4-stroke engine options?

[13brv3]

Greetings,

It looks like I'll have a year to come up with an engine, and I'm really only considering 4-strokes, or rotary engines. The obvious option is the ThumpAir, but I don't think many are flying yet. By the time I need one, there should be a lot more known about it. I have one of the Predator 670s on a test stand that I'm planning to test, but I'm not sure I'd be able to get that down to the necessary weight limit. I do enjoy alternate engine projects, so I'm open to any promising option.

Aside from the ThumpAir, what other 4-stroke options are out there? There are some neat small rotary engines, but not a lot of them flying that I can find. The Aixro is particularly interesting.

Rusty

[LA F2 Flyer]

Have the rotary engines started to come down in price? I recall Chris having one on his site, and it was priced more than the kit.

I'd love to have a scaled down 5 cylinder radial, myself. Now THERE'S an alternate engine project I can get behind. haha

You are asking the same question I've been asking since I first started my build. I think the UL world is still waiting for the perfect engine - reliable, powerful and light.

[Badland-F5 Pilot]

So far, the only 4 stroke I see that's been able to beat down the weight issue of a 4 stroke is the ThumpAir. Other's have brought down weight, but only a little. Scott has done a great job with the ThumpAir. I would love to have a rotary, but as Peer mentions, the price is crazy and so is the TBO, very low. I'm also waiting for Scott to give us a better TBO on the ThumpAir. It's 300 hours. I'd like to see it more in the 500 hour range, but testing has to be completed before that can happen. Have you heard the ThumpAir? It sounds exceptional, both at idle and power. I'd like to see more produced to give us more of a selection, but so far I've only seen the ThumpAir as an alternative for me, instead of a two stroke motor.

Todd

[13brv3]

The rotary that's on the Badland site is crazy expensive. The Aixro XP40 is around $7k US.
https://www.aixro.co.uk/aixro-xp40-data/

I fear 38HP for an extended duration on the 460cc ThumpAir may never have a long life. We won't know until there's more data though. I may be able to get the 670 down to a usable weight, but I'd much rather have a rotary since I'm a big fan of them.

It's been suggested that a water cooled option will be more likely to keep up with cooling, but I don't really see suitable water cooled 4-stroke small enough for part 103.

[LA F2 Flyer]

Hmm. All things considered, the Aixro isn't THAT crazy priced. Todd, how much is the Hirth twin you were originally looking at?

I like the Thump-Air both for power and for the sound. But I don't really need to worry about TBO, at least in my current situation. Getting in the air won't be nearly an every day occurrence for me due to the lack of convenience.

[fly44d]

I have the ThumpAir 460 flying in my F1.
You can see a couple videos at my YouTube playlist that is in my signature below. I’ll try to add some more I know I have.
So far I like it, it’s very torque-y and I get off the ground really quick.

[13brv3]

Good to hear a first hand report Tom. It certainly looks and sounds good. Can you share any numbers, RPM for climb and cruise, prop size, climb rate, etc?

[fly44d]

Numbers!

Redline is 5600.
Climb at 54/5500 and 50mph
Cruise about 55 at 4500 to 5000
(I am just starting and haven’t had a lot of flight time in calm air.)
Climb rate seems to be at least 500 fpm, I have some GPS data I need to analyze to get an accurate number.
The propeller is E-Props three blade 160cm prop.

[LA F2 Flyer]

Awesome! Thanks for posting this!

[13brv3]

Excellent. Thanks Tom

[Badland-F5 Pilot]

Hmm. All things considered, the Aixro isn't THAT crazy priced. Todd, how much is the Hirth twin you were originally looking at?

I like the Thump-Air both for power and for the sound. But I don't really need to worry about TBO, at least in my current situation. Getting in the air won't be nearly an every day occurrence for me due to the lack of convenience.

The F23 from Hirth runs in the $7,000 to $9,000 range (OUCH!) based on options like fuel injection, oil injection...etc. The big advantage to it is you'll most likly never have to rebuild the engine. It has a 1,000 hour TBO!

I plan to fly my behind off. I have about 30 years left on this earth, and now that I'm retired I want to enjoy every minute of it

Todd

[Badland-F5 Pilot]

It's been suggested that a water cooled option will be more likely to keep up with cooling, but I don't really see suitable water cooled 4-stroke small enough for part 103.

I would love to have a 4 stroke water cooled, but you're absolutely right. I don't see it happening because of weight limitations.
Todd

[Badland-F5 Pilot]

I have the ThumpAir 460 flying in my F1.
You can see a couple videos at my YouTube playlist that is in my signature below. I’ll try to add some more I know I have.
So far I like it, it’s very torque-y and I get off the ground really quick.

I think I love that engine!

Very nice video's. Thanks for posting them.

Todd

[13brv3]

The F23 from Hirth runs in the $7,000 to $9,000 range (OUCH!) based on options like fuel injection, oil injection...etc. The big advantage to it is you'll most likly never have to rebuild the engine. It has a 1,000 hour TBO!
Todd

I have a quote for the F23 from Feb 2023. For the engine, basic exhaust, belt drive, and shipping, the carb version was $11,172.69 and the FI version was $13,485.97. Used F23s can be had in the $5k-7k range though.

Nice engine though (for a 2-stroke). The weight is going to probably be too high for anything but the titanium frame.

Rusty

[Badland-F5 Pilot]

Wow, the price has gone way up. Thanks for looking it up and posting, and yes, I have an F5 on order I wonder if the F23 can be used on the F1 through the F4 series if a BRS is added to the plane?

Todd

[13brv3]

I exchanged a bunch of emails with Chris a while back about the F23, and it's just too close to call. Basically he said it would be an interesting challenge The total weight of the engine (no prop) was around 82 lbs if I recall correctly. I'm looking at 80 lbs as the upper limit.

As for the chute, a lot of people want to believe that you get an automatic 24 lbs for any chute, regardless of the actual weight. I don't think that's what part 103 says. It says they "may" allow that much without weighing, maybe if they're feeling lazy Most BRS chutes I've seen list the weight on them, so I wouldn't want to try to convince an FAA guy that it was 24 lbs, if the label says 16. Of course realistically, you've got to already be in trouble by the time anyone would actually check that.

[Badland-F5 Pilot]

It certainly would be a challenge. No doubt about that.

As for the FAA, there is the other side of the story. I don't believe the FAA has the desire to even look at the weight of the chute in the calculations. Instead they would go with the full allowance. They like simple, simple is easy. Plus there's no money in it or I should say not enough for them to even bother. The FAA is too busy with other issues. Just look at how many people pilot planes each year with expired or even revoked licenses. As long as no one goes crazy claiming there 152 is an ultralight (gross over exaggeration, but you understand where I'm going with this), the chance of getting an inspector that's going to make an issue of the weight with/without a chute most likely would be very small.

I've also found that there are plenty of fat UL's. I've only heard of two UL ramp checks ever, and those were back in the 80's. The FAA is more concerned about the safety of the plane rather than being slightly fat. Having the chute meets that need.

Of course this doesn't mean hang a monster rotax on the front end either. There are limitations to deviations. In the end as we know, it's up to the pilot to decide if the allowance would or would not be a problem. I decided to go with the F5 just for the reason we're talking about. I want the maximum usable weight, and dropping 25 or so Lbs with titanium gives me a big advantage on meeting weight without too much trouble, all be it at great expense! Since I can't take my retirement with me when I eventually leave this earth, I'm at least going to have some fun with it! I have seen some UL's with the F23, so it has been done.

Though I really like the F23's boxer design, I also really like the ThumpAir for the 4 stroke, torque it produces, better economy, lower cost, and of course it doesn't sound like a chainsaw

Actually I would love to have both and be able to directly compare performance. Scott was estimating he could get 40~45 horse out of a ThumpAir. That's not far from the 50 out of the F23. It would be an interesting comparison.

Todd

[13brv3]

On another forum I read some info about the Aixro XP40 rotary from an actual owner. It does seem to be a unique engine with it's own cooling limitations. It might still work well, but so far I haven't been able to contact the two main sites that seem to sell and support it. That's a bad sign when it comes to being able to get parts and support if you bought one. Unless you're just a rotary engine nut like me, you'd probably do well to just forget about these.

With the green squad outlawing 2-strokes, there seems like a lot of small 4-strokes being used in jet skis, ATVs, etc. I wonder if all those are just too heavy for us? I've heard of a few of the Yamaha engines being used for aircraft, but I don't think any of those are under 80 HP. There has to be something else out there. Motorcycles seem to be a good option, but I've read that their configuration makes them hard to adapt.

[13brv3]

Another forum mentioned this little 4-stroke screamer that I hadn't heard of. No word on TBO, and the price is supposed to be around $4300. 30 HP (3 min limit) and under 40 lbs complete.

https://www.eos-engine.com/index.php?lang=1&hID=71

It seems low on power, and overworked, but maybe something to consider vs a Hirth F33.

Rusty

[LA F2 Flyer]

Not a bad looking little unit, but as with most para engines, it needs to be confirmed that the main bearings can withstand a tractor configuration versus a pusher set-up...one of the advantages that Polini has over the other para engines.

The price and weight of the Eos definitely has appeal though!

[Dobie]

Anyone heard much about the Helvenco AG Aero 1000? EFI, water cooled, and expensive. Also 80lb.

[Badland-F5 Pilot]

Anyone heard much about the Helvenco AG Aero 1000? EFI, water cooled, and expensive. Also 80lb.

Not heard of it. Do you have a URL or more info? The 80Lb is do-able but may need some trickery to get it there.

Todd

[Dobie]

You may get a warning about this site but it looks legit. US distributor Blackhawk Paramotors

https://www.helvenco.com/aero1000.htm

[13brv3]

You may get a warning about this site but it looks legit. US distributor Blackhawk Paramotors

https://www.helvenco.com/aero1000.htm

BitDefender certainly doesn't like that link, but I found plenty of others just searching for the Aero 1000. None of the links I found gave a weight though. Looks like it's probably around $9300, though often prices were packages for a particular aircraft.

[Badland-F5 Pilot]

You may get a warning about this site but it looks legit. US distributor Blackhawk Paramotors

https://www.helvenco.com/aero1000.htm

BitDefender certainly doesn't like that link, but I found plenty of others just searching for the Aero 1000. None of the links I found gave a weight though. Looks like it's probably around $9300, though often prices were packages for a particular aircraft.

It looks like something is wrong with the cert for the site, but I agree it appears legitimate. Nice engine and water cooled too boot. Really nice HP for the weight. The price kills it for me, but if I had the cash laying around, it most likely would be in the running. Thanks for posting the info and link.

Todd

[13brv3]

My Predator 670 project is running well, but as expected it's overweight. With a starter, PSRU, lighter flywheel, and nothing it doesn't need, the weight with oil is about 97 lbs. It should produce plenty of power, but wouldn't make weight. There are others testing direct drive version, and that would save 10-15 lbs, but still not enough, and only around 30HP at a lofty 4000 RPM. It could possibly work, but it will almost certainly get sold to someone who doesn't care about making 254 lbs on their "ultralight", or maybe a mini-airboat guy.

I have a new project that's using the Briggs and Stratton Vanguard 23HP engine that's a 627cc v-twin. This is an engine that's flying on lots of planes around the world with little modification. I'm very hopeful for the weight since it known to be considerably lighter. I'm just getting started with the teardown and weighing.

I'm still quite interested in the AIXRO XP-40, but communication is lacking with the German place that makes/sells them. Other dealers don't reply at all.

Rusty

[LA F2 Flyer]

Rusty, out of curiosity, what is the "starting" weight of that B&S Vanguard?

Looking forward to seeing where you go with this one. And would love to see pics (of this AND the Predator build)!

[13brv3]

The Predator was 109 out of the box, but had an almost 10 lb muffler. The Vanguard 23HP was 88 out of the box. Current estimates put the Vanguard more than 20 lbs lighter. Still estimates though.

I just sent my first deposit to Chris today, so I've got some time to come up with an engine I like.

Rusty

[13brv3]

Ordered the AIXRO HP-40 today. It came to about $7700 with some options and spare parts. Should be here in maybe 6 weeks I'm guessing, which will put it about 8 months ahead of the kit Who knows what it might go on in the mean time...
Rusty

[LA F2 Flyer]

Very nice! Give us an update when you receive it...would love some details.

[Badland-F5 Pilot]

Ordered the AIXRO HP-40 today. It came to about $7700 with some options and spare parts. Should be here in maybe 6 weeks I'm guessing, which will put it about 8 months ahead of the kit Who knows what it might go on in the mean time...
Rusty

Nice and as Peer mentioned, please, pictures and let us know what you think when it arrives and you've looked it over a bit!

[fly44d]

I got some more Thump-Air time yesterday!

trim_A1F6E85D-B0AC-4015-BC7C-E324C6D989DC.mp4

(9.01 MiB) Downloaded 266 times

[Badland-F5 Pilot]

Thanks Tom for posting. I'm really liking that engine, but of course you're already flying with it Have you run into any issues or surprises with it?

Todd

[LA F2 Flyer]

Ha! I just watched this video on Youtube earlier before I checked the forum. Love it. Looks like a good flight!

[13brv3]

Sounds good. What climb rates are you getting? Climb and Cruise RPM?

I've got a Briggs Vanguard 38 (627cc) v-twin on the test stand now and it's doing well. At 75 lbs I'm getting 38HP at 5500 using a test club. This engine definitely needs to find a plane, but my eventual Badland will almost certainly be getting the AIXRO XP-40 engine.

[fly44d]

Thanks Tom for posting. I'm really liking that engine, but of course you're already flying with it Have you run into any issues or surprises with it?

Todd

So far the only problem has been the bracket that holds the exhaust pipe on the right side. I am on the third iteration of it but this seems to be holding. There is a lot of vibration it has to hold up to on the ground, in flight it is much smoother. A pair of lord mount like vibration isolators between the engine and bracket is doing it.

[fly44d]

Sounds good. What climb rates are you getting? Climb and Cruise RPM?

I've got a Briggs Vanguard 38 (627cc) v-twin on the test stand now and it's doing well. At 75 lbs I'm getting 38HP at 5500 using a test club. This engine definitely needs to find a plane, but my eventual Badland will almost certainly be getting the AIXRO XP-40 engine.

I’m at sea level and I’m guessing 500fpm, climb at 50 at 5600rpm to get away from the airport, cruise about 55 at 5000 rpm.

[Badland-F5 Pilot]

Thanks Tom for posting. I'm really liking that engine, but of course you're already flying with it Have you run into any issues or surprises with it?

Todd

So far the only problem has been the bracket that holds the exhaust pipe on the right side. I am on the third iteration of it but this seems to be holding. There is a lot of vibration it has to hold up to on the ground, in flight it is much smoother. A pair of lord mount like vibration isolators between the engine and bracket is doing it.
IMG_4505.jpeg
IMG_4506.jpeg

Excellent, thanks for the info. I'll have to watch how I mount that exhaust. Is there the possibility of using the flexible exhaust coupler like this-

https://www.amazon.com/Autarboor-Exhaus ... rce=1&th=1

? or not a good idea because of weight?

Todd

[13brv3]

UPS brought me a couple of boxes today. I did some quick weighing and came up with 61.5 lbs for everything except coolant. I won't likely do anything with it for some time, since I have other projects ongoing, and the kit won't be here for a long time.

[LA F2 Flyer]

This is the AIXRO HP-40, yes? Very cool.

So, what is the expected benefit of this over a piston engine? (I don't know much about Wankel engines.) Right off the bat, it seems pretty light, comparatively (or am I mistaken?).

[13brv3]

I love rotary engines, so that's a big part of the benefit to me. I've owned a couple RX-7s, and put a Mazda rotary in an RV-3B. The engine should be very smooth, and lighter than a normal 4-stroke making similar power. I'm all about making legal weight, so being light weight and not 2-stroke is a requirement. It's a MUCH less known option of course, but I never seem to use the correct engine in anything, including my current 912 powered Onex.
Rusty

[Badland-F5 Pilot]

The big difference besides the large weight savings that I remember from high school auto shop is the rev time is nill compared to a piston engine. Hit the throttle and the rotation increase is just about instant! This is why the little Mazda RX-3 pickup truck could run aways from many higher HP cars. The time it took for the little rotatory to rev to full HP was much quicker than the standard reciprocating engine. The reason you heard that high pitch whine is because that little motor was at max RPM and HP and was pulling away quickly! It'll be interesting to see if you'll get a quicker jump into the air. Looking forward to seeing your eventual flight.

Todd

[LA F2 Flyer]

Ah. I hadn't considered how smooth it might be over a reciprocating engine. That is definitely a big plus!

[Badland-F5 Pilot]

Yep, spin up is very quick, at least in the pickup truck it was. It'll be interesting to see if that's going to be similar with a prop.