Scota4570

I did a weedie 21cc mostly for experience and fun. It will spin a 15 x 8 at a usable rpm. It is heavy for what you get. The thing with all the smaller conversions is the power to weight ratio is very poor. I tried the weedie on a Nosen 1/4 scale aeronca champ. It wallowed off the runway. After a bit it begn to fly and eventually it got to decent altitude. I did not crash but I did not fly it with that engine again. I installed a CRRC 40 kit engine. It now climbs like a rocket. I have a Big Bee, (22# 8" span airplane) with a 56cc echo conversion. That engine spins a 21 x 8 at 6800. That one has unlimited vertical, even more fun. Thing is power to weight is important. Small gassers fall short in this respect. Here are some examples of purpose made gassers:

..................displacement... HP........HP per Pound with muffler and ignition
RCS...........17cc................1.6............. 0.61
.................26...................2.8......... ....0.74
Aerovate....20...................2.2.............0 .94
................26...................2.6.......... ...1.04
DL............30...................3.7............ .1.6

Conversions are even more so. You will put a bunch of time and money into your model. Why have it fly poorly? I suggest more cc's to get the power up and the weight down. At very least get very agressive at removing everything you don't need from the engine. Be thoughtfull about how you make the mounts. Loose the steel can muffler. Use an electronic ignition. Make the prop hub as short a possible. I have had all kinds of vexations getting prop hubs true. Making is short makes that job easier.

I was recently looking at a well done article on conveting a Echo. http://tech.flygsw.org/echo_conversion.htm I notticed that the echo could probably loose everything aft of the PTO shaft. I would look at amputating the shaft and housing and use standoffs if I did one. Run the ignition magnent off the prop hub, you get a dual purpose part that way. The person made a tidy looking disk and housed the sensor inside the rear housing. It was a testiomonal to his maching skills but it costs weight with no advantage. I know it is simmilar to a BME but more modern designs are done differently. Look at some internet picture of engines like the DL-55 and emulate it in your conversion. Look at this one, http://www.maxfordusa.com/crrc26ccgasengine.aspx See how the back plate is under cut below the jug?

rangerfredbob

My Feedback: (13)

that's why I'm carefully choosing components and the way I set things up, giving myself a few options, the attached pictures are of the 18cc engine I'm thinking of using, I made the prop hub about a year ago, now realizing that I don't need that much material behind the prop I will probably turn it down some more and shorten it a little. I don't use the flat plate mount like most people, on an engine this size extra weight is your enemy like you said, studs out the back is a better setup. I plan on using a 40 size "ultra easy stick" from ebay at about $65 shipped, I believe I have everything but a switch harness to assemble it including reciever and servos, so I'll have under $100 as I consider my spare parts free. the plane has about the same wing area as a trainer and instead of adding tail weight I am going to shorten the nose. I figure I can experiment with $100... the engine most likely won't fly in stock form, the exhaust port timing on the poulan engines is pretty lame, somewhere between 95 and 110* if I remember right from the last 18cc I degreed, I'll get it up to the 150* range that is much better for power. Being a non cowled plane I am going to keep the stock muffler but simply remove the two baffles and put it back together as shown in the pictures, light, simple and should work fine. if I can get away with it I am going to keep the magneto ignition, if not it's not that big of a deal to stick one of my EI units on it. The engine as it is is about as stripped as it can get, I can chuck the cylinder up in my lathe and turn the fins down but that will only gain me a few grams.

like I said, it'll be an experiment, I'm sure I can get it to work and once I have a decent setup I'll share my findings

Ralphbf

My Feedback: (27)

I think the solution is a little simpler than that.

Any thing less than 25cc just isn't big enough, power to weight ratio wise.

You need to pick a short nosed radial engine airplane.
They need a lot of nose weight anyway.

With them and a new low weight engines you will have to add weight anyway.

Choose the plane and find out what engine people are using and if they are adding extra weight
then pick the engine that best suits the plane.

I had a 25cc Toro in a 60 size Tiger Moth that truely was the perfect setup.

If it wasn't for a low battery and a light pole I'd still be flying it.

rangerfredbob

My Feedback: (13)

just realized I left the fender washer prop washers in that setup, I just got some hole saws today so it will have an aluminum washer when complete.

I'll see if I can get a thrust bench setup and get a few engines running and compare their thrust and weight, throw in the .46 nitro I have to get a comparison

rangerfredbob

My Feedback: (13)

I guess I just wanted to see for myself, and figured a stick type plane can generally handle whatever you throw at them, if I can get it to work it'll be super cheap to run and easy to haul around, if it doesn't work, I have a .46 nitro sitting around not being used...

the reason for choosing the poulan was because it is the lightest in it's size range that is readily available, it's lighter than an echo, if a .40 size twist can be flown on a mitsu 19cc on magneto this setup should work, if the 18cc isn't enough, I'll port up one of my 21cc jugs and size up from there, I have 18cc to 25cc jugs to fit on the same crankcase...

ok, just pulled out my magnum .46, didnt realize those things were only a pound! doesn't mean I'm not going to attempt it...

rangerfredbob

My Feedback: (13)

turns out what is pictured is a 21cc, oops. Took it apart to do some porting and measured it and it's not what I thought it was, while I was there I ported it anyway. Exhaust timing was at 135 stock and intake is at 110. While there I widened the exhaust port a little for good measure.

now to grab an actual 18cc and get it fixed up

EDIT:

pulled out a real 18cc, their intake duration is the same as the 21, but the exhaust duration is a pathetic 120*. I had thought the 18cc and the larger engines were the same jug casting, but the 18cc is built lighter, the cooling fins are smaller in some areas. I'll get a comparison in weights tomorrow.

w8ye

My Feedback: (16)

I found a Weed Eater 21 in the trash back in 1998 and fixed it up for a model engine. It was a little down on compression. It turned a 14 X 10 at 7000

av8tor1977

My Feedback: (6)

It's really hard to beat the little Echo engines, though they are not as easy to find as some others.

The last Echo 21cc I built for my brother weighed just over 2 lbs. (with EI) ready to fly, and turned a 15 x 8 prop at 9000 rpms. (I ported the engine and added a larger carb, gutted stock exhaust.)

AV8TOR

rangerfredbob

My Feedback: (13)

I just got a SRM200 echo, will tear into that here soon, maybe tonight

man, jumping from 120* to 150* duration can REALLY try your patience... the port height nearly doubled. I will see if I can get a stand made up so I can test run these things.

Scota4570

You have lots of extra material on that engine. It needs to go on a diet, like the one I attached as a photo. Even so it is to heavy. I am thinking of suing mine on a swamp buggy.

av8tor1977

My Feedback: (6)

You've got me worried about that comment.... raising the port so much. It shouldn't be that much. Each little bit you take off the top of the port raises the timing "double" because the port opens sooner AND closes later. Are you sure you are measuring the full duration from opening to closing??

AV8TOR

rangerfredbob

My Feedback: (13)

well, maybe it seemed like doubling it's size, probably 1.25 to 1.5 times but it started out TINY, yes I did go from open to close. it didn't even go to the piston top at BDC. It probably just seemed further because I was using a chainsaw file (to go slow and more accurately) instead of the dremel... and I checked my progress like 4 times...

rangerfredbob

My Feedback: (13)

got the plane, and decided to go EI on this thing... engine with MA prop and ignition (with battery) came to 2lb 6oz with a little more aluminum to shed if I feel like it, should be easier to balance.

winter project...

w8ye

My Feedback: (16)

Your weight is not too bad

rangerfredbob

My Feedback: (13)

got the ignition timed and finished mounting everything to it, will try to get pics soon, looks like it was supposed to be built like this, I broke off the extra metal and didn't clean up all of the edges, but close enough...

also ended up with a big trainer type plane with one of these poulans on it for cheap, figure it will be a nice lazy flier and trainer for teaching others to fly (a lot cheaper to fly than the nitro it will replace), I'll try to get it up setup how it is, but will probably end up moding it, I think it has a 21 on it right now, I have a couple 24's on hand...

rangerfredbob

My Feedback: (13)

finally got that trainer thing home, it's got a few things done WRONG for a gasser, first it has a wire throttle linkage, second the kill switch was 1/4" from the throttle servo, third they used one of those sliding adjustable length servo horns that HiTec sends with their servos for the throttle and they put the outer part on upside down so a smooth surface was next to the rippled side, and I haven't spent too much time looking into it. it looks like it has a 21cc engine on it (will measure to be sure), which would be a dog on that 12lb plane... I might try to fly it stock and see what it does, it's got about 1100 square inches of wing area, I'll have to find some of my bigger versions of this thing...

I just moved starting last weekend so things are still getting organized, although I'm organizing better than I was before... once I get my computer at my new place I'll get pics of both planes.

AA5BY

Hope it works out well and it likely will. My only experience with a conversion was on a heavy plane for a conversion at around 12.3 pounds, a Phaeton 90. It flew scale like and was a delight on take offs and landings. The engine was 100 percent reliable, can't recall it ever hiccup except at the end when the ignition unit failed and it wouldn't start, but it gracefully did that in its sleep so posed no threat to the plane. It would pull a loop if some speed was gained going down hill and the loop not too big. It was a pleasure to fly although the aerobatic potentials only came alive after replacing with a Zenoah G-20 after the ignition failed.

Good luck with the project.

rangerfredbob

My Feedback: (13)

my computer fried, going tomorrow to try to get replacement parts for the burned hard drives to try to salvage stuff off of them... will be getting a new computer in the next month sometime...

ok, just need to get the trainer type plane programmed in a radio now and get a battery mounted in it, still unpacking so I haven't found all of my parts yet, but I've done most of the reworking on this one. It turned out to be a 19cc engine on it, that got pulled in favor of a ported 21 I had already converted (have 5 spare 18-21cc of these things already trimmed down, a couple have prop hubs and all need modified mufflers), the engine on it had the newer style spring attached muffler that's heavy to start with then they soldered a 3/8" copper tube to the back of it for an outlet, that muffler wasn't light... so I opted for the old style that is held together with the springs without the baffles, welded the stock outlet closed then welded two 1/2" tubes to the outside, looks kinda cool if I do say so myself... throttle rod is now goldenrod. need to stiffen up the tail wheel spring then it's ready mechanically.

haven't gotten any further on the stick yet, engine is in place but still have yet to add the wing extension in the middle.

rangerfredbob

My Feedback: (13)

I'm tired of unfinished projects so I'm working on the half started things I have around (endless supply, but it's a start), I've been getting a lot done in the last couple weeks.

anyway, dusted the stick I got off and am reworking my ideas. I'll try to get some pics soon of where I'm starting and what I'm planning. Being a 40 size stick it's kinda small but it should fit my needs fairly well. The wing was supposed to be attached with rubber bands but I'm moding it to attach like my USL 120 with two dowels and two bolts, also adding about 3" to the center of the wing along with removing the dihedral. It came with wire landing gear but it's not sufficient for the engine, have some aluminum gear to try to scab on.

might pull out that big trainer and see why it wouldn't run the last time I tried it.

rangerfredbob

My Feedback: (13)

Plane is mostly assembled and modified, I don't have all servos needed yet, haven't installed the control linkages yet either or the switches. It's at 5lb 5oz with engine, ignition, both batteries, tail wheel, receiver and fuel tank so it'll probably gain another couple ounces with extensions, switches, two more servos and control linkages.

635 square inches wing area, 5.5 pounds, assuming the engine puts out about the power of a .46 (if it doesn't, a 21cc cylinder that's ported should do the trick without changing the weight) it should fly with the power of a trainer, shouldn't be too horrible.

rangerfredbob

My Feedback: (13)

There she is, the LHS was out of white monocote so I haven't been able to cover the center of the wing or tail supports yet

The throttle linkage should be interesting since the engine is wider than the airframe but I have an idea...

gr8flyer55

I'm trying almost the same thing with a Big Stik 60 kit.. Firewall is 3 3/4" wide and maybe will use a Shindaiwa 21cc...belcrank for throttle linkage already on the carb.

John

rangerfredbob

My Feedback: (13)

Worked on this thing a little more, once EVERYTHING was installed it's at 5lb 13oz (using a different scale than last time though) and balances perfectly at the spar with no extra weight.

Now I just need a bigger memory card for my transmitter so I can put all of my planes in it...

Part of the point of this is to get a gasser that I can fit in the back of my tiny little Subaru Justy and go flying... we'll see if it works