Dan Thomas
Touchdown! Greaser!
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Dan Thomas
Three things you get wrong right away:If you read the FAR 33.37 is states "or have an ignition system of equivalent in-flight reliability." With the reliability being subjective for the FAA to decide. Remeber, by LAW (or AD) you have to rebuild a tractor mag every 500 hours. Who is replacing an car alternator that often? Reliability?
1. There is no law or AD that mandates magneto overhaul every 500 hours. These are manufacturer recommendations, not laws, and while Champion/Slick is 500, TCM/Bendix is 400 hours.
2. Alternators on aircraft engines do not last anywhere near as long as in cars, and there is a completely understandable reason for that. The alternator in both cars and airplanes is geared or belted at a ratio to have it reach its redline RPM at the engine's redline. This protects it from overspeed damage or failure, while having it spin fast enough at idle to generate useful output.
A. The car's engine cruises at around a third of its redline RPM. My Hyundai and Ford ranger both cruise at about 2100 on the highway, and their redlines are both around 6500.
B. BUT the airplane's engine typically cruises at around 2500 RPM, and redlines at around 2700. So that alternator is running at 93% of its redline RPM, far higher than the car's 30% of redline.
Why does that matter? Because the carbon-block field brushes that run on the rotor's slip rings to bring the power to the field coil in the rotor are wearing faster at higher RPMs. It's as simple as that. Cessna, for instance, recommends a 500-hour inspection interval on those brushes. I did that on all our aircraft; at 500 the brushes were about a third gone, and at the next inspection, 1000 hours, 2/3 gone, I replaced them. If the airplane uses more power, as the 172SP did with its glass panel, the brushes were mostly gone by 600 or 700 hours. More current causes more electrical erosion.
And so, alternators on auto conversions will also have much shorter lives than they did in the car. If there is no redundancy for EI and EFI power supply, you're in trouble pretty quick. Those auto conversions won't be cruising at 2100 RPM. I cruised the Subaru EJ22 at 4600 RPM, in the Glastar I installed it. Redline was 5600. Its alternator was then at 82% of its redline.
On POA we read constantly of alternator failures, and it's mostly due to running alternators to failure. No preventive maintenance. No brush checks.
pfarber: If you read 33.75 it gives guidance that I bet almost no one designing a car motor package ever read. While (to me at least) its all common sense, being in EAB land means that you can legally ignore all the common sense and do whatever you want.... and give the nay-sayers of auto conversions more fodder to blather on about.
You'd be surprised at the stuff auto designing engineers go through. No automaker wants a reputation of many their cars sitting broken down at the side of the highway.
Yes, go ahead and do what you want with your experimental, but it's all on you, then. Not on anyone else at all.
3. Significant mortality of overhauled certified engines? Where do you get your numbers? Please post the source. Ron Wanttaja has actual numbers on that, and he posted this chart in Post #42 of this thread:Even though a newly overhauled certificated engine has significant 'infant mortality'.... so much for those properly designed and freshly rebuilt motors done by trained professionals.
One also has to remember that a Lycoming or Continental in a homebuilt has often been "rebuilt" or at least opened up by the homebuilder, often without any reference to the overhaul manual for the engine. They often use the wrong sealants (RTV being a common mistake), over- or under-torque hardware, and do not maintain proper tolerances, and the engine fails. That's supposed to be Lyc's or Continental's fault?
Then add in the improperly designed fuel systems that fail. Those aren't included in that chart, which lists only mechanical failures of the engine itself.
Understand that chart. It's not saying that 58% of traditional engines are failing by 400 hours. It's saying that those that DO fail are failing at the rates shown. And not many fail. This is in homebuilts, too, not certified aircraft maintained by certified professionals.
I bought many Lycoming factory overhauls in my time as a director of aircraft maintenance for a flight school (and flew them as an instructor, and also as the test pilot after installation. Not one of them ever failed me. None, in about 35,000 total hours of operation to TBO). Your assertions of high infant mortality are just made-up baseless accusations. If it was actually true, all the aircraft engine manufacturers would have been sued out of existence long ago.
Edit: Re the SDS stuff, yes, it's good stuff. But like all electronic fuel injection and ignition, it relies completely on electrical power from sources outside itself; the alternator and battery, and if one wants reasonable reliability, one needs redundancy, such as a second alternator and battery. Lycoming's certified iE2 engines are fully EI and EFI, and have a built-in redundant alternator and would also require a separate battery. FAR33.37.
Magnetos and carbs or mechanical fuel injection need no electrical supply, which is why we still see them on brand-new airplanes. Cheaper, no extra weight or complexity of redundant systems. They just need to be maintained properly, and the SDS stuff will be no different. Ignore it or its electrical supply, and it will eventually cause problems. In any aircraft (or car), electrical problems make up about 90% of engine problems.
SDS has some dandy stuff to improve the legacy engines. O-ring seals in rocker covers instead of gaskets. O-rings in induction flanges instead of gaskets. Other such stuff. Lycoming has no incentive to change it, because the recertification costs might never be recovered. Their volumes are too low. But it's great stuff for the homebuilder.
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