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<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.
Sorry if I didn’t explain well. If you fill your
cooling system completely to the top, no air in sys anywhere, then warm up
engine to 200F will result in seeing a pressure reading of 16 psi. Fluid will
leave sys and go to your overflow. Now, let the engine completely cool. But this
time, remove 2 cups of fluid. This time when you warm up engine, you see max
pressure of 10 psi. No fluid will overflow. This is because that two cups of air
acts like a giant spring. Air molecules are far apart and easily compress.
I fly with 2 cups of air in system. It’s so much
safer. Since the air is above the engine, out of the block flow, it has no
significant neg affect on cooling. The safety is due to flying way more
informed. A cooling sys completely stacked full of fluid will mask a number of
common problems, but the two cup of air method makes all the problems clear with
gobs of advance notice. For example, I was deliberately very abusive to my first
engine. Overheated it time and again during ground run experiments. Suddenly I
saw 22 psi, where normally it never exceeds 12 psi. That pressure only lasted
for 5 seconds. Sure nuff, I review the data, this was caused by my finally
getting a compression leak into cooling sys. It only leaked for 5 seconds after
applying full throttle. Would not have been able to observe this if I had a
fully filled cooling sys.
<are you using an
unpressurized reservoir
Not needed with two cup of air sys. Coolant never
exits sys unless temps exceed 220 F or so.
<purpose of the cap's
setting
Cap pressure rating gives you boil over protection.
Boil over is a big deal. When it happens it’s darn near irreversible. Pretty
remarkable actually. You reduce power and it continues to boil fluid out of
system. I use 22 psi cap. With my 50/50 mix, that means boil over can’t occur
until I reach a whopping 289F. 16 psi cap is a good choice. That would boil over
at 252 F and you are not running at needlessly high pressure.
<Would it not be true
that in an "Ideal" system that a popoff cap would theoretically not be
necessary?
I need that popoff on the off chance that I achieve
very high operating temp. If I reach 289F, then my 2 cups of air are fully
compressed. I would then have completely fluid filled sys that needs to dump
water to prevent rad from blowing open.
-al wick
Sent: Wednesday, May 06, 2015 11:25 AM
Subject: [FlyRotary] Re: Return to Flight
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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This message, and the
documents attached hereto, is intended only for the addressee and may contain
privileged or confidential information. Any unauthorized disclosure is
strictly prohibited. If you have received this message in error, please notify
us immediately so that we may correct our internal records. Please then delete
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