|
You are absolutely correct, Al. Just did not
think, gas/air compresses, liquids do not (to any appreciable
amount).
If by failure, you mean my system would not have
supported flight in that condition (air in coolant) , then you are quite
correct, on the other hand, one of the reasons to test (as you have often
pointed out) is to discover problems before flight inorder to preclude failures
in the air. In this case, I discovered my configuration requires a bit
more effort to remove the air. Once that is done, the system is operating
within its intended parameters. So does that make the overheating a
system failure or a failure to configure the system to the proper operating
parameters.
Even in the automobile, the rotary is known for trapping
air and requiring burping although many can get away without it because most
autos don't operate anywhere near WOT for more than a few seconds, so the
effects of trapped air (depending on amount) may not be
noticed.
I think there is a difference, for instance
you could design the perfect coolant system but fail to put sufficient coolant
into the system resulting in overheating - is that a system failure or a
operational failure? or perhaps more accurately - operator failure?
I am always impressed by your meticulous attention
to detail and systematic approach. I could certainly have benefit
from your knowledge/approach back 10 years ago in my initial
design{:>). But at that time, there appear to be bigger issues - such
as trying to understand how a rotary engine really worked
- sorted out. Air flow and cooling were just vague notions back
then and I just assume they would naturally fall into place {:>)
I still have a photo of my first oil cooler
installation to remind me of how ignorant of those matters, I was back
then . It had a "Plenum" that conformed to the area of the core - and its
wall stood 2" way from the core face at every point. Then I had a 2 1/2"
dia hose piping air to one corner. Is there small wonder that my first
flight was limited to once around the pattern due to oil temps{:>).
Here is a photo - the oil cooler plenum and inlet are the brown colored box on
the left side of the engine (facing the engine) close to the firewall with the
large back hose (that one is 5" in dia as one of my several early attempts
to address the oil temp problems). The radiator plenums were only slightly
better.
No question, knowing what I now know, I would
have done some things different. As you know, Al, I may
sometimes take issue with your characterization, but not your approach and
insight. A valuable contribution to say the least.
Best Regards
Ed
----- Original Message -----
Sent: Friday, October 05, 2007 12:49
PM
Subject: [FlyRotary] Re: Coolant Water
Pressure
>at 22 psi the air would likely occupy even more
space
The opposite is true. Air space reduces substantially
when you increase pressure. Think of coolant as a solid, and air as a
giant spring. When you add btu's to coolant, it immediately responds by
expanding. This causes the air molecules to compress...a lot.
There are some fun exhibits at the science museum I
volunteer at we use to demonstrate air/ water compression. An eye dropper
inside a pop bottle. When you squeeze the pop bottle, the eye dropper plummets
to the bottom. When you let go, dropper rises to surface. The pressure
increase when you squeeze bottle causes air in dropper to compress(less air
volume). This allows water to displace that air....dropper is now heavier and
falls to bottom.
It's really ironic. You can design a system that seems
to work fine. You fly with that for years with no failures( well I guess you
could call the pinging and high temps a failure). But a small change can
greatly reduce your risk. There's a lot of value to changing your air bleed
design to one that dynamically removes air. No shrader valve, no repeated
cooling cycles to remove air. By "dynamic" I mean that it automatically
removes air from the system. No muss no fuss.
As you design system, just pretend their is air at the
top of each component. Then find simple way to allow that air to move to
higher component in system. So, my radiator is lowest point in system. If I
place my radiator tube near the top of radiator, then all air naturally leaves
rad and flows to engine. Next I look at highest coolant passage in engine. In
my case I had to drill and tap a little 1/4 npt into the coolant manifold,
then run tube from there up to highest point in system. Suddenly I've got a
system that automatically removes all air immediately. Try as I can, I can no
longer trap air anywhere. If I develop compression leak that pumps air into
system, it has much less effect, because it rises out of the coolant flow.
When I drain and refill, it all immediately and rapidly fills, I can get every
drop back into the system.
Dynamic air bleed is a safety advantage, easy to
accomplish. Each of these safety advantages adds up mathematically.
Likewise, contrary to popular theory, leaving two cups
of air under cap increases safety. You can prove that to yourself with simple
experiment I described earlier.
-al wick
----- Original Message -----
Sent: Friday, October 05, 2007 9:07
AM
Subject: [FlyRotary] Re: Coolant Water
Pressure
Yes, at 22 psi the air would likely occupy even more
space, but since I do my runup with the cap off or loose, there is no
pressure during the process. So while I have never measured it and it
probably varies from one time to the next, there appears to be approx 1/3 of
the top part of each core which has air on the initial fire up of the engine
- after a complete drain and refill of the coolant system.
I must admit that the first several times of draining
and refilling coolant, not being as knowledgeable as I am now, I almost
cooked the engine, because I assumed that when the header tank was full -
the engine had all the coolant it could take. Rapidly climbing coolant
temps and pinging of hot engine cooling off soon make it clear that just
because the header tank was full didn't mean a whole lot. Of course, I
noticed after each run up that the coolant level in the header tank would
decrease permitting me to put more coolant in. That finally made me
realize what the problem was - would have been very nice to have this list
around back then {:>)
After burping the system there is still small amount
of air left, but the overflow tank set gradually removes the remaining air
over a couple of flights. Then the hydraulic "lock" phenomena starts
with initial pressure of 21-22 psi immediately on engine start, dropping off
quickly to zero and then gradually climbing back to 5 - 7 psi as
the coolant heats up.
But, other than having to "clear" the air out with a
couple/three run ups to 5000 rpm, it works just fine and has since 97.
I occasionally toy with the idea of putting in simple small air bleed on the
top of each - but, like I said, it works fine and other things to do
{:>)
Sounds like your approach will avoid my burping
problem. However, Lynn has mentioned that even in the car installation
it often takes burping the engine to get the air out.
Ed
----- Original Message -----
Sent: Friday, October 05, 2007 11:12
AM
Subject: [FlyRotary] Re: Problem?
[FlyRotary] Re: Coolant Water Pressure
Hi Ed,
for sure I saw your installation before (numerous times...), but I do
not recall your exact pluming.
Your description below sounds like inlet and outlet are facing
down.
At 22psi it should even be more like 1/2 the radiator with air
:)
Anyway, I assume waterflow is radical enough to strip the air out in
3 trials.
My system will have a bottom inlet and a top outlet. If it doesn't
fit the outlet may exit the bottom of the tank but will have an internal
standpipe - this way there is next to no space where air can get trapped,
just a small bubble atop the standpipe, won't be big enough to cause any
cooling detriment.
I still see BMW motorcycle oil-coolers mounted this way. Don't know
the exact make-up today, but the earliest ones where simple single pass
bottom feed bottom exit (cheapest solution and esthetically least
disturbing), a big problem to purge. 1/4 was useless because of trapped
air...
Furthermore, if the pump had a little leak or just a long time
between runs would drain the oil fro mthe cooler and at start-up you had a
fresh load of cold air inthe cooler! As it heats up the air-bubble expands
and reduces cooler volume even more...
Best Regards,
TJ
snipped..
In my case, if I do a complete drain
and refill of the system, on the first run up the core's tanks
will be hot approx 2/3 of the way up and then they are much cooler
- indicating that the remaining 1/3 of my core is filled with air.
It generally takes me 3 runups reaching 5000 rpm before I can
touch the core tanks and find them hot all the way from top
to bottom. So depending on your radiator set up that might be
something you can quickly check.
snipped....
|