Great Planes J3 Cub


Legendary member
Well, I've been totally backlogged with mostly non-RC related crap to do for a while and so haven't had much time for this until now. Took the time today to pull the door back off - I both don't like the Robart hinges and the now-broken pile of crap known as the door latch. I don't know why exactly I believed it would work. So, I removed all of it.

Here's the new latch. A much nicer brass piece with a linkage retainer so I can easily adjust the deadbolt clearance. The eyelet is where the rubber band ties off.


Installed with JB weld. Just gotta wait for it to cure tomorrow. The cut down nyrod is just to make sure the spacing remains correct. I'm gonna cut down the wire and dremel most of it off anyway.


I also did some work on the skylight, painting the edges of it to match the windshield and securing it in place with canopy glue. I wanted to use screws to install it but sadly I had none narrow enough to drill into the sides of the plastic without causing the edges to crack, because the overlap is so narrow.



Active member
After seeing your post directly above mine when checking new posts I had to read some more, hey @speedbirdted I like iike this thread even more and I will study your build compared to mine.



Legendary member
Door latch, take 2. I like this a lot more. It's much smoother and a lot less, just, janky I guess. The adjustable deadbolt is real nice too.

I have yet to hinge it, and I might also paint over the little bits of Robart hinge still in there. As for hinging now, I think I'll use polypropylene hinges (if I still have any) since they'd look much cleaner.


And here's everything back in place. I went in and reinstalled the servos, and most of the electronics though I didn't bother taking pictures of that cause it's pretty boring. Minus installing the door hinges, it's now on hold until I get my glass cloth. It's good to fly now actually, and I might fly it again. I cut the old cowling a bit so the engine and exhaust would fit, for now. Though, there's no longer really a good place to put the nose weight because I previously had it in the bottom of the cowl and now the engine is occupying that space. So, I'll have to think about that a bit...

I think I'll also add a little magnet or a clip or something in the wing to hold the window open, and something similar for the door. The door is just held on here by some tape, but this looks about right if it opens like that when the hinges are installed.

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Legendary member
A little something more, not physically about the plane:


Not sure if I mentioned the tires, or how the ones I currently have on it are rock hard and useless. Not sure why either - apparently they work well according to other people... guess I must have gotten a bad batch. So, why not 3d print some stuff? I measured the rim and designed a tire accordingly. I'll have to do some modifications to this model to be able to print it - I think what I'm going to do is print them in two halves then either glue them together - or, because I'm thinking of using TPU so that they can actually have some shock absorption capability, coat them in some sort of rubberized stuff so that they're durable but still flexible and just use that to hold the halves together. Glue might cause stiff sections in the tires and make them deform weirdly. I think I can either adjust the infill if I use the right pattern or just stuff some foam in it to control the hardness of the tire.
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Master member
Door latch, take 2. I like this a lot more. It's much smoother and a lot less, just, janky I guess. The adjustable deadbolt is real nice too.
Now I see how the latch works. I never figured out the previous wire contraption from the photo. Doors and windows are hard. 👍
I'm thinking of using TPU so that they can actually have some shock absorption capability, coat them in some sort of rubberized stuff so that they're durable but still flexible and just use that to hold the halves together.
Plasti Dip? It comes in black.
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Legendary member
So, after the propeller attempted to delete my finger today for the millionth time after trying to pull the glow igniter off, I caved and bought an onboard glow driver. I was hoping I could get away without needing one, though to make the glow driver easier to put on, I would have to remove a considerable section of the firewall. There's also no way to move the propeller...


So, you think this would be simple, right? Well, no. The answer to that question is always no. For some weird reason, I could not get the driver to ground properly with the ground attached to any part of the engine crankcase or head, other than the plug itself. Normally, aluminum is conductive to a limited degree, but I think this engine is coated with something that prevents any conduction from occurring. (Every Saito I have does it fine - maybe they're coated with something else) So I had to get creative and the solution I came up with was:


Yeah, I just snipped the little eyelet off and soldered the ground wire onto the plug washer. It is copper after all. There's enough room in the plug cutout for the wire to fit, and I doused a little water on the plug and spun it over, and saw no bubbles, so it must still seal well enough. Now, we get to see if the tin-lead solder I used will melt down from the engine heat. I'm betting no, but I guess if it does I can just redo it with silver solder.

I set up the driver to a switch in my radio and also set up a second throttle curve with the same switch, so that when the glow heat is engaged the engine cannot rev past idle. I did that to prevent me accidentally leaving the heat on and killing the battery. Half the innovation I do is to protect me from my own stupidity :ROFLMAO:

I was skeptical to whether this was actually going to work at first, so I stole the cylinder head from a random engine and installed the plug in it. Victory!


Joker 53150

Mmmmmmm, balsa.
Nicely done. When I was trying my hand at glow engines I picked up the onboard glow drivers like you're using, but never got around to actually using them before I decided to just stick with gas and electric. The plan I had was to have the glow driver activate anytime below about 20% throttle, which (in theory?) would help keep the engine running when the plane is coming in for a landing with low throttle.


Legendary member
Nicely done. When I was trying my hand at glow engines I picked up the onboard glow drivers like you're using, but never got around to actually using them before I decided to just stick with gas and electric. The plan I had was to have the glow driver activate anytime below about 20% throttle, which (in theory?) would help keep the engine running when the plane is coming in for a landing with low throttle.
As much as I absolutely hate having to apply glow heat to keep an engine going at idle I think it might be what I need to do here. After playing with the LSN a little more today, I came to the conclusion that this engine really hates being inverted. The lowest I could get it to consistently idle with no heat was 3500 rpm whereas when it was side mounted and running upright on the stand it would put along at 2600 rpm all day perfectly happily. If that ends up not being a low enough idle to land at then I might have to add heat at idle. Don't know yet as I haven't flown the plane, and can't again until I get the new cowl done and install the weights in it. I ordered the fiberglass stuff a month ago, what the hell is Sig up to?

Strange as none of my other 4 strokes do this, at least not this badly. They tend to be pretty unaffected regarding orientation. It tends to be only 2 strokes that have significant problems with fuel pooling at idle when inverted. I'm thinking maybe I'll try running the engine with the plane flipped upside down, to see if the engine runs better. If it doesn't it might actually be some sort of fuel system problem.
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Legendary member
Weather was nice and the wind was dead today so I decided to try and put a flight or two on the plane now that I've got the engine bugs worked out. Or, I thought I did. The reason you're not seeing any pictures of flying airplanes here is because I don't. I got it to idle somewhat consistently at about 3500 without it loading up too bad, however unfortunately this is not low enough for the plane to not pull itself around on the ground while idling. I have found from flying this plane previously, if it pulls itself at idle, it won't be able to land with power on. Even after consulting some of the other people at the field with vastly more engine experience than me and letting them have a go at trying to tune it, nothing anyone did managed to get it to idle at lower than 3500, at least stably. The engine often loaded up and died at less than this, even with the heat on in some cases. Flying the plane would have been a terrible idea.

So, this leaves me with two ideas. Either this engine is mechanically incapable of maintaining a correct idle mixture at an acceptable idle speed while inverted, or I have a fuel system problem. I'm really, really, really hoping it's the second one, because what I really don't want to do is either revert the engine to its original side-mounted configuration, or just get a better engine. I have a Saito 56 on hand which I've test ran inverted and idles much better, though I don't want to mount it because it's taller than the current engine, by enough that mounting it inverted would cause not just the rocker covers but the entire cylinder head to stick out the bottom of the cowling. I already don't really like the rocker cover of the current engine sticking out the bottom but it's the best I could do.

If I do end up going the most drastic route and getting another engine, perhaps I'll get a twin? It would negate the need to build dummy engines and also just be really kick-ass in general. Though there are some issues with this idea. The only twin small enough to fit on this airplane that I can think of is a Saito 60T, which weighs nearly twice as much as the engine currently on it, and also has less power. Right now it's not really underpowered but subtracting power is not something I would want to do. They're also out of production, incredibly hard to find, and when you do find them, you'll have to sell some organs on the black market to have enough money to buy one. But hey, at least I won't need nose weight! Maybe a 90TS might work too, and it would certainly have enough power, though I think that might be a little too heavy.

As for fuel system issues all I can really think of is the tank height may be a little off. It's not leaning out, so it doesn't have a pressure leak, and there was no gunk in the carburetor when I took it off to clean it, just to be safe. I have the tank bung about 1/4 inch below the carburetor, which in theory should be enough to prevent fuel being siphoned into the carb and flooding it out. I sure hope that's enough - I've only really got enough clearance to lower the tank maybe another 1/4 inch, and if it has to go any lower I'll need to start cutting things up.

Anyways the engine came off the airplane and got mounted back on the test stand. Tomorrow I'll see whether this engine actually can idle inverted, with a fuel system I know works. I'll get it set right running upright then turn it over and make low speed adjustments as needed, and add spacers under the tank to see if lowering it has any effect. I'm fairly sure the muffler's effect on performance at any speed is negligible because it has no baffles and it isn't tuned, but it won't hurt anything so I reinstalled it.

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Legendary member
Test ran the engine today. Luckily, my hunch that the fuel system was causing the poor behavior at idle appears to be true - here the engine is clearly inverted and I managed to shave the idle speed down to about 2900-3000 rpm, while still maintaining a good transition, which I'm satisfied with. I also ran it with the jug pointed upwards just to see how much better it is, and in that configuration 2500 rpm was no problem and I still had the low speed set rather poorly :ROFLMAO: The low speed is incredibly sensitive at idle inverted, both to manual adjustment and depending on the level of fuel in the tank. You can see here the fuel level is very low - the engine was pretty happy here, even with no spacers under the tank. However, with the tank full the engine richened up and died every time at idle regardless of the LSN setting. I put one 1/2 inch thick spacer above the tank and that seemed to make it much happier. The idle speed figure I stated above was obtained using this setup and about a 1/2 tank of fuel.

The reason I think this is happening is because the fuel must always travel downwards relative to the tank to get to the combustion chamber with the engine inverted, which will produce a siphoning effect when the engine is running regardless of the tank position, unless the entire tank is below the cylinder head which is clearly impractical. This issue almost never happens when the engine is pointed the other way, because then the fuel is travelling upwards, negating the siphon.

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I'm still dissatisfied with these results for a few reasons, however the most severe one is the way the tank sits here cannot be replicated in the airplane without having to cut things up. The fuel tank floor in the airplane is actually a structural piece and so I'm not at all willing to cut it up to lower the tank. Also, an engine unable to idle when the tank is filled all the way? That's just pathetic. Well, back to the drawing board.

So, now that I know that nothing is mechanically stopping this engine from idling inverted except the inlet manifold, I have a few more ideas. One, I could turn the manifold itself upside down and lower the carb below the cylinder head, but this has the problem of the carb sticking out the bottom of the airplane and making the engine susceptible to inhaling all kinds of crap from the ground, plus the carb could be physically damaged in a nose-over. There's also no way to really secure it there; I would likely have to machine some parts.

I think what I'm going to do is get a fuel regulator. Cline made a great example which I've used on a couple engines to great effect, however unfortunately it appears Cline is either outright no longer in business or no longer manufactures regulators. I have another idea though - Walbro carburetors by nature have built-in fuel regulators, because they're almost entirely meant to be used with gasoline (gasoline having a much narrower stoichiometric range in which it'll burn correctly, as opposed to methanol) and I wondered if perhaps I could use a Walbro regulator. Apparently some digging around revealed people have done this, though I'm not sure how parts meant to be used with gasoline will tolerate being exposed to methanol. If it's anything like fuel tubing, methanol will destroy gasoline fuel tubing and vice versa, so that obviously won't work if it's the case. It's worth a shot anyhow. So far my only experiences with Walbro equipment is repairing an old weed eater engine, so it's mostly uncharted territory for me.


Legendary member
Nevermind what I said earlier: I just found a guy on eBay who has a few NOS Cline regulators for sale. I bought a couple for 30 a pop. They used to sell them for $70 new, what a steal! They aren't shipping from too far away so that shouldn't stall this project too bad.
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Legendary member
A little progress on the engine and regulator, though nothing substantial. Firstly, I think this as good as this engine can do inverted. I've given it every bit of help I can here - a regulator, a heavy APC propeller, and even a Saito P2 plug, as I think they hold heat a little better than an OS F. The LSN is still incredibly sensitive, and it still seems like the sweet spot normally present when upright has been outright deleted.

It's idling here, low enough to land (probably) but as you can see in the video it picks up from about 2700 rpm to over 3000 and then plateus, which is a sign that it's too lean and will die soon. Which it did almost as soon as I cut the video. Further running without changing the LSN setting revealed that it would not transition, either smoothly or at all, unless the throttle servo speed was slowed way down. I consider that a failure. Keep in mind that this is with the LSN as rich as it will go while still actually running without loading up and dying, which it did when adjusted even 1/100 of a turn richer. In short, to a lesser extent it's the same behavior the engine was exhibiting when I was running it inverted with no regulator on it. However, now the tank placement no longer matters, and it behaves the same no matter the height of the tank or the fuel level, which is at least an improvement, if not the one I wanted.

I think this engine has something fundamentally wrong with it that doesn't let it run inverted. It was suggested to me last time I was out flying with folks that the engine may still not be fully broken in yet, however I probably put about half a gallon of fuel through it both on the stand and flying when it was side mounted, so it's probably loose enough now. That was my last idea as to why it doesn't work inverted. I'll admit defeat at this point...

This is what the regulator installation looks like. I mounted it directly behind the engine, on a plywood brace, with a 1/16 inch thick section of fuel tubing on the screws in between the engine mount and the regulator mount. You always want to soft mount something like this, as these things have very delicate little components in them that will easily become screwed up by engine vibration. The plumbing setup which before was rather straightforward has now become a disgusting mess - but, due to the tank pressure system, both lines to the tank now need a T-joint and a corresponding plug for fuelling/defuelling.


So this leaves me with the question of what do I do now? Obviously, if I want to build realistic dummy engines, keeping the current engine is not going to work. Additionally, I am not sure how many 4st glow engines will reliably function inverted at idle with no glow heat applied, while also fitting in the cowling. I know from experience that a Saito 56 will do it marginally better, however that comes with the problem of the entire cylinder head sticking out of the bottom of the cowling, which I don't want to do. From measuring someone else's engine, a Saito 45S will fit, however the engine on it right now is already edging on underpowered, and an even smaller engine won't help this. I believe the 45 and 45S were also never sold in the US and so finding one of them is incredibly hard. I did think for a second that perhaps I could use a YS FZ53 since they both have a pump and regulator system built in, though I don't know if that would work either since I already tried a regulator system on the current engine with little luck. Also, this engine will not fit either, as it's taller.

Should I give up on inverted engines I have some more ideas. One, the obviously most expensive route, is buy a twin. This has the advantage of instantly making the airplane 100 times cooler plus negating any need for nose weight. They also sound really cool. My ideas for which engines here were the Saito 60T and 90TS as stated earlier, and I think I'm gonna go with the 90TS because some consulting of the user manuals revealed to me that the 90TS actually weighs less than the 60T despite being 33% more displacement, and as such appreciably more powerful. Speaking of power, the 60T would be woefully underpowered, so much so that it's not even worth discussing, while the 90TS would have a little more power than the current engine, though surprisingly not a huge amount more. I get about 4.5 lb of thrust out of the current engine, on a 12-6 at 9500 rpm, which is fine-ish for a 7 lb plane as long as you never try anything stupid. I lack personal running experience with any Saito twins but I know a guy that has a few and he said that his 90TS will spin a 14-6 at about 8000 rpm, which according to some crappy math should make about 6 lb of thrust, give or take a few ounces depending on the propeller used. I'm not sure about running a 14 inch prop on this plane, though I'll test fit it on the current engine once I get home to see if the clearance is okay. A 13-8 would probably be enough load for the engine, but that might be a little steep.

Another idea I have is instead of buying an actual twin, just build a fake one. This would be a hell of a lot cheaper as well as negate the need to buy another engine. I figure if a twin is an acceptable appearance while not being totally scale, and a side mounted single is basically just half of a twin, just build a fake cylinder to the exact same dimensions and appearance of the actual engine, and then just mount it on the other side. It would be a lot less cool than an actual twin, and getting a convincing metal finish on balsa especially for an engine is quite tricky, but it could probably be done. Though, I've been given an excuse to buy an actual twin cylinder now, and I believe not using it is a crime against humanity or something :ROFLMAO:
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Legendary member
Well, I didn't know the postal service was capable of operating so slowly. But hey, the glass stuff is here now...


Almost immediately a problem presented itself. I thought initially I would make the plug out of 4" thick XPS foam board, so I made a little half sphere out of it to test whether the foam would react to the resin and also whether the cloth was light enough to stretch around a compound curve. Normally I buy the .56 or .75 oz stuff but that was all out of stock when I bought it (It isn't now, but I bought it 2 months ago :rolleyes:) In conclusion, I can't use foam. The resin has a similar effect on it to CA, which is dissolving it into a useless sludge. I didn't get to test the stretching capability. I will make another test plug out of balsa tomorrow...

I spent a good chunk of time measuring out the firewall of the plane trying to figure out what dimensions to build the plug to, but then I remembered I literally have the exact plans the cowl is built to. So I just cut out all three views of it and measured some pieces based off that. I didn't have a big enough slab of balsa to make a solid plug out of but I did have some sheets of 1/2 inch balsa that I got for another project years ago. When I got them, they were way too heavy so they just sat for a while. Weight doesn't matter here so they were perfect to use, it'll just make sanding it down later a little annoying.

Here are the side and top pieces cut out. The bottom piece will not be cut to shape, I will just glue it on and cut it to shape there.


Gluing it up. I wanted to use Super Phatic glue on it because it sands really well, but I have almost none left and didn't want to use the rest of it. The sheets are a little warped, so making sure they were at 90 degrees wasn't entirely effective, so when I glued the top piece on I cut out another piece to wedge in between the two sides to hold it straight.


After a little bit of sanding. After this I glued a block onto the front, to make the nose cone thingy out of.


Tomorrow I'm gonna sand it to its final shape, outdoors. I'm pretty sure if I did this inside the amount of sawdust generated would guarantee my slow death.

Not sure though if I'll do the fiberglass layup with a mold or not. I might make one if I have any plaster lying around or can be bothered to buy some more. Usually I prefer vacuum bagging when doing glass layups though my vacuum pump broke due to my own stupidity so I guess I'll either just use a brush or make a mold though both of these are considerably more effort...
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Legendary member
Major sanding on the cowling plug today. As I said before I moved the work outside, not just cause it was a really nice day (for some reason Indiana in August is cookin in the 90s and it goes away as soon as September comes around as it was 74 degrees today) but because the amount of sawdust generated would be ridiculous. I was going to try and use saws much more to carve a lot of balsa away instead of just sanding it, but I've never been real good at using them for whatever reason.

My technique for creating large objects out of balsa while still maintaining symmetry is to cut large flat sections away at once. It's much easier to take away the same amount of material on each side if you are doing it on a polygon one side at a time. I ended up not using the templates all that much to create the rough shape as I remembered I was actually going to shape the cowling a little differently. At this stage, this had been done mostly with the saw, and the faces had been finished with 60 grit paper as outstanding smoothness is not needed here.


Gradually adding curvature, in the form of more flat faces...


The curvature started at the bottom and would need to be more or less maintained all the way around the cowling; so I increased the grain to 100 and used an electric sander at this stage which if done carefully actually helps immensely. I also began to narrow the front underside of the cowling at this stage; some Cubs have a cowling that's shaped like that, and I like them much more.


Here it is at probably 70% done. I've actually used no paper higher than 100 grit; if you're just light with it it actually smooths out pretty nicely. As you can see the top curvature is still a little rough looking. I have a few spots that need filler in that area - mainly the seams between the 1/2 inch sheets and the block - so I might as well just complete the sanding there when the filler dries and is ready to be sanded off.

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Filler added. The stuff is cheap so there's no need to be conservative using it. And now we wait...