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----- Original Message -----
From: "David Carter" <dcarter@datarecall.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Thursday, March 04, 2004 12:41 AM
Subject: [FlyRotary] Re: Ideal Cooling System Plumbing (was Re: [FlyRotary]
Re: overflow connections
> Mark, I can't see why you think that. If there's a leak anywhere, and if
> the level of coolant goes down 2-3 inches, then the coolant temp sensor at
> the end of the block gets uncovered by coolant and the combustion chamber
> there ceases to be cooled and that temp sensor will cause a temp gage
start
> climbing (now reading CHT instead of coolant temp) and an alarm from the
> engine monitoring system will go off. So, I should know there's a leak
> before the engine block empties and the seals are damaged. I don't see
how
> the plumbing I described makes the system more susceptable to uncovering
the
> water pump inlet. If the leak is so bad the level falls below the inlet
of
> the water pump, it ain't gonna pump, period, any system design. Help me
if
> I'm missing something.
>
> David
It is clearly prudent to plan your cooling system for as many adverse
eventualities as feasible. However, I think there are some expectations
about the coolant system that may not be quite correct. First, there
appears to be a belief that the coolant level in an engine behaves similar
to a pot of water. That if there is some reduction in the amount of water
then the water level in the pot decreases thereby uncovering more of the top
of the side of the pot. This does not appear to be a true representation of
the dynamic coolant flow in an operating engine. I have measure the
temperature variations at the rear rotor housing (using the block sensor
installed there) and call tell you that even with the coolant at low levels,
some of it is still being forced through the upper coolant channels. True,
it is not a steady flow but surges as the pump apparently loses and regains
pump "suction". You can observe the temperature variations quite easily as
the surging coolant covers and uncovers the temperature sensor.The coolant
is forced through the relatively narrow coolant passages like water from a
fire hose. I have a "Plugs Up" installation which would supposedly be the
worst situation as the plugs are of course at the highest point of the block
in that orientation. So even a small loss of coolant should be a disaster -
but it just hasn't work out that way.
I have (particularly after draining and refilling the coolant system) come
out after the first flight or two and checked the coolant level to find it
down by a cup to a pint as the remaining air in the system is expelled.
Under the level water pot theory, that should be sufficient loss of coolant
fluid to uncover the hottest part of my engine (Plugs up) at its hottest
point, so therefore my engine should have been fried several times over by
now, but fortunately with 90% of the fluid remaining the water pump forces
that coolant through all the channels sufficient to keep the engine cool.
True at some point, there will not be sufficient coolant to provide any
significant cooling benefit. I recall Tracy Crook having a leak when one of
his tension bolts broke. In addition to his quick reaction to the cooling
problem, he believed that the actually boiling of the coolant in the coolant
chambers removed sufficient heat to preclude engine damage before he sat it
down on the runway.
If you have a leak that severe - then you are going to have to set the
aircraft down as soon as possible. Actually, if you have lost coolant
sufficient to reach this stage, then you may find your coolant temperature
sensor will erroneous display a lower coolant temperature rather than
higher. If the coolant temp sensor element is not bathed in coolant but
surrounded by hot air/steam then the heat transfer to the sensing element is
less from the air/steam than from the denser water/coolant. Therefore, it
is highly likely that your coolant temp sensor will not convey the true
condition of your cooling system, but will show a lower temperature not the
CHT effect as you might imagine since the coolant element is not touching
the metal but would be suspended in the void of the "empty" cooling
galley..
It is my opinion that a coolant pressure gauge is a better early indications
of a leak rather than coolant temperature. By the time your coolant temps
start to climb (even if the temp gauge is reporting the condition accuracy)
you are already in sufficient trouble that probably only turning the engine
off and becoming a glider is going to save the engine from damage. The
pressure gauge may provide an earlier indication as the system fails to
pressurize (after engine start) or pressure starts to fall from levels
normally seen for that stage of flight.
So while I think it is prudent to plan your coolant system to alleviate the
risk of low coolant levels, I think we shouldn't fool ourselves that any of
it is going to make a significant difference in extending our flight time.
But, then 2-3 minutes difference can make a difference in some cases.
FWIW
Ed Anderson
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