POTENTIAL CONFLICT IN A NIGHT OP. NON ADHERENCE TO ATC CLRNC INSTRUCTION.

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I WAS AT AN UNFAMILIAR ARPT TAXIING TO RWY 21 (CHS). COMPLETED RUN-UP AND WAS SETTING UP FOR MY DEP BACK TO GMU. I CALLED TWR SAYING I WAS READY TO GO AT RWY 21. I NOTICED A C-17 AIR FORCE TRANSPORT ON FINAL (SHORT FINAL) FOR LNDG. I HAD BEEN GIVEN THE DEP INFO TO TKOF MAINTAIN RWY HDG (21). THEN I CONTACTED TWR - READY TO GO RWY 21. HE CALLED BACK CLR FOR TKOF 'NO DELAY.' (C-17 ON FINAL) IN THE RUSH BTWN GETTING THE VORS AND RADIOS SET UP I THOUGHT I COULD TURN ON COURSE AFTER TKOF. CONFUSED ON IF HE HAD SAID CLR ON COURSE. TOOK OFF; AT ABOUT 500 FT AGL TURNED TO MY COURSE 342 DEGS. THE C-17 WAS (HAD BEEN MOST OF THE AFTERNOON) DOING TOUCH AND GOES IN THE PATTERN. I TURNED TO 342 DEGS AND HE HAD TAKEN OFF AND WAS DIRECTLY BEHIND ME (LOWER - MAYBE 100 FT OFF THE GND. I WAS AT ABOUT 700 AGL NOW; STILL IN CLB) C-17 (BEHIND BY MAYBE 1/2 MI). TWR CONTACTED ME AND INFORMED ME THAT I WAS SUPPOSED TO FLY RWY HDG. THEN HE CALLED THE C-17 ASKED IF HE HAD ME IN SIGHT. THEY DID AND DEVIATED TO THEIR R AND AWAY FROM ME. THE TWR GAVE ME ANOTHER HDG TO FLY SAID SOME MORE ABOUT MY BEING ASSIGNED RWY HDG; THEN TURNED ME OVER TO DEP. I APOLOGIZED REPEATEDLY. DURING THAT I HAD NOT RESET MY DIRECTIONAL GYROSCOPE TO CORRESPOND WITH MY COMPASS. I HAD A PROB WITH DEP -- ASSIGNED A HDG; I WAS FOLLOWING IT BUT MY DIRECTIONAL GYROSCOPE WAS OFF. HE CALLED BACK AND SUGGESTED MY DIRECTIONAL GYROSCOPE WAS IN FACT OFF A FEW DEGS AND THAT I SHOULD ALIGN IT WITH THE COMPASS. I DID THAT THEN TOOK THE HDG AND THERE WAS NO PROB. THE FLT WAS AT NIGHT. I WAS GETTING TIRED FROM THE DAY'S ACTIVITIES. WHAT I SHOULD HAVE DONE WAS ASK TO STAY HOLDING SHORT AT RWY 21 FOR THE LNDG TFC. AND WAIT UNTIL THE C-17 HAD LANDED; TAKEN OFF; AND HIS WAKE TURBULENCE DISSIPATED.