 To cater for the increased cooling requirements of the Merlin, a new fuselage duct was designed.This housed a larger radiator, which incorporated a section for the supercharger coolant, and, forward of this and slightly lower, an oil cooler was housed in a secondary duct which drew air through the main opening and exhausted via a separate exit flap.A "duct rumble" heard by pilots in flight in the prototype P-51B resulted in a full-scale wind-tunnel test at NACA's Ames Aeronautical Laboratory.