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Date: Sun, 11 Mar 2012 08:42:32 -0500
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Subject: Re: [FlyRotary] Thermostat and coolant pressure oscillation
From: Kelly Troyer <keltro@gmail.com>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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Steve,

    Excellent report...........Great detective work (which you are known
and appreciated for)............My gut reaction is I would
bet that your cooling system is not the only one with this
problem...........If it were not for your repeated heater core failures
this problem would have probably gone
totally unrecognized.............IMHO.....................<:)

Kelly Troyer
On Sun, Mar 11, 2012 at 4:11 AM, Steven W. Boese <SBoese@uwyo.edu> wrote:

>  This winter I have made more flights than usual with OAT in the range of
> 10-30 degrees.  Since the temperatures here are generally not considered
> hot, I had retained the bypass thermostat in the 1986 13B and used the
> heater core from the donor car for cabin heat.  Recently the heater core
> developed a slow leak so it was replaced with a new one.  The new
> one developed a leak in a short time too.  While it is nice to have heat in
> the cockpit, obtaining it from hot coolant dripping onto the center floor
> section wasn't the preferred method.
>
>
>
> Finding the source of the leaks in the heater cores proved difficult until
> the inside was rinsed several times with alcohol and dried with compressed
> air.  Then the cores were pressurized to 10 psi with air and immersed in
> water.  The leaks in both cores were from cracks in the end tank where it
> was bent to form the double pass function.  Soldering the cracks only
> stopped the leaks for a short time after which the same area failed again.
>
>
>
> My cooling system uses an expansion tank with a 14 lb cap which has never
> had a problem containing the system pressure.  The expansion tank is
> connected to the area of the water pump just below the thermostat with
> a 1/4" hose with a 1/16" diameter constriction.  Another 1/4" hose connects
> the expansion tank to the water pump inlet.  This allows air to be purged
> to the expansion tank since the expansion tank hose connections are below
> the liquid level.  During phase 1 testing, measurements of pressure in the
> area just below the thermostat showed pressures up to 40 psi under
> conditions where the thermostat was modulating the system pressure by being
> only partially open and engine RPM over 4500.  The constriction in the line
> between the high pressure area below the thermostat and the expansion tank
> prevents the expansion tank from seeing that high pressure and also limits
> the coolant flow that bypasses the radiators.
>
>
>
> Similarly, a 3/16" constriction was placed in the 5/8" hose to the inlet
> of the heater core to limit the pressure in the core.  Another 5/8" hose
> connects the outlet of the heater core to the water pump inlet.
>
>
>
> Since it was possible that the pressure was still too high in the heater
> core and causing the leaks, a pressure gauge was connected to the heater
> core inlet with a 4 ft section of 1/8" id Teflon tubing.  Under cruise
> conditions where the thermostat was controlling the coolant temperature by
> being partially open, the average pressure at the core inlet was about 10
> psi which should be reasonable.  The term "average" is used because the
> pressure on the gauge was observed to be oscillating 2 to 3 times a second
> from about 5 to 15 psi.  The coiled Teflon line to the gauge was also
> observed to be squirming in sync with the gauge pressure changes.
>
>
>
> The conclusion is that the heater core tanks fail not because of the
> average pressure but because of fatigue due to the pressure oscillations.
>
>
>
> So why is the coolant pressure oscillating?
>
>
>
> One possibility is that the water pump is unstable.  It is known that
> centrifugal pumps can exhibit behavior where they can have two different
> flow rates at the same head.  Systems with these unstable pumps can
> sometimes vibrate due to oscillation between the two flow rates.
>
>
>
> Another possibility is that the bypass thermostat is oscillating in the
> partially open position.  Calculations of the area of the bypass
> passage combined with the measured pressure difference across
> the thermostat bypass blocking plate and the measurement of the thermostat
> spring force show that this is possible.  The oscillation could occur
> because the velocity of the coolant between the blocking plate and the
> bypass inlet increases as the thermostat opens and Bernoulli lowers the
> pressure in this region.  The blocking plate is pulled down against the
> seat at which time the bypass flow is blocked, the pressure under the
> plate rises and the plate opens again.  The sequence of events is then
> repeated.  It should be noted that the oscillation would be due to pressure
> and flow, not temperature change since the thermostat cannot respond to
> temperature changes fast enough to open or close 2 to 3 times a second.
> Bernoulli would be working at the top of the thermostat, too, contributing
> to the problem.
>
>
>
> A possible solution is to block the bypass passage and install a non
> bypass thermostat.  A flight with this configuration on Friday showed that
> the pressure oscillations had been eliminated.
>
>
>
> Another possible solution is to block the bypass passage and eliminate the
> thermostat completely.  A flight today showed that the pressure
> oscillations had been eliminated with this configuration, too.
>
>
>
> If the coolant temperature doesn't remain too cold, the preferred solution
> would be to eliminate the thermostat.  Otherwise, the coolant temperature
> could be controlled by throttling the cowling exit area or with a non
> bypass thermostat, the choice being a compromise between the reliability of
> the two methods.
>
>
>
> In hindsight, the coolant pressure oscillations may have been the cause of
> several other issues.  One is that I always had a problem with chaffing of
> the coolant hoses between the engine and the radiators.  Tying the hoses
> down and installing supports to them was somewhat effective, but resulted
> in having to replace the supports made from 3/8" aluminum tubing
> periodically due to cracking.  I had attributed this to air flow through
> the cowling and engine vibration, but seeing the writhing Teflon line to
> the pressure gauge has caused me to rethink this.
>
>
>
> Another issue was a clicking noise of several times a second frequency
> that sometimes would occur during cruise at OAT's where cooling was more
> than sufficient.  This may have been due to the movement of the coolant
> hoses or the sound of the thermostat motion itself.
>
>
>
> A third issue was an airframe vibration with a frequency similar to the
> clicking noise.  I searched for a cause of this with a possibility of an
> oil canning surface or engine roughness, but could not isolate the cause.
>
>
>
> I have not been able to initiate the pressure oscillations, clicking
> noise, or airframe vibration since removing the bypass thermostat.  With
> only two flights so far, though, it is too soon to draw definite
> conclusions on the noise, vibration, or chaffing, but the coolant pressure
> oscillation is definitely gone.
>
>
>
> This is experience is reported in the light of the recent thermostat
> discussion, with the caveat that my problems may be unique.
>
>
>
> Steve Boese
>  RV6A, 1986 13B NA, RD1A, EC2
>
> --
> Homepage:  http://www.flyrotary.com/
> Archive and UnSub:
> http://mail.lancaironline.net:81/lists/flyrotary/List.html
>
>


-- 
Kelly Troyer
Dyke Delta_"Eventually"
13B_RD1C_EC2_EM2

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<div>Steve,</div>
<div>=A0</div>
<div>=A0=A0=A0 Excellent report...........Great detective work (which you a=
re known and appreciated for)............My gut reaction is I would</div>
<div>bet that your cooling system is not the only one with this problem....=
.......If it were not for your repeated heater core failures</div>
<div>this problem would have probably gone totally=A0unrecognized..........=
...IMHO.....................&lt;:)</div>
<div>=A0</div>
<div>Kelly Troyer<br></div>
<div class=3D"gmail_quote">On Sun, Mar 11, 2012 at 4:11 AM, Steven W. Boese=
 <span dir=3D"ltr">&lt;<a href=3D"mailto:SBoese@uwyo.edu">SBoese@uwyo.edu</=
a>&gt;</span> wrote:<br>
<blockquote style=3D"BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px 0.8ex;PA=
DDING-LEFT:1ex" class=3D"gmail_quote">
<div>
<div style=3D"FONT-FAMILY:Tahoma;DIRECTION:ltr;FONT-SIZE:10pt">
<p>This winter I have made more flights than usual with OAT in the range of=
 10-30 degrees.=A0 Since the temperatures here are generally=A0not consider=
ed hot, I had retained the bypass thermostat in the 1986=A013B<a></a><a></a=
> and used the heater core from the donor car for cabin heat.=A0 Recently t=
he heater core developed a slow leak so it was replaced with a new one.=A0=
=A0The new one=A0developed a leak in a short time too.=A0 While it is nice =
to have heat in the cockpit, obtaining it=A0from hot coolant dripping=A0ont=
o the center floor section wasn&#39;t the preferred=A0method.</p>

<p>=A0 </p>
<p>Finding the source of the leaks in the heater cores=A0proved difficult u=
ntil the inside was rinsed several times with alcohol and dried with compre=
ssed air.=A0 Then the cores were pressurized=A0to 10 psi=A0with air and imm=
ersed in water.=A0 The leaks in both cores=A0were from cracks in the end ta=
nk where=A0it was bent to form the double pass function.=A0 Soldering the c=
racks only stopped the leaks for a short time after which the same=A0area f=
ailed again.</p>

<p>=A0</p>
<p>My cooling system uses an expansion tank with a 14 lb cap=A0which has ne=
ver had a problem containing the system pressure.=A0 The expansion tank is =
connected to the area of the water pump just below the thermostat with a=A0=
1/4&quot;=A0hose with a 1/16&quot; diameter constriction.=A0 Another=A01/4&=
quot; hose connects the expansion tank to the water pump inlet.=A0 This all=
ows air to be purged to the expansion tank since the expansion tank hose co=
nnections are below the liquid level.=A0 During phase 1 testing, measuremen=
ts of pressure in the area just below the thermostat showed pressures up to=
 40 psi under conditions where the thermostat was modulating the system pre=
ssure by being only partially open=A0and engine RPM over 4500.=A0 The const=
riction in the line between the high pressure area below the thermostat and=
 the expansion tank prevents the expansion tank from seeing that high press=
ure and also limits the coolant flow that bypasses the radiators.</p>

<p>=A0</p>
<p>Similarly, a=A03/16&quot; constriction was placed in the 5/8&quot; hose =
to the inlet of the heater core to limit the pressure in the core.=A0 Anoth=
er 5/8&quot; hose connects the outlet of the heater core to the water pump =
inlet.=A0 </p>

<p>=A0</p>
<p>Since it was possible that the pressure was still=A0too high in the heat=
er core and causing the leaks, a pressure gauge was connected to the heater=
 core inlet with a 4 ft section of 1/8&quot; id=A0Teflon<a></a> tubing.=A0 =
Under cruise conditions where the thermostat was controlling the coolant te=
mperature by being partially open, the average pressure at the core inlet w=
as about 10 psi which should be reasonable.=A0=A0The term &quot;average&quo=
t; is used because the pressure on the gauge=A0was observed to be oscillati=
ng 2 to 3 times a second from about 5 to 15 psi.=A0 The coiled=A0Teflon<a><=
/a> line to the gauge was also observed to be=A0squirming in sync with the =
gauge pressure=A0changes.=A0 </p>

<p>=A0</p>
<p>The conclusion is that the heater core tanks fail not because of the ave=
rage pressure but because of=A0fatigue due to the pressure oscillations.</p=
>
<p>=A0</p>
<p>So why is the coolant pressure oscillating?</p>
<p>=A0</p>
<p>One possibility is that the=A0water pump is unstable.=A0 It is known tha=
t centrifugal=A0pumps can exhibit behavior where they can have two differen=
t flow rates=A0at the same head.=A0 Systems with these unstable pumps can s=
ometimes vibrate=A0due to=A0oscillation between the two flow rates.</p>

<p>=A0</p>
<p>Another possibility is that the bypass=A0thermostat=A0is oscillating in =
the partially open position.=A0 Calculations of the area of the bypass pass=
age=A0combined with the measured pressure difference=A0across the=A0thermos=
tat bypass blocking plate=A0and the measurement of the thermostat spring fo=
rce show that this is possible. =A0The oscillation=A0could occur because th=
e=A0velocity of the coolant between the blocking plate and the bypass inlet=
=A0increases as the thermostat opens and=A0Bernoulli lowers the pressure in=
 this region.=A0 The blocking plate is pulled down against the seat at whic=
h time the=A0bypass flow is blocked, the pressure=A0under=A0the plate=A0ris=
es and=A0the plate=A0opens again.=A0 The sequence of events is then repeate=
d.=A0 It should be noted that the oscillation=A0would be=A0due to pressure =
and flow, not temperature change since the thermostat cannot respond to tem=
perature changes fast enough to open or close 2 to 3 times a second.=A0 Ber=
noulli would be working at the top of the thermostat, too, contributing to =
the problem.</p>

<p>=A0</p>
<p>A possible solution is to block the bypass passage and install a non byp=
ass thermostat.=A0 A flight with this configuration on Friday showed that t=
he pressure oscillations had been eliminated.</p>
<p>=A0</p>
<p>Another possible solution is to block the bypass passage and eliminate t=
he thermostat completely.=A0 A flight today showed that the pressure oscill=
ations had been eliminated with this configuration, too.</p>
<p>=A0</p>
<p>If the coolant temperature doesn&#39;t remain too cold, the preferred so=
lution would be to eliminate the thermostat.=A0 Otherwise, the coolant temp=
erature could be controlled by throttling the cowling exit area or with a n=
on bypass thermostat, the choice being a compromise between the reliability=
 of the two methods.</p>

<p>=A0</p>
<p>In hindsight, the coolant pressure oscillations may have been the cause =
of several other issues.=A0 One is that I always had a problem with chaffin=
g of the coolant hoses between the engine and the radiators.=A0 Tying the h=
oses down and installing supports to them was somewhat effective, but resul=
ted in having to replace the supports made from 3/8&quot; aluminum tubing p=
eriodically due to cracking.=A0 I had attributed this to air flow through t=
he cowling and engine vibration, but seeing the writhing Teflon line to the=
 pressure gauge has caused me to rethink this.</p>

<p>=A0</p>
<p>Another issue was a clicking noise of several times a second frequency t=
hat sometimes would occur during cruise at=A0OAT&#39;s<a></a> where cooling=
 was more than sufficient.=A0 This may have been due to the movement of the=
 coolant hoses or the sound of the thermostat motion itself.</p>

<p>=A0</p>
<p>A third issue was an airframe vibration with a frequency similar to=A0th=
e clicking noise.=A0 I searched for a cause of this with a possibility of a=
n oil canning surface or engine roughness, but could not isolate the cause.=
</p>

<p>=A0</p>
<p>I have not been able to initiate the pressure oscillations, clicking noi=
se, or airframe vibration=A0since removing the bypass thermostat.=A0 With o=
nly two flights so far, though, it is too soon to draw definite conclusions=
 on the noise, vibration, or chaffing, but the=A0coolant pressure oscillati=
on is=A0definitely<a></a> gone.</p>

<p>=A0</p>
<p>This is experience is reported in the light of the recent thermostat dis=
cussion, with the=A0caveat<a></a> that my problems may be unique.</p>
<p><font face=3D"Tahoma"><font face=3D"tahoma"></font></font>=A0</p>
<p><font face=3D"Tahoma"><font face=3D"tahoma">Steve Boese</font></font></p=
>
<div>
<div>
<div><font face=3D"tahoma">RV6A, 1986 13B NA, RD1A, EC2</font></div></div><=
/div></div></div><br>--<br>Homepage: =A0<a href=3D"http://www.flyrotary.com=
/" target=3D"_blank">http://www.flyrotary.com/</a><br>Archive and UnSub: =
=A0 <a href=3D"http://mail.lancaironline.net:81/lists/flyrotary/List.html" =
target=3D"_blank">http://mail.lancaironline.net:81/lists/flyrotary/List.htm=
l</a><br>
<br></blockquote></div><br><br clear=3D"all"><br>-- <br>Kelly Troyer<br>Dyk=
e Delta_&quot;Eventually&quot;<br>13B_RD1C_EC2_EM2<br>

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