REFLECTOR: updraft cooling
Chuck Jensen
cjensen at dts9000.com
Sun Feb 18 09:46:30 CST 2007
Ronnie,
Have you ever heard the theories/facts about why the shape of the EZ and
LEZ is so different that the top NACAs won't work? Does the roofline of
the EZ drop away more sharply and a low pressure is created there unlike
the long tapered, shallow roof line of Velo? I imagine some EZ owners
have tried it and confirmed it didn't work since there was 1) the known
cooling problems and, 2) the salulatory effects of top NACA in another
pusher (Velo).
Or, maybe it was just those theorists at it again?
Chuck Jensen
-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Ron Brown
Sent: Sunday, February 18, 2007 8:43 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: updraft cooling
Dave,
I don't have any other details about the CHT locations.
Here's what I know about the cooling systems on Velocities from my 10+
years
of following, building and flying Velocities. I'm sure there is more
information out there that I am not aware of.
The Long EZ's use updraft cooling, scoop on the belly and baffling that
forces air up through the engine and out the back. The baffles have
holes
through them for the exhaust pipes. My understanding is that most Long
EZ's
run hot and cannot stand much idling on the ground (like Oshkosh). Down
draft cooling with NACA scoops were considered for Long EZ's, but every
one
said this would not work. But the fuselage shape is different for a
Long EZ
than a Velocity.
The original Velocities had a similar arrangement which did not cool
very
well. The thinking was that the Lycomings had to have down draft
cooling,
so the inlet scoops were connected to large tubes that fed into the top
of
the cooling plenum, then air went down through the engine and out the
back.
Most still ran hot. In fact, a friend of mine had continuing problems
with
seized valves after running 400 degree plus CHT's.
By the way, Dean's engine has the Lycoming crossflow heads used on some
twins. The combustion intake air enters the bottom of the cylinder
heads,
the exhaust goes out the top. His Velocity has upflow cooling air and
exhaust pipes exiting through the top of the cowl. I have not seen any
other Velocities built this way.
I have heard that the early XL's also had arm pit cooling scoops with
large
tubes feeding air to the top of the engine. One of these XL's had a
small
NACA scoop on top of the fuselage to feed over head cabin vents for the
rear
passengers. Then someone noted that cigarette smoke was going out the
panel
mounted vents meaning there was more pressure at the top of the fuselage
than there was at the front of the fuselage. That led to the
installation of
the top mounted NACA inlets for down draft cooling which has proven to
be
the most successful cooling arrangement for Velocities.
OK, much of the above are recollections and third hand information from
the
Reflector.
I have three holes in my firewall left over from relocating my oil
filter.
I intend to connect some tubing to the top of the plenum, the cowl area
and
the inlet to my fuel injection servo and use manometers to get better
data
on what is really going on back there!!! I promise to do this during my
annual this spring.
Ronnie
----- Original Message -----
From: "Dave Philipsen" <velocity at davebiz.com
<mailto:velocity at davebiz.com> >
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org
<mailto:reflector at tvbf.org> >
Sent: Saturday, February 17, 2007 6:35 PM
Subject: Re: REFLECTOR: updraft cooling
> Where were the CHT thermocouples on his engine? Did he move the
> thermocouples when he converted to downdraft?
>
>
> Ron Brown wrote:
>> And, the downdraft NACA cooling for some unexplainable reason, runs
>> about 40 degrees cooler than the updraft cooling. Mark Machado
>> converted what is now the factory trainer from updraft to downdraft
>> and says the heads ran 30-40 degrees cooler. My 173 Elite RG runs
>> 360-370 max on a long climb out and 320-340 degrees during a 2600
>> rpm/155 kt cruise. I highly recommend the NACA cooling system.
>>
>> Couple of other recommendations to get good cooling:
>>
>> 1. Make sure you make the extended wraps to go on the plenums that
>> wrap all the way under the cylinders and heads - such that cooling
air
>> is forced to go through the fins - and exit through a gap which is
>> 1.5" to 2" wide at the bottom. If you look at the Lycoming supplied
>> inter cylinder baffles, you want to match them on them on the
outside.
>>
>> 2. Close up the factory plenum to the crank case gaps. I put duct
>> tape on the crankcase, then laid up two BID on the plenum onto the
>> duct tape.
>>
>> 3. Keep the gap at the rear of the cowl to prop clearance small -
Alan
>> Shaw recommended 1". That is what I did - with reliefs at the top and
>> bottom to allow the cowl to slide by the prop (with a rag to protect
>> the prop finish).
>>
>> My cooling concerns while building wound up being absolute non
>> issues. Even on the ground - with long taxiing - like Oshkosh are no
>> problem at all. The only temperature issue I have is a landing -
shut
>> down - and start back up and climb out (like young eagles flights) on
>> a hot day, I'll get 360 degree CHTs on climb out. I put the nose
down
>> and pull back on the throttle a bit - temps start coming back down.
>>
>> Ronnie Brown
>>
>>
>>
>> ----- Original Message -----
>> *From:* Douglas Holub <mailto:doug.holub at tx.rr.com
<mailto:doug.holub at tx.rr.com> >
>> *To:* reflector at tvbf.org <mailto:reflector at tvbf.org>
<mailto:reflector at tvbf.org <mailto:reflector at tvbf.org> >
>> *Sent:* Saturday, February 17, 2007 1:28 PM
>> *Subject:* REFLECTOR: updraft cooling
>>
>> I can understand why Burt Rutan and Nat Puffer are proponents of
>> updraft cooling. From an engineering point of view, it has a lot
>> going for it. You need more cooling when the airplane is
climbing.
>> If the cooling intake is below the wing, the pressure is higher
>> during a climb so you automatically get more cooling during a
>> climb. Similarly, it would be nice if cooling was minimized
during
>> descent. The pressure is reduced under the wing during descent,
>> and so there is less cooling to the engine. Also, you've got
>> convection working with you instead of against you with an
updraft
>> system.
>>
>> That all adds up to more drag with down draft cooling, because
the
>> NACA scoops have to be large enough so that there is adequate
>> cooling during climb out, when the pressure at the NACAs is at
its
>> minimum.
>>
>> But the down draft is a lot simpler to implement, and that's
>> probably going to be the deciding factor for me.
>>
>> Doug Holub
>>
>>
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>
> --
> Dave Philipsen
> Velocity STD-FG
> N83DP
>
>
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