Compact Radial Engines

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DaveK

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This is probably a question for Mstull, but does anyone have experience with Compact Radials single cylinder engines? I'm monkeying around with a twin engine ultralight design that would require two smallish engines 15-20ish hp each. Weight obviously would be a big issue, but also vibration. Any other good choices in this power range?

DaveK
 

Dana

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The MZ100 and MZ34 are very popular for PPG use. There are a lot of other engines in this range used on PPG's; google "PPG engines". The small 100cc engines have very good power to weight ratios but reilability suffers. The most reliable are probably the 210cc Solo 210/Hirth F36 (15HP) and the 175cc Cors-Air Black Devil (25HP).

-Dana

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mstull

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Dave,

The MZ34 has the same 313 cc piston, cylinder, and head as what's on my MZ201 twin. It is almost identical in displacement, size, and power, as the Hirth F-33 (which I've owned 2). Big bore singles have TREMENDOUS vibration... enough to vibrate your plane apart, quickly destroy mufflers, and froth the fuel in float type carbs. I wouldn't go there. Anything over 250 cc falls into this category.

Smaller displacement singles have less vibration. Their RPM is high enough that they commonly come with expansion chamber type tuned exhaust systems... which are fairly heavy, but are the only way to get decent Hp out of such small displacement. I was amazed with the CorsAir M25Y that I had. I'd consider owning another one. It has an ingenious belt reduction set up that's part of the crankcase casting, saving weight.

There's a couple problems with most all singles. To save weight and cost, they usually don't come with a spark advancing ignition. So the timing is way too advanced at idle, creating a rough idle (unless you set the idle real fast), that causes extra vibration and belt wear.

They also usually come with a cheap Walbro diaphragm carb. Dana has good luck with his, but most everybody else hates them. They only have full throttle and idle mixture adjustments. If you plan to throttle back in cruise, the mixture is often way off. I stuck a float/slide carb on mine after I scorched my upper connecting rod bearing from lean running because of that problem. But that wasn't easy. If I were to get another small engine like that, I would probably leave the Walbro on, and add a control cable to control the high speed mixture in flight... no matter what it took to accomplish that.

Yet another problem with most all the singles... is the reduction belt. Because of the aforementioned lack of spark advance, the engine tries to kick backwards at idle every revolution, changing the rotational velocity of the crankshaft enough that the prop is helping push the engine through. The result is extra loads on the drive belt, and extra vibration. If you have brakes and can set your idle fairly fast, it's not such and issue.

Adding to this problem is a lack of flywheel mass. To maximize their power to weight ratio, they minimize the mass of everything on the engine. For example, CorsAir "upgraded" their engines with a lighter, internal magnet magneto. But that decreased flywheel mass (compared to their old external magnet magneto). That increased the kick back loads on the belts, so they had to upgrade to a wider, multi-V belt.

No matter what, I'd recommend getting the largest crankshaft pulley they offer. The larger pulley has more surface area touching the belt, making the belts and pulley last longer.

Yet another problem with the belt drives on these engines is the crankshaft pulley tends to get hot enough to scorch the belt. Most of the heat conducts from the crankshaft. But the friction with the belt contributes. Pushers are the worst for this problem, since the engine blocks the cooling air, and heats the air there too. I think the most scorching happens when you shut down, and one part of the belt just sits against the hot pulley. A few engines put an aluminum heat sink disc on the back of the pulley (like the CorsAir), and it may be possible to add that to some engines.

Frustrated by all the above, led me to experiment with running the MZ201 twin direct drive. Stripped down, it only weighs 70# including muffler, carb, prop, and mounts. At direct drive RPMs, it puts out about 28 Hp. It is the most reliable alternative to singles that I've found. I get good thrust, and love the engine sound with direct drive. Vibration is high for a twin, but way lower than singles with that power.

The only small single I've tried was the CorsAir. It puts out an honest 24 Hp. I like that it comes with exhaust system mounts that really work. Most engines don't come with mounts at all, and leave you to struggle with broken exhausts. (Although their exhaust mounting puts the exhaust system perpendicular to the wind.) That engine likes to rev freely, so don't overload it with a big prop. One advantage of the CorsAir: parts for it are dirt cheap. Hirth parts are astronomical.

Good luck with your project.
 

Dana

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Mark,
Are you sure about the spark advance thing? I don't have any personal knowledge, but adding advance to an electronic ignition package is such a trivial thing that I can't imagine they'd leave it out to save a few pennies. I do know that Walbro (which also makes ignitions for OEM's) offers it, and customizes it to the customer's application.

The Walbro carbs are marginal, but they work... if they're selected properly. A lot of the 100cc engines use the WG8 carb, which doesn't even have a high mixture adjustment (for emissions reasons on the chainsaws and weed whackers they were designed for). The only mixture adjsutment is playing with the popoff pressure. The WB32 and WB3A (nearly identical for all intents and purposes) do have a high mixture. A lot of ppgers modify these, either notching the butterfly or (as I did) blocking off one of the midrange fuel passages. Some have added inflight mixture adjustments, as simple as brazing an arm to the main needle, and attaching a cable... I don't need to as I can reach the carb in flight on my PPG.

The small Hirth engines use Tillotson carburetors, which like the Walbros, are diaphragm carbs, but I have no experience with them. OTOH, the Fresh Breeze paramotors use a Bing [float] carburetor, on the end of 6" or so of hose to damp the vibrations... with great success.

The Hirth F36 (the 210cc 15hp engine) IS the Solo 210... they buy the bare engine from Solo, and add the carburetor and redrive. I don't know about the F33. I get Solo 210 parts from either Mojo's Gear or my local lawnmower shop (since the Solo is an agricultural sprayer engine, and Solo also makes weed whackers and such).

You're right about the single cylinder vibration, no way around it. Good vibration mouts are a must. The redrive belts you just replace regularly, treat it as a routine maintenance item (50 hours or so, not a big deal). Keeping a Poly-V belt tight is important, they need to be a LOT tighter than a conventiona V-belt. I will say that vibration related problems (e.g. muffler cracks) on my Solo powered machine are more caused by out of balance wood props than anything else.

My UltraStar (with the Cuyuna engine) was derived from the earlier Kolb Flyer, which had twin Solo engines. I would feel perfectly comfortable with twin Solos (or twin Cors-Airs, if I could afford it).

-Dana

"Hiking is just walking where it's ok to pee.....
Sometimes old people hike by accident....."
 

mstull

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Dana,

I agree that the cost of a spark advancing ignition isn't that much these days. And it wouldn't add much weight either. But yes, I'm sure. I don't know of any single cylinder U/L engine that has a spark advancing ignition. There might be one, but I haven't heard or read about any. I would be glad to pay $100 or more extra for one. I have asked manufacturers to offer them. No luck yet.

When you look at the engine specs sheet, engines that have spark advancing ignitions advertise that proudly. They use different names, like "variable timing ignition", depending on the translation. Some twin cylinder U/L engines have them. The Kawasaki twins do. My MZ201 doesn't. Rotax twins don't.

The Walbros, that don't have high speed mixture adjustment needle valves, do have a (tiny) main jet that can be changed. I think it's possible to stick a larger jet in, and add the needle valve so you can adjust them. The one I saw just had a machine screw plugging the place where the high speed needle valve goes.

Tillotson carburetors are supposed to be better at mid range accuracy, but they are larger, more complex, and more expensive.
 

mstull

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Dana,

You probably know, but didn't mention it... so for others' sakes... Not only do you have to replace the belts every so many hours, you have to replace the crankshaft pulley every so many belts. I was surprised how short lived that pulley can be. And the wear isn't visible or even measurable.

If your belts are gradually lasting fewer hours, or to get the same hours, you're having to make the belts tighter and tighter, your crankshaft pulley is worn out. The big prop pulley will last almost forever.

You're right, you do have to keep the belts adjusted tight. But there is a limit. Not realizing my crankshaft pulley was worn on my Hirth F-33, I kept making the belts tighter until I overloaded the front crankshaft main bearing... which resulted in a catastrophic engine failure in flight.

That engine was particularly hard on belts and crankshaft pulleys.
 

Dana

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I don't know... I have about 350 hours on the Solo engine right now. When the redrive failed and the prop/shaft/big pulley were lost in flight (not the fault of the belt) at around 250-300 hours, there was no discernable wair on the small pulley, which I replaced when I replaced the entire redrive with used parts from another manufacturer. The old belt was inadequate; I know from running the numbers according to the manufacturer's reccommdations (I didn't design it). The new one is stronger (14 ribs vs. 10) but I don't have enough time on it to have a history yet. Unlike the old one, the new pulley is anodized, so any significant wear will be visible.

I did run the numbers on the bearings... the belt's gotta be pretty tight, but well within the limits of the crankshaft and prop shaft bearings (at least on the Solo, I can't speak for the F-33).

-Dana

"Hiking is just walking where it's ok to pee.....
Sometimes old people hike by accident....."
 

mstull

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Dana,

Your anodized pulley may very well last much longer, and may show the wear. The pulley on the F-33 was/is soft, bare aluminum.
 

DaveK

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Touched off a good exchange there, just what I was hoping for! I'm playing around with a twin engine pusher design with the engines mounted out on the wings. Two small engines would give me adequate power and wouldn't be mounted very far from the centerline so single engine operation shouldn't produce too much of a yaw. But if the engines of this size range vibrate as much as you are saying I might just drop the idea. I doubt there are any lightweight twins that would fit the bill, unless you guys know of some. Looking for a combo that would be about 70-80 lbs total. Thanks guys.
Dave
 

mstull

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Dave,

Try designing your pusher with twin empennage booms, rather than twin engines. All of my designs have had that. It comes out very weight efficient, and works out well with twin rudders.
 

Dana

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Well, the Lazair used that configuration (first 9HP West Bends and later Solos).

There are some giant R/C model engines in this size range, in twin opposed, flat-4, but people who've attempted to use them on PPG's have had problems with cooling. But then there's this: Picture a pair of these babies on your plane! A bit on the pricey side but Oh, the lovely sound! (check out the video).

-Dana

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mstull

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Another option to consider might be using 2 electric motors. Electric power, with its limited endurance and/or lower power, generally favors an efficient, motor-glider type design.
 

DaveK

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That little radial is amazing and has features most of our normal engines don't have like coil on plug ignition. A bit pricey though.

The design idea came up from some sketching I was doing and it just seemed "right". The fuselage came out as a simple and lightweight triangular truss (one top longeron and two lowers. Mounting the engines on the wings as pushers keep them relatively close to the fuselage. The wings could also be taken off relatively easily without much rigging if strut braced. The fuselage truss just seemed to fit without much effort with good support locations for gear and struts. And it looked good. But without the right engines it isn't worth messing with.
 

mstull

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Dave,

Don't give up yet. I like the triangular truss fuselage. I've pencilled that out a few times. One solution is to make the fuselage sit low to the ground, and put the wing, with a single pusher engine, up above as a parasol. It comes out pretty efficient, with very short main gear and good visibility.
 

DaveK

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I promised a nice sketch, but heres a cleaned up version of the original sketch to give you an idea of what I was talking about. If the weight can be kept under control I would cover the fuselage with light dacron. Doesn't have to be a tailwheel, but a tailwheel pusher seems interesting for some reason.
 

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Dana

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Hmmm... except for the tail structure, reminds me of the Kolb Flyer (like my UltraStar but with two engines):






-Dana

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mstull

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Dave,

That's a nice, clean, efficient design. Some of the smaller (less than 250 cc) engines we talked about, don't vibrate too much. Your design looks pretty sleek and clean. More than about 30 Hp total, and you'll break the U/L speed limit. So you could get by with pretty small (100 cc to 125 cc) engines, which might make your weight goal.

There are other reasons to prefer a single engine, like cost, weight, complexity, and the double maintenance of two engines. You'll have to beef up the wings to take the vibration and G loads at both engine mounts if you have two engines, adding weight.

My U/L climbs great with just 28 Hp, and cruises with about 19 Hp, for example.
 

DaveK

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Thanks for the kind words Mark and I completely forgot about the Kolb Flyer Dana; I got a brochure from Kolb on that plane back in the early 80's. I would imagine there would be a small benefit from span loading, but haven't actually done any analysis on the design at this point. If I decide to proceed, I would probably go through all the iterations of single strut, dual strut, etc...

I've gone through many iterations (always sketching and a few through much more detail) of different designs, but I kind of like the double engine, at least in theory, to hopefully get me out of trouble over rougher terrain. There are many areas around here that if you lost the engine, even at U/L speeds, it wouldn't be a pretty site after attempting a landing. And those spots are the prettiest to fly around.

The benefit of having my private ticket is if it does come out more then 254 or a bit fast I could go experimental. On the other hand I'm always battling the dual urge to go low and slow and high and fast. I find I have to just start something or I will endlessly change plans!
 
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