Sig 1/4 Scale Clipped Wing Cub, Bringing it Back to Life!

[Joker 53150]

Ok, time for a new plan. Originally the goal was to burn a gallon of gas before mounting the engine in the Cub, but at that rate it'll be Christmas before that can happen! It took at least 20 minutes of running just to go through maybe 3 ounces of gas with the test stand set up in the garage. The new plan is to get the fuselage ready and mount the engine once it's good to go. With the other Cub, the engine ran smoother and with less overall vibration once mounted in the plane. I assume it's because the airframe just doesn't amplify the vibration of a gas motor like a metal sawhorse and plywood. Hopefully that's the case with this engine as well.

For the break-in procedure, I'm running the engine for around 7 minutes at a time, varying the throttle from idle up to about 5,000 RPM for a few seconds, then back to idle. Throttle response has been very crisp so far and it holds a steady speed. After the short run time I let it cool down for around 10 minutes and then repeat the process. As it gets more run-time the break-in will include some wide-open-throttle runs. There are tons of different ways to break the engine in, but this method makes sense to me.

 

[Joker 53150]

It was a pain to install, but the 26cc is mounted. The top bolts were the worst, since they mount into a closed-off cavity. I had to create an access hole and fish the bolts through with very thin wire, while the bottom bolts were easily accessed through the interior of the fuselage. The stand-offs give me 1" total clearance between the firewall and carb intake, which I've read is the minimum for good airflow. Unfortunately, that puts the prop about 1/2 to 3/4" farther forward than I'd like and the prop sticks way out in front of the cowl. There are a few different approaches to fix this problem...

1. I could just mount the cowl in the stock location, which would put the prop far forward, and also may put the cylinder head in contact with the cowl. Test fits just show me it looks funny like this.
2. I could try recessing the motor about 1/2" or so which would have it at the right spot for the stock cowl mounting. This would require completely re-building the entire firewall, which is more surgery than I want to do on the Cub.
3. I could split the difference by moving the cowl further forward than normal. This means the cowl would need some surgery to narrow it by about 1/8", and would also leave a gap along the bottom edge.

The plan right now is Option #3. If I don't point out the slightly longer nose people will probably never notice it. Narrowing a plastic cowl while it's in two halves won't be difficult, and the gap along the bottom will promote airflow. In the other Cub I cut this area open to make sure air could escape the cowl, and now it's done for me.

If this bird flies well and eventually gets to the point where I want to re-cover it, I may then think about recessing the engine a bit for a proper appearance. For now, I guess I'm taking the easy way out.

 

[Joker 53150]

Looks like I now have Option #4 as my main choice... The stand-offs can be replaced with shorter pieces, or these may be able to be shortened as well. That would put the carb too close to the firewall, but I didn't think about simply cutting a hole in the firewall or creating a recessed area to help supply air to the carb. A little digging tells me this is fairly common, so now the research will start so I can figure out how best to accomplish this. The original thought was recessing the entire engine assembly with stock stand-offs, which may have been overkill.

 

[Joker 53150]

The original standoffs that came with the Turnigy engine are 70mm long. Test fitting a cowl with these standoffs shows me that they are 25mm too long. Horizon sells 45mm stand offs which look like they'll work fine, so the order was placed. The engine has tons of room for the standoof, and the firewall should also give me more than enough room. Well, once I cut a hole in it for the carb that is...!

 

[Joker 53150]

The new 15mm shorter stand-offs for the engine arrived today, and it looks like they'll work perfectly! I'm able to use the original mounting holes for the cowl, which is nice. However, I'm still planning to replace the cowl with a new one ASAP since the current one has a little hole in the right side from a former nitro motor. But for now, it's good enough and confirms everything will fit. A hole will probably be needed for the spark plug cap, which is no big deal.

To finish off mounting the engine I'll need to hit the hardware store to pick up a few more bolts. The new stand-offs only come with 4 bolts total when 8 are needed (4 for the firewall and 4 for the engine). Doesn't matter, I'm just happy it all fits well!

 

[BobK]

Wow those standoffs did put it right in the sweet spot. Looks good!

 

[Joker 53150]

Thanks. It looks like the cowl could even be pushed back a little further if needed, but either way it did the trick. The firewall still needs to be carved out a bit to make sure airflow isn't an issue. By this weekend it will hopefully be ready to fire up the engine to see how it all works.

 

[willsonman]

If its jut that little hole... put some tape on the BACK side, apply some 5-minute epoxy on the outside and apply some saran wrap over it to smooth out the blob. Hole=fixed. You can then use some sandpaper to blend the seam if you need to prior to painting.

 

[Joker 53150]

If its jut that little hole... put some tape on the BACK side, apply some 5-minute epoxy on the outside and apply some saran wrap over it to smooth out the blob. Hole=fixed. You can then use some sandpaper to blend the seam if you need to prior to painting.

The LITTLE holes would be no problem, it's the BIG hole above them which is the issue! I could probably patch it without much difficulty, but the time & effort to do so would best be spent elsewhere. New cowls aren't too expensive and it'll allow me to start with a completely clean slate.

 

[The-One-Who-Never-Crashes]

Won't get to make fest this year Just no budget for a trip this summer since we really need to replace the car ASAP. Really wish we could, but every other year is the most we can really even hope to pull off. But we have a hard enough time pulling off Ohio visits to see family every other year so flitefest may end up like the olympics for us - something that happens every 4 years (every other family visit.)

I'm still hoping you try the medicine man glider I sent you, only seen 2 or 3 of them built and would love to see another

Why don't you host a "jhitesma Flite Fest" in your backyard? I'd love to come!

 

[Joker 53150]

With all four stand-offs in place it looks like everything should work well. I've opened up the firewall hole a bit to make sure airflow isn't impeded, and now need to figure out the routing for throttle and choke pushrods. The choke pushrod might be done with a manual lever, but if I can't route that easily enough I'll just use a servo as planned. The downside in that is that it just adds to the complexity of controls inside the fuselage.

So the gas tank needs to go in first, followed by the electronic ignition and spark plug wire. Then the throttle (and choke?) pushrod over the top of those. Hopefully I won't need to remove the EI or gas tank any time soon, since they'll be buried inside the fuselage pretty well.

 

[jhitesma]

Why don't you host a "jhitesma Flite Fest" in your backyard? I'd love to come!

Well maybe in my "backyard" but my actual backyard we'd all have to fly micro planes/multis and pray for no wind - and hope no more than about 10 people show up

I'm actually going to be heading up towards you later this month for the David Gilmour concert at the Forum, but it's going to be a quick one day trip with no time to even hit any decent hobby stores let alone get together to fly with anyone

 

[Joker 53150]

It's been slow work, but the guts of this plane are slowly coming together. Servos are mounted, pushrods installed, and programming done. The electronic ignition module and optical kill switch need to go in and then it is about done. Weather should keep me out of the sky tomorrow so I may be able to finish it up so ground tests and further engine break-in can continue!

 

[Joker 53150]

Here's a good reason to really check over work done by other people (and yourself). The Turnigy engine is mounted to the fuselage and I'm running it to test out the throttle and choke linkages. After about 1/4 tank of gas I noticed the braces for the vertical stabilizer had come loose due to a nut falling off the screw. After finishing up the tests I took it back to the workbench and added LockTite to all of the screws/nuts to make sure they wouldn't fall off in flight, because that could get ugly fast!




The engine runs well and has a pretty crisp throttle response. I had some problems getting it to stay running today, with temps around 33 degrees, so I richened up the low-side needle about 1/16th of a turn and it settled down nicely. Below is a link to the engine testing. Some weight also needed to be added to the nose to get the balance right. I ended up adding about 5 ounces of lead which is a LOT more than I expected. Either the tail on the plane was built very heavy or the engine is light, or both. Judging by how the other Cub flies I don't expect this extra weight to be much of an issue, even with the shorter wing. Depending on the weather, I may take this Cub out to the field tomorrow for a maiden. Hopefully!

 

[earthsciteach]

Sounds nice! Good advice, there. You could have checked the nut, it might have seen tight at the time, but could have quickly unthreaded itself during the vibrations in flight. Thread lock everything!

 

[Joker 53150]

Absolutely! I checked them all and thought they were already good with LockTite, but that one must have only been tight. The other braces all might have had some on them, but it was old and they came apart with a little effort. No more trusting for me!

 

[Joker 53150]

Success!

I loaded up the Clipped Cub and headed north to the field by my dad's place. Before flying we did a little unscheduled field maintenance, pulling and resetting about 25 fence stakes, rehanging 100' of fence, and replacing another 50'. The fencing is basically plastic snow fencing used to protect the pit area from the runway. It was falling apart and the freeze-thaw cycle had the stakes all jacked up. We wanted to do something nice for the club so we did it prior to the Spring club work day.

Then, on to the maiden! I ran the Cub up and down the runway a few times until I was sure it was running well. A quick check of the control surfaces, and UP it went! A little trimming was needed, but it handled well and the weight I added to the nose appears to have been correct for CoG. After about a minute the engine quit. Panic time, but I was downwind and had altitude so I scrubbed speed and altitude and cruised in for a safe landing.

Tweaking the carb needles I think it's running better so up it goes again. This time for about 7 minutes before the engine quits once again! Same deal, scrub altitude and land, this time just past the runway in a mowed field, nice and safe.

More carb tweaking follows, and the next two flights were uneventful. Flight 5 I cut short as the motor just didn't seem to be running well and I didn't want to risk it. The landing was fantastic, and I then drained the tank and cleaned it up.

So it flies nicely and is very aerobatic feeling compared to the full wing Cub, and I can't wait to get the engine running well so I can really wring it out!

Pictures to follow.

 

[Joker 53150]

First up, the new fencing. It isn't pretty, but it works. Dad did the extra effort and brought his lawnmower along to clean up the area along the bottom.




Fired up and ready for action. This work bench has the tallest restraining posts at the field and it's just barely tall enough to handle the Cubs. A small project we're doing next is extending and padding the posts about 8" taller so I can more safely test the engine.




Good roll-out leading up to the maiden flight. No cowl was installed yet, as I needed easy access to the carb needles. Plus I haven't taken delivery yet on the cowl ordered for this Cub. The cowl that came with it would need a lot of cutting to fit and I don't feel like doing it twice.






A safe landing. Note that the vertical stabilizer bracing came loose, just like it did when I was testing the engine yesterday. The LockTite didn't work - I assume the screw was grimy? I'm now going to do what should have been done yesterday - replace it with new hardware and LockTite.

 

[HilldaFlyer]

I've been following... Well done.
Bring it to FF2016!

 

[Joker 53150]

I'd love to bring one of the big Cubs to FTFF, but it's an electric-only event and would take up way too much room in the truck for a plane I can't fly there.