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