Posted for "Al Gietzen" <alventures@email.msn.com>:

There are certain advantages to using oil/ water heat exchangers, among
them some thermal balance advantages Leon pointed out, and that they are
compact.  In-radiator coolers are typically much lower capacity than we
need, but a custom design may do the job. There is a performance disadvantage of more significance to aircraft
because you want to limit coolant temps to about 200F, whereas you might
let the oil run to 220F (cooler inlet).  The lower rejection temp means
more air flow - generally means more drag.  But the issue that ended my
studies of the oil/water approach is that the available exchangers
(Fluidyne, Setrab, others) had large pressure drops at the flows we need
in the rotary; at least for the 20B in my case.  These exchangers are
for the engines that go clunkety-clunk instead of hum-m-m-m-m, but have
lower heat loads and lower flows in the oil circuit.  Again, this can be
overcome with a custom design.

Al