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Tracy (still dreaming of drag free cooling)
Awhile back I did some rough calculations on cooling the engine using the wing surfaces. While it might be possible, it didn't look very promising. The main trouble is that the flow over the wings is laminar. Heat you move from the wing surface to the air then hugs the wing. The heat only slowly conducts through the layer of air surrounding the wing. The same thing is true for the engine cowling.
Having said that, it is probably worth the effort to attempt to transfer heat from the oil to the returned fuel. It is low risk, low weight, does not require power, and will add some degree of "free" cooling.
An oil-to-fuel heat exchanger would be less risky than a water-to-fuel heat exchanger. Since the oil is at higher pressure than the fuel, oil will tend to leak into the fuel if something goes amiss. Getting oil in the fuel is not a big deal. Getting water in the fuel is a different story.
Your highest load on he cooling system is during departure taxi and climb-out, when you have the most fuel. Thus, the fuel system can absorb (and reject) the most heat when you need it to do so most. Even if you are not rejecting that much heat from the wing surfaces, the thermal mass of the fuel and tank structure might help significantly during transient peak engine cooling while taxing and during climb-out.
It would be prudent to monitor the temperature of the fuel exiting the heat exchanger as well as in the tank.
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