Sonex Onex

Lowflynjack

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Jack Fleetwood
Anyone here own or fly a Onex? I've seen one in person and was impressed, but not interested at the time. I'm thinking it's a cheap way to just fly around very economically, do some aerobatics, and the folding wings mean I could rent out half of my hangar. They seem to stay listed for awhile when they come up for sale. Is this because they're single seat planes or something else?

How reliable are the Aerovee engines?
Any concerns or issues with the design of the plane?
 
My biggest concern now would be about how long the company will last now that it’s up for sale.
 
I've been sort of fascinated by the Onex as well. It'd be fun to fly my own single-seat mini-"fighter" and do some mild aerobatics. I peer at the Sonex website every once in a while, trying to convince myself to buy the tail kit and have a go at it. Also, I didn't know Sonex was for sale - interesting. They seem to be one of the more successful and innovative companies in the business.
 
My dad has a partially built one, that he hasn't touched in a couple years. I'm pretty sure he'd sell.
 
I've got a friend [stop laughing] who is close to completing his.

He's 2 hangars down from me, so it's been fun to stop by and see what he's up to. He's going with Skyview, and I gave him my old wing leveler A/P. Should be a fun little birdie when he's done.
 
My question with all of the VW and VW derived powerplants is "Why don't I see more of 'em out and about?"

My understanding (from R.A "Bob" Hoover) is that the VW is heat limited and if you want more than 60 hp on a continuous basis, you're looking at a lot of head and valve maintenance. He generally had disparaging remarks for those 80-100 hp engines you read about.

All of which ties back to the Onex. The VW derived engine is cheap - <$10k. The other options (UL, Jabiru, Rotax, etc) all start in the $20k range. So if you go with one of the alternate engines, you could come out cheaper buying a used RV-4, and you'd have a lot more airplane (and 1.5x the fuel burn).

Just a thought.
 
I was curious about the engines. That may explain why most of the planes for sale have very low time engines. Some of the Sonex or Waiex for sale have the Jabiru and they're not that much more money, but honestly I've never liked the looks of them. I know the Onex is based on a scaled down Sonex, but it looks a lot better to me.
 
While support is nice, I guess it's not quite as concerning with an experimental? I have a buddy that just bought a Sonex, they do seem pretty neat.
 
So Aerovee says they don't publish a TBO, but you can expect 700 - 1200 hours on an overhaul. The whole engine costs $7495, so I could keep a spare! The only reference I can find so far to overhaul shows $50 - $250 overhaul costs. That data is from Sonex in 2011.

The plane looks like fun, the engine concerns me.
 
It’s funny, I had a similar reaction when I saw the first Onex at OSH a few years ago... “I want one!”. I looked it over from nose to tail, sat in it, watched the wing fold demo, read all the website info on it, figured it’d be a relatively easy build. It just seems like it’d be a blast, cheap to fly and keep, but... I already have an RV-8. Everything the Onex can do my RV will do better, and almost as economically. Even with a 180hp O-360 and constant speed prop, the RV is just stupid cheap to fly and maintain while still cruising at 165 KTAS. So, methinks I’ll skip the Onex for now.

The folding wings on the Onex are pretty cool though. I could probably fold my RV-8 wings if I really wanted to... just once.
 
I liked Aeromomentum as possible solution.

Tim
 
I have not heard of any in flight engine failures of the Aerovee engine. There have more than a few of the prop hub, something with the original thrust bearing they were using. However they changed the thrust bearing out and I haven't heard of anything more. This is pretty well documented in home built discussion groups.

VW engines are air/oil cooled. Get good airflow through them, make sure you have oil to circulate, cool the oil and they can run a long time at a high RPM. If you don't get good airflow through them, you're going to have overheating issues, but isn't that true of any engine?

For a Onex, the VW engine seems to be a good fit. I've heard that a full Sonex with a VW is a tad underpowered.

Last, what do you want the plane for? If you're going to do aerobatics, you want the strongest engine you can manage, which probably means more than 80hp. I don't know what engines they recommend or a max HP, but maybe a Jabiru or UL would be a better choice for aerobatics based on the hp ratings.
 
Everything the Onex can do my RV will do better, and almost as economically.
The RV-8 does everything better, except the impact it would make on my wallet!! I'm looking at $24K airplanes.
 
I have not heard of any in flight engine failures of the Aerovee engine. There have more than a few of the prop hub, something with the original thrust bearing they were using. However they changed the thrust bearing out and I haven't heard of anything more. This is pretty well documented in home built discussion groups.

VW engines are air/oil cooled. Get good airflow through them, make sure you have oil to circulate, cool the oil and they can run a long time at a high RPM. If you don't get good airflow through them, you're going to have overheating issues, but isn't that true of any engine?

For a Onex, the VW engine seems to be a good fit. I've heard that a full Sonex with a VW is a tad underpowered.

Last, what do you want the plane for? If you're going to do aerobatics, you want the strongest engine you can manage, which probably means more than 80hp. I don't know what engines they recommend or a max HP, but maybe a Jabiru or UL would be a better choice for aerobatics based on the hp ratings.
I want something cheap to fly around. The fact that I can fold the wings and put $157 a month back in my pocket by sharing a hangar is a plus too. I like doing aerobatics. I think 80HP is all they recommend on this plane and looking at videos, it looks sufficient for aerobatics. If all I wanted to do was aerobatics, I wouldn't even consider this plane, but for a few loops and rolls, it looks like a blast.
 
I’m building a Onex with VW power. I’ve not decided between engine suppliers, Sonex, Hummel or Great Plains.

Really like the airframe and based on what I’ve read, if you maintain the engine properly including valve adjustment, oil changes etc, it’s reliable. The turbo version, not so much. With the really cheap overhaul and availability of parts, the VW option looks good to me.

Cheers
 
That VW power plant is the most bulletproof ever invented by anyone. They powered umpteen million bugs despite the efforts of empties million ham handed teenagers. I'd happily fly behind one of them. I'd worry about the reduction drive way more than the engine.

The Waiex also has a folding wing, and is just a Sonex with a Y tail. Sonexes, Onexes, and Waiexes come up on the used market all the time, and they're quite reasonable. A lotta airplane for the money, says me. That said, I had a good friend die in one in what we think was a density altitude related accident. Monet's kid died in one too.
 
I’m building a Onex with VW power. I’ve not decided between engine suppliers, Sonex, Hummel or Great Plains.

Really like the airframe and based on what I’ve read, if you maintain the engine properly including valve adjustment, oil changes etc, it’s reliable. The turbo version, not so much. With the really cheap overhaul and availability of parts, the VW option looks good to me.

Cheers
Thanks for the info! I will research something to death before pulling the trigger, but I'm liking what I'm hearing so far. I will be on a lot of homebuilt forums soon.
 
That VW power plant is the most bulletproof ever invented by anyone. They powered umpteen million bugs despite the efforts of empties million ham handed teenagers. I'd happily fly behind one of them. I'd worry about the reduction drive way more than the engine.

The Waiex also has a folding wing, and is just a Sonex with a Y tail. Sonexes, Onexes, and Waiexes come up on the used market all the time, and they're quite reasonable. A lotta airplane for the money, says me. That said, I had a good friend die in one in what we think was a density altitude related accident. Monet's kid died in one too.
There is no reduction drive on this one. It's a 1:1 ratio. I understand the argument about it powering a ton of bugs, but I've heard the same argument on Subaru engines. My counter would be there aren't many bugs running at 3200 RPMs for hours on end.

The Sonex CEO you're speaking of died in a Sonex due to loss of power. The NTSB said he only used a small fraction of the runway for an intersection departure and would have had plenty of room to land had he used the whole runway. This plane had the Aerovee Turbo on it. No issues were found with the engine. I believe they looked at the turbo charger as a potential cause.
 
Thinking seriously about building a Waiex when I retire.
Is the only difference on the Waiex cosmetic? I understand there's no performance difference between the Waiex and Sonex. Is that what you've found? I like the look of the Y tail.
 
I think the y-tail improves the looks, especially with conventional gear. Probably trickier to assemble, though. There's a two-cylinder O-100 variant that might be a reasonable alternative engine. It's been discussed on Homebuiltairplanes.com.
 
I have not heard of any in flight engine failures of the Aerovee engine.

ERA13LA024 (Loss of power for undetermined reasons)
ERA13LA254 (Loss of power for undetermined reasons)
CEN16LA273 (Separated rocker arm)
ERA15FA003 (Loss of power for undetermined reasons)
ERA15LA306 (Loss of power for undetermined reasons)

When a homebuilt is powered by a traditional engine (Lycoming/Continental), about 14% of the accidents are due to either mechanical failure of the engine or an undetermined engine failure. For Volkswagen and derivative engines (including the Aerovee and Great Plains) the percentage is more than twice as high... 33%.

Ron Wanttaja
 
There is no reduction drive on this one. It's a 1:1 ratio. I understand the argument about it powering a ton of bugs, but I've heard the same argument on Subaru engines. My counter would be there aren't many bugs running at 3200 RPMs for hours on end.
From what I've seen of the way folks fly these things, the engines don't run for hours on end. Still, what Ron posts is indeed sobering.
 
From what I've seen of the way folks fly these things, the engines don't run for hours on end. Still, what Ron posts is indeed sobering.
Yeah, I guess that is probably true too. There are a few for sale, with the highest time one being around 350 hours. I would probably make some 3 or 4 hour trips in it, but I'm not looking for a cross-country machine. However, running at 3200 RPM for that long, I hope to have an engine I can believe in.
 
ERA13LA024 (Loss of power for undetermined reasons)
ERA13LA254 (Loss of power for undetermined reasons)
CEN16LA273 (Separated rocker arm)
ERA15FA003 (Loss of power for undetermined reasons)
ERA15LA306 (Loss of power for undetermined reasons)

When a homebuilt is powered by a traditional engine (Lycoming/Continental), about 14% of the accidents are due to either mechanical failure of the engine or an undetermined engine failure. For Volkswagen and derivative engines (including the Aerovee and Great Plains) the percentage is more than twice as high... 33%.

Ron Wanttaja

What the source of finding this data? You've quoted 4 incidences, I'd like to see the whole data set.

Of the 4 you quoted
- The first was a problem with oil circulation, unknown reason.
- The second was the separated rocker arm, which I read was almost certainly due to a builder error in engine assembly (drilling an extra hole).
- The third was probably a fuel system problem on a first flight. This is not engine related at all.
- The fourth was inadequate power on a first flight, probably due to builder error in putting the engine together. The builder admitted that he had problems assembling the engine.

Three of these are almost certainly builder errors and not problems with the engine per se. All occurred during the 40 hour flight test period.

Now that still gives pause in asking "do you really know how to properly assemble the engine?" But it doesn't make me think the engine itself is faulty, it's just a less forgiving part of aviation experimentation.
 
When a homebuilt is powered by a traditional engine (Lycoming/Continental), about 14% of the accidents are due to either mechanical failure of the engine or an undetermined engine failure. For Volkswagen and derivative engines (including the Aerovee and Great Plains) the percentage is more than twice as high... 33%.

Ron Wanttaja
So a little over 10% of the accidents in an aircraft with a traditional aircraft engine are due to mechanical or engine failure. Is it possible that the accident rate is higher, thus the number that fall out of the sky from miscontrol or other factors is that much more common? The Aerovee powered aircraft are mostly slower and perhaps more forgiving, thus crashes due to mechanical failure could be higher just because rates of miscontrol are lower.

Not saying that's how it is, but that is another way of interpreting what you posted, Ron.
 
People with certified aircraft seem overly wary of alternate engines in experimental aircraft. The are a lot of automotive conversions flying quite successfully (Corvair, Aerovee, Hummel, Subaru, Viking-Honda, etc.). Just make sure the engine is built right and the fuel system is well constructed. And yes, there is more risk during the 40 hour phase 1 testing.

Chalk me up as another that thinks an Onex would be a lot of fun. However, if I were to buy a single-seater I would probably be looking for something with a smaller engine and less fuel burn. Mooney Mite, anyone? There was also a Minicoupe for sale on barnstormers a few months back. Not sure if it's still there.
 
While the folding wing is neat they are just so ugly! An RV4 would probably only cost 4-8k more to build and is twice the airplane and a much better resale.
 
An RV4 would probably only cost 4-8k more to build and is twice the airplane and a much better resale.

I'm not building a plane, but I think the average engine on an RV-4 would cost more than the whole Onex.

Average cost of a built Onex is $24-28K. There is no doubt an RV-4 is a much more capable plane. On Barnstormers right now there are a lot to choose from: $40K, $53K, $45K, $45K, $49K, etc. Not a fair comparison!
 
What the source of finding this data? You've quoted 4 incidences, I'd like to see the whole data set.
Not sure what you mean by "whole data set". The basic information comes from the NTSB accident database. I have analyzed homebuilt accidents from 1998 through 2016, read the narrative on each, and assigned what I considered to be the main cause of the accident.

Here's a screen grab from my database, showing a summary of the results. The data I gave before was for all homebuilts; this is just fixed-wing homebuilts.
engines.jpg

The "Cases involving engine problems" includes both identified mechanical issues, as well as cases where the cause of failure was undetermined. Some percentage of these undetermined failures was due to issues not related to the powerplant.

The "Auto Engines" includes the VW and derived engines listed later, as the Rotax 912 numbers are included in the "Non-Cert Four Strokes" section.

Here's a breakdown I performed awhile back, showing an estimate of the installed engines in the homebuilt fleet.
engine_breakdown.jpg


Three of these are almost certainly builder errors and not problems with the engine per se. All occurred during the 40 hour flight test period.

Now that still gives pause in asking "do you really know how to properly assemble the engine?" But it doesn't make me think the engine itself is faulty, it's just a less forgiving part of aviation experimentation.

Allow me to quote your posting, which mine was a response to: "I have not heard of any in flight engine failures of the Aerovee engine." I provided several.

As for these cases being builder error...well, seems to me that those who install Lycoming or Continentals would be prone to the same types of errors, yet the failure rate is much lower.

Certainly, there's a difference between installing a purpose-built aircraft engine and a converted automobile engine. The ability to install such a conversion is a cherished part of the homebuilding world.

However, from the SAFETY viewpoint....what the data says it's more difficult FOR THE AVERAGE BUILDER to field a reliable auto-engine conversion.

Ron Wanttaja
 
The VW Type I/Type III engine that all these conversions are based on maxed out at 1600 cubic centimeters, and 60 SAE gross horsepower, as they came out of the factory. This was developed at a propeller unfriendly 3000 rpm. They were intended for automotive use, no thought was given to having one power an airplane. If you're at all familiar with these engines, they have a very restrictive intake and exhaust, and the power made per cubic inch is low. This makes them ripe for hot rodding, and racers get a surprising amount of power out of them, at a cost of vastly decreased longevity and reliability.

Most of the experience I have with VW engines is with friends who raced Formula Vee. In that application, they use a nearly stock intake and get maybe 45 hp from 1200 ccs. Not much output, but adequate for an 800 lb race car. They're reliable in that application, but the top ends get rebuilt frequently because like any other engine, they lose a little power once the valves are worn in, and in racing every bit counts.

@kyleb mentioned R. A. Hoover's writings, and I think Hoover is correct. All engines are cooling limited, and that is usually manifested in the cylinder heads. if you look at the last air cooled Porsche racing cars, they had liquid cooled heads, as Porsche couldn't get competitive levels of power with air cooled heads. Rotax has the same setup in the 912 series, air cooled cylinders, liquid cooled heads. As it came from the factory, the VW engine made around 52 net horsepower at its peak, so Hoover's contention that you can reliably get 60 horsepower and no more sounds reasonable. That should be enough for a light, clean single seat airplane. It would be helpful if that 60 horsepower came in at a more propeller friendly engine speed, with the VW spinning as fast as it does at maximum power, the prop has to be small, and I think that thrust would suffer somewhat, particularly during takeoff and initial climb.

The Aerovee folks claim 80 hp at 3400 rpm Great Plains claims 80 at 3600 rpm. Both builders have increased the displacement of their engines to over two liters, but IMO haven't dealt with the limited amount of cooling that can be done with the stock heads. What is propeller efficiency at that speed? I can't think it's good. How much thrust will you have for the takeoff run? You're going to have to run a very small prop on either of those engines. I also think you're not going to want to try to get 80 hp out of a VW Type 1 engine for very long, I'd be very concerned about the cylinder head temperature while burning that much gasoline.

If it were me, I'd give the Revmaster engine a look. Joe Horvath has designed some custom cylinder heads that are installed on that engine, and they've held up well in the racing world. It also makes its rated 82 hp at a more reasonable 2950 rpm. If you're going to be doing aerobatics, you're going to be climbing a lot, and the cylinder head temperatures are going to reflect that.

I'd still keep my eye on the cylinder head temperatures.
 
Certainly there ARE accidents involving VW engines. The question I have is why.

By the small amount of data that you’ve shown, the overwhelming reason seems to be that pilots don’t assemble the engine correctly. Instead of 33% of accidents being engine failures, it could be broken down and said that about 8% are engine failures and 25% are assembly failures.

Certainly that is to be taken into account in terms of overall safety. Like all things building, if you don’t know what you are doing, get help.

However, based on the evidence thus far, that is not a condemnation of the engine, it is a condemnation of the building process. The engine itself is not the issue, the builder is. That is not new.

Would a builder using a lycoming engine have different results? Yes, there are far better instructions and better knowledge out there.
 
I'm not building a plane, but I think the average engine on an RV-4 would cost more than the whole Onex.

Average cost of a built Onex is $24-28K. There is no doubt an RV-4 is a much more capable plane. On Barnstormers right now there are a lot to choose from: $40K, $53K, $45K, $45K, $49K, etc. Not a fair comparison!
I know several people who have built RV-4's for less than 35k by purchasing second hand kits and overhauling the engine their self. But if building isn't for you then yes the Onex will be cheaper as the RV-4 has a better resale value.
 
So a little over 10% of the accidents in an aircraft with a traditional aircraft engine are due to mechanical or engine failure. Is it possible that the accident rate is higher, thus the number that fall out of the sky from miscontrol or other factors is that much more common? The Aerovee powered aircraft are mostly slower and perhaps more forgiving, thus crashes due to mechanical failure could be higher just because rates of miscontrol are lower.

Not saying that's how it is, but that is another way of interpreting what you posted, Ron.
Certainly a valid point.

However, it's hard to pin down the rate of miscontrol, since lower-experienced pilots tend to gravitate towards the simpler airplanes like the Sonex. Here's a summary of the median total time and median time in type for a number of homebuilt aircraft (1998-2016 data):
median_hours.jpg

As you can see, the median pilot total time in RV-7 accidents is 1301 hours, vs. less than five hundred for the Sonex accidents. So it's hard to make any conclusions relative to the engine-failure data and miscontrol.

(By the way, is there any way to build tables in the forum software? It'd work better than posting the screen grabs....)

Ron Wanttaja
 
The VW Type I/Type III engine that all these conversions are based on maxed out at 1600 cubic centimeters, and 60 SAE gross horsepower, as they came out of the factory. This was developed at a propeller unfriendly 3000 rpm. They were intended for automotive use, no thought was given to having one power an airplane. If you're at all familiar with these engines, they have a very restrictive intake and exhaust, and the power made per cubic inch is low. This makes them ripe for hot rodding, and racers get a surprising amount of power out of them, at a cost of vastly decreased longevity and reliability.

Most of the experience I have with VW engines is with friends who raced Formula Vee. In that application, they use a nearly stock intake and get maybe 45 hp from 1200 ccs. Not much output, but adequate for an 800 lb race car. They're reliable in that application, but the top ends get rebuilt frequently because like any other engine, they lose a little power once the valves are worn in, and in racing every bit counts.

@kyleb mentioned R. A. Hoover's writings, and I think Hoover is correct. All engines are cooling limited, and that is usually manifested in the cylinder heads. if you look at the last air cooled Porsche racing cars, they had liquid cooled heads, as Porsche couldn't get competitive levels of power with air cooled heads. Rotax has the same setup in the 912 series, air cooled cylinders, liquid cooled heads. As it came from the factory, the VW engine made around 52 net horsepower at its peak, so Hoover's contention that you can reliably get 60 horsepower and no more sounds reasonable. That should be enough for a light, clean single seat airplane. It would be helpful if that 60 horsepower came in at a more propeller friendly engine speed, with the VW spinning as fast as it does at maximum power, the prop has to be small, and I think that thrust would suffer somewhat, particularly during takeoff and initial climb.

The Aerovee folks claim 80 hp at 3400 rpm Great Plains claims 80 at 3600 rpm. Both builders have increased the displacement of their engines to over two liters, but IMO haven't dealt with the limited amount of cooling that can be done with the stock heads. What is propeller efficiency at that speed? I can't think it's good. How much thrust will you have for the takeoff run? You're going to have to run a very small prop on either of those engines. I also think you're not going to want to try to get 80 hp out of a VW Type 1 engine for very long, I'd be very concerned about the cylinder head temperature while burning that much gasoline.

If it were me, I'd give the Revmaster engine a look. Joe Horvath has designed some custom cylinder heads that are installed on that engine, and they've held up well in the racing world. It also makes its rated 82 hp at a more reasonable 2950 rpm. If you're going to be doing aerobatics, you're going to be climbing a lot, and the cylinder head temperatures are going to reflect that.

I'd still keep my eye on the cylinder head temperatures.

I thought I read a while back that someone had developed water cooled heads for the VW engines. But yes, VW powered aircraft without a reduction drive suffer in takeoff and climb though they do have reasonable speed. That's why they are mostly used on very small air frame light airplanes even though the net HP could pull something bigger and heavier. They have their application but for the cost you could overhaul an 0-200 with careful parts scrounging and have a better performing longer lasting engine.
 
The RV-8 does everything better, except the impact it would make on my wallet!! I'm looking at $24K airplanes.

Oh yeah, good point! I tend to forget about the acquisition/build costs on my RV as I finished it in 2006... but for the mission you describe, the Onex would be a hoot. Looks like it would be fun to build as well. I’ll admit it; I still kinda want one...
 
Certainly a valid point.

However, it's hard to pin down the rate of miscontrol, since lower-experienced pilots tend to gravitate towards the simpler airplanes like the Sonex. Here's a summary of the median total time and median time in type for a number of homebuilt aircraft (1998-2016 data):
As you can see, the median pilot total time in RV-7 accidents is 1301 hours, vs. less than five hundred for the Sonex accidents. So it's hard to make any conclusions relative to the engine-failure data and miscontrol.

Ron Wanttaja

I see. My biggest reservation is those numbers are so low that it is difficult to make solid conclusions. The other is that the overall accident number really isn't very informative. There are a lot more RV6 accidents than Sonex accidents. Does that make the RV6 more dangerous? Of course not! There are a lot more RV6's, it is the most successful of all the kit aircraft. For all I know there are so many more RV6's that its actually safer.

What is really valuable (and nearly impossible to ascertain) is the accident/time flown rate. That relates both to fleet size (which can be gleamed from FAA records) and time flown, which can be gleamed from your local neighborhood gypsy, perhaps. You can get a bit of a clue from fleet size. I imagine time in flight is less for the Sonexes and their brethren, since they aren't very big, don't have a lot of creature comforts, and I imagine aren't mostly used for travel.

And I do have to give a vote of thanks, Ron. These are really enlightening discussions, impossible without your superlative level of expertise.
 
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There are a lot more RV6 accidents than Sonex accidents. Does that make the RV6 more dangerous? Of course not! There are a lot more RV6's, it is the most successful of all the kit aircraft. For all I know there are so many more RV6's that its actually safer.
I remember a magazine did a story on aviation statistics. One could conclude that none of us should ever fly in a 172. One of the deadliest planes ever made... or maybe the highest produced plane!
 
Certainly there ARE accidents involving VW engines. The question I have is why.

I took a quick look at the cause data. The rates of engine internal failures is almost the same. VW conversions stand out under Ignition Failure.

But, like Steingar says, it's a relatively small data set. Roughly 200 accidents involving VW-derived engines, and only a third of those can be attributed to the engine.

Ron Wanttaja
 
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