Perry Mick wrote:
Flew this morning with the cowl piece removed, so I had a big
hole in the cowl. That is the configuration that allowed me to
fly to KS. Took pressure data from inside cowl. Then put the
cowl piece back in to close off the hole, so only the 16 slots
1/4" x 5" for oil cooler exit. Flew again. Here is the data:
Open + slots:
-80mph at 80 knot climb = 0.114 psi = [-] 3.15 in. H2O
-75mph at 130 knot cruise = 0.100 psi = [-] 2.77 in. H2O
Slots only:
-55mph at 80 knot climb = 0.054 psi = [-] 1.59 in. H2O
-50mph at 130 knot cruise = 0.044 psi = [-] 1.25 in. H2O
So now I'm going to mod the cowl to put a circular hole
around the fan hub, and hopefully maintain the lower pressures.
I mean more negative pressures.
--
Perry J. Mick
I see. You had me confused there for a minute.
That's not bad. The oil cooler should be working fine
if indeed that is the negative pressure all over the back
side of the cooler.
Sorry 'bout that. It does work ok until ambient temps
are above 70 F, then oil temp goes to 220 and even 230.
Seems to be about the best I can do.
Where was the tap? Porous foam on end?
It is a small 1/8" tube tie-wrapped to a duct strut above
the oil cooler exit. No porous foam but the end is somewhat
blocked.
Could be air is going in those slots unless they are sealed
to the top of the oil cooler. In other words is the oil cooler
outlet duct sealed straight up to the cowl? Is there an outlet duct?
There could be a dynamic pressure effect of air flowing in the slots
off setting the lower static pressure on the back side of the cooler.
There is no outlet duct, oil cooler exits into the cowl. The inlet
side is sealed to the oil cooler.
Ideally the cooler should have an outlet duct going straight
up to the skin with either closely spaced louvers or venetian blind
cowl flaps. That is the simplest and most straight forward
way of handling that oil cooler position.
Try it again with the cowl piece removed and tape up the slots.
Paul Lamar
I've already started the cowl mod to put a 10" diameter hole
in the cowl in front of the 8" diameter fan hub. Hopefully that will
work better and have less aero drag than the hole above the hub.
--
Perry Mick
Just be be sure you are not reading stray
air currents glue a bit of porous kitchen foam to the end of that
tube as in this picture.
Paul Lamar
OK. Another thing I forgot to mention. Those inside
cowl pressure measurements are negative relative to static.
Previous data seemed to indicate that before I opened up
the oil cooler duct inlet, that pressure in the inlet side was
actually lower than the exit side, even though the exit side
is negative relative to static, and the inlet side should be
seeing pitot-type pressure. So apparently air was flowing
through the oil cooler backwards. Increasing the opening
from 5x2.5 to 5x4.5 caused the airflow to stop and it cooled
worse!
So I will have to take more data on the inlet side, as that is
where the real mystery is.
--
Perry Mick
Duckt N7XR
http://www.ductedfan.com
When you measure the pressure on the inlet side did you use
the foam?
It is possibly the air is deciding to go around the
inlet because of the flow restriction. If that is the case
an angled inlet might help. Back at the top.
There is room in there to actually measure the velocity of
the air entering the duct with a pito tube flush with the
inlet. That and the static pressure is the bottom line.
The air velocity is the dynamic pressure minus the static
pressure in the inlet.
It seems to me you are going to have to bite the
bullet sooner or later and devise a method of taking more
than one pressure at a time. Should have room in there
for at least one U tube.
BTW if you use this scheme with a dynamic pressure (air speed)
reference and a U tube water manometer you will have to pinch
off the dynamic pressure line when you change ports as the
dynamic pressure will blow all the water out of the U tube.
Paul Lamar
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