SMA TRAINING FLT. STUDENT PLT HAVING PROBS DIRECTIONAL CTL ON TKOF WANTED TO PRACTICE ON A GRASS STRIP. INSTRUCTOR ADVISED STUDENT PLT TO USE MORE R RUDDER ON TKOF RESULTING IN LOSS OF CTL AND R WINGTIP BRUSHING A NEARBY CORNFIELD. INSTRUCTOR TOOK OVER; BUT UNABLE TO REGAIN CTL AND CAME TO REST IN THE EDGE OF THE CORNFIELD. MECH CHKED THE ACFT AND IT WAS FLOWN TO BASE ARPT FOR MINOR REPAIRS.

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I FLEW WITH ANOTHER CFI STUDENT FOR THE FIRST TIME. HE HAS 40 PLUS HRS AND IS 'GETTING READY' FOR HIS CHK RIDE. HIS NORMAL INSTRUCTOR WAS NOT AVAILABLE AND HE WANTED TO WORK ON SOFT-FIELD OPS. WE PRACTICED 2 TKOF AND 2 LNDGS FROM THE ASPHALT STRIP HERE AT OUR HOME BASE. LNDGS WERE EXCELLENT. I NOTED ON BOTH TKOFS HAD A BIT TOO HIGH NOSE ATTITUDE AND DIRECTIONAL CTL PROBS DUE TO NOT ENOUGH R RUDDER. HOWEVER; IT WAS NOT NECESSARY FOR ME TO TAKE THE CTLS. AFTER THESE 2 CYCLES; WE FLEW TO A GRASS STRIP NEAR HERE. THE STRIP MEASURES 2575 FT BY 145; FT AND APPROX 20 FT FROM THE R EDGE OF THE STRIP THERE IS A CORNFIELD; WHICH IS 8-10 FT TALL THIS TIME OF YR. BEFORE OUR FIRST TKOF ATTEMPT AT THE GRASS STRIP; I CRITIQUED HIS 2 PREVIOUS TKOFS AND EMPHASIZED THE NEED FOR MORE R RUDDER. AS WE BECAME AIRBORNE ON THE SOFT-FIELD TKOF ATTEMPT; THE STUDENT USED ENTIRELY TOO MUCH R RUDDER AND WE DRIFTED QUICKLY TO THE R; STILL IN GND EFFECT. THE R WINGTIP BRUSHED THE TOP OF THE CORN JUST AFTER I TOOK THE CTLS. THAT ADDITIONAL DRAG ON THE END OF THE R TIP YAWED US FURTHER TO THE R AND I WAS UNABLE TO RECOVER. THE ACFT ENDED UP APPROX 1 WINGSPAN INTO THE CORNFIELD; APPROX 1000 FT FROM THE BEGINNING OF THE TKOF RUN. AFTER ONE OF OUR MECHS FLEW OVER AND RAN IT UP; ETC; I WAS ABLE TO FLY IT HOME. THE ACFT SUSTAINED ONLY MINOR CRACKS TO THE COWLING AND A SMALL DENT TO THE R WING LEADING EDGE. THE OBVIOUS MISTAKE I MADE WAS NOT TAKING THE CTLS SOONER. WE; AS CFI'S; TALK QUITE A BIT ABOUT HOW ONE NEEDS TO LET A STUDENT MAKE MISTAKES TO LEARN FROM THEM AND TAKE CARE TO NOT TAKE THE AIRPLANE AWAY FROM THE STUDENT PREMATURELY. THIS EVENT ILLUSTRATES THE IMPORTANCE OF KNOWING WHEN YOU DO NEED TO TAKE CTL AND RETURN THE ACFT TO A SAFE FLYING CONDITION. THE NOT-SO-OBVIOUS MISTAKE I MADE IS PERHAPS AN INCOMPLETE CRITIQUE. I FOCUSED ON THE FACT HE HAD NOT USED ENOUGH R RUDDER ON THE 2 PREVIOUS ATTEMPTS. A BETTER CRITIQUE MAY HAVE FOCUSED MORE ON HOW TO USE THE RUDDER FOR DIRECTIONAL CTL; AND THE VISUAL CUES TO USE IN THAT NOSE-HIGH ATTITUDE TO DETERMINE WHAT RUDDER PRESSURES ARE NECESSARY.