Had a pair of Clamshell doors on a Chinook come open one afternoon over a very unfriendly neighborhood...I can assure you the Boeing-Vertol version of speed brakes are not as stout as the Boeing style. While in cruise at 120 knots....there was a wee bang sound....followed by some really weird air flow in the cabin and cockpit. The clamshell doors being on the leading edge of the aft pylon....and opening rearwards from the front....well I guess they must have swung pretty hard there for a moment...like a New York Instant really....and just like the old country western song...."and psssst....you were gone!" As the air pressure increased inside the aft pylon....the two aft work platform doors decided to open as well...and they did so with one heck of a bang! It was about then we sensed something was amiss.....that and seeing a crewie with eyes the size of dinner plates speaking in gibberish while pointing a very shaky finger towards the rear of the aircraft. The work platform doors were of better quality than the pylon doors.....we at least came home with them.