This. Side climb, and learn to energy fight if you're using the Hellcat. It's pretty good at it, for reasons already mentioned. Great dive speeds. Great energy retention in turns. Most of the time teams go three ways. The real dumbasses go in low and suicidal. The players using planes that can climb rapidly go head on in a steep climb. Others will side climb steeply, or in the case of American planes, a more shallow climb to the side. Do not try to climb and go head on into the fray in most of the American arsenal. You'll die. You'll be on the edge of a stall, or even if climbing properly you'll be well below the other guys and they'll whack you with no problem at all.

The biggest thing is don't get into a rush. Learn what your plane can do. In this case, learn energy fighting or squad tactics. Do not ever, under any circumstance, let someone pull you into their fight style. If someone is trying to suck you into a low altitude turning match for example, leave them be and let someone else get it. Know how to pick and choose a fight. Don't go after everything you see. And when you go after something, do it smart. Do not sacrifice altitude and energy on a "maybe". Make them play your way, or stalk them from above and dive at an opportune time. Always keep some altitude. Sometimes other players will end up above you. Sometimes a plane that can outturn you will climb to your level. You always want a way out. Dive speed is useful again, your plane is fast in a dive. Keep that option open always as a possible evasive maneuver. Run away, climb back up using the momentum you gained, start again.

If you're wanting to play as a B-17, learn to squad up. It's just a simple fact. People squeal with glee when they see bombers as they are a guaranteed kill no matter what, unless you're an absolute noob. Bombers can be very effective but not often alone. Likewise, if you see a bomber, look at the situation first. If they're flying together or have a few fighters sticking close, do not go after them alone, I don't care what you're in. That goes back to square one, learn to pick your battles.