As usual, Ed does a good job of explaining this.  Ed, this was exactly why I was interested in how much the wedge shaped duct would slow the air down.  If I can't get the air slowed down, I need to find another way to mount my oil cooler and radiator.  It would seem that whatever our ducting scheme we need to slow the air down and then limit the amount of airflow thru the radiators to match the cooling requirement in our quest for the lowest cooling drag - most of us would probably chose to use cowl flaps to limit the airflow thru the radiator.  Lastly, a 'too large' radiator intake doesn't appear to cause alot of drag since external diffusion will be taking place on the front of the aircraft anyway.  Of course, this last point only seems to apply to tractor aircraft.  Many thanks to Bill Freeman for educating us on external diffusion.

Ken Powell
>
>
> ----- Original Message -----
> From: <peon@pacific.net.au>
> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> Sent: Thursday, February 05, 2004 5:14 PM
> Subject: [FlyRotary] FD Rad Cooling Capacity was Re: [FlyRotary] Re: Engine.
>
>
> > Hi Guys,
> >
> > On 4 Feb 2004, at 1:39, Finn Lassen wrote:
> >
> > > Russell Duffy wrote:
> > >
> > SNIP>>
> >
> > .  The radiator in the car would be too small for aircraft use.
> > > > Some people are using a large aftermarket radiator, though
> > most are
> > > > using air conditioner evaporator cores.
> > > >
> >
> > Hmmmmmm, ... I was discussing this the other day with "Wally
> > the muddle headed wombat"  and we were just pondering why on
> > earth an FD radiator wouldn't cool a 13B turbo in an aircraft?
> >
> > It goes without saying that providing of course, you fed it sufficient
> > air in a properly designed dukt - that man at that "other place"
> > knows how to do it - he's been preaching it for years - nice big thin
> > rads).
> >
> > Wally reasoned that as they seem to work OK on 300 + neddie
> > ekranoplans (aka porsche beating FD RX7's) down Conrod straight
> > at Bathurst (130 - 140 + MPH,  albeit in ground effect),  they
> > should work in an "ordinary" aircraft as well.
> >
> > They also seem to work OK at idle and heavy traffic, even on our
> > hottest Sydney daze.  Only downside is that the plastic header
> > tanks are known to split,  which can be interesting!  So can you
> > please enlighten us??
> >
> > Cheers,
> >
> > Leon
> > (aka The Rotary Duck - ordinary ducks just go "quack,  quack" -
> > this one's a bit "daffy").
> >
>
> Hi Leon,
>
>     Don't know if it  would cool an aircraft application or not, but I
> strongly suspect that if it can cool a racer full out then it probably can.
> Given sufficient surface area and air mass flow through it almost anything
> will cool.  I suspect that the reasons a lot of evaporator cores are used
> include 1:  They do cool the 13B NA just fine,  2.  Their small size
> provides flexibility in placement compared to one large radiator
> (particularly for tractor installations). 3 They are designed for much more
> pressure than would ever see in a radiator application  4 They are very
> cheap relative to a custom radiator  5 There are plenty of them around in
> the junk yards 6 They have no plastic or epoxy
>
> A somewhat more debatable view  is - that since drag is proportional to
> frontal area -  that a larger radiator may incur more drag.  However, that
> view is offset by the fact that a smaller radiator must have higher air
> velocity through it to get the same mass flow and therefore cooling effect
> and drag is proportinal to the square of the air velocity.  So with perfect
> ducting/diffusion  in both cases the larger radiator supposedly wins with
> less cooling drag, since the same cooling effect can be had with lower flow
> velocity through a larger radiator.   However, if you ducting is less than
> optimum and your velocity through your larger radiator is higher than
> optimum then the larger radiator can indeed be a higher source of drag than
> a smaller radiator.  Also a smaller radiator might enable packaging with
> reduced external (airframe) drag.  In fact, they are just the right size to
> stick under the cowl behind the traditional twin ducts each side of the
> propeller.
>
>
> If cooling drag is a large part of total drag (as it frequently is in higher
> performance aircraft) then ducting diffuser and core size all become very
> important - on the other hand if cooling drag is a small part  (like perhaps
> on a liquid cooling bi plane) of the overall drag, then I would think
> cooling drag is less of an consideration - just cool it.
>
> FWIW
>
> Ed Anderson
>
>
>
> .
>
>
>
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