To climb in any plane, just press alt x. This will engage auto-climb and you will automaticlly climb out. Every plane has different climb speeds, some can climb 4,000 ft/sec and others (such as bombers) can only climb 1,500 ft/sec. Yak, 109, spit, and P-38 are some of the faster climbers. To see what climb rate you have look at the climb gauge in the cockpit (don't have a picture of it to post right now but it's the one with 0 to 4+ and 0 to 4-).