This left turning squirrel was made by sliding the wing to the left several inches, and taping the the fin 1/2 inch to left.
The following video shows it flying. We found the modifications gave a fairly tight and (flat – meaning it did not bank much) left turn.
The standard Squirrel, carefully built, will normally turn left. The motor torque rolls it into a left bank (hold a wound plane by the prop and see which way it rolls), which makes the wing lift pull partly to the left. It also produces a left sideslip. The wingtip fins in sideslip produce a bit of right roll, enough to keep the left bank and sideslip from turning into a spiral dive. (Dihedral does the same thing.) Without the wingtip fins, the Squirrel will bank and sideslip left into the ground. Try a wing without the tip fins. The propeller slipstream hits the tail fin from the left, producing a force pulling the fin to the right. The propeller slipstream produces some right roll on the wings, but not enough by itself to overcome the motor torque roll.
Other things may be done to control bank and turn. Moving the wing to the left puts more lifting area to the left, producing a right roll, countering the motor torque roll. This roll will remain when the motor torque runs down and may produce a right spiral dive in descent. It also puts more drag out to the left, increasing the left turn. Bending the fin to the left also will produce a left yaw. Bending the prop bearing to the left will produce a left turn under power. Twisting the wing to raise the left leading edge at the tip and depress the right leading edge at the tip will produce a right roll and also a left yaw. Similar effects may be produced by gluing paper ailerons on the trailing edges of the wings near the tips, bending the left one down slightly and the right one up slightly. Tilting the tailplane will also produce a turn, depending on whether the tailplane lift is up or down.
The goal of trimming is to get all these forces and moments to produce a consistent flight pattern with a minimum of control forces through all three phases of flight: the power burst climb, the level circling cruise and the gliding descent. The method is to use the minimum control force that will balance the upsetting force. Some of these mechanisms are more effective with more power and can be used to control the effects of power. Some will work consistently at all speeds.