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The title of this section says it all. There are two ways we
can fly faster, either develop more power or reduce the airframe
drag. Today's air-cooled engines require a substantial volume
of airflow that must be passed over a large and complex structure.
Passing this much air past such a complex structure as an engine
creates drag and that drag adds to the overall parasitic drag
of the airframe. This type of drag call cooling drag. In addition
it is very difficult to get the airflow over the engine
to pass evenly by each of the cylinders. The result is eneven
cooling of the cylinders with the front cylinders generally running
much hotter than the rear cylinders. Using an air-cooled engine
means that there are few options on how to pickup cooling air
and were the heated waste air can be dumped.
A liquid cooled engine offers several benefits to reduce cooling
drag. The first benefit is that we now only need sufficient airflow
to pass over a high density radiator. Such a radiator is designed
to provide substantially more thermal conductivity thereby reducing
the amount of air required to remove the same amount of heat.
A radiator can be located in more aerodynamically suitable locations
on the airframe. Air pickup can also be located in more optimum
locations and internal ducting offers the opportunity to create
a far more efficient air stream to the radiator. Slowing the incoming
cooling air and allowing it to expand towards the radiator surface
allows much more efficient cooling. More heat is dissipated to
a smaller air volume.
A radiator presents far less drag on the passing air flow than
does a complex structure such as an engine with baffles. Reducing
the air flow volume while at the same time reducing the drag as
the air passes thru the radiator means an overall reduction in
cooling drag.
It used to be that the convention wisdom was that a low drag
radiator with a reduced number of fins per inch was the most aerodynamically
efficient way to reduce cooling drag. This allowed a high volume
of high speed air to pass through the radiator with little pressure
differential (aka drag). Today we have learned through high speed
automotive racing that the opposite is true. It is better to have
a small volume of air passing at relatively low speed through
a highly efficient radiator structure (ie: high pressure differential
across the radiator) and to allow the incoming air to back up
at the air inlet.
The result of this approach is that a far smaller air inlet is
required and that the smaller volume airstream carries off far
more waste heat. The smaller volume of air is handling a higher
heat load. It also means that the cooling air inlet can be located
in shaped in such a manner as to take advantage of the pressure
dome that will be created at the inlet. This pressure dome can
be viewed as almost a flat surface which can be blended into the
surrounding structure to produce a smooth aerodynamic contour.
The overall result is substantially reduced cooling drag.
The much more efficient radiator also means that far less airflow
is required for cooling on the ground - reducing the need for
supplemental boost fans in most installations.
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