Pilots with Sonex experience - Talk me out of (or in to) a Sonex

[MarkH]

I'm planning to buy a plane to replace the Yankee I sold in the next 4-8 months.

I have a few models in mind, but I keep seeing Sonex Legacy planes with Jabiru engines in my budget and they seem like fun, low cost, VFR airplanes. I am having trouble finding planes in my budget that fit all of my goals, but if cross "fly to Mexico" (something I want to do, but have never actually done) off the list, a few small experimentals fit the bill.

I'm 6' 240lbs, and fly 90% of my missions solo (+25lbs baggage), but I would like to have the option of carrying a passenger. Budget is up to $30K, but I am hoping to keep it closer to $20K.

But, I don't have any experience with Sonex planes, so I would like to hear opinions from people who have flown them.

 

[Daleandee]

IIRC the Sonex and the Yankee have the same airfoil (NACA 64-415) but that info likely don't tell you much. They are built like a tank though!

The Sonex is lighter in pitch than roll and has good rudder authority (standard tail - I have no experience with the "Y" tail). They are sporty with neutral stability and go where you point them but are not a "trim it and forget it" aircraft although in smooth air they tend to stay where you aim them for a few moments. I have a Trio AP that's keeps it going where it should while I eat lunch or take a nap.

With the size & weight you gave I'd suggest that you find someone with a Sonex close to you and get a test fit. I'm 5'9" at 190 lbs. and the fit for me is good but "cozy" is a polite word to describe the cockpit. Most with a larger engine should have ~400 lbs. useable with full fuel. That would leave you with 160 lbs. for a passenger. Less fuel = more passenger (as you already know).

I use a Clarity Aloft headset to give me more side clearance to the canopy (I really like this headset for other reasons). If you get a center stick you can fly from left seat or center seat. I've done both and the center makes it a large single seater.

Mine has a 3.0 Corvair conversion engine that gets it up and going in a hurry. I wouldn't recommend the VW conversion as IMHO there's not enough power for the airframe. The Jabiru 3300 engines make good power and some have gotten good reliability from them but the company has had issues with various generations of their engines and some complain that parts and service can be difficult to come by.

More opinions and information can be found here for the planes: https://www.sonexbuilders.net/

Here's a good source for Jabiru engine information: https://jabcamit.groups.io/g/main

Dunno if this helps ...

 

[MarkH]

Thanks, that’s a great start.

 

[O2 Birddog]

Thanks, that’s a great start.

Ran into a guy flying one last week and got a good look at it. He was at least 6'2 and didn't seem to have any issues with comfort as he'd flown it nearly five hours straight to get to the local airport.

They definitely look like they could be a whole lot of fun!

 

[Chicago Bearhawk]

Mine has a 3.0 Corvair conversion engine that gets it up and going in a hurry. I wouldn't recommend the VW conversion as IMHO there's not enough power for the airframe. The Jabiru 3300 engines make good power and some have gotten good reliability from them but the company has had issues with various generations of their engines and some complain that parts and service can be difficult to come by.

I really like the idea of a VW engine. I have a 73 Kombi, and am really familiar working with the engine...
do you have any statistics on this or is it just a perception?

 

[Daleandee]

I really like the idea of a VW engine. I have a 73 Kombi, and am really familiar working with the engine...
do you have any statistics on this or is it just a perception?

Great perception from personal aircraft experience. BTW I currently own a couple of VW Beetles …

 

[kyleb]

I really like the idea of a VW engine. I have a 73 Kombi, and am really familiar working with the engine...
do you have any statistics on this or is it just a perception?

Think about how underpowered your Kombi is. Apply that to an airplane.

Seriously, look at the aircraft which are successful with 65 horse engines...things that go slow, spin slow turning (and large diameter) props, have big wings, etc - Cubs, Taylorcraft, Aeroncas, etc. The VW drives a small prop at high RPM. It is gutless on takeoff and climb. Decent for speed on limited HP, but a little prop at high RPM is terrible at low speed acceleration and climb.

One of the locals had an 80 horse Jabiru powered Sonex. He didn't like it - thought it was underpowered. As he was selling it, he loaded up a prospective buyer, lifted off the ground and climbed to about 10' high, semi-stalled it, sank back down to the ground, accelerated some more, then lifted off and slowly climbed out.

That prospective buyer walked away.

For what it is worth, he 80 HP Jab is a better motor than the VW...

 

[Chicago Bearhawk]

Think about how underpowered your Kombi is. Apply that to an airplane.

Seriously, look at the aircraft which are successful with 65 horse engines...things that go slow, spin slow turning (and large diameter) props, have big wings, etc - Cubs, Taylorcraft, Aeroncas, etc. The VW drives a small prop at high RPM. It is gutless on takeoff and climb. Decent for speed on limited HP, but a little prop at high RPM is terrible at low speed acceleration and climb.

That is a good analogy, and really makes sense. a sonex airframe is not an Aeronca, and not the same mission.

 

[kmacht]

I own a VW powers sonex that I scratch built. It performs just fine. More power is always nice but not always necessary. The only downside is climb rate when fully loaded on a hot day. Solo in the winter I have seen climb rates in the 800 to 1000 rpm range. Hot summer day with a passenger at gross it drops to around 300fpm. Cruise is a little faster but not by much with the 6 cylinder jabiru but it also sucks more fuel. The gas tank is only 16 gallons. That’s plenty for a VW who is only using 4 gallons an hour but starts to get a bit small as you step up in engine size.

The big advantage of the VW is its cost to maintain. Spark plugs are $5 each at th local auto store, a set of 4 brand new pistons, cylinders, and rings can be had for under $200, etc. You can do a full overhaul on the aerovee motor for less than just the inspection cost on a certified engine. Of course the downside is that you need to be mechanically inclined. You can’t be afraid of things like valve adjustments and oil changes. Most A&Ps aren’t going to work on VWs so you will need to learn to do them yourself.

One thing to note between your yankee and the Sonex is in pitch stability. Most planes are have positive stability so if you point the nose up and let go of the stick it will eventually come back down and fly however you have it trimmed out. A sonex has neutral stability. If you point the nose up or down it stays pointed up or down and doesn’t self correct. This makes it a lot of fun to fly (especially aerobatics) but makes it a horrible ifr platform. The Sonex is strictly a day vfr airplane. It requires your constant attention while flying.

Things to look out for when shopping would be the fuel system. The sonex line of aircraft have had a high number of engine outs on takeoff and also suffer something called “the burps”. Many (jabiru and vw configurations) use the aerocarb instead of a regular carburetor. The aerocarb is an updraft slide carb that doesn’t have a float bowl. It is essentially a controlled leak into the intake. It is simple and works well but does not tolerate any disruption to fuel flow. The carb and fuel lines run right by the exhaust and if not very well insulated gets vapor bubbles in it which can cause the engine to stumble. The way to spot a sonex with a good fuel system is to look for one that has a straight run from the tank under the glare shield directly to the carb and that line is fully insulated. This is now the recommended installation from the factory. If it still has a gascolator or anything but a straight flow from tank to carb to let the vapor bubbles escape back to the tank don’t fly it until it’s fixed. I personally don’t care for the aerocarb even after flying behind one for a few years and am switching mine out for a fuel injection system.

As mentioned above, for a larger person you will likely want one with a center stick configuration. When solo this allows you to fly in the middle of the seat with your feet on the two outside runnder pedals.

Check on insurance rates before you get too enamored with a Sonex. As I said, the Sonex has had a number of engine outs including on two aircraft owned by the factory. Some are seeing very high rates compared to the value of the plane. If you are a higher time pilot you will likely find an acceptable rate but if a newer pilot or buying a taildragger with little taildragger experience expect to pay a really high fee compared to your yankee for at least a couple of years.

Don’t let the taildragger scare you off. The Sonex has got to be one of the easiest taildraggers to land. The tail wheel has a direct connection to the rudder pedals. There are no springs in the system. Where your rudder points is where your tail wheel points. As soon as that back wheel is on the ground it goes exactly where your feet tell it to. The only downside to direct steering is the limited turning radius. The tail wheel doesn’t unlock so you can’t pivot the plane on one wheel. For 99% of flying this is fine. The only place I have found it an issue is on really narrow strips where you need to turn around and back taxi to get off the runway.

keith

 

[MarkH]

I own a VW powers sonex that I scratch built. It performs just fine. More power is always nice but not always necessary. The only downside is climb rate when fully loaded on a hot day. Solo in the winter I have seen climb rates in the 800 to 1000 rpm range. Hot summer day with a passenger at gross it drops to around 300fpm. Cruise is a little faster but not by much with the 6 cylinder jabiru but it also sucks more fuel. The gas tank is only 16 gallons. That’s plenty for a VW who is only using 4 gallons an hour but starts to get a bit small as you step up in engine size.

The big advantage of the VW is its cost to maintain. Spark plugs are $5 each at th local auto store, a set of 4 brand new pistons, cylinders, and rings can be had for under $200, etc. You can do a full overhaul on the aerovee motor for less than just the inspection cost on a certified engine. Of course the downside is that you need to be mechanically inclined. You can’t be afraid of things like valve adjustments and oil changes. Most A&Ps aren’t going to work on VWs so you will need to learn to do them yourself.

One thing to note between your yankee and the Sonex is in pitch stability. Most planes are have positive stability so if you point the nose up and let go of the stick it will eventually come back down and fly however you have it trimmed out. A sonex has neutral stability. If you point the nose up or down it stays pointed up or down and doesn’t self correct. This makes it a lot of fun to fly (especially aerobatics) but makes it a horrible ifr platform. The Sonex is strictly a day vfr airplane. It requires your constant attention while flying.

Things to look out for when shopping would be the fuel system. The sonex line of aircraft have had a high number of engine outs on takeoff and also suffer something called “the burps”. Many (jabiru and vw configurations) use the aerocarb instead of a regular carburetor. The aerocarb is an updraft slide carb that doesn’t have a float bowl. It is essentially a controlled leak into the intake. It is simple and works well but does not tolerate any disruption to fuel flow. The carb and fuel lines run right by the exhaust and if not very well insulated gets vapor bubbles in it which can cause the engine to stumble. The way to spot a sonex with a good fuel system is to look for one that has a straight run from the tank under the glare shield directly to the carb and that line is fully insulated. This is now the recommended installation from the factory. If it still has a gascolator or anything but a straight flow from tank to carb to let the vapor bubbles escape back to the tank don’t fly it until it’s fixed. I personally don’t care for the aerocarb even after flying behind one for a few years and am switching mine out for a fuel injection system.

As mentioned above, for a larger person you will likely want one with a center stick configuration. When solo this allows you to fly in the middle of the seat with your feet on the two outside runnder pedals.

Check on insurance rates before you get too enamored with a Sonex. As I said, the Sonex has had a number of engine outs including on two aircraft owned by the factory. Some are seeing very high rates compared to the value of the plane. If you are a higher time pilot you will likely find an acceptable rate but if a newer pilot or buying a taildragger with little taildragger experience expect to pay a really high fee compared to your yankee for at least a couple of years.

Don’t let the taildragger scare you off. The Sonex has got to be one of the easiest taildraggers to land. The tail wheel has a direct connection to the rudder pedals. There are no springs in the system. Where your rudder points is where your tail wheel points. As soon as that back wheel is on the ground it goes exactly where your feet tell it to. The only downside to direct steering is the limited turning radius. The tail wheel doesn’t unlock so you can’t pivot the plane on one wheel. For 99% of flying this is fine. The only place I have found it an issue is on really narrow strips where you need to turn around and back taxi to get off the runway.

keith

What Carb are you considering to replace the aerocarb?

 

[kmacht]

What Carb are you considering to replace the aerocarb?

I’m going with a dual carb/fuel injection setup. I’m putting in a SDS fuel injection systems the main system while still keeping the carb and using that as a throttle body for air control and a throttle position sensor. Should the fuel injection system or fuel pump fail all I have to do is turn on the gas to the aerocarb from a valve in the cockpit and open the mixture on the aerocarb and I can continue flying with no electrical system. The downside is that I also need to upgrade the alternator. The 20 amp alternator isn’t going to keep up after adding in the SDS system and fuel pumps. I am retrofitting a 50 amp alternator similar to the one the current aerovee uses, just a bigger diameter.

I looked hard at other carbs before going with an injection system. The rotec looked promising but had some issues with the fuel metering system in the early versions. I would love to put an Ellison on as it is the best throttle body carb out there but they stopped making the smaller version of it so they are hard to come by and rebuild kits don’t exist. I also looked at a standard carb but would have had to put in a carb heat system as well as still deal with the fact that it hangs right next to the exhaust pipes and may have vapor lock issues with a float bowl.

 

[schmookeeg]

One of the locals had an 80 horse Jabiru powered Sonex. He didn't like it - thought it was underpowered. As he was selling it, he loaded up a prospective buyer, lifted off the ground and climbed to about 10' high, semi-stalled it, sank back down to the ground, accelerated some more, then lifted off and slowly climbed out.

That prospective buyer walked away.

And it sounds like he was lucky to do so!

 

[Daleandee]

I like your approach to the Aerocarb concern. I had one, and as you say, they can be made to work, but there are a few things that made me decide against them for my plane. The burps (stumbling when hot), and the initial difficulty when setting it up was aggravating. But on my earlier version, slide sticking was a real concern. When a friend ended up in a field and did major damage to his Sonex over a stuck slide in his Aerocarb I knew I had to do something else.

The answer was to build a taildragger with more power and use a real carb. This wasn't exactly the plan but it was the how the events unfolded at that time. I was looking for an aircraft carb to use on the VW Sonex and considered a small Marvel Schebler or Stromberg from an engine of similar displacement. Some were using Zenith carbs but the jury was out on how well those might have performed.

I pondered then, and even now, how it is that Sonex sells a carb (for many years) that was designed to feed an 80 HP VW engine yet the consistency when bolted on is not there. Seems that the carb (back before the turbos were added) would have come jetted and set up to bolt on and run with little tweaking. Such was not the case as it came with three different needles and was finicky to get correct.

Peter van Schalkwyk's EFI conversion for his VW Waiex was clean and very well done. It also gave the engine a bit more power. My little VW Sonex would give me (on a not too warm day) a climb rate of 750 fpm solo & ~300 dual. The problem was that once I reached pattern altitude here in hot Carolina, the CHTS were at or above the limit so step climbing was required.

But I agree with kmacht ... build a very light taildragger Sonex and fly it when it's not too warm and they do pretty well. But for me having flown a Corvair powered Sonex, I wouldn't consider anything else. Yulp ... I am biased!

 

[Propwasher]

I'm looking to sell my 85hp Champ soon and have been looking at Sonex and Vans. Given the price difference between a -6 vs a Sonex with the Jab, is it worth it?

 

[MarkH]

I flew a Sonex last weekend and it was a very nice plane, a trigear with a 95hp Type 4 VW (it’s for sale, message me if you are looking and I’ll put you in touch with the owner/builder).

Flying out of the Kingman airport (5900’ DA) it still managed to pull off 4-500 ft per min initial climb (with me (250) and the 170lb owner/builder). It is a dual stick setup with hand brakes. Light on the controls and fun to fly. It had the original plans seat pan, which I have read is worse for larger guys.

But it was not for me. At 6’, 250lbs it was not a a comfortable plane with 2 people in it (I didn’t expect it to be a Bonanza, but I would like to be able fly an hour with a passenger).

After flying that plane, I would not consider an 80hp Sonex at all. But, the 100hp turbo or the 120hp Jab would be a fun plane if I found one built by a larger guy that I could fit in (but at the price of those, I would look more at a Cessna 120).

As for building, if I’m going to build a plane I would build something bigger. Climbing in and out of that plane at 35 is tough, I don’t want to be climbing in and out of it at 75 (and if I build a plane, I would plan for it to be my last plane)

 

[kyleb]

I'm looking to sell my 85hp Champ soon and have been looking at Sonex and Vans. Given the price difference between a -6 vs a Sonex with the Jab, is it worth it?

I think you should get a ride in both. The RV-6 should be a relatively easy get. An RV-7 would be very comparable if that was easier. Then you need to find someone with a Sonex and get a ride. Only you can put a value on the respective differences. One thing I will say is that an RV-6 can be a true 2 person X/C airplane with a real amount of baggage. I'm not sure the Sonex can do that kind of thing. For what it is worth, my -6 has a 650 lb useful load. Which is full fuel (600 NM worth), two 180 pound people, and 65 pounds of baggage. Or some 650 pound combo of those three items...

 

[Daleandee]

One thing I will say is that an RV-6 can be a true 2 person X/C airplane with a real amount of baggage. I'm not sure the Sonex can do that kind of thing. For what it is worth, my -6 has a 650 lb useful load. Which is full fuel (600 NM worth), two 180 pound people, and 65 pounds of baggage.

The Sonex can't compete on the long cross country comparison with the RV-6. I can get a couple of 180 pounders with ~40 lbs of baggage and a bit over two hours fuel with reserve. Even solo the distance is limited by fuel (factory gives the tank at 16 gallons but the tail draggers can get close to 17 with the tail down - just burn that extra gallon in the initial climb). But the cost of the RV-6 is quite a bit more than a Sonex.

I'm OK with setting it down after a couple of hours in the air. Horses for courses.

 

[kmacht]

I wouldn’t recommend the aerovee turbo unless you live at a higher elevation. It’s a normalized turbo so there isn’t much advantage at sea level. I also think that their turbo design isn’t well thought out. It had all sorts of problems with heat dissipation and coking when it first came out not to mention that the turbo configuration was the one that crashed at the factory during an engine out at takeoff killing John Monnets son and an employee passenger. They still don’t know why it quit. You now need a coolant system for the turbo to try and prevent the coking issues but even some are still having problems. If you can afford it the rotax 912 seems to be the new popular engine for a sonex and the factory now sells mounts and cowels for them. If you can’t afford a rotax then the 80hp VW motor works well unless you are at a high density altitude or fly at gross weight regularly. There is also the corvair option which is a great fit for the sonex but not factory supported.

The sonex line of aircraft unfortunatly isn’t made for people in the 250lb range if you have any plans to take passengers. Based on your numbers above it is very likely you were over the 1100lb gross weight limit that Sonex recommends at 250lbs with another 170lb pilot. The plane tends to loose a lot of performance quickly as you get over 1100lbs. I tested mine up to 1200lbs but would not want to fly regularly at that weight. It’s a great plane for a larger pilot if you only want to fly solo though. It performs pretty well on only 80hp if you keep to the weight limits. Many heavier pilots go with the center stick option and fly sitting in the middle with your feet on the outside rudder pedals. It’s like a very roomy single place plane in that configuration.

Keith

 

[Mxfarm]

Great perception from personal aircraft experience. BTW I currently own a couple of VW Beetles …

I’ve always admired the VW engine - especially how easy it is to change the belt on.

 

[MarkH]

I wouldn’t recommend the aerovee turbo unless you live at a higher elevation. It’s a normalized turbo so there isn’t much advantage at sea level. I also think that their turbo design isn’t well thought out. It had all sorts of problems with heat dissipation and coking when it first came out not to mention that the turbo configuration was the one that crashed at the factory during an engine out at takeoff killing John Monnets son and an employee passenger. They still don’t know why it quit. You now need a coolant system for the turbo to try and prevent the coking issues but even some are still having problems. If you can afford it the rotax 912 seems to be the new popular engine for a sonex and the factory now sells mounts and cowels for them. If you can’t afford a rotax then the 80hp VW motor works well unless you are at a high density altitude or fly at gross weight regularly. There is also the corvair option which is a great fit for the sonex but not factory supported.

The sonex line of aircraft unfortunatly isn’t made for people in the 250lb range if you have any plans to take passengers. Based on your numbers above it is very likely you were over the 1100lb gross weight limit that Sonex recommends at 250lbs with another 170lb pilot. The plane tends to loose a lot of performance quickly as you get over 1100lbs. I tested mine up to 1200lbs but would not want to fly regularly at that weight. It’s a great plane for a larger pilot if you only want to fly solo though. It performs pretty well on only 80hp if you keep to the weight limits. Many heavier pilots go with the center stick option and fly sitting in the middle with your feet on the outside rudder pedals. It’s like a very roomy single place plane in that configuration.

Keith

Thanks for the great information.

According to the owner/builder, we were right at the 1150lb gross weight of the plane with him, myself and partial fuel. Most of my flying is done alone, and that is what attracts me to the Sonex. I would fly it mostly as a single seat, centerline airplane and occasionally take along a passenger for a short ride or a quick lunch run. The mission profile I am planning for is 40% solo just to fly, 30% solo with baggage for traveling, 10% with a passenger, 20% rent for everything else.

 

[Daleandee]

I’ve always admired the VW engine - especially how easy it is to change the belt on.

Never seen that before but I do carry a spare belt with me. Now I know how to change it quickly ... not sure I would do it that way though!

I have a '74 auto-stick beetle that I'm gonna get rid of. The engine needs to be put back in it and a few other things before it's ready to ride. Gettin' older and don't need so many projects ...

 

[Daleandee]

The sonex line of aircraft unfortunatly isn’t made for people in the 250lb range if you have any plans to take passengers. Based on your numbers above it is very likely you were over the 1100lb gross weight limit that Sonex recommends at 250lbs with another 170lb pilot. The plane tends to loose a lot of performance quickly as you get over 1100lbs. I tested mine up to 1200lbs but would not want to fly regularly at that weight. It’s a great plane for a larger pilot if you only want to fly solo though. It performs pretty well on only 80hp if you keep to the weight limits. Many heavier pilots go with the center stick option and fly sitting in the middle with your feet on the outside rudder pedals. It’s like a very roomy single place plane in that configuration.

I'm gonna make a couple of exceptions to the rules ...

The 1100 lbs. gross is listed for 80 HP versions. Sonex allows 1150 lbs. for 100 HP or greater engines. Although not factory approved a number of 120 HP Jabiru versions are setting the gross at 1200-1250 lbs. I set my Corvair powered Sonex at 1250 lbs. and test flew it to that during the 40 hour phase one.

I have seen and flown in a Sonex that had the gross weight set at 1320 (LSA limit) and it flew very well off of a short grass field. It had a 120 HP Corvair conversion on it. It was a nose roller plane and had additional wing tanks as well as most of the comforts of a larger plane. I actually did one of my flight reviews in that plane.

Poor handling because of a lack of power is one thing but with correct CG & more available power the plane performs very well. This isn't just one man's opinion as there have been a number of Sonex aircraft built and flown to a gross weight of 1250 lbs. or even more. Some of them are Corvair powered.

Here is a less than two minute video with me and my son taking off on a warm day in Carolina. Gross was at 1210 lbs. It flew just fine. Just takes a bit more power in the steep turns, a smidge higher approach speed, and two folks sharing the smiles!

FWIW ...

 

[brcase]

I have giving some dual instruction (checkout) in a Waiex with Jabiru 3300 engine. It is a fun airplane to fly. I really like the Jabiru engine, however we have been have to workout some issues with the Aeroinjector Throttle Body. This mostly may be the owner(s) and I learning how it works and in part by the builder not really ever getting it set up properly. Mostly have had issues with it running rich or not leaning the way we think it should. Have also had some issues with the Air filter, I have read the stock air filter is a bit to small for the 3300 engine, I believe this is likely true.

It is a nice easy airplane to fly, it only has about 60 hrs on it and the current owners have probably less than 15hrs in it, and are still working out some issues. Like one owner is to small to actuate the manual flaps. So until we can get either an Electric Flap system installed or modify the manual system, she is flying it without flaps. which is perfectly doable, but makes speed control on approach more difficult as the airplane changes speed quickly with small pitch changes. She regularly flies a Citabria so is have to adjust to the reduced forward visibility and not sitting on the centerline of the aircraft.

She has been making some videos of some our flights...

Brian

 

[MarkH]

At 1150 gross, with 95hp Type 4 motor, at a 5900 ft density altitude, I have no complaints about the performance we saw. It was better than I expected (and better than most small certificated planes I have flown in similar conditions).

My concern was the space. With dual sticks and center flaps/brakes I had to anticipate his actions to ensure that I was not blocking the stick when he was maneuvering or the flap handle in the pattern (because I was always blocking one of them).

I would still like to fly a center stick version (ideally with the lower seat pan), but I wouldn't buy one unless the experience is very different.

I know I don't want to build one, because I am just too big and as I get older I don't think I will want to climb in and out of it.

Having said that, I am keeping an eye on the Sonex High wing. If it's a little bigger, can still fly center stick, and is as easy to climb in and out of as other cantilever high wings then it might be what I want to build.

 

[The Master]

IIRC the Sonex and the Yankee have the same airfoil (NACA 64-415) but that info likely don't tell you much.

So the Sonex then has the same airfoil as the original (unmodified) Yankee airfoil, which is known for its bad stall (which is entirely a function of the airfoil) Should I assume the Sonex stalls the same then?

 

[Daleandee]

So the Sonex then has the same airfoil as the original (unmodified) Yankee airfoil, which is known for its bad stall (which is entirely a function of the airfoil) Should I assume the Sonex stalls the same then?

I wouldn't make that assumption. I've never stalled a Yankee (or flown one) but aircraft stall characteristics are not solely dependant on wing airfoil. FWIW, I find the Sonex stall similar to a Cherokee i.e. a bunch of wing rocking before the break. Lot of U-Toob videos of guys doing stalls in Sonex aircraft if you want to view them ...