The Harrison built (same as Corvette radiator) used in air conditioning in full sized GM cars are designed to flow a major amount of air. They are cheap even when bought new. They transfer heat as good as anything you can find. They are compact. Two of them can dump the heat from most any rotary installation. Two of them for coolant and one for oil has been proven in a number of installations.

 

If you want less cooling drag build the radiators into the wing skins like a Supermarine racer.

 

I raced for years with one Harrison core for oil cooling in front of the water radiator. A poor installation with heated air going through the water radiator, but it worked just fine.

 

Lynn E. Hanover