Avonds cut 1 NACA shaped openings (as they are not true NACA ducts) on each side of the intake ducting to let air spill into the fuse for pipe cooling. The single wall pipe is short of the tail cone by about 1/2" to create the venturi effect that Matt was talking about. Using a single wall pipe, heat proofing using BVM heat shield or ceramic blanket is a must for ground ops as air is not flowing well through the fuse on the ground.