Rotaries are great, if fuel inefficient engines. The challenge with them remains exhaust system temperature and making a PSRU live, including handling both engine torque reversals during the compression and firing process and dealing with the thrust and gyroscopic loads from the prop.

Most auto conversion failures aren't of the "engine" as we think of it. They are failures of engine support systems - cooling, ignition, PSRU, etc.

I called on a rotax 915 just yesterday. ~60 days lead time. I just ordered a Kitfox with a partner to build last week. Given the mkts I thought long term we might save some money by ordering the engine now and having it sit in the hangar given the ever increasing cost on everything. One thing I hadn't thought about is that the factory warranty on the engine starts on either the first start or ~ a couple of months after delivery (can't remember the exact time frame). We are probably 2 years out from needing it and that is optimistic so I would have used up the full warranty with it sitting.

I wonder why there are so many PSRU failures in auto-engine conversions and not on the engines for the Twin Bonanza.

Dan

Because when those engines and gearboxes were designed, the world still had a ton of experienced engineers who'd done that kind of thing in WWII when there was a near-infinite supply of $$ to conduct research and build test equipment for that kind of stuff. Both the engineers who did that and the test equipment they used are long gone. The Auto PSRU guys are doing it (largely) by trial and error. The errors are pretty obvious at the end of the day...

Years ago I had invisioned an all aluminum block auto V-8 converted for experimental aircraft. The engine would be mounted in the same orientation as in an auto with a primary PSRU in the rear. A drive shaft would run in the engine "V" up to a secondary PSRU driving the prop and mounted to the frame (somewhat similar to the P-39). The driveshaft would be designed as a torsional damping unit (lots of interesting ways to do this).

In the end, I determined that this idea was too heavy and impractical but I still think of it from time to time. A high performance V-12 (think BMW) would be a lot more fun though.

Dan

Because when those engines and gearboxes were designed, the world still had a ton of experienced engineers who'd done that kind of thing in WWII when there was a near-infinite supply of $$ to conduct research and build test equipment for that kind of stuff. Both the engineers who did that and the test equipment they used are long gone. The Auto PSRU guys are doing it (largely) by trial and error. The errors are pretty obvious at the end of the day...

Back in my power plant days (early eighties), I had the opportunity to watch a massive steam turbine/electrical generator being tested for sub-synchronous oscillation. That's where the turbine/generator can set up an oscillation with the electrical grid and possibly snap the main 24" diameter turbine shaft in two. What they did was inject low level pink noise into the generator exciter and monitor several speed sensors on the turbine shaft. The device they used to monitor was an HP Structural Dynamics Analyzer and they let this setup run for several days because the signal they were looking for was buried down in the noise. If the Analyzer detected any resonant frequencies (poles), they would insert a compensating function into the generator exciter. For a young engineer, I was in heaven watching all this. It was really high tech for the early eighties. It probably still is.

Because when those engines and gearboxes were designed, the world still had a ton of experienced engineers who'd done that kind of thing in WWII when there was a near-infinite supply of $$ to conduct research and build test equipment for that kind of stuff. Both the engineers who did that and the test equipment they used are long gone. The Auto PSRU guys are doing it (largely) by trial and error. The errors are pretty obvious at the end of the day...

This is the issue with experimental. You see a prototype. Even from a known designer. The glowing magazine reviews look good. Kit preorders start so jump in and order a kit.

Kits are delivered and you start building. Then you notice the designer is working on a new version "Upgraded" to fix the handling. So you end up cutting the wing saddle to move your Glasair IIS wing back to fix the CG problem even with a constant speed prop.

Or you buy a model II Kitfox kit and after building discover that it's unstable in pitch, has no yaw centering and flaperons that give full nose down trim when extended but there is no pitch trim system to counteract. By the time your finished they are up to the Model IV with 1/3 larger tail and moving on. Huge stick travel and arm fully extended for neutral stick.

Buy a X-Air LSA kit and after assembly find out the importer lied about the gross weight instead of 1320 it was 1100lbs. Then find out divergent in pitch. No yaw centering, Aileron reversal at high speed due to deforming wing battens and no torsional strength in the wings to keep the ailerons from being trim tabs for the wing. Then find out the airframe failed static testing in the UK due to fuselage cage collapse. Upgrade fuselage with parts but this should have been tested before kits were sold.

Buy a new SLSA approved Flight Design CTSW LSA and when it's delivered they are also unloading a shipment with a tail that is 1/3 larger. Guess what no yaw centering. Heavy ailerons and super light pitch. Designer says the europeans like them that way because they fly like a ME109.

Buy a Aerolite Ultralight and find out the factory built one is too heavy to be legal with the MZ201 engine they supplied. Even though the EAA press has been touting about the girl flying to Oshkosh in the same one with the same engine. It has an engine EIS system that when an alarm triggers you have to push a button to cancel. But the instrument panel is too far away to reach in flight. You have to land and unstrap to reach the panel. Gearbox was the wrong ratio so the prop needed a large amount of pitch to keep the RPM in the right range. Factory said "Oh you can buy a different engine for 13K". Remember this was supposed to be factory built and tested not a kit.

Then find out it can't accept full throttle due to the single carb so they say to just limit throttle travel as a fix. (My friend died in it due to engine failure on takeoff on his third flight).

We have had others come through over the years. It's just amazing what ends up on the market.

The point is all of the designs above were not the prototype or even the first version of the design and still they had issues which I would say should have been fixed before ANY kits were sold. How many ultralight accidents were due to lack of carb heat. We figured this out in the 1920s. The LSA market and experimental market continue to not look to the past and make repeat fatal mistakes that were long known to be just not how to build an airplane and it's documented why. Not even expensive to fix in most cases. just a little care and effort to understand could have saved many unsuspecting people.

I'm not anti experimental. Glad they exist. I own a Bowers Flybaby with A-65 built in 1967. Yes I know they have had issues also. But I have known this particular airframe all my life so very familiar with it.

I am just really saddened by the lack of effort to provide even the most basic engineering to unsuspecting kit buyers. And especially the many current buyers who did not build the airplane or know anything about the construction.
Even many of the racers in the 1930s had independent stress analysis done for their designs. If you're building your own design great. But if you're going to market it there is a greater responsibility. Many examples of other designs with issues but the old Happy Miles Adventure Air amphib stands out.

https://www.aero-news.net/subscribe.cfm ... 484917b91f

Sling and Vans do seem to do well with testing. Issues come through but they are proven in numbers. Cub clones are pretty well proven also. A few I wonder about the rib spacing with high horsepower. Cub Crafters early sport cubs and carbon cubs have inexcusable engineering screwups that were later put on the owners as "Service Bulletins:" Simple things like lack of Static system makes for inaccurate airspeed. Well how did you flight test the design for LSA approval then?

The reviews always sound the same. Oh it's a light airplane so it was very "Active in turbulence". Oh your feet will be "busy" but it was fine. We had trouble landing but the dealer/designer helped and said it will be ironed out in the production kits.

Designers will say " Oh it's short coupled so that is why it's not stable.

I have a Piper Vagabond which is very fat and boxy with a shorter fuselage length than all of the designs above and it's dead stable in pitch and yaw. No excuses. Piper figured it out. The new designs should be able to also. Funny the Vagabond has a mass balanced elevator trim tab hinged back from the leading edge on each end to give a slot that opens with deflection for more authority and resistance to flutter. Simple and easy to make. Yet the Wag Aero homebuilt version just was piano hinged. Also simple but not as good. Piper felt that was needed even when making the cheapest airplane possible while bankrupt. Only 65hp. Wag Aero Wagabond Traveler could have 115hp! Things to consider, WHY did Piper feel that was important.... It was a unique part for the design.

Actually most old (over 45 years old) GA airplanes are very nice to fly. Terrible cockpit ergonomics but they handle well. It's only the Aeronca Champ and descendants that are weird in yaw. All the production light airplanes I have encountered designed after 1980 or not US designed all seem to be toads for handling. The old Stearman has almost no fixed vertical fin yet it is just fine in yaw. There are others to compare. It's tail and or control surface design not area lacking. It was like the WWII era trained designers retired and we lost something.

The experimental designers need to look back and see how solutions were done and build / improve from there rather than making the same mistakes over and over again.

Thank you for the link, good reading!

On the topic of PSRU design considerations, here's another page to get lost in for a while:
http://www.epi-eng.com/propeller_reduct ... ntents.htm

Oh, yeah, the ÉPI web site is a good place to spend a few hours if you want to learn something.

That’s some incredible stuff from Charlie, what a wealth of knowledge he is. Which I think makes the point…to go EAB and be successful you have to know a lot and that starts with the build process. There are no short cuts, none that are worth pursuing anyway.

Bryan, I see No one has mentioned Glasair Sportsman 2+2 located in Arlington Washington, they also offer a Two weeks to Taxi program You go and help them Build your Plane, there used to be one Hangared right next to me.....it was a Great Looking Airplane

Check them out

Mark, I had no Idea it was Chinese owned To bad a US company couldn't have bought them out And as of writing that post, they still have (had) a website