From alot of the topic in this forum, I see alot of " what plane for this engine" or "what size engine for this plane" . For people new to coversions like me, is there a rule of thumb to go by? Or is it hit and miss till you find the right combo? Me for example, Im converting a 38cc poulan. What lbs size plane max will it fly with vertical authority? What size for scale? And prop sizes, do you choose props according to RPM? Whats a guide to go by there? Would love to hear from any and all who know the secrets to these puzzles! Thank You! AL

that pc is not a poulan is it? I don,t recall ever seeone like that or maby Im not old enough. A 38cc poulan should fly a 12# plane but remember there are alwas veribles such as drag and style of flying you want to do, I built a 38cc poulan for a freind and he flies it on a hanger 9 talorcraft it is on the thrird one and still never misses a beat and has plenty of power

Go-Verticle, Not for sure it is a Poulan as any I have looked at or converted used a beam mount. Anyway, I did a Poulan 42 for my son in his first gas plane. We had it on an old Sig Extra we poicked up. The plane had been repaired many times before we got it and with the engine it weighed almost 17 pounds. Still the engine with an 18/8 MA prop turned 8000 and pulled it straight up. I sold the engine to a guy here who put it in a U-Can-DO that weighed much less and it was unlimited. Your engine would do well in any type of aerobatic airframe in the 10 to 15 pound range provided you can balance it. I put an Echo 44 in a Wildhare 73" Edge and it was unlimited and weighed 12.5 pounds and turned a 20/8 Menz prop about 7400 RPM. I have put an Echo 21cc weedeater motor in a Seagull Models 60 size Edge and it flew fine. Wouldn't hang on the prop but I was trying to prove a point that a gas engine on a 60 size plane will work. Way cheaper than a 1.20 OS 4-stroke too. Carb and prop with a good muffler are the things to concentrate on.

Tim

Tim: Just wondering.....how is the air-frame holding up on the gas pwered .60 size plane? Thanks Capt,n

Well, it was holding up fine. Those little motors really don't vibrate all that much. Not much more than the .91 4-stroke OS motor that was on it. That was until my son found that gravity always wins if you are not careful. He just lost it and pancaked it it. Boy was he mad. First this [X(] then this [>:]

My rule of thumb is two times weight equals cc to fly with reserve power. A 31 cc engine will fly a 15 lb. airplane. I am currently flying a 35 lb. airplane with a 57cc engine. A 69cc engine flew the airplane with more authority. Personal experience says a 25 lb. airplane will barely fly with a 25cc engine.

However I have no idea what “with vertical authority� means. The term is simply too ambiguous and open to multiple interpretations.

25 cc engines usually use 16x8 props.
31 cc engines usually use 18x6 or 18x8 props.

Hope the information helps.

Bill

Hi Tim

I have an older Sr, Kadet float plane. +1300 sq.in. wing
I've been wantung to put a 25cc toro on it and have been told that I'd have to re-do the fuse and the wings.

The fuse I can see, but the wings too? What do you think.

Ralph,

I have a Sr. Kadet with three 45 engines (10 1/2 lbs.) and believe that it will be plenty strong. The gear will be weak and I installed the next size wire. Some tail weight will probably be required. It would not hurt to reinforce the firewall or at least watch it closely.

Bill

Ralpbf, My Balsa USA Eindecker 90 has 1155 square inches of wing area. I have an Echo 23.6 blower motor on it with a CH ignition. I use a 16/6 MA prop and it turns about 7500 or so. Flying is very scale with average power. But that is the way it is supposed to fly, nice and slow. I have a Toro 25 I could use on it and I know it will be way over powered. That motor with the same prop turns almost 7600 sor I thionk it will fly your Kadet jsut fine too. The Echo motor has a lot of time on it and is kinda tired but does well in the plane so I am leaving it alone.

Tim

This is kind of off-topic, but an old guy once told me that .10 cubic inches for every pound was a good rule of thumb for sport flying. He was talking about glow engines though. Gas engines aren't as powerful as glow--so I'd expect you need a bit more cubes for every pound of airplane. Maybe .15 -- .20 cubic inches per pound for gas?

Ken.... From all the info I could get, it is a craftsman 2.3/16 made by poulan manuf. date oct. 1983. Figured 2.3{38cc} / 16 {bar size} I ve made a mount out of threaded rod through the 4 holes where the cases are bolted together , double nuted then sleeved each rod with warshers and rubber bushings. Ive got it ready to fire up but need a hub and muffler. can you help? What measurments do you need ? And what the cost for hub and muffler? Thanks! Al Ps Does this engine look like something else to you?

Tim220225... Im planning on putting this engine on my GeeBee. Plenty wing area, wingspan;97in weight will be 14lbs or under. I think it will balance ok. For my byrons eagle I figure Ill need at least 40cc for it.

Thanks Bills... THe 2 times the weight to cc seems like a good starting point. What about prop size? what rule of thumb do you use to determine what prop, 16, 18, etc. Are you looking for one to turn a specific rpm range?

Tim.. Im lookin for a 40cc engine for my eagle. Thought about buying a new weedy motor, but hear they are so hard starting. I heard put the elect. ignition on and solve that problem. Then I hear complaints about battery ignitions interfering with radio reception. So what do you do? Whats the best recommendation? Thanks! Al

Go-vertical, I think the 38 would do well on the Gee Bee. As for the Christian Eagle a guy at our field has one and it has a Brisson 3.2 I think he said. It flew very well. I would use at least a 50 on it. You can get a Sears saw (Poulan) over 50 cc. 55 I think it is and pretty reasonable. There was one at my local store that was returned and it was $99.99. Looked almost new. Check your Sears stores out and see if they have a returned merchandise area. Home-Depot here has the same area. Echo saws there for the most part.

Tim

it doesn't look like a poulan to me but I can make a hub and muffler for it . need the muffler port size and bolt center to center. need the thread size and pitch and the shaft size next to the housing.
If it is a poulan it is vary old type I haven't seen before
A few more pics at different sides and angles could help also. Kenneth

Thanks Tim. So you think a saw engine is the way to go huh? Ill go check out sears. What about blower motors? Seems like they might have good potential.

Thanks Ken ... Ive got the engine down at the body shop, Ill get some more pics monday. Ill get the measurements too. What do you mean by the pitch? The crank is keyed and tappered through the fly wheel.

the pitch is like on a bolt size 5/16 x 24 the 24 would be the trhead pitch and the 5/16 would be the siza , the pitch is how many threads there are per inch or in mm it would be per mm.

Reading the results of a search for Ryobi in ‘Engine Conversions’ or ‘Tachometer Readings’ will give yield a wide range of props. The prop sizes mentioned are the most common sizes used on 25 and 31 engines.

I measure the RPM in the original blower or weed eater installation (electronic tack). The measured RPM is assumed to be the design speed the engine. Then I attempt to prop for the design speed. A Ryobi trimmer will turn 8,000 RPM. The MB 290 25cc blower also turns 7500/8000 RPM depending upon Hi needle setting. Stihl blowers are set to turn 8000 when they are sold.

Bill

Go-vertical, The saw engines (Poulan and Homelite Powerstroke) are the easiest to convert as they already have the beam mounts like a glow engine. The Echo CS-4400 and CS-510 are the same way but they cost about $175.00 or so. I have done several of them as they are the basis for the BME 44 and 50. Low weight and tons of power. I have done several blower and trimmer motors as well but you have a fair amount of trimming to do of excess metal. Not hard but it just taked maybe an hour to do with filing and grinding to make it look pretty when done. Then I bead blast the case halves to even out the color and texture. Yea, I am anal! In my opinion anything over the 30cc mark and I like the saws for conversion best. The ports are designed to scream in the saw all day long with minimal care on the operators behalf. But I have to say if you can find a good engine and the price is right then use what you have and can afford.

Tim

Ken... here are a more pics you asked for. And matched up a die to the threads on the crank, it was 8mm 1.0. Measured center of holes to center holes on muffler side. Across the bottom it was 11/2in. from bottom holes on both sides, they measured 11/4in. to the top hole. Is this what you need? thanks! Al

Bills ...thanks .. this is what I was looking for! Your info on the prop selection is right on. answers my questions! Thanks again. Al

Tim... thanks for the info on the poulan and homlite. Ill start lookin for one. I always thought the 50size and up would be to bulky and heavy for a 15lb bird. Hey thanks again to all for your input and help! AL

Looks like reed valve induction. Nice big exhaust port....good for rear tuned exhaust. Looks in good shape. Good luck with the project. Capt,n