Hey all,
I was just toying around in my head with the idea of getting our converts into the higher RPM ranges. Would not shortening the stroke (if possible) be an easy way to increase RPM? It seems to my layman eyes that we are not hurting for torque as we swing the mighty 18" and 20" props. Would it even be useful to make a higher RPM engine? I wonder if all we would need would be a shorter crank rod, and to modify the cylinder? Perhaps we could even bump compression in the process to burn either the 50/50 gasahol or diesel? Any considerations that I havent made?

Shortening the rod would not shorten the engines stroke which I think is what you're driving at. A slight torque advantage can be gained by lengthening the rod and moving the wristpin higher and to the left in the piston but this is a lot of trouble for little gain, plus it increases the side thrust on the piston and increases wear.

Actually increasing the rod to crank ratio reduces the side load on the piston it also moves the point where the piston hits maxium speed as well which can be a good thing
it tends to let the piston dwell on tdc for longer alowing the engine to build pressure before the piston comes down

Looks like we need a 3rd or 4th opinion on the rod lenght and the thrust thing. Also the crankpin determins the lenght or piston travel....not the rod length. Capt,n

Don't I feel sheepish I really shouldn't post at 3am What I realized this morning is that I meant that we need to mod/switch the crank. As for the rod, I am not even sure one would need to goof with it as the crank ultimately dictates the stroke where the rods sole purpose is positioning the piston in the cylinder which can be modified "potentially" easier than the rod. Food for thought!

Jet, I should have been more clear, it reduces the side load during compression but increases it during the combustion stroke because of the steeper rod angle. My explanation involved lengthening the rod accompanied with moving the wristpin to the left. Sorry

It is commonly thought that short stroke equals high rpms. What is not commonly understood is the reason for this thinking. With a longer stroke, an engine will reach it's critical piston speed at a lower rpms. If you try to twist a long stroke engine at high rpms, reliability will suffer due to the high piston speeds. But decreasing the stroke will not automatically make the engine turn higher rpms. The rpms an engine turns is relative to the port size and timing, the exhaust and carb, and the load.

These "conversion" engines, are not really rpm limited by their stroke practically speaking. If the port timing and size is increased, along with the potential flow of the muffler and carb, and you use a smaller prop, they will turn high rpms. I have a Homelite 30cc that reliably turns 9400 rpms, and the boat guys turn these Homeys at well over 10,000. The Ryobi is a different case, as it's stamped steel rod is not intended for such high rpm operation, but most of these conversion engines will support quite high rpms.

However, we're missing something important here. It is much more efficient to turn a large prop slowly than to turn a small prop at high rpms. That's why we get such great thrust, or pull, from these motors; their ability to swing large props due to their inherent torque characteristics.

The original Ultralight airplanes had small engines turning small props at high rpms and were very inefficient. The ultralight airplane movement really "took off" (sorry), when the manufacturers developed reduction drives that enabled the use of large diameter props that created much more thrust and performance. Another example of this is the antique aircraft. It takes about a 200 horsepower modern engine to equal the performance of an antique engine of only 80 horsepower. Why? Because the antique engines swung a huge 8 foot diameter prop at only 1000 to 1200 rpms. Much more efficient.

AV8TOR

Hmm, most of that I knew already, but some of the perspective with the aviation side is nice. I am used to tweeking with VW and Porsche engines and understand how to fit them to a certain requirement. I figured that a higher RPM engine/shorter stroke would be more responsive and a bit more powerful. If our converts are not RPM limited, though I agree that there is little merrit to shortening the stroke. I am sure it would not be easy/cheap. Honestly, the only experience that I have with these type of engines on aircraft is the video of the Ultra SPAD with a 31cc Ryobi. It looked docile (week) and did not have the verticle capabilities that I am desiring. I was wondering if it was because of the low RPM note that I heard coming from it. I am sure it is because of the 13.5 lb weight, but I would have guessed a 31cc to pull a big booger like that into the air with no prob. I am admitedly a layman, though. I apreciate all of the the info contributed on this site, and the member responses.


Consequently, I love ultralights! I wish I could have one....soemday.

Ideal would be to soup one of these suckers up to the hilt and wind up to 15000 THEN put a reduction drive on it to swing a humongous prop It's called Clydesdale power.

The longer connection rod offers better top end and midrange power as it dwells longer at TDC, allowing the engine to build better pressure to push the piston harder. Please make a note that either a longer rod or a shorter stroke crank will lower the squish velocity.
So there for you will have to recut the head
which you would be doing anyway if you were modifying the engine in such a way

The benefit of the long rod was shown in the jetski engines i build to race
there is less side load at any time and this amounts to less friction and better mechanical efficency
the upside of all that = Horsepower

Moving the gudegon pin in the piston
wow thats some work i dont know i would want to try unless you are making a new piston ??

Nope, you need to order those especially made and I doubt you could get them for models. I'll bet MotorBoy or AJ could make one thoughbut for a model I doubt it would be worth the expense.

Pistons are not generally round. They are cam ground.

Enjoy,

Jim

You are correct they are cam ground to help with heat expansion
thats why its hard to make a piston due to the fact you dont know how each piston will expand until you run it
and if you dont cam grind it then you will have larger clearance on the non thrust areas
where the gudgeon is

Dr Evil,

My hopped up Homelite 30cc on my 11.5 lb. Ultra Lite Stick 120 has unlimited vertical. Or at least it will go straight up until it's too small to see what it's doing anymore, rolling all the way!!

AV8TOR

Well now that is good news

Hi,

My custom "Phoenix", with a modified Homelite 25cc has a pretty impressive vertical as well. (10.5 lbs.)

8700 rpms with an APC 16 x 8 prop.

AV8TOR

And the latest project. A GSP Katana 72" with a souped up Kioritz 23.6cc running on glow with 40% ethanol/60% gasoline. 8600 rpms with a 16 x 8 and the engine is not broken in. Flight test soon. (I hope.) Engine weighs only 2 lbs. 6 oz. including muffler, prop adapter, etc.

Note: A lot of guys like to run an 18 x 6 on this size engine, which would increase the thrust and the vertical performance at the expense of some speed. I prefer the 16 x 8 myself.

AV8TOR

Nice! I just posted anotehr thread about where to get conversion parts for the 23.6 that I just picked up. I was in teh garage this evening converting the 14cc and the 23.6 and now I need adapters. Am I understanding correctly that the smaller 23.6 can perform as well as a bigger engine?

Hi,

Answered your other thread also. Kioritz/Echo 23.6cc is a really neat engine. More power than a Homelite 25cc, and a good bit lighter. You are going to want to gut the exhaust and provide two 1/2" i.d. outlet tubes, and install about a 10mm carb. Then if you install a CH Ignitions electronic ignition and optimize the timing, you are ready to go. Don't expect quite the numbers I'm quoting however, as mine are all ported and thoroughly hot rod "massaged".

If you want to get into hopping up the Homelites or Kioritz engines, do a search in this forum as there is a lot of info posted both by myself and others.

AV8TOR

I already copied the info on how to sup up the engine. Thanks! I will likely run it semi stock first. I don't want to get caught up in the tinker and no fly vortex....if I can I will fly in winter here. I am not used to winter, but I will just have to make skis for the plane I guess.

av8tor1977,
i just read your post about engine efficiency and prop size, i dont mean to be adverse here at all but i just thought i would give my two cents. the word efficiency as you are using it sounds like static thrust efficiency. it is true that as far as static thrust numbers go, a heavy, low horsepower motor can make more thrust than a lighter high horsepower motor. emphasis on the heavier and lighter wich has to do with torque at the crankshaft. but in defense of the sweet little modern engines, they are designed to be efficient along a wide range of flight envelopes, not just static thrust wich is mostly felt during takeoff or any other low airpseed situation(hovering). try this next time you are out on the field so you dont have to take my word for it. take any aircraft and put a small high pitch prop on it fly it around a little, then put a big low pitch prop on it. the small prop sure did go fast, but the big prop doesnt go as fast, which one is more efficient? it all depends on the way you use the word, if i am a speed man then the small prop is efficient. but anyhow i just wanted to make my wife mad by sitting here and writing this. but physics in general is no walk in the park much less the part of it that is aerodynamics, heres some useless info for ya did you know that out of all the engineering fields aeronautical engineering uses the most math! thats scary, i am glad i am just a user and a fixer.