# Blue-skying about lawn trimmer engines



## Kludge

Okay, its blue-sky time.

As some of you know, Ive got a small project sitting on the sidelines in the form of a Nick Ziroli 101 wingspan B-25. For this, I need a pair of fairly decent engines. According to the design spec, the Ryobi 26 & 31cc 2 and 4-stroke lawn trimmer (weed whacker) engines are good which is convenient since theyre cheap, available and pretty cool. 

Ahhh, but its max gross is 35 pounds and I fear the take off weight may well exceed that. It will be getting some of the more advanced lift and performance improving tricks (civilianized with no turrets plus other airframe changes for streamlining and weight reduction, wing tiplets, improved flaps, etc) which may net be another couple pounds of lift with the same power but pumping the engines up will help some as well. So that what this is about  pumping up the engines so they can give me a bit more power. 

Increased compression is one path. Since the heads and cylinders are all one piece, this means shaving the base of the cylinders instead of the top. Im not sure how far this can be pushed but the engines are cheap enough to find out. There are also porting and relieving, variable ignition timing, modified cams or electrically operated valves, and tuned intake and exhaust systems. (This was covered in an earlier thread.) Ram intake would also be good though it will only help with increased airspeed. In other words, on takeoff when the added power would be really nice, its not there. I also thought about a supercharger (mechanical or turbo-driven) but came to the conclusion it would take more to drive it than Id gain.

The 4-stroke engines may benefit from fuel injection, either throttle body or direct injection at the intake valve. (Ive thought about injection directly into the cylinder but never came up with a way to handle that due to the pressures and heat involved.) The floatless carburetors used on these engines are fairly close to the pressure carbs used on WW II aircraft engines with the exception that there is no altitude compensation. (Admission time: My favorite of those was the Holley variable venturi carb  one huge variable-width rectangular hole that lead straight to the intake manifold. Beautiful!) The concept isnt far off from that of the body part of a throttle body injection system so that wouldnt be a difficult conversion. 

Stepping up to direct injection takes a bit more, though aircraft systems when I was still an aircraft mechanic were pretty simplistic. Even so, not much would have to be sensed in comparison to an automotive system  rpm, demand (throttle position), intake air temp, ambient air pressure, and either cylinder head or exhaust gas temp. With radio control available, there would be three mixture conditions  full rich, automatic and idle cutoff. Full rich (still kind of automatic) would be for take off & climb out, landing and other times when I may wind up needing full power in a hurry. Automatic would be an economy setting controlled by ambient air temp, CHT or EGT, demand vs. engine speed and probably one or two other variables. Idle cutoff  well, thats how we shut down piston pounders, starve them of fuel. All of this would be under the control of a microprocessor with the default in case of failure being full rich.

On the other hand, 2-stroke engines present a few problems as far as the fuel systems go. The most common system is a floatless carb like the 4-stroke engines use only with a fuel/oil mix instead of plain fuel. Like with the 4-stroke engines, these lend themselves well to throttle body fuel injection, though I cant say Ive ever seen a system like that on a lawn trimmer. (On the other hand, lawn trimmers are designed to be inexpensive hence easily replaceable.) At the same time, Ive never seen anything close to a direct injection system either though I think it would be possible. Sort of.  A constant oil feed for the bottom end for a given engine speed and an independent fuel feed as defined with the 4-stroke direct fuel injection would, to my mind, be an ideal solution but not one easily achieved with current designs.

Okay, so what might work? And I do emphasize might most strongly. The first step would be to separate the fuel and oil feed to the engine. The second would be to remake the carb (such as it would wind up) so the oil feed is directly controlled by engine speed not fuel demand but is still misted into the air feed The third would be to install an injection nozzle at the intake port controlled by the same type of processor & sensors as the 4-stroke engine so metered fuel is introduced sometime during the intake portion of the cycle. When? Im not sure though it would be convenient if there was minimum fuel loss out the exhaust port for economy purposes. 

Since the engines will be operated at pretty constant speeds and loads for the greater part of their operating life rather than blipped as they would be as lawn trimmers the fuel, ignition, intake, exhaust and lubrication systems can all be modified to suit. This greatly simplifies things as far as design goes since it can be tuned to cruising conditions  about 75% throttle, straight and level, 100-500 absolute altitude  with what adjustments are needed to provide full power when required plus to prevent overheating in other than cruise conditions. Another factor is that the airplane will be spending most of its time right side up like a proper (civilian) airplane rather than designed for aerobatics including inverted flight even though the engines were originally designed for operating in any position. 

Ill cover the on-board electronics another time but its sufficient to say theyre a bit more than most R/C aircraft fly with, even giant scale which this will be. 

Anyway, this is just in the idea stage right now. Im in no position to even consider the merest thought of starting this project and wont be for quite a while.


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## kf2qd

Why don't you do a bit of testing of the weed eater engines. You will be running them at a fairly constant setting so carburation should be pretty reliable and simple. Those weedeater engines use pumper style carbs so engine position is not critical. One hop-up for 2-strokers is to go with a bigger bore carb and a reed valve to make the induction a bit crisper. All that other stuff just adds complexity and weight and unreliability. Keep it simple and you stand a much better chance of being able to enjoy the plane.


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## Kludge

kf2qd  said:
			
		

> Why don't you do a bit of testing of the weed eater engines.



At the moment, I have a stack of priority projects to clear before I can get to them, hence the blue-sky time. Were this an active project, I'd have them on the bench (all three varieties - one 4-stroke and two 2-stroke) torn down and the internal cleanup (porting & relieving et al) well under way.



> You will be running them at a fairly constant setting so carburation should be pretty reliable and simple. Those weedeater engines use pumper style carbs so engine position is not critical.



Yep, as far as the fuel system, position is pretty much a don't care situation. (Oil too, though that's another issue.) What I don't like about carbs is their inefficient use of fuel hence the idea of switching to fuel injection of some sort. This will allow better fuel management with fewer compromises. 

At the same time, though, I'll be looking at bumping the carbs up in an alternate path to improved fuel management. I suspect the engines are undercarbureted to act as power limiters which, in turn, would louse up a whole bunch of things. The same may hold true of the exhaust systems - restricted some to reduce power so people don't blow the bottoms out of the engines or cause the cylinders to become unguided missiles.

Speaking of carbs, I've been known to do a little re-jetting in my sinful past, some by simple replacement and some by drilling. If I can set up a carb so it gives me the nice rich mix I want at takeoff et al then can take control of the mixture in flight to lean it out for economy, I may skip the injection entirely except for experimentation purposes. (THis goes to a lengthy discussion in a Yahoo group.) 

One thing that absolutely has to go is the magneto. That's a whole lot of weight spinning around that could be more easily managed with a more conventional coil & points (magnetic or otherwise) ignition system which would be significantly lighter plus the timing can be tuned as needed. 

As to the rest ... experimentation once I can get back to the B-25 project (which is a whole lot of little projects all in one) will be in order. I suspect the local Ryobi rep and I will be on real good terms before it's all over. 

You're right about KISS, though. Simple is good. 

BEst regards,

Kludge


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## PTsideshow

Check out, the model boat RC speed sites, as they have the weed wackers in a two cylinder model and plenty of hop up parts for them there is a company that makes them for the RC boating (speed) crowd.
http://www.rcgroups.com you want racing boats gas.
glen


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## Kludge

PTsideshow  said:
			
		

> Check out, the model boat RC speed sites, as they have the weed wackers in a two cylinder model and plenty of hop up parts for them there is a company that makes them for the RC boating (speed) crowd.



Oi vey! With that kind of ooph, I could upgrade to the larger version (118" wingspan vs 101") and have enough weight forward of the center of lift that the usual tailheaviness found in the model is no longer a problem. (It's a problem in the design of the model that usually necessitates fancy footwork in locating radio parts.)

I guess I should cruise a bit lower at rcgroups ... I lurk there but always in aircraft. Never thought about boats as a possible source of power. Thanks!

BEst regards,

Kludge


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## PTsideshow

Here you gohttp://bonzisports.com/zenoah/
and herehttp://search.horizonhobby.com/index.jsp?N=866&sid=11E2BB3AF71D and I see they mention giant scale air racers, I assume planes. Or a super fast weed wackers :big: I have one of their engines on my 1/6 ankle breaker(off road buggy)
glen


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## Kludge

Oh, yeah. Zenoah makes excellent engines. They make expensive engines that do what I want to do but that means I don't have the fun of making it do what I want to do. 

On the other hand, I also know that if I wind up needing more than I can make an engine deliver, I can buy one. Of course, even a Zenoah can be hottest up so ... hmmm ... I wonder how much power can be gotten out of a 38 before it becomes a lot of little bits of metal.

So much fun, so few straightjackets ... 

BEst regards,

Kludge


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## bentprop

Hi,Kludge.I used to own a Quadra engine,which was modded by the factory for model use.
I don't know if Quadra are still around,but mine was 50-odd cc iirc.But I think there was also a smaller one.There was also a kit available to run them on glow fuel.This involved some mod to the carb jets as well.I think drilling out was the usual mod.
For the type of a/c you're using,your mods really need to be geared towards torque,rather than out and out speed.
Similar mods could of course be made to any engine.The flywheel can be chucked,since the propeller takes over that function.Various bits of the engine casting can be removed to save weight.
i would stick with spark ignition though,running these engines on glow fuel gets expensive very quickly!
However ,this means you also need to consider it's effect on the radio gear,and ways of reducing the "noise"factor of the ignition.I would imagine you will need to make special mufflers to suit your cowls anyway,so the standard exhaust system will need to be scrapped.
Whichever way you go,there's "no substitute for cubic inches",as the old hot-rodders cry goes.
You can always throttle back a larger engine,but if you don't have enough power,your flights will be short and memorable ;D


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## Kludge

bentprop  said:
			
		

> I don't know if Quadra are still around,but mine was 50-odd cc iirc.



I googled. They are as are several outfits that make ever so slight mods to them. Their smallest (40cc) is plenty to fly the 118" wingspan version. 

The smaller version will fly with 18cc engines but only by definition. 26cc allows enough reserve to actually do something and going much over 31cc means getting to a situation where prop ground clearance can become a problem. That is, of course, if I insist on having the prop shaft coming out the _center_ of the nacelle. 



> For the type of a/c you're using,your mods really need to be geared towards torque,rather than out and out speed.



There's a tradeoff somewhere between prop size & speed and using the engine performance curves efficiently. For any given prop diameter, the general rule is lower pitch for increased power & higher pitch for increased speed. A higher pitch prop results in a longer takeoff roll and a flatter climb out angle but the cruising speed is higher. For low pitch it's shorter, steeper and slower respectively. For any given airplane using that diameter prop there is a compromise pitch that offers a reasonable tradeoff between the two. 

I don't care about winning races or aerobatics, just about getting cameras in the air and over where they want to be long enough to get the pics and video I want. (This gets into the on-board electronics I'll cover elsewhere.) It would be seriously convenient if the plane could fly on one engine as well; an engine out condition isn't a nice one and makes one work harder than usual. With models, it's usually also sufficient cause for a crash which ruins a perfectly good day. 



> Similar mods could of course be made to any engine.The flywheel can be chucked,since the propeller takes over that function.Various bits of the engine casting can be removed to save weight. i would stick with spark ignition though,running these engines on glow fuel gets expensive very quickly! However ,this means you also need to consider it's effect on the radio gear,and ways of reducing the "noise"factor of the ignition.



Hmmm ... reading my mind again, are you? It's a short read so it's cool. ;D

Yeah, ignition noise will be a bear. I need to do some reading on filtering and consider the possibility of a separate power source for the ignition ... perhaps overkill but much better than underkill.



> I would imagine you will need to make special mufflers to suit your cowls anyway,so the standard exhaust system will need to be scrapped.



Several people have pointed out the direction to tuned exhausts which I can run along side the wheel wells in the nacelles kind of like Convair did with some of their aircraft. Tuned intakes and a few other breathing improvements have also been mentioned which I had in my original post. The all mighty cube (echoes of being a muscle car era mechanic ) is good but letting it breath is more gooder. 



> You can always throttle back a larger engine,but if you don't have enough power,your flights will be short and memorable ;D



Let's see ... engine failure on takeoff ... been there, done that (1:1 scale airplane) ... not a fun experience but one of several I walked away from without a whole lot of damage to either airplane or me. In a few of them, I can't say the same for either the pilot's seat or my pants. ;D

BEst regards,

Kludge


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## Kludge

Hmmm ... not quite lawn trimmer but ... 

We have some rather annoying mopeds around which I would love to take off the road through the simple expedient of removing the engines. (This does not extend to all of them by any means! Of course, "annoying" probably refers tro the riders instead.) These critters have 50cc 2-stroke engines that I'm sure could be dressed up for other applications ... like a certain bomber project. Anyway, has anyone got an experience with them who may know a reason they wouldn't work?

BEst regards,

Kludge


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## dwentz

Put them 4 strokes on Methanol, increase the compression by shaving the head and you should get the power you need simply. You will have to re-jet the carbs to a larger size also. My kids raced karts and I would take a stock briggs tiller motor, that should turn at about 3500 RPM, and after about 20 hours in the shop we would spin them at 7000 RPM on Methanol no problem. The big gains came in making sure the engines had good flow, and to bring the compression up. You could probably lighten the engine castings some also.

Dale


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## artrans

Well here's a subject i no a little bit about I been in r/c for about 20 years no time of late but that's life and it sucks. anyway props should be chosen to put in in the max power band of the motor. So let say the motor max h.p. is 6500 rpm and you put a 14 x 5 prop I am making up the numbers here for example well a 5 pitch prop is to little but take a 14/9 now that could be to much and you can not get up to 6500 rpm so it takes some testing to get the best set up. Now in my opinion I would go with the largest motor that will fit and I can afford and go with what the engine states as a starting point prop wise and then adjust from there
also i found this site that lets you convert string trimmer to r/c motor.http://jagengines.com/2instructs.htm If I where building this I would go with the new 4stroke trimmer motors but generally 4 stroke have a little less power then 2 strokes. 1 rule I try to stick to in r/c is power is your friend and can save the plane the more the better. 2 build as lite as possible 3. No matter how good you are or lucky one day it will crash that's the r/c hobby.


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## 1Kenny

Sounds like you are making a spyplane.That would be a large area wing for lift. High pitch propeller for power. A low RPM engine for linger and distance.

Here is a site with good information about propellers.
http://ntrs.nasa.gov/search.jsp?N=17&Ntk=all&Ntx=mode matchall&Ntt=propeller

Keep in mind that the propeller tip needs to stay sub-sonic. Taking the tip sonic will cause the propeller to stall like a wing. Another thing is the faster you turn the propeller, the more noise you make. 

Kenny


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## 1Kenny

Here is a RC plane with infrared cameras.

http://www.imaging1.com/nightvision/UAV.html

Kenny


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## Kludge

dwentz  said:
			
		

> Put them 4 strokes on Methanol, increase the compression by shaving the head and you should get the power you need simply.



Farglegrump! :-[ I forgot about alcohol injection. That was one of two ways used to pull extra power out of recips for short periods of time - like takeoff. (The other was water injection.) 

Both 2- and 4-stroke lawn trimmer engines (and moped engines, if all are typical of what I saw on a couple) have one piece heads & cylinders so shaving has to be at the base of the assembly instead. 

I'm not concerned about cranking up RPM (except on takeoff and similar situations), but torque is good. The prop size I can turn is limited by ground clearance but being able to twist a higher pitch prop is a really good thing.

THanks for reminding me about the alcohol injection, Dale. I had forgotten.

Best regards,

Kludge


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## Kludge

artrans  said:
			
		

> anyway props should be chosen to put in in the max power band of the motor.



Yep. It's a balance between diameter (determined by ground clearance if nothing else), pitch and optimizing the engine's power curves. As Kenny mentioned, keeping the tips subsonic is an issue as well - a problem encountered during WW II in a few cases. With even a 14" prop (on the 118" version of the B-25), that won't be an issue keeping the engine under around 10,000 rpm. 



> also i found this site that lets you convert string trimmer to r/c motor.http://jagengines.com/2instructs.htm



If you check the same site, they show the Nick Zirolli B-25 for which I have the plans. Mine will have a few changes - essentially be a post war civilian version (no gun positions, etc) with a few things like tiplets for lift enhancement (slows the possibility of tip stalls and helps make the ailerons more effective), improved flaps and some other really cool things. 



> If I where building this I would go with the new 4stroke trimmer motors but generally 4 stroke have a little less power then 2 strokes.



Like you, I prefer 4-stroke engines. While they develop less power per cc, they also don't drink fuel as fast and that means more flight time. THis assumes the engines aren't modified, of course.



> 1 rule I try to stick to in r/c is power is your friend and can save the plane the more the better. 2 build as lite as possible 3. No matter how good you are or lucky one day it will crash that's the r/c hobby.



That's also 1:1 flying. I'd like the '25 to have enough power to be able to continue flying on one engine if possible. It might be a real pig in the air but it's better than a pile of debris on the ground. As to crashing ... hmmm ... yep, I've wound up on the ground a little earlier than anticipated once or twice. 

BEst regards,

Kludge

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## Kludge

1Kenny  said:
			
		

> Sounds like you are making a spyplane.That would be a large area wing for lift. High pitch propeller for power. A low RPM engine for linger and distance.



Not really a spy plane, just for aerial photography. The exhaust note alone will be enough to wake everyone up, and the polished bare metal with International Orange markings airframe (Think: USN trainer) with nav lighting and strobes might also be a give away that it's there. ;D

We have a lot of areas that are completely inaccessable here on Oahu that are beautiful - forest preserves that occupy something like half the island but are blocked off from ground traffic. Model aircraft are restricted to an altitude of 500' without FAA notification. However, there are several helicopter tours that fly at that altitude or lower so the possibility of collision exists. The Zirolli designs are big enough and fast enough that they could cause some real damage if they encounter a 1:1 aircraft so having them bright and visible is a Very Good Thing. 

The plane will also go out over the ocean looking for dolphins (on a par with dogs as my favorite people) or flying the beaches looking for ... ummm ... scenery.  Anyway, nothing spyish, just putting cameras in the air since I can't be there myself anymore.



> Here is a site with good information about propellers.
> http://ntrs.nasa.gov/search.jsp?N=17&Ntk=all&Ntx=mode matchall&Ntt=propeller



Cool! I used to get the tech briefs when they first started printing them and mailing them to any interested party as well as any other publication I could get my hands on. Thanks for the URL; it will come in handy!



> Keep in mind that the propeller tip needs to stay sub-sonic. Taking the tip sonic will cause the propeller to stall like a wing. Another thing is the faster you turn the propeller, the more noise you make.



I covered the tip speed problem in another response but you're right. THere's another thing that can happen when a prop tip approaches Mach 1 and that's flutter which can tear a prop apart. Most people agree that this is pretty much a not so very good thing to have happen.

As I mentioned, the sound level isn't an issue (keeping in mind that I do need to be a good neighbor so an overabundance of it isn't good either) although it would be cool to make those engines sound like the radials that the original airplane had. 

BEst regards,

Kludge


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## Kludge

1Kenny  said:
			
		

> Here is a RC plane with infrared cameras.



Hmmm ... it's not that difficult to convert a number of the current digital cameras & camcorders to IR - mostly take off the existing IR blocking filter and add a filter to block (or reduce) visible light. The sensors already are IR sensitive which can make things interesting if one's looking for heat signatures. 

Something to consider. Thanks!

Best regards,

Kludge


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