From: "Marvin Kaye" <marv@lancaironline.net>
Subject: Re: [FlyRotary] Temp control with airflow
To: flyrotary
Date: Tue, 22 Oct 2002 20:25:34 -0400
Message-ID: <web-1842847@logan.com>
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Posted for "Tracy Crook" <lors01@msn.com>:

> I've read over and over again, about people who have trouble getting
proper
> airflow through their radiator.  This usually leads to several revisions
of
> cooling ducts, radiator size placement, etc.  After reading all the right
> things to do when designing radiator ducting, look at an FD (93-95 RX-7).
> The radiator is leaning forward at a 60 degree angle to the airflow, which
> just can't be optimal.  It is a thin radiator, which helps airflow, but
the
> real solution seems to be two, 3 speed electric fans.

I think most people grossly underestimate the power required to ram the air
through heat exchangers that are cooling an engine running at full tilt
boogie.
A commonly accepted figure for cooling drag on a relatively clean airframe
at top speed is 30%.  Think about this for a minute.  That means that 30% of
the energy delivered to the airframe by the prop is absorbed as cooling
drag.  This is one area where turboprops and jets have a real advantage over
us.  If it were not for this factor, their fuel economy would look even
worse than it does.

Another way to look at it is volume of air delivered at the required
pressure.  A 180 HP engine requires around 2500 - 3000 CFM of air squeezed
through the heat exchangers.  The pressure required is usually around the
same value as dynamic pressure at the top speed of the aircraft.  This
represents a lot of energy.  More than could be delivered by the average
electric fan.   The fan would work fine on the ground at taxi speeds (just
like it does on a car) but my guess is that it won't do much for you at
flight power settings.

But I'd love to be wrong about this :-)

Tracy