X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from outbound-mail02.dca.untd.com ([64.136.47.36] verified) by logan.com (CommuniGate Pro SMTP 5.4.4) with SMTP id 5437315 for flyrotary@lancaironline.net; Sun, 11 Mar 2012 10:45:47 -0400 Received-SPF: pass receiver=logan.com; client-ip=64.136.47.36; envelope-from=alwick@juno.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=juno.com; s=alpha; t=1331477111; bh=47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=; l=0; h=Message-ID:From:To:Subject:Date:Content-Type; b=J4rVzI1wMaB6m03xtCa3R05hsZN+1DpDF4RVBTQ9XF633JxqlyBfKD7exhhhfgqgq bBqGLkSXxNMULrb87dA9qhiKvr9Kq3JAIL8JzESpc9Lih6i2Nko/uXQ7EDaOIblcvm ok659Nsldby/+hQ4ClwBgoyQWWDElNWtsltr/1Sc= Received: from Penny (50-39-190-32.bvtn.or.frontiernet.net [50.39.190.32]) by smtpout02.dca.untd.com with SMTP id AABHX3QVEAH2DSQS for (sender ); Sun, 11 Mar 2012 07:44:52 -0700 (PST) Message-ID: <23DD3A710FAB45F2A0D998A06B360B85@Penny> From: "Al Wick" To: "Rotary motors in aircraft" References: In-Reply-To: Subject: Re: [FlyRotary] Thermostat and coolant pressure oscillation Date: Sun, 11 Mar 2012 07:44:50 -0700 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_02BD_01CCFF5A.D7163C80" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Windows Mail 6.0.6002.18197 X-MimeOLE: Produced By Microsoft MimeOLE V6.0.6002.18463 X-UNTD-BodySize: 17633 X-ContentStamp: 47:23:2763667420 X-MAIL-INFO:447d192d192dbd082d29bd2129bd1d486cec554c899cb1556cd5e8d545b89ce5d5bd31ed11ed115cb911f55cbcf55c95e58d79294c954121f929014985999981393d2cb5edb8116875edf5a8a81575289818a9dcb9dcc959f9419d885c4818c1d81d4de81c910848485d6cbcb9619115ccaca1ec68ecfc7c0de8555d1c6c19b1b1e5787dcc3145d5bd X-UNTD-OriginStamp: L941HVjjYzDhN3itp//mkKM3V62Hb/zR/8iSVIv25BUDoE6p0D26xA== X-UNTD-Peer-Info: 10.171.42.32|smtpout02.dca.untd.com|smtpout02.dca.untd.com|alwick@juno.com This is a multi-part message in MIME format. ------=_NextPart_000_02BD_01CCFF5A.D7163C80 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Absolutely superb analysis. Converting theory to facts. Open to = alternate hypotheses. Fact gathering methods so excellent. Love reading = stuff like this. Congrats congrats.=20 -al wick ----- Original Message -----=20 From: Steven W. Boese=20 To: Rotary motors in aircraft=20 Sent: Sunday, March 11, 2012 2:11 AM Subject: [FlyRotary] Thermostat and coolant pressure oscillation 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. =20 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. =20 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. =20 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 ------=_NextPart_000_02BD_01CCFF5A.D7163C80 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Absolutely superb analysis. Converting theory to facts. Open to = alternate=20 hypotheses. Fact gathering methods so excellent. Love reading stuff like = this.=20 Congrats congrats.
 
-al wick
 
----- Original Message -----
From:=20 Steven W. = Boese=20
Sent: Sunday, March 11, 2012 = 2:11=20 AM
Subject: [FlyRotary] Thermostat = and=20 coolant pressure oscillation

This winter I have made more flights than usual with OAT in the = range of=20 10-30 degrees.  Since the temperatures here are = generally not=20 considered hot, I had retained the bypass thermostat in the=20 1986 13B and used the heater core from the donor = car for=20 cabin heat.  Recently the heater core developed a slow leak so it = was=20 replaced with a new one.  The new one developed a leak = in a=20 short time too.  While it is nice to have heat in the cockpit, = obtaining=20 it from hot coolant dripping onto the center floor section = wasn't=20 the preferred method.

Finding the source of the leaks in the heater cores proved = difficult=20 until the inside was rinsed several times with alcohol and dried with=20 compressed air.  Then the cores were pressurized to 10 = psi with=20 air and immersed in water.  The leaks in both cores were = from cracks=20 in the end tank where it was bent to form the double pass = function. =20 Soldering the cracks only stopped the leaks for a short time after = which the=20 same area failed again.

 

My cooling system uses an expansion tank with a 14 lb = cap which has=20 never had a problem containing the system pressure.  The = expansion tank=20 is connected to the area of the water pump just below the thermostat = with=20 a 1/4" hose with a 1/16" diameter constriction. =20 Another 1/4" hose connects the expansion tank to the water pump=20 inlet.  This allows air to be purged to the expansion tank since = the=20 expansion tank hose connections are below the liquid level.  = During phase=20 1 testing, measurements of pressure in the area just below the = thermostat=20 showed pressures up to 40 psi under conditions where the thermostat = was=20 modulating the system pressure by being only partially open and = engine=20 RPM over 4500.  The constriction in the line between the high = pressure=20 area below the thermostat and the expansion tank prevents the = expansion tank=20 from seeing that high pressure and also limits the coolant flow that = bypasses=20 the radiators.

 

Similarly, a 3/16" constriction was placed in the 5/8" hose to = the=20 inlet of the heater core to limit the pressure in the core.  = Another 5/8"=20 hose connects the outlet of the heater core to the water pump = inlet. =20

 

Since it was possible that the pressure was still too high in = the=20 heater core and causing the leaks, a pressure gauge was connected to = the=20 heater core inlet with a 4 ft section of 1/8" id Teflon=20 tubing.  Under cruise conditions where the thermostat was = controlling the=20 coolant temperature by being partially open, the average pressure at = the core=20 inlet was about 10 psi which should be reasonable.  The term = "average" is used because the pressure on the gauge was observed = to be=20 oscillating 2 to 3 times a second from about 5 to 15 psi.  The=20 coiled Teflon line to the gauge was also observed to=20 be squirming in sync with the gauge pressure changes.  =

 

The conclusion is that the heater core tanks fail not because of = the=20 average pressure but because of fatigue due to the pressure=20 oscillations.

 

So why is the coolant pressure oscillating?

 

One possibility is that the water pump is unstable.  It = is known=20 that centrifugal pumps can exhibit behavior where they can have = two=20 different flow rates at the same head.  Systems with these = unstable=20 pumps can sometimes vibrate due to oscillation between the = two flow=20 rates.

 

Another possibility is that the bypass thermostat is = oscillating=20 in the partially open position.  Calculations of the area of the = bypass=20 passage combined with the measured pressure = difference across=20 the thermostat bypass blocking plate and the measurement of = the=20 thermostat spring force show that this is possible.  The=20 oscillation could occur because the velocity of the coolant = between=20 the blocking plate and the bypass inlet increases as the = thermostat opens=20 and Bernoulli lowers the pressure in this region.  The = blocking=20 plate is pulled down against the seat at which time the bypass = flow is=20 blocked, the pressure under the plate rises = and the=20 plate opens again.  The sequence of events is then = repeated. =20 It should be noted that the oscillation would be due to = pressure and=20 flow, not temperature change since the thermostat cannot respond to=20 temperature changes fast enough to open or close 2 to 3 times a = second. =20 Bernoulli would be working at the top of the thermostat, too, = contributing to=20 the problem.

 

A possible solution is to block the bypass passage and install a = non bypass=20 thermostat.  A flight with this configuration on Friday showed = that the=20 pressure oscillations had been eliminated.

 

Another possible solution is to block the bypass passage and = eliminate the=20 thermostat completely.  A flight today showed that the pressure=20 oscillations had been eliminated with this configuration, too.

 

If the coolant temperature doesn't remain too cold, the preferred = solution=20 would be to eliminate the thermostat.  Otherwise, the coolant = temperature=20 could be controlled by throttling the cowling exit area or with a non = bypass=20 thermostat, the choice being a compromise between the reliability of = the two=20 methods.

 

In hindsight, the coolant pressure oscillations may have been the = cause of=20 several other issues.  One is that I always had a problem with = chaffing=20 of the coolant hoses between the engine and the radiators.  Tying = the=20 hoses down and installing supports to them was somewhat effective, but = resulted in having to replace the supports made from 3/8" aluminum = tubing=20 periodically due to cracking.  I had attributed this to air flow = through=20 the cowling and engine vibration, but seeing the writhing Teflon line = to the=20 pressure gauge has caused me to rethink this.

 

Another issue was a clicking noise of several times a second = frequency that=20 sometimes would occur during cruise at OAT's where cooling = was=20 more than sufficient.  This may have been due to the movement of = the=20 coolant hoses or the sound of the thermostat motion itself.

 

A third issue was an airframe vibration with a frequency similar=20 to the clicking noise.  I searched for a cause of this with = a=20 possibility of an oil canning surface or engine roughness, but could = not=20 isolate the cause.

 

I have not been able to initiate the pressure oscillations, = clicking noise,=20 or airframe vibration since removing the bypass thermostat.  = With=20 only two flights so far, though, it is too soon to draw definite = conclusions=20 on the noise, vibration, or chaffing, but the coolant pressure=20 oscillation is definitely gone.

 

This is experience is reported in the light of the recent = thermostat=20 discussion, with the caveat that my problems may be = unique.

 

Steve = Boese

RV6A, 1986 13B NA, RD1A,=20 EC2

--
Homepage:  http://www.flyrotary.com/
Archive and=20 UnSub:  =20 = http://mail.lancaironline.net:81/lists/flyrotary/List.html
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