On the other hand, it has occasionally been claimed that cooling can produce negative drag. I recently had occasion to edit the Wikipedia article — any idiot can — on a skimpy 1920s radial-engine cowling called a Townend Ring to challenge the preposterous claim that it produced thrust. The same claim was made, more plausibly, for the cooling system of the P-51 Mustang. The idea is that, like the combustor of a jet engine, the hot radiator expands the cooling air, increasing its velocity through the outlet. Since the same mass goes out as comes in, an increase in exit velocity would result in a forward thrust. It's questionable, however, whether enough thrust can be generated, given the relatively small heat rise in the cooling air, to overcome the internal resistance of the duct and radiator. Disagreements about this point simmered for years after the war, and when I interviewed several veterans of Mustang development at North American 16 years ago some residual heat was still detectable.