Flaps do several things: increase lift, decrease stall speed, increase drag, and in some planes increase turn rate.
Some planes have multiple flap settings, others only one. The F4U and P-38 have "maneuvering flaps" that will deploy at a higher speed than most.
You can use flaps to increase lift for takeoff. Drop a notch of flaps just before you get pull off the runway. Get enough speed to maintain level flight without flaps, then retract them.
To use flaps for landing, slow the plane to a speed where the flaps will deploy, then gradually drop them a notch at time as your speed decreases. This will slow the plane and decrase stall speed so you can touch down at a speed slow enough to stop on the runway.
You can use flaps to increase lift to "get over the top" of vertical maneuvers.
You can use flaps to increase turn rate to get the nose on the bad guy for a shot. However, flaps also increase drag and will cost you speed (energy), so only use one notch, and only when you are pretty certain of a kill. Retract them as soon as possible.
The rudder also has a few effects: steering on the ground, compensating engine torque, increasing roll acceleration, pointing the guns, improving a vertical reversals, spin recovery, deliberate slips or skids, flat "rudder turns", and coordinating banked turns.
To increase roll acceleration, give the rudder a kick in the direction of the roll as you begin the roll.
You can point the nose for a snap shot with a kick of the rudder.
At the top of a zoom, you can use a kick of rudder (along with ailerons) to bring the nose around for a reversal.
To recover from a spin, power off, nose down, and rudder opposite to the direction of spin.
You can use the rudder to deliberately skid the plane (so that the nose is not aligned with the direction of travel). This will create a LOT of drag and slow the plane quickly. Useful for landing, in a steep dive, or as a last ditch maneuver to force an attacker to overshoot.
You can turn with rudder input only. This is a very slow turn mostly useful for bombing.
A very important use of rudder is to coordinate a banked turn. If you watch the slip indicator (the "level" near the gunsight), the ball will go off center in turns. By using the rudder to keep the ball centered, drag is minimized and energy retained.
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