I'd love to see more types of planes, it makes the game a lot more interesting. Oh, and an option to replay battles would be fun too.