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Hi Nigel,
> The purpose of the divergent duct (a diffuser) is to slow the air
> down and raise it's pressure, before presenting it to the radiator
> at a speed that can be passed through the radiator matrix that is
> fast enough to absorb the heat, but not so fast that the airflow
> stalls and causes drag (like the standard Europa).
You describe a divergent diffuser. Later it was discovered that you
can also make a wedge diffuser, by placing the radiator at a steep
angle, and a horizontal sheet of metal meeting the end of the
radiator. This accomplishes the same effect, slowing the air down and
increasing the pressure. The air goes indeed into turbulent flow, but
this is just what you want inside the radiator; turbulent air carries
heat more easily away. One of my experiments was to use the original
radiators but in a wedge setup and it gave a huge improvement, while
the required radiator opening was much less.
A later discovery (not by me but in general) was that you can also
create a natural wedge diffuser by placing the radiator at an angle...
and leaving the horizontal separator away. This simply means you can
mount the radiator inline with the angled shape of the lower cowling.
It sounds a bit strange at first sight, but once you realize that the
function of a diffuser is to convert a small opening with high speed
air to a large opening with low speed and higher pressure, you will
see that the same happens with a very steeply angled radiator in free
air. The frontal surface is much smaller than the surface of the radiator.
Thinking like air molecules: hitting the radiator they are scooped up,
to pass through them, but when there is no more room for air molecules
the remaining air molecules just see a solid angled "plate" and just
follow the shape of the lower cowling as if there is no radiator at
all, so it causes no drag.
Anyway, it works perfectly well. And it is done this way in many
modern aircraft.
Hope this gives further insight.
Greetings,
Frans
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