If you're doing badly and your plane is heavily shot up, land in a field and keep the engines running. Drive to a friendly air base avoiding any trenches. Enemy pilots try to shoot you but they always miss, but do however fly in front of you so you can zap them.

After takeoff, hit 'A' to accelerate time. You'll now be immune to enemy gun fire. Now climb well above the clouds, until a certain altitude is reached. Soon, all the enemy aircraft will hang motionless in the air, allowing you to shoot them all down.