We have flown about 130 hours on our mono-wheel XS, with Rotax 912S,
since first flight last June. Last fall we noticed high temperatures in
the lower cowl area, melting tie wraps, taking the temper out of muffler
retaining springs, etc. We instrumented the lower cowl area with four
temperature probes that are sampled and logged once every 10 seconds
(along with all engine parameters such as fuel flow, rpm, MAP, etc.). We
measured winter time temps (OAT = 40 degF) of up to 400 degF at
different phases of flight, the worst phase just after leveling off from
a climb and beginning cruise. Extrapolating to summer time temps gets us
to 450+ degF, enough to melt nylon tie wraps, etc.
Over the past several months we have tried about 15 different cooling
modifications/combinations and then tested them in flight, logging data
and then plotting the results graphically, cowl temps vs fuel flow,
where fuel flow provides the best indication of flight phase, i.e.
runnup, takeoff, climb, cruise, etc. We tried opening up the rear duct
area, closing off most of the upper cowl louvers, cutting additional
exit holes in the lower cowl and installed reverse scoops for them and
the exhaust hole. We added cooling holes in the cooling duct, just in
front of the muffler. The best of these configurations reduced max temps
to about 290 degF (OAT of 40 degF), although the results were not very
repeatable.
Finally, last week we built a baffle behind the engine, between the
tops of the foot wells. This, in combination with blocking off 3 of the
4 upper cowl louvers, and cutting the rear of the cooling duct forward 5
inches, gets a very reliable max temp of 215 degF in the lower cowl. The
muffler is the biggest source of heat in the cowl, and this modification
forces the cooling air around the sides of the engine, down past the
muffler, and then out the rear of the cowl, instead of allowing it to
pass over the top of the engine, around the back, and then out, allowing
air to stagnate in the muffler area. Note that in all tests, the upper
cowl area (between the carbs) has remained below 100 degF, exceeding 70
degF only on the ground.
We know that some planes have no problems with cowl temps. We also
believe there are others, like ours, that do. If the spark plug wire
markers in the lower wires shrivel up to little brown strings, you may
want to consider our cooling mod.
regards,
Terry Seaver
A135/N135TD
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