To:
Rotary motors in aircraft
Subject: [FlyRotary] Re: Diffuser Configuration Comparison
I
especially liked the comment in the report where it says that the
higher
the pressure drop across the core, the higher the diffuser
efficiency.
I interpret this as 'thicker core is better than
thinner!'
Of course there is a point of diminishing returns where
flow
is simply too low as Ed has pointed out but in an application
where
diffuser efficiency is such an important factor, this pushes
that
point in the direction of 'thick'.
Tracy
(back from the dragon and sorry to bring up the thick vs thin
thing
again :>)
Again I feel compelled to caution against
over-simplification and generalities (I’m not just trying to be a
contrarian; really!). In a myriad of factors, drawing that conclusion based on
one configuration and one parameter can certainly be misleading. What is your
criteria? I can say that generally increased pressure drop equates to
increased drag. Also “thickness” and “pressure drop”
are not equatable (is that a word?). The very thick P-51 rad with large
hexagonal passages is in no way comparable to the same thickness of modern radiators with
½” between tubes and 16 fins/in.
The most detailed,
sophisticated analysis of radiators for our application that I have seem was
done by Fred Moreno back in ’99; but unfortunately it also was done for a
specific case of 220 KTAS and 10 fins/in. What it suggests is there is an
optimum thickness (minimum drag), and that is roughly 3”. That’s
for 10 fins/in.; I concluded that my 16 fins/in rad should be thinner, and went
with 2.5” thckness. It works exceptionally well. The 2.5” to 3”
thickness seems common for rads made by Ron Davis and Griffin for
racing applications. That seems consistent.
But likely, obtaining
minimum drag is beyond the sophistication that most of us can achieve in our applications.
If we can get it to cool adequately we’ve done a good job; and that is
probably mostly a function of scoops, diffuser and exit configurations.
FWIW,
Al G