NMAC BTWN A C172 TRANSITING THROUGH CLASS D AIRSPACE AND A SHORTS SHERPA CLBING OUT. BOTH ACFT TOOK EVASIVE ACTION UPON OBSERVING THE OTHER.

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I WAS FLYING SBOUND FROM MADRAS; OR (S33) TO BEND; OR (S07); LEVEL AT 4500 FT; ABOUT 1200 FT AGL. AT 12 MI DME N OF DSD; I RADIOED RDM TWR AND REQUESTED TO TRANSITION THROUGH CLASS D AIRSPACE AT RDM TO S07. RDM TWR CLRED ME TO TRANSITION AND TOLD ME TO RPT WHEN PAST THE RDM ARPT. I HAD RPTED MY POS AS 10 MI N OF RDM AND MY ALT AS 4500 FT. A MOMENT LATER; RDM TWR CAUTIONED ME ABOUT SKYDIVERS THAT WERE OUT ABOUT 10 MI E OF MY POS. I CALLED RDM TWR AND CLARIFIED MY POS AS OVER CROOKED RIVER RANCH; A RESORT AREA ABOUT 10 MI NNW OF THE ARPT. TWR ACKNOWLEDGED. I CONTINUED SBOUND AT 4500 FT MSL; AT 120 KIAS. AS I ENTERED CLASS D AIRSPACE; I HEARD RDM TWR TALKING TO A PLANE INBOUND NEAR DSD. SOON AFTER; I HEARD THE TWR CLR A SHORT SHERPA TO DEPART RDM. AS I ENTERED THE AREA ABEAM RDM; I LOOKED FOR THE SHORTS; BUT DIDN'T SEE IT. THEN I LOOKED W TOWARD DSD TO CHK THE AIRSPACE FOR INBOUND TFC. WHEN I LOOKED L (E) TOWARD RDM; I SAW THE SHORTS AT MY 11 O'CLOCK POS AT 200-300 FT. THE SHORTS WAS IN A CLB ATTITUDE AT MY ALT. I IMMEDIATELY BANKED L AND DOVE SO AS TO PASS BEHIND THE SHORTS. I BELIEVE THE PLTS OF THE SHERPA SAW ME ABOUT THE SAME TIME AS I SAW THEM BECAUSE I SAW THEIR ACFT BANK SLIGHTLY L; AWAY FROM ME. I BELIEVE OUR 2 ACFT PASSED WITHIN 100-200 FT OF EACH OTHER. IMMEDIATELY; I CALLED RDM TWR AND TOLD THE CTLR WE HAD A 'CLOSE ONE' WITH THE SHORTS. AFTER ASKING ME TO SAY AGAIN; RDM TWR ASKED ME TO CALL THEM WHEN I REACHED S07. I CALLED AND GAVE THE CONTRACT TWR EMPLOYEE A DESCRIPTION OF THE EVENT. CONTRIBUTING FACTORS: 1) I BELIEVE THE TWR DID NOT WARN THE SHORTS OR THE INBOUND PLANE OVER DSD THAT I WOULD BE PASSING PERPENDICULAR TO THEIR FLT PATHS. NOR DID THE TWR WARN ME OF THE SHORTS' DEP PATH. 2) IN RETROSPECT; I SHOULD HAVE ASKED TWR FOR A POS RPT ON THE SHORTS WHEN I DIDN'T IMMEDIATELY SEE IT. 3) THE 2 PLTS ON THE SHERPA SHOULD HAVE SEEN ME EARLIER SINCE I WAS INITIALLY ABOVE THEM AND HAD MY LNDG LIGHTS ON. 4) IF THE SHERPA PLTS HAD SEEN ME; THEY SHOULD HAVE CHANGED COURSE; SINCE I WAS TO THEIR R AND UNDER RULES OF RIGHT-OF-WAY (FAR PART 91.133(D)); HAD THE RIGHT-OF-WAY. 5) IN PEACE TIME; IF ACFT FLY INTO CIVILIAN ARPTS; THEY SHOULD HAVE SOME BRIGHT ORANGE OR WHITE PAINT. THE SHERPA'S FLAT GRAY PAINT BLENDED IN WITH THE SAGEBRUSH AND JUNIPER OF CENTRAL OREGON'S HIGH DESERT; MAKING IT DIFFICULT TO SEE FROM ABOVE.