I have been experiencing high CHT readings on my Rotax 914 since I started
flying my Europa (XS Monowheel). The Europa people and Rotax seem to think
it is an instrumentation problem, but I am not so sure, so I thought I
would solicit the opinions/advice of you out there.
At cruise with an OAT of 27C (82F) at about a 65% power setting (about 27"
and 4800rpm) I consistently read about 130C (266F) for CHT. I have checked
this several different ways:
1. I am measuring CHT with a Rocky Mountian Instrument engine monitor,
which requires a J thermocouple input for CHT. My initial installation
used a J thermocouple designed to mount under the spark plug. This gave
very high readings - up to 140C.
2. The Rotax rep at Sun N Fun said this was improper, and said I must use
the well in the cylinder head provided for CHT. So I purchased a J
thermocouple probe designed especially for the 912/914 and installed it.
It read somewhat less, but still settles out at 130C at cruise as described
above.
3. I have checked the readings using an infrared thermometer supposedly
accurate to 2C and it agrees completely with the CHT from the RMI, at
least at the temperatures I can check (It is impossible to get the
temperature up to 130C on the ground with the cowling off).
4. I reinstalled the original Rotax CHT sensor, which has a resistance
inversely proportional to temperature. At the flight conditions described
above, I measure 23 ohms with two different digital ohmmeters, which
corresponds approximately to 130C per the graph in the Rotax installation
manual.
As a result of all the above, I am now convinced that my CHT is running
130C at low power, compared to what I understand is normal for higher power
of 100C - 105C (212F -221F). Although this is not over the absolute limit
defined by Rotax of 135C, it leaves no margin for higher power operation or
climbing.
I have received a plethora of advice on how to correct this. I have
flushed my radiator and it seems to be free and unblocked. If the problem
is the water pump I have not figured out a way of determining this other
than disassembly, which requires removing the engine from the aircraft. I
have increased the air outlet at the lower rear of the duct to provide for
more air flow over the cylinders. Although the air flow over the number 3
cylinder seems to be greatly impeded due to the engine mounting bracket and
the air plenum from the turbocharger, I see no way of correcting this.
Note that even though my CHT is consistently high, the oil temperature
remains normal or below normal.
Any advice/suiggestions would be greatly appreciated.
Bill Stewart, N6LB (A098)
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