Is it possible to make your outlet too big, thus
causing turbulence and poorer air flow out the exit?
Walter
-----Original Message-----
From:
murphy-rebel@dcsol.com
[mailto:
murphy-rebel@dcsol.com]On Behalf Of
klehman
Sent: Friday, October 11, 2002 4:24 PM
To:
murphy-rebel@dcsol.com
Subject: Re: Rotax 912 oil cooler exit air
Hi Ian
I thought I'd throw a little theory at you. Theory
can't hurt when you
don't have a successful model to copy. As always I
defer to any pratical
experience that others may offer.
My understanding is that simply reducing the intake
area can not
possibly increase cooling unless it is done in such a
way to capture
additional air. Air does love to bypass intakes and
reverse back out
again, but just reducing the size of the intake is not
likely to help
that. The reason for minimizing intake area is to
reduce drag. The
guidelines for intake to exhaust area are also intended
to minimize
drag.
The longer story is that you can't really force 100mph
air through a
radiator. The pressure drop would be much higher than
what is available.
The airflow through the core will likely be much less
than half that
speed regardless of what the intake and exhaust look
like. Therefore one
can aim for a small air intake that opens (gradually if
possible) into a
plenum. The air slows in the plenum and the pressure
increases. The air
pressure then drops as it flows past the rad. Ideally
the air then
converges in another duct where it speeds up again
hopefully exiting at
close to ambient pressure and speed.
So you need at least several inches of duct between the
intake and the
rad to get any improvement at all (preferably more like
a foot or
more). Adding an inch simply won't make any
difference.
It sounds like you really only have the exit to play
with. Again try to
arrange it so that the air smoothly accelerates without
any sharp
corners like at the bottom of the firewall. Exhaust
parallel to the
fuselage as much as possible. Exhaust air will not
exhaust with any
significant downward velocity unless you have a large
high drag lip that
causes many times higher drag than necessary. Even then
the exhaust will
be turbulent slow vortices rather than smooth and fast.
A cowl that
drops a couple of inches lower than the fuselage bottom
is preferable to
a large lip. If you have to have a large lip to make
things work you
could try a moveable cowl flap if you want to cruise a
few MPH faster.
Ken
Drew Dalgleish wrote:
Hi Ian
I'm just passing on what I read in Tony Bingeliss'
book. I belive he was
talking about the total area. When I was trying to
get more air out of my
cowling I made a lip for the bottom of the cowling
with cardboard and duct
tape to see if it would help. Maybe you could
temporarily block part of
your inlet to see if it helps. That way you could
reduce it a little at a
time to find the optimum size instead of reworking
your cowl and finding it
didn't help. Good Luck
Drew
At 11:20 PM 10/10/2002 +1000, you wrote:
G'day Drew
You can see that I never thought things through!
In your last email you suggested that the inlets
should be 3" x 6" times
two. Do you think that
that area would include the openings for the oil
cooler and water radiator?
What I am trying to establish is to whether I have
more entry air than I
need. If the entry area was
reduced there would not be such a problem getting
the hot air out.
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