PIPER J3 CUB PLT EXPERIENCES RWY EXCURSION INTO ADJACENT GRASS AREA AFTER ABRUPT RUDDER INPUT.

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THE PLANE JUST CAME OUT OF RESTORATION. I RECEIVED ABOUT 6 HRS OF DUAL IN IT; MOSTLY ON GRASS FIELD AND GOT SIGNED OFF BIANNUAL/TAILWHEEL ENDORSEMENT. THE PLANE NOW HAS A FULL SWIVEL (MAULE) TAILWHEEL. WHILE I WAS INSTRUCTED IN THE OP OF THE TAILWHEEL (I HAVE ONLY OPERATED NON-SWIVEL TAILWHEELS) AND PRACTICED AT THE GRASS FIELD IN HAMPTON I WAS NOT FAMILIAR ENOUGH WITH FULL SWIVEL TAILWHEEL OP ON HARD SURFACE RWYS WITH SLIGHT XWIND. I LANDED 3 POINT AND USED FULL RUDDER TO CTL THE PLANE -- UNKNOWINGLY UNLOCKING THE TAILWHEEL. AT APPROX 25 MPH THE PLANE SWERVED OFF THE RWY INTO THE GRASS. I WAS NOT QUICK ENOUGH TO USE DIFFERENTIAL BRAKING. THE PLANE CAME TO A STOP APPROX 30 FT FROM THE SIDE OF THE RWY. RATHER THAN TAXI BACK ON THE ACTIVE I TAXIED APPROX 35 FT TO THE NEAREST TXWY. NO DAMAGE WAS INCURRED. I FEEL MY ACTION TO LET THE PLANE TURN INTO THE WIND AND SETTLE TO A STOP IN THE GRASS WAS CORRECT. I DID NOT; AT THE TIME; UNDERSTAND WHY THE RUDDER/TAILWHEEL DID NOT RESPOND TO MY RUDDER PEDAL INPUTS. I HAVE SINCE RECEIVED GND INSTRUCTION AND DEMO ON THE OP OF FULL SWIVEL TAILWHEELS. IN ADDITION I WILL PRACTICE GND TAXI IN VARIOUS SITUATIONS TO LEARN HOW THE TAILWHEEL REACTS AND; SHOULD I UNLOCK THE TAILWHEEL WITH FULL RUDDER DEFLECTION HOW TO 1) RECOGNIZE EARLY; 2) BE PREPARED TO IMPLEMENT DIFFERENTIAL BRAKING.