A Corporate pilot reported taking evasive action after experiencing an NMAC with a group of J3 aircraft near PDK.

Narrative

IFR flight plan. Performed all normal preflight checks including the TCAS system which tested normally. Performed radio check and received airport advisory on UNICOM (122.725). Was informed that Runway 30 was in use. Wind was favoring Runway 30 at FDK. Received the following clearance from Potomac Approach: 'upon entering controlled airspace proceed direct MRB VOR then STILL Intersection then direct; climb and maintain 3000; expect 4000 10 minutes after departure; departure 126.1; squawk xxxx; hold for release.' Released at XA30 local time. Monitoring UNICOM (122.725) and visual scan of airspace yielded no traffic. Announced the following on UNICOM (122.725): 'Frederick traffic; Aircraft X taxiing into position Runway 30 for takeoff.' 'Frederick traffic; Aircraft X rolling Runway 30; Frederick.' 'Frederick traffic; Aircraft X turning left crosswind; Runway 30; Frederick.' No response was heard to any of the 3 transmissions. A normal takeoff was accomplished. Aircraft accelerated to 140 KTS. Climbing through approximately 800 AGL aircraft was turned left toward the crosswind leg of the traffic pattern and MRB VOR. Objects thought to be large birds were observed moving from 10 to 12 o'clock approximately 200 FT above the aircraft altitude. The objects were then recognized as 6 or 7 Piper J-3 Cubs in loose formation. The 4 lead J-3 cubs continued straight-ahead flight. The trailing aircraft turned toward my aircraft and began a descent. Rapid and positive evasive action was taken. I descended and turned sharply to the left to avoid a certain midair collision. No TCAS alert sounded. No targets were observed on the TCAS. No radio transmissions were heard from any of the aircraft. No report of the near midair was made during the initial airborne call to Potomac Approach. This event could have been avoided had there been an Air Traffic Control Tower in place and operating at the FDK.