I agree, Steve. There is no question each part of
the cooling system is critical and the total results is no better
than the weakest link.
Most studies I have read indicates that after a certain
size in inlet area (from 25-35% of core frontal area) - the outlet size becomes
the determining factor and further increases in intake provide no additional
benefit and can hurt by increasing cooling drag. Adding such things as
cowl flaps can reduce the pressure in the outlet region and promote more airflow
and cooling but naturally at the cost of more drag. But, then at higher
speeds with plenty of dynamic pressure, you can retract the cowl flaps and
reduce the drag.
NACA ducts have been made to work with radiator
cores - no question about that. The question is would a different
approach have produced a "better" cooling system. Again, I think it
depends on your intended operating environment.
For a high speed cruise environment, I would think cooling
drag might be of more importance than say perhaps a few pounds of additional
weight, on the other hand if you are flying an already draggy biplane for
example, cooling drag is probably a very small part of your over all drag, but
getting cooling with low airspeed might be the system driver.
Its all about compromises - space, weight, flow, drag,
etc. - oh, yes! - and cooling of course {:>) all matched to your
constraints and operating environment.
Ed
Sent: Friday, April 29, 2011 8:48 PM
Subject: [FlyRotary] Re: Cooling Inlets
Hi Ed and Dwayne
I'm working on my inlet and outlet for Renesis powered Glasair SIIRG.
It seems whenever we turn our attention to air, it is not about inlets or
outlets but pressure differentials and the whole system.
A great inlet is killed by a lousy outlet, and both made mute by inadequate
diffusion.
Perhaps a NACA would work adequately given a system with good diffusion
that SUCKED well.
Cheers
Steve Izett
Not flying, so maybe completely deluded.
Continues to more than respect Tracy's thoughts and practices.
On 30/04/2011, at 7:26 AM, Ed Anderson wrote:
Dwayne
There is a NACA study on NACA ducts
which in essence found that while they were excellent for feeding an intake
(an duct with no internal resistance such as a heat exchanger core) such as an
engine intake, that their performance suffered relative to other duct
configurations - where you had a radiator core installed. The
reason appeared to be that the pressure build up before the core hindered the
airflow into the duct and caused a lot of the air to flow around the opening.
On the good side, they were relatively low drag ducts.
Now that being said, several
approaches have been found that seems to offset the problems. One that
comes to mind is the placement of vortex generators which guide more airflow
into the ducts and the other one is the placement of the inlet in a high
pressure area. Folks have used them successfully for cooling - so long
as sufficient airflow can be achieved through the duct the core doesn't care
what kind of opening is used.
Ed
Sent: Friday, April 29, 2011 5:05 PM
Subject: [FlyRotary] Re: Cooling
Inlets
OK, I gotta ask. Does anyone use NACA ducts for
cooling inlets? Why or why not?
From: Tracy <rwstracy@gmail.com>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Fri, April 29, 2011 9:49:00
AM
Subject: [FlyRotary] Re: Cooling
Inlets
Some questions:
Prior reading seemed to indicate that
the oil cooler did ~1/3 of the cooling, implying a 2/1 ratio on air
requirements. This setup seems to have a significantly higher percentage
allocated to oil. Is this a byproduct of heat exchanger differences, or the
less efficient heat transfer ability of oil, or....?
2nd, assuming
similar inlet & diffuser efficiencies, could the inlet areas mentioned be
reduced by roughly 1/3 with reasonable expectation of cooling a 2 rotor
Renesis?
On the subject of exit area: Does either heat exchanger have
an exit duct? The RV guys with really fast Lyc powered planes all have some
variation of exit ducting to smoothly re-accelerate and redirect exit air
parallel to & at or above the slipstream. Even the stock RV-8 has a
rounded lip at the bottom of the firewall (which the really fast guys say is
much too small a radius...). And there's always the near-mythical P-51
system...
Thanks,
Charlie
The inlets were originally closer
to the 2 - 1 area ratio but many experiments (mostly failures) ended up with
the current sizes. I just don't have it in me to go back and un-do them
all. Also wish I had tried these inlets with my original oil cooler
which had about 1/3 more core volume and much thicker. Might have
been able to do the oil cooling with less CFM airflow. But, I don't
think there is much penalty for having more than enough (but properly faired)
inlet area and throttling the airflow with a cowl flap.
Yes, I do think
both inlets could be scaled down in area for a 2 rotor.
Neither of my
heat exchangers have exit ducts. Just not enough room to do this in
their current locations.
Tracy
On Thu, Apr 28, 2011 at 4:23 PM,
Charlie England
<ceengland@bellsouth.net> wrote:
On 4/28/2011 8:07 AM, Tracy wrote:
Finally got around to finishing my cooling inlets. (pictures
attached) Up until now they were simply round pipes sticking out of
the cowl. The pipes are still there but they have properly
shaped bellmouths on them. The shape and contours were derived
from a NASA contractor report (NASA_CR3485) that you can find via
Google. Lots of math & formulas in it but I just copied the best
performing inlet picture of the contour. Apparently there is
an optimum radius for the inner and outer lip of the inlet.
There was no change to the inlet diameters of 5.25" on water cooler and
4.75" on oil cooler.
The simple pipes performed adequately in level
flight at moderate cruise settings even on hot days but oil temps would
quickly hit redline at high power level flight and in climb.
The significant change
with the new inlet shape is that they appear to capture off-axis air
flow (like in climb and swirling flow induced by prop at high
power) MUCH better than the simple pipes. First flight
test was on a 94 deg. F day and I could not get the oil temp above 200
degrees in a max power climb. They may have gone higher if
the air temperature remained constant but at 3500 fpm the rapidly
decreasing OAT kept the temps well under redline (210 deg F).
I
have an air pressure instrument reading the pressure in front of the oil
cooler and was amazed at the pressure recovered from the prop wash.
At 130 MPH the pressure would almost double when the throttle was advanced
to WOT. That did not happen nearly as much with the simple
pipes.
These inlets
ROCK!
Tracy
Crook
& replace
it with James' rotary cowl, or just modify the existing cowl.
Some
questions:
Prior reading seemed to indicate that the oil cooler did ~1/3
of the cooling, implying a 2/1 ratio on air requirements. This setup seems
to have a significantly higher percentage allocated to oil. Is this a
byproduct of heat exchanger differences, or the less efficient heat transfer
ability of oil, or....?
2nd, assuming similar inlet & diffuser
efficiencies, could the inlet areas mentioned be reduced by roughly 1/3 with
reasonable expectation of cooling a 2 rotor Renesis?
On the subject
of exit area: Does either heat exchanger have an exit duct? The RV guys with
really fast Lyc powered planes all have some variation of exit ducting to
smoothly re-accelerate and redirect exit air parallel to & at or above
the slipstream. Even the stock RV-8 has a rounded lip at the bottom of the
firewall (which the really fast guys say is much too small a radius...). And
there's always the near-mythical P-51
system...
Thanks,
Charlie
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