Norbert the Champ has been ailing a bit in the last few months. I've been flying every few weeks, as the weather allows, occasionally letting a whole month pass between flying dates. The problem is, when the engine sits like that for long periods, it gives condensed water a chance to attack the innards and start creating rust.
The accepted remedy for this problem is to fly more often (how convenient!). The idea is that by flying, you warm up the oil, and encourage the water to evaporate out, as well as getting a fresh coat of oil in all the places it's supposed to be. Ideally, you want the oil to be 180-200°F for at least half an hour to get the water out.
Fortunately, I haven't noticed Norbert's ailment in the sense of feeling like anything's wrong as we fly. Rather, I've been noticing that the crankcase breather tube is drooling a bit of oil/water mixture after flights. I'll come back a week or so later, and there's a 2" pool of mocha-colored oil-water emulsion sitting under the engine, almost exactly like it had a little potty training accident.
The plane is equipped with an air-oil separator, which is a little thing the size of a beer can which is supposed to condense the oil out of the crankcase breather tube, and let it drain back to the oil tank, rather than sending it out over the belly of the plane in flight. It seems to work pretty well, but this new pool of oil was worrying.
Did it mean the separator needs to be cleaned? Did it indicate some other problem inside the engine? The oil on the dipstick came out looking like oil (good) and not like mocha foam (which would be bad), so I wasn't sure.
Finally this last weekend, I got a chance to chat with the local mechanic about it, and his recommendation was to go fly the plane a bit to warm up the engine, then do a compression test. This would confirm whether any of the cylinders were leaking more than they should. Previous compression tests (we do one at least every year) have been good, but this one showed that the #4 cylinder was down a little bit. Apparently the ideal number is 79 out of 80, with a full 80/80 indicating a problem, and anything down to about 40/80 being in the acceptable range (this is hard for my perfection-oriented brain to comprehend, but apparently is true).
After the compression test, the A&P mechanic said, "Frankly, what I'd recommend is that you go out and fly for a while at higher power, like a high-power cruise. That'll probably improve this, though even 72/80 is pretty good." This actually aligned well with my thoughts on boiling the water out of the oil, so I set out to see what I could do.
Norbert and I launched into the warm May day (it was over 80°F that day), and I set out to fly it like I basically never do.
The first order of business was not to climb too high. Normally I'm in the "altitude is insurance" game: the higher you are, the more gliding distance you have if anything goes wrong with the engine. However, the air is thicker and hotter down low, so I mentally plotted a course over a set of flat fields through the Snoqualmie Valley.
The next thing was to push the engine faster than normal. I've settled on a fuel-sipping 2200 RPM cruise (2500 is the maximum, or nominally 100% power), which probably represents around 60-65% power. I've been burning about 5.5 gallons per hour at this setting, which seems like a nice level. I have no idea how much fuel we'd actually burn at higher power, but presumably around 8-10 GPH, which is a lot for a 90 HP engine on a plane like this.
So I launched from Harvey and aimed myself southeastwards. It was interesting to see what happened.
I set myself up for about 1700 feet of altitude, which puts me safely over the legal limit, but not so high that I was losing much heat from the ground-level air. I pushed the power until it was just shy of the 2500 RPM redline limit. The plane made a constant shimmy and judder feeling, very light, but enough to communicate to me that it wasn't happy. The louder engine noise combined with increased wind noise to give a sonic edge to the plane's discomfort. We ended up cruising around 105-110 MPH, vs. my normal 85 MPH at 2200 RPM. Gratifyingly, the oil temperature kept rising, finally stabilizing just below the 200°F mark -- I haven't seen over 150° since last summer. Maybe I have been under-working the engine.
I flew most of the way to North Bend, then turned around over Carnation and flew back, circling once over a friend's house, and then looped back around to Harvey Field. I briefly lowered the engine back down to 2200 RPM and let it settle into its happier cruise speed. It was remarkable how much more comfortable the plane felt. Then it was back up to nearly 2500 for the return to Harvey, and an uneventful landing.
In all, just shy of an hour's flight time, almost all of it spent at just shy of full power. Out of curiosity, I checked the fuel left in the tanks -- I'd taken off with around 21 gallons -- and found there were about 14 left. 7 GPH for nearly full throttle. I had expected more, and would probably plan on at least 8 if for any reason I had to fly for any distance at full throttle; part of my hour's flying time included taxiing on the ground. My fuel dipstick measurement technique is fairly crude, and will never be more accurate than within about a gallon or two (gas cans always seem come in frustrating "gallon plus 3 ounces" sizes to accomodate people mixing 2-stroke fuel, making accurate measurement very difficult).
I was able to visit the plane Tuesday night after the flight on Saturday, and found a small puddle of discarded oil, fortunately not as mocha-colored. There is a distinct trace of oil running down the belly, but it's coming from somewhere in the engine compartment rather than from the breather tube.
A very interesting experiment in Going Fast with my pal Norbert. My key takeaway is that I should probably be running the engine harder for its own good health. The slightly increased fuel burn is a fair trade-off for not having to overhaul the engine (a $25,000 proposition) early.