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Glad to hear you were able to bring the temps down,
Dennis. Controlling the airflow so your large radiator (which is more than
adequate) gets the maximum benefit is clearly the need. The photos of
your turning vanes looks to me to have a nice curve to them and are
clearly helping the air flow.
If I were going to suggest any change it would be to make
the lowest vane a bit shorter than the middle vane and the upper
vane a bit longer than the middle vane. The rationale for this
suggestion is that the bottom portion of your radiator is likely to be getting
the major portion of the air flow as it is from the straight-in approach from
the inlet. It just needs a bit of a vane to get it turned into the
core at a better angle. The upper segment of your duct likely has
less favorable air flow - so having its vane a bit longer poking out into the
inlet air will capture more of the high velocity air and direct it to the upper
portion. But you don't want it so long that it shadows your two lower
vanes.
But, this is just a SWAG on my part if you are intent on
making any changes. On the other hand, if you got a 16 deg drop in
oil and cooling temps with your current vane set up -then unless that still is
inadequate for your cooling needs - why mess with success?
Regarding interpretation of air pressure in a duct - it
can get counter intuitive. Pressure tends to increase in two
circumstances 1. where there is a sudden change (enlargement) in
cross section area (like a diffuser) and 2. where airflow is
restricted (like a core its encountering or having blockage at the
rear of the core). It generally means that the air arriving in that region
of lower pressure has a lower velocity such that the conversion of its low
amount of kinetic energy (dynamic pressure) does not produce much of an
increase in static pressure.
The maximum pressure drop is across a solid core - which
has no airflow – the minimum pressure drop occurs with no core - which has
unrestricted airflow. Neither approach is optimum in removing much heat – but
one is much lighter and has less cooling drag
So since the bottom of you radiator is pretty much seeing
a straight shot from the inlet the velocity of that air flow is probably
considerably higher than the air forced to make the turn to the upper end of
your radiator. Therefore you get higher pressure at the bottom due to the
large amount of kinetic energy being converted to static pressure.
Ideally, you would like you pressure distribution
across the core to be equal - but the idea is rarely
achieved.
Sounds like you are on the right track to me based on the
improvement you got. You may have found the "magic" formula for
successfully using the "James Cowl".
Keep going!
Ed
----- Original Message -----
Sent: Thursday, May 31, 2007 11:38 PM
Subject: [FlyRotary] RV-7a cooling -
continued!
>I took out the first
splitter/duct that was shown in my 5/27 posting
> photo 1678.
Cooling was than back to slightly marginal not critical!
>
> I
than built a rig with 3 turning vanes to direct air into the radiator
>
near the front of the wedge. (See attached photos) I achieved about a
> 16 deg. F drop in oil and water! This told me that most of the
inlet
> air was going through the radiator at the small end of the
wedge. Bobby
> Hughes said he had seen data indicating a wedge duct
had the highest
> pressure near the end of the wedge and lower pressure
near the opening.
> I now am considering modifying the lower cowl -
inlet duct bottom -to
> include a curved ramp to direct air upward over
the first 30 - 40 % of
> the radiator and than transition into a wedge
the rest of the way back
> to the trailing edge of the radiator.
>
> I have not found any information I feel comfortable to use to
design
> the curved ramp and may try to build a wind tunnel in the cowl
on the
> plane. I should be able to insert several ramps and wedge shapes
in the
> test area and measure air pressure on the back side of the
radiator
> using a sensitive manometer. I have two electric leaf
blowers that may
> provide enough air to run it. I'm planning on a
3 in wide x 4 in high
> inlet size. Any comments or
suggestions?
>
> Radiator to Duct Sealing:
> My radiators are
sealed to the duct with silicone baffle rubber strips
> used on
cowl-to-baffles on spam cans. I believe my seals are very
>
good. But - after several E mails concerning sealing the duct I
looked
> more closely at the total air flow sealing package. On the
Griffin
> radiator the fins between the tubes do not extend to the water
tanks.
> There is gap of 1/4 to 1/2 inch!! This is on both
ends of the radiator.
> The area of the holes is 0.375 in X 12.5 in
X 2 ends = 9.375 sq in of
> holes just in the one radiator! The
Mazda oil cooler also has gaps but
> they are only about 1/8 inch wide. I
also found about 2 more sq. in of
> other potential leaks I can fill
using Leons light testl Thanks for
> the info. to make me
look harder at this area! If I get the wind
> tunnel working
I will first test with the holes and no ramp/wedge - fill
> the holes and
retest to determine how much the holes decrease the outlet
> air
pressure.
>
> Air Speed in Duct at Radiator:
> The inlet duct
is 51 sq. in. (per James Cowls) and expands to about
> 17.25 X 8.5 inchs
or 146 sq . in. at the start or forward end of the
> radiator
wedge. This gives a velocity of about 42 mph for the cooling
> air
at the front entrance of the wedge at the radiator at a climb speed
> of
120 mph. The duct is a wedge from that point on the the rear of the
> radiators.
>
> --
> Homepage: http://www.flyrotary.com/
> Archive
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