All of this has been good advice.
The P-39 in game has very benign departure characteristics. It doesn't snap roll in wing stalls, and is very controllable at very low speeds. It can kick the tail around better than the famous 109 move all the K4 jockies love because the plane has a lot of rudder. I tend to suck the faster planes into turn fights by progressively turning tighter, then make aggressive moves into rolling scissors. It's easy to get most late war birds into an overshoot, because of how well the P-39 slows down, and flies well slow [stable, good roll rate, lots of rudder]. This will generate slow crossing shots which are an ideal use for the 37mm because the range is close, and the plan view of the bandit provides the largest possible target to hit. If you miss the shot, you're toast as most bandits can extend quickly because the P-39 accelerates pretty poorly, and once the bandit is 600 and beyond, he's very tough to hit with the big gun. But alas these maneuvers leave you easy pickins for the second and third guy in, which is why my K/d in a P-39 was always around 1.0. In a furball do the opposite. Get high and dive through because the the P-39 is one of the best diving planes in the game. it'll do 550mph without compressing so it turns well at high speed for BnZ attacks. It won't however maintain it's E so get back to vertical as quick at possible, then climb out to regain the energy you lost and repeat your diving attack. I have landed 7 kills in this fashion on 1 ammo load. Good luck!
The views are less of a problem with Trackir. The fixed view will leave a lot of blind area when looking backwards. there's no avoiding it with fixed views.
I've always found I can win 1v1 with most of the late war planes because the P-39 turns pretty well, but it will struggle against the better turning planes with better Power-to-weight ratios like zekes, spits, n1kis. The trajectory on the 37mm is something you can get used to, but the scatter on successive rounds is the problem. If the bandit is maneuvering, best to keep shots inside 200 yards. If he's flying straight and level, 400yrds is max.
The plane is a de-acking monster. you can de-ack almost any field on one ammo load with a little practice. to do this you have to practice lobbing 37mm at the guns from 1.5-1K out and breaking before the auto ack gets you because the P-39 is pretty vulnerable to auto ack fire and bursts into flame easily.
