If you have enough water under your keel - Dive and turn at ahead flank, changing the direction of your turn as soon as you hit periscope-depth and are no longer leaving a visible wake on the surface.
If you can't dive - Ahead flank and do what you can with the flak guns. Twist and turn to present the least favourable target to the attacking plane - running at flank broadside on to the line of attack is often the best choice agaisnt bombs. Head on agaisnt a strafing attack presents the smallest target to the plane's forward-pointing guns but if they get lucky they walk their rounds the full length of your boat - So, even though it's a bigger target I usually try to get broadside on to the attack - but only when I have no other choice and only until I can dive.
Against aircraft, my rule is avoid if humanly possible.
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