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Thanks Ed,
My biggest concern on pressure
differentials through the Rad is cowl pressure. If cowl pressure is
the biggest culprit ( in some installs) I say isolate the Rads. If you isolate
the rads you can use an eductor but an eductor need s an
inlet.
So the inlet area is therefore enlarged ,
because you now have two - but as you say just enlarge the rad inlet.
However the bigger inlets also cause drag, I guess drag is unavoidable but can be minimized.
I guess that answers my question.
George ( down under)
Hi
George,
You may have missed
the e mail where Tracy described his “Eductor” experiment –
one reason his drag was so high was he was flying without a cowl – or part of
one. In any case, cooling was great but drag was horrendous. You
could certainly do better on drag if you designed and build a slightly more
“sophisticated” one than Tracy quickly came up with. However,
the question still remains as to whether the benefits vs cost equation is on
the right side of the line using an eductor as opposed says to
larger inlet opening? Its not a question of can it be done –
its been done, the real question is if it offered some cost effective benefit
why are not more folks using it?
I’ve attached a
couple of discussions links related to augmentation that you might want to
read. Basically, it appears that the reason most folks are not using
them more are due to it simply not being worth the effort – unless you are an
all out air racer. That’s my take on it.
http://aafo.com/racing/news/98/intrepid.htm
http://www.homebuiltairplanes.com/forums/firewall-forward-props-fuel-system/395-exhaust-augmenter-cooling-system.html
http://www.homebuiltairplanes.com/forums/design-structures-cutting-edge-technology/4897-exhaust-augmentoed-cooling.html
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of George Lendich
Sent: Wednesday, April 28, 2010 6:00
PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Eductor
scavenging of radiator outlet, WAS 20B RV-8 cooling
results
Like yourself I have been
considering the benefits of an Eductor for some time, but can't understand why
it would cause extra drag.
As a matter of fact I thought it
might decrease drag by speeding up the rad exit air back up to outside air
speed as it exits the cowl.
Can you explain the reasons
behind the increased drag issues.
"Since the
draw of air via low pressure on the output side seems to be key, I wonder if
an eductor type of scenario would work."
Sometimes I
doubt my ability to get a point across clearly :-) An eductor can be
made to help (but very hard to do as Ed pointed out) but the point I
was trying to make in my original post is that the draw of air on the low
side is NOT key. You will never get a fraction of the pressure delta
with low side help (even with an eductor) that you can with the proper inlet
and diffuser. This is especially true on faster airplanes.
A Pietenpol might be an exception.
The inlet is what fixed my
problem. This is an extreme example but when I used low side help, it
did cool but the drag caused the fuel consumption to increase by 50 - 60% !
at the test speed of 130 mph. That's not a price you want to
pay.
Tracy
On Wed, Apr 28, 2010 at 9:19 AM, Chris Owens -
Rotary <rotary@cmowens.com>
wrote:
You know, I don't know if this
has been discussed, but the whole pressure differential thing got me
thinking of something that I'm surprised I hadn't thought of earlier.
Since the draw of air via low pressure on the output side seems to be key, I
wonder if an eductor type of scenario would work.
Back in my Navy
days, we used to use a device called an in-line eductor for dewatering
flooded spaces. Similar to a venturi, more or less, you pumped water
through it, it created a suction, and it was designed to suck as much water
through it as you put into it. 100 gallons per minute input would
dewater at 100 gallons per minute with 200 gallons per minute flowing
through the output. A representative device is here (perhaps not for
fluid use, but the concept is similar):
http://www.1877eductors.com/eductor_gas_dimensions.htm
I
presume a similar approach could be taken with a radiator setup, would you
think? I imagine it would work well for a center mounted radiator with
a centerline, below-the-nose scoop, so one could utilize the cheek inlets to
provide source air for the outlet
side.
~Chris
From: "Ed
Anderson" <eanderson@carolina.rr.com>
Sent: Wednesday, April 28, 2010 7:08
AM
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: 20B RV-8
cooling results
Hi
George,
As you know, taking
heat away from your radiator cores requires sufficient air mass flow - a
number of factors affect this - one of the principle factors is pressure
differential across your core. No pressure differential = no
flow. The primary positive pressure on the front side of the core
comes from converting dynamic energy of the moving air into a local static
pressure increase in front of the core. This is basically limited by
your airspeed and efficiency of your duct/diffuser. The back side of
your core air flow (in most installations) exits inside the cowl.
Therefore any positive pressure above ambient under the cowl is going to
reduce the pressure differential across your core. So once you have
the best duct/diffuser you can achieve on the front side of the core - the
only thing left to increase the pressure differential is to reduce the
pressure under the cowl.
An extreme example
is someone who flies with an opening (such as one of the typical inlet holes
beside the prop) exposed to the air flow. In effect this hole with
little/no resistance to airflow can "pressurize" the cowl and raise the air
pressure behind the radiator cores reducing the pressure differential and
therefore the cooling. Exhaust augmentation is theoretically a way to
reduce the under the cowl pressure by using the exhaust pulse to "pump" air
from under the cowl, thereby improving the Dp across the core
and therefore your cooling.
While exhaust
augmentation can apparently work - there was a KITPLANE issue back several
years ago on the topic showing several installations where this was
used. However, from what I read (and think I understand), it takes
some carefully planning to get an installation to work correct and the
effort is not trivial. Give the challenges you may encounter (such as
motor mount struts, etc), fabrication of the augmentation exit, the
need to have the exhaust pulse exit at or inside the cowl (or construct an
extended bottom cowl tunnel) means you would have the bark of a rotary in
front of your feet. Also, to gain maximum advantage of these
techniques, it is desirable to have the exhaust velocity at the maximum -
which implies little/no muffling. Having had my muffler back out one
time (at the cowl exit), I can tell you that you do not want to position the
pilot behind the exhaust outlet (in my opinion). It is much quieter
when you have the exhaust exit behind the position of the pilot
{:>).
Some few people
seem to have been able to achieve some degree of success, but even in
aircraft where you have an engine without the aggressive bark of the rotary,
you seldom see it used. The basic reason (in my opinion), is that it
offers few advantages (cooling wise) that can not be achieved easier and
more reliability by other methods. For an all out racer where noise
and discomfort is secondary, it may have some
benefit.
Having said that,
it's clear that in some installations it appears to work well (see KITPLANE
issue), but if it were the magic solution, I think many more folks would be
employing it - but, again, just my opinion.
Ed
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On Behalf Of George Lendich
Sent: Tuesday, April 27, 2010 9:41
PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: 20B RV-8
cooling results
Can't
say as I understand Tracy's set- up completely, other than it's
toward the lower end of Rad sizes. I was thinking to myself how I could
create a -ve pressure in the rad outlet to create a suction on the Rad. We
all know how the exhaust augmentation works and I was wondering why we can't
do the same thing with the rad outlets by running the rad outlets inro a
larger outlet fed by outside air. At idle the air is fed by the prop air
stream and at level fight it is fed by outside air
stream.
The
outside air could be could controlled by a butterfly - simple enough. I
know there emphasis on using shutter /flaps to control the cowl outlet and I
believe their good at restricting air flow, but I don't know if this equates
to a good -ve pressure behind the Rad. This presupposes the Rads are
completely enclosed for both inlet and outlet
air.
75% of my
cooling problems were solved with the oil cooler change I did but still
needed more margin for hot weather climbs. Made the decision to not
change or enlarge the cooling outlet (that adds drag) so went ahead
and butchered the pretty inlets I made.
Ed Anderson's
spreadsheet on BTUs & CFM cooling air required was instrumental in
deciding to go this way. It showed that without negative
pressure on the back side of the rads, there would never be enough cfm to
do the job during climb at full throttle. Negative pressure is what
I had when I flew without the cowl on but oh what a draggy condition that
was.
The old inlets were 4.5" diameter for the radiator and 4.125"
diameter for oil cooler.
New inlets
are 5.190" for the rad,
and 4.875" dia for the oil.
This may not sound like a lot
but it represents a 36% increase in inlet area.
Results were
excellent. Oil temp went down 19 degrees at the test speed (130) and
water temp dropped 9 degrees. On 80 degree day and 500 ft msl the
oil temp maxed out at 194F at 210 mph which is way faster than I would
normally go at this altitude. Temp was around 175 at 130.
Oil Temp in climb remained below redline (210) but the
temperature lapse rate today made results not very meaningful. OAT
was dropping 14 degrees a minute at 3000 fpm climb rate.
now back
to that nasty composite work to pretty up the inlets again. They
look like large stubby pitot tubes now.
I hadn't thought of a good
name for the RV-8 but a friend in California recently came up with the
winning idea which fit it well. "Euphoriac" It's a term from a
Sci Fi book (Vintage Season) meaning something which induces
euphoria.
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