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Interesting discussion, Al.
I am a little confused about the system pressure being able to read what
the pressure cap setting is, and somehow adjusting the pressure, under normal
operating conditions.
My understanding and evaluation of the mechanics of the cap is that If and
only if the system pressure exceeds the cap setting,coolant is blown
overboard or into the reservoir until pressure goes down to the setting and then
that stops. It seems to be, in effect, a popoff safety valve and not a
controller.
In the above (below) example, are you using an unpressurized reservoir
(such as is used in automotive applications) or an air buffer at the
top of a sealed cooling system, or both.
Other than with extreme temperature and the ability to blow off coolant
with high pressure, due to this excessive temperature, to prevent
blowing off hoses and gaskets (and in our case seals), what is the purpose of
the cap's setting assuming that it is rated above, (if only
slightly) normal operating pressure.(assuming the setting is
high enough to allow the pressure created by the normal expansion of the fluid
as it heats? )
Would it not be true that in an "Ideal" system that a popoff cap would
theoretically not be necessary?
Thanks for your input
Rich
In a message dated 5/6/2015 10:16:33 A.M. Central Daylight Time,
flyrotary@lancaironline.net writes:
Hi Jeff. I’ve done lot’s of experimenting with cooling sys. Tough to
explain all this, but I’ll give it a try:
Pretty clear you saw 6 psi only because you took off with 3+ cups of air
in the system. If you’d taken off with 2 cups of air, then pressure would have
been 9 psi. 1 cup, 14 psi. No air in system, then you would have seen rated
cap pressure (16 psi in your case).
Sorry to say, the pressure you see has no significant effect on cooling
efficiency (heat transfer). The next time you fly, since you’ve removed most
of that air, you will still see 210F. Just like before. There’s one huge
exception to that statement, that’s if you have air in sys that can’t move to
a high point out of the flow. In that case air in sys has huge negative
effect. Causes local boiling when it passes hot areas and inflates cooling
temp.
You don’t have to fly to prove these concepts. Ground running at idle is
all that’s needed. Let’s assume all air is removed. Then letting engine warm
up to 180 F will result in rapid pressure rise to 16 psi (rated cap pressure).
Fluid will exit system. With 2 cups of air in sys, that same 180F will yield
slow rise in pressure to only 9 psi. No fluid will leave sys.
A good cooling design pretends air is stuck in block, so you add a path
for that air to rise out of the block coolant flow. This is called a dynamic
bleed. Air is automatically removed from engine coolant flow. Super low risk
way to fly as you no longer care if air is in system. It’s can’t affect
cooling.
It’s a bit higher risk to fly without dynamic air bleed, you just make
darn sure you purge all air from block sys prior to flight. Applying vacuum to
rad cap is great way to remove air.
One of the ironies about cooling design is that air that resides above
engine flow is a safety asset. For example, your cap is highest point in sys
and you have 2 cups of air under cap. Big safety advantage simply because your
pressure gage is then a great predictor of how well your sys is doing. A leak
will be detected long before overheating. A bunch of other assets to this
design.
Conversely, air in engine flow has overwhelming negative affect. Temps
soar and risks boil over.
Clear as mud eh?
Fwiw
-al wick
Sent: Wednesday, May 06, 2015 4:54 AM
Subject: [FlyRotary] Re: Return to Flight
Dave,
I can’t argue with your adequate theory; I have that for my oil cooling
system, along with the temperature-controlled Mazda oil cooler, I never worry
about oil temperature.
It’s
the practical application of that theory to the radiator installation that has
plagued me from the beginning … after the next flight I’ll know where I stand
and if there’s still a problem, then radical change may have to be the
recipe.
Jeff
From: David Leonard
[mailto:flyrotary@lancaironline.net]
Sent: Tuesday, May 05, 2015
9:31 PM
Subject: Re: [FlyRotary] Re: Return to
Flight
I agree with Bobby, this may (hopefully) be the cause, but
if so it just means that you didn't design in an adequate way to fill the
coolant without trapping air. You would hate to have to do a
coolant-prep flight every time you wanted to drain and refill the coolant.
(but not the end of the world).
I originally designed my system like Ed's. Minimal
drag (just the 2 stock RV-6 inlets) but cooling was like you and Ed
describe... OK, but you always have to keep and eye on it and often
reduce power or AOA in order to keep coolant temps in check. Then I made
a major change to provide TONS of air to an adequately sized radiator.
Since, I never even think about cooling. Yes, I do probably burn a
little more fuel but it is well worth it to never worry about cooling
again. Its not rocket science. Adequate air to an adequate
radiator equals adequate cooling. Change adequate to plenty in that
sentence and now your are rocking!
On Mon, May 4, 2015 at 12:37 PM, Bobby J. Hughes <flyrotary@lancaironline.net> wrote:
Jeff,
Congratulations on getting back in the air. That ½
quart of coolant may have been trapped air that purged during flight.
Bobby Hughes
N416AS-
RV10 223 hrs
From: Rotary motors in
aircraft [mailto:flyrotary@lancaironline.net]
Sent: Monday, May
04, 2015 7:53 AM
To: Rotary motors in aircraft
Subject:
[FlyRotary] Return to Flight
Yesterday 3-May,
my bird got airborne once again on its maiden flight after 2+ years
rebuilding. I had a friend test-pilot fly it while I was flight engineer
watching the gauges. Flight controls and rigging were reported to be
good and the engine performed flawlessly. The oil temperature was stable
around 160F (Mazda oil cooler in series with a Fluidyne) but again the water
temperature was high reaching 210F by circuit height so we backed off the
throttle, did a touch and go with a 500’ circuit and parked it. One
unusual result was the cooling system pressure was only at 5-6 psi; with the
high temperature it should have been 15-16 psi. Removed the cowlings and
found no leaks of either oil or glycol. I checked the coolant level and
it was down, requiring 1/2 quart to top it up. I also looked at my pinched
duct and think it needs more pinching, so am already starting to modify that –
hope to get another test flight in before the weekend.
Jeff
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