PROs and CONs of Engine Choices

[airlina]

Amen Brother, its all fun and games until you gotta fix something-in my simple mind simpler is better. I have an IO-240 up front of my Series 5 . So far 15 years and 850 hrs later its been bullet proof . No regrets here , but thats the beauty of a homebuilt, you build it the way YOU want it , to meet what YOU think an airplane should look like.Bruce N199CL

[nyrikki]

I should clarify that I was referencing guessing on Rotax and other S-LSA industry trends and motivations. The exponential pricing seems to indicate that they don't worry too much about the increased price and CV props are fairly popular in Europe where the rules are different.

My budget is below S-LSA prices but sufficient enough to avoid hand propping J3/7AC. I won't go with a 915+CV but I doubt the LSA rules will ever allow for a three lever config. "pilot's improper use of the mixture control" is pretty common on crash reports. So I doubt that adding a 3rd lever will happen for S-LSA. Note that is a guess and not based on any knowledge.

I don't own a car without a clutch pedal and haven't for decades so I get the sentiment about simplicity.

While I have no experience with CV props and can't even comment on that topic. On the subject of the rotax ignition. A shift in complexity can actually lead to less complexity for the system as a whole.

While my opinion won't be true for everyone. IMHO it would be easier for me to troubleshoot the 912/15iS than the 912ul(s) Ducati "Dual Capacitor Discharge Ignition". Despite being "electronic" it is not digital and while it may have a lower part count it is not necessarily simpler.

The magneto generator stator assembly is complex and the e-boxes are hard to test and the shared nature of components makes things challenging IMHO. Like a typical CDI system it is mostly passive components which sounds simpler until you have multiple functions on one stator and two potted black boxes without enough connectivity to fully test the thyristor circuit.



While the value obviously needs to be evaluated by each user the increase number in sensors on a computer controlled system can be easier to test in isolation.

With simple dual coil packs and hall effect crank sensors I wouldn't even need a schematic to troubleshoot the major components on a 912is/915is. Which is a good thing because I don't think Rotax has release a schematic but that is partly because they don't have to.

Obviously this is dependent on a builders preferences, budget, and experience. Personally I would find the shift in complexity of a computer controlled ignition to actually result in a lower overall complexity compared to the 912ul(s) dual Ducati DCDI system.

I wouldn't back away from a newer flyEFii or SDS system on a traditional aero engine but people will want to understand fuel maps etc...which does pose an additional barrier.

Your mileage may vary.

It is really cool to have the ability to build it the way YOU want.

[tjentzsch]

For me, when I get to it, the 912is will most likely be the one I choose. I live in Utah, so already I am high elevation. I would LOVE to put on the 915is, but 38k for just an engine YIKES!. My entire brand new 2016 Tacoma cost under that!

[nyrikki]

As to not include others in what probably edges on paranoia on my part I should point out that people do fine with the 914 and even the lower power 912ul or even the two strokes in Wyoming and Colorado.

I had a bad experience coming back west bound across the divide South of Lander a few decades back J-3 (pre-gps days). With the typical Westerlies it is not uncommon for there to be a sink on the leeward side and there is a gradual rise that can be very deceptive especially in the winter with flat lighting or fresh snow.

This has lead to several crashes in the area but a kitfox should do better than that old J-3. The slight increase in power at altitude from the iS plus Bing carbs tending to the rich side is a concern for me. Note this is a extremely conservative stance. The J-3 didn't even have a mixture control or the altitude compensation and had less HP to start with so...

I lost the link to the great web page that graphed historical DA. The concerns about the 8000' critical altitude for takeoff power on the 914 was the inability for official sources to describe how power fell off . If you watch the DA for EVW you will see that it cycles between 7200' and 10,000' daily for most of the year.

If you are down in the valley in Utah at a ~4,200 elevation this is less of a concern and the performance benefits may be worth it for you if cost is not a barrier.

My grandfather built a glider in the 1930's and had no problem flying it at 8000' despite the fact that he launched with his brother towing it with a motorcycle and even 45HP would be a stretch. So I chose to error on the side of known values and constancy.

None of these limitations I mentioned would pose a problem if someone simply knew their aircraft. It is simply past my personal comfort levels because I want to do the math.

I am only offering my opinions because this is a hard decision and most threads tend to be focused on single choices. Hopefully my posts help others evaluate their options but please temper them with with the above personal limitations and conditions. I am in no way an authority or subject matter expert.

If my home airport below 5000' I would personally consider the risk vs. exposure time to be small and less critical than other factors.

A 915 or x340 and potentially Vx being primarily limited by pilot comfort would be pretty cool though.

It would be nice if people circle back an update what personal choices or assumptions they would have changed after they have some flight time. I'll try to remember to point out my mistakes were.