David Renaker wrote:

If you have looked in your computers lately you will notice that they are
gluing ever larger heat sinks to the CPU.  Has anyone ever considered using
that large aluminum wing hanging in the breeze to dissipate heat instead of a
radiator?

Russell Kent wrote:

David,
Many engineers have had the same thought.  Some early racers actually used this
idea, but eventually found that radiators work better.  The problems are:

1. Complexity.  Attempting to turn the wing or fuselage into a radiator means
plumbing the interior with coolant lines.  Because of the number of lines, and
the number of plumbing connections, these lines are a significant point of
failure.

2. Weight. All of those coolant lines, plus the lines to and from the engine, are
filled with coolant.  Coolant isn't light (at roughly 8 lbs. / gal.)

3. Efficiency (area).  The transfer of heat from the coolant to the airstream is
a function of the area (among other things).  In order to transfer the heat
energy, a relatively large area of aircraft skin must be used, aggravating the
complexity and weight problems (see #1 and #2).

4. Efficiency (transfer mechanism).  The transfer of heat from the coolant to the
airstream is affected by the nature of the transfer mechanism.  For a
metal-to-air transfer, we want a turbulent airflow to maximize the volume of air
contacting our radiator area.  But for an aircraft wing or fuselage, turbulence
means wasted energy (ideally we want laminar flow).

The reasons a radiator works best are:

1. Close to engine.  This reduces the coolant run lengths, helping to save
weight.

2. Fabricated as a unit.  This reduces the chances of a discreet radiator weld
failing.

3. Multiple corrugated fins.  This increases area and also induces turbulence,
both working to increase the radiator's efficiency.

4. Fed reduced-speed air.  This works to minimize the energy wasted in
"turbulating" the air.  Reducing the airspeed is accomplished by using an
expanding-volume duct. Downstream of the radiator, any ducts used should be
reducing-volume ducts, to return the air back to free stream velocity, in order
to eliminate unwanted drag.

Russell Kent
[BTW, please feel free to post this on the FAQ site.]

Excellent job Russell! Thanks.

Paul Lamar
 
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