NMAC IN THE UNCTLED TFC PATTERN BTWN A NORDO BIPLANE AND A CHEROKEE.

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WE WERE MAKING TKOFS AND LNDGS AT THE PETALUMA ARPT IN THE STUDENT/OWNER'S SKYBOLT -- A 2 SEAT (TANDEM SEATING) 4 CYLINDER BIPLANE. THE STUDENT (A PVT PLT)HAS ABOUT 200 HRS TOTAL FLYING TIME; MOSTLY TAILWHEEL BUT VERY LITTLE BIPLANE TIME. I (ACTING AS CFI AND SAFETY PLT) WAS IN THE FRONT SEAT AND FLYING THE AIRPLANE AT THE TIME OF THE INCIDENT. THE ACFT WAS NOT EQUIPPED WITH A COM RADIO. AS A RESULT; WE WERE NOT TALKING WITH ANYONE IN THE PATTERN AT PETALUMA AND SIMPLY RELYING ON VISUAL SEPARATION AND USING THE AIRPLANE'S SHADOW ON THE GND AS A DOUBLE-CHK OF PROX OF 'UNSEEN' ACFT. THIS WAS THE FIRST TIME THE OWNER HAD EVER FLOWN A R HAND PATTERN IN A BIPLANE AND HAD STRUGGLED THROUGH SEVERAL LNDGS. I WAS IN THE PROCESS OF DEMONSTRATING AN APCH AND LNDG. WE WERE ON DOWNWIND AND SAW WHAT WE BELIEVED TO BE THE TFC WE WERE FOLLOWING 'OVER THE NUMBERS.' I CALLED OUT THE TFC TO THE STUDENT PLT AND THAT I SAW NO OTHER TFC AHEAD OF US IN THE PATTERN. I FLEW A CIRCULAR APCH FROM DOWNWIND TO FINAL WITH A R WING LOW SLIP. THIS PUT THE ACFT IN A POS THAT WAS 'BELLY UP' TO A CHEROKEE THAT WAS ON A NORMAL FINAL. (I HAVE NO IDEA HOW OR WHEN THAT ACFT ENTERED THE PATTERN.) AS I LEVELED THE SKYBOLT OUT ABOUT 100 FT AGL; R OF THE EXTENDED CTRLINE; I SAW THE CHEROKEE BELOW AND SLIGHTLY AHEAD OF ME APCHING TO LAND ON RWY 29. I APPLIED PWR; DIVERTED TO THE R AND WENT AROUND. THERE WAS ANOTHER ACFT HOLDING FOR DEP AT THE APCH END OF THE RWY AND IT IS POSSIBLE HE ALERTED THE CHEROKEE PLT TO THE CONFLICT. AS I WAS MAKING MY GAR; THE CHEROKEE PLT MADE A GAR ALSO. WHILE THE TFC PATTERN WAS NOT PARTICULARLY BUSY (NO MORE THAN 3 ACFT DURING THE TIME WE WERE THERE) BEING ABLE TO MONITOR AND XMIT ON THE CTAF WOULD HAVE PROVIDED ONE MORE POTENTIAL COLLISION AVOIDANCE METHOD. IT REINFORCED (FOR BOTH ME AND THE STUDENT) THAT THE APCH REQUIRED FOR A BIPLANE TO MAINTAIN VISUAL CONTACT WITH THE RWY CAN BE DIRECTLY IN CONFLICT WITH THE NEED TO MAINTAIN VISUAL CONTACT WITH POTENTIALLY CONFLICTING TFC; IE; SCANNING THE TFC PATTERN. A SLIP POTENTIALLY MAKES THAT SIT WORSE! THE 'STUDENT' PLT/OWNER SAW AND CALLED OUT THE CONFLICTING TFC AT ALMOST THE SAME TIME I SAW IT. FOUR EYES ARE ALWAYS BETTER THAN TWO. INSTRUCTING SHOULD NEVER INTERFERE WITH TFC SCANNING BUT IN THIS CASE IT PROBABLY DID -- OR AT LEAST DEGRADED THE QUALITY AND FREQ OF TFC SCANNING. IN OVER A DECADE OF FLYING SMALL BIPLANES; THIS IS THE SECOND INCIDENT OF THIS TYPE. THE BAD NEWS IS GETTING THIS CLOSE. THE GOOD NEWS IS THAT I NEVER STOP LOOKING FOR TFC UNTIL THE PLANE IS PARKED. BETTER LATE THAN NEVER.