Wasn't the Mustang alleged to obtain some thrust from the radiator exit due to thermal expansion effects?
It depends on your viewpoint. Yes the radiator produced thrust, but not sufficient to overcome the cooling drag. Lee Atwood, Vice President of North American during the P-51 days, gave a speech in which he said,
In the case of the Mustang, the air duct pumping system at full speed at 25,000 feet was processing some 500 cubic feet of air per second, and discharge speed of the outlet was between 500 and 600 feet per second relative to the airplane. This air jet counteracted much of the radiator drag and had the effect of offsetting most of the total cooling drag. To offer some approximate numbers, the full power propeller thrust was about 1,000 pounds and the radiator drag (gross) was about 400 pounds, but the momentum recovery was some 350 pounds of compensating thrust--for a net cooling drag of only some 3% of the thrust of the propeller.
ie total drag of the cooling system was reduced from 400 lbs to 50.
A search on the web has failed to find the documents, but should anyone be interested they can PM for a full copy of the rather lengthy speech, and also a copy of Merediths original paper.