IS-4 Jastrzab

[stevekremer]

Looking amazing Michael. I am impressed!

[mlachow]

Thanks. Weather hasn't been the best for outdoors so time to build. Construction is pretty typical of oldgliders builds. Similar stuff for my Orlik, and Bocian. The Pirat had similar surface construction but it has a fiberglass fuselage. They are a bit more work than laser cut stuff.

Lots of page views.

[mlachow]

Progress being made on the second wing panel. Webs in place ready for spar wrap before getting sheeted.

[mlachow]

The second wing panel build is mostly done now. Time to look at figuring out how to complete the wing mount on the fuselage. This one is an interesting one to align. Gull wing and different sweeps in the panels. Not too much in the way of straight lines so you have to pick your alignment locations.

One nice part is the wing is a 3024 / 3024 airfoil. So that is virtually a flat bottom. So it is easy to clip some carbon rods to the wing panels in a few places.


The root ribs match up to the wing ribs. There is one nice hole through there for the wing wires. I can get my smallest C-clamps through the hole to clamp the wing root rib to the fuselage root rib. After that I should be able to put in some shims around the support rib in the fuselage to lock the joiner box in place before gluing.. It is going to be a bunch of checking and moving things. I also need to put on the final pieces of fuselage sheeting near the tail to lock the stab and fin into alignment.



The joiner rod is straight. I took care in the wing panel builds to get the dihedral angle and to keep the joiner rod square with the root ribs. The fuselage bulkheads and support ribs all lock together and that were kept square when it was built up initially.

So far I like what I see. Things look pretty good visually. I will make some spacers for root for when it gets glued up. I don't want to glue the wing to the fuselage. The joiner box will get glued to the support ribs first. Gluing to the outside rib will be done after removing the wings and joiner.

[mlachow]

Plenty of checking and double checking and now the box in the fuselage is glued in.

https://forum.scalesoaring.com/forum/tugs-and-stuff/building-tips-techniques/43060-aircraft-alignment-when-building?p=43143#post43143

When I built the wings, the joiner box at the root rib was at the top, rear of the oversized rib holes. I tried shimming the boxes in the fuselage the same way and found alignment was pretty close. Dihedral was right on. The wing was skewed less than a half inch so the front shim was sanded just a little to get the wing straight.

The 3d laser level dropped from the ceiling on a metal angle bracket worked fine. I used a metal bracket since tge fine adjuster bracket with the level has a metal mount. The wireless buttons to turn beams off is a must have. You dont have to touch the level and you can turn off the vertical beams when walking around the model. The horizontal beam is above head height. The other useful thing is marking at least two reference points on each wing panel.

I am still using multiple carbon rods attached to the wing to sight for wing angle matching between the two wing panels. The only glue up remaining is the brass alignment pin tube in the fuselage.

Fin is really close to vertical. I will just have to measure when I glue the fuselage sheeting from the stab back.

[mlachow]

Joiner boxes and pin are now all glued in. Epoxy with milled glass added for additional fillet. The wing joiner was actually right against the fuse wood parts on two sides with shims on the other two sides. So without epoxy they were in place. The extra epoxy probably wasn't necessary. Still it's a fuselage I would worry about hitting a tip. Top sheeting here will be a place to use the one c-grain sheet of balsa I have from the pack. Near the front of the wing, there is some level sheeting to the first curved ply strip. Starting there and over the rest of them, sheeting will follow the rounded shape of the fuselage.

Lots of balsa fitting....


Stab platform area is a real joy to sheet. I added an extra piece of balsa half way up for the sheeting to attach to. Then more solid balsa that can be carved will be glued above that at the front of the stab mount.

[mlachow]

I am still working on little pieces of fuselage sheeting that remain to be done. The tail area takes time. I am also finishing up the sheeting around the wing mount.

The other bit of work is the canopy frame. This requires a bunch of laminated pieces to get the frame to 6mm thick. The outline interlocks. There is also a middle part of the frame that needs to be glued in. I had to figure out how the cross pieces interlocked ans then fit the ends to the rest of the canopy frame.

To do the glue up I added some tape to the fuselage to provide some space for paint, and covering on the fuselage. I used small pieces of release film over the tape in the corners that were being glued. Just some medium epoxy and then some small fillets in the corners. I used some West Systems 405 mixed in. with the epoxy. It is also nice for wood since it is tan.



The canopy install will be weird. They just give you some canopy that is close and you have to cut it up into two pieces. The two pieces meet at the middle canopy frame piece.. The frpont of the frame also needs some thickness added to the top so that it can be angled to the canopy. It's a small canopy but it is a lot of work.

[mlachow]

The canopy frame is glued up. After gluing, I added all the fillets around the frame.


At the back end, servo wires need to go to the stab for the two elevator servos. I put in a short section of rocket tube to guide the wires down to the bottom of the fuselage so things don't bump into the pull-pull rudder cables and so the cable can't just drop into the fuselage at the wrong time.

[mlachow]

I made some really wide strips of balsa for the bottom, about 4 1/2 wide. Much of the bottom is wider than the sheets of balsa so it is either make wider sheets or glue two pieces. I still have a few pieces at the tail remaining.


In the wheel area, I made some filler pieces out of foam to fill out around the wheel area and close up the lightening holes in the wheel mount to keep from getting dirt in from the wheel well. Also a few side balsa strips to make gluing the sheeting easier along the side.



Next decision is what to do with the nose and tail skids. The kit just laminates some layers of light ply for the nose skid and the tail skid. Just solid ply and nothing functional. The tail skid does not have a lot of support for a landing where you spin the model and it might catch and break. So that area will need some fiber glassing to strengthen it.

The last 6mm square pieces were also added to the bottom front of the fuselage.

[mlachow]

Yes building is getting a little slower. It has been cold and the basement is getting cooler. September through November is usually the best temps in the shop.

[mlachow]

Checking the fit of the wheel and wheel mount. Everything fits and looks good. I will have to make some spacers between the side and the hub.

[mlachow]

I made a sheet of fiberglass over a round tube to glue inside the wheel well. It was cut to width and then epoxied over the foam.


After that cured, I bolted in the wheel supports before finally finishing the fuselage sheeting in that area. I am using a long M4 bolt as the axel.

[mlachow]

I previously made a nose block lamination from pink foam to get the rough shapes. I used those to cut out a stack of 12 pieces of balsa for the nose block. I glued up three pieces at a time.


This thing is going to need a bit of nose weight. Aloft Hobbies has some 25 x 38mm tungsten slugs. Each one is about 12 ounces. I drilled out the middle two layers for four of them plus a 25 x 25mm piece. I also drilled for the TopModel tow release.


After some rough sanding I have four pieces ready for some final shaping and for attaching to the fuselage.


I will probably still need some additional nose weight. This will depend on the batteries and pilot and cockpit work plus how heavy the finish is on the model. This weight should be a good start and it is located about as far forward as you can get. 3.7 pounds is a good start. The wing shape sweeps forward in the mid section. That places the wing joiner behind the CG. The notes say the CG is around 60mm. The one retroplane build used an estimator to come up with 70-75mm range. They had 2Kg in the nose. That would be another 12oz above what I have right now..



I did assemble the model and finished gluing and rough shaping the nose block. With the block taped on and two batteries and release servo sitting in the fuselage, the uncovered model is pretty close to the CG point. I still need to through the servos in the tail cover and finish, and finish up everything else. But things are in the right ballpark.

[mlachow]

On to some work on the tail hinge lines. The back edge of the fin is not a straight line.It does cover some of the gap but not exactly. To make it look a little like it is supposed to, I added some really thin ply onto the back edge of the fin. I used a small plane and a chisel and some sandpaper to make a little recess in the balsa for the plywood. The grain is oriented so the outside layer runs nose to tail. It is stiffer that way and bends nicely to the shape of the back of the fin.



The elevators also need a little addition. Again, the edge from the lasts hinge to the tip sweeps back a bit. I added a little tapered spacer for the tip. The LE of the stab is also tapered back in addition to rounding it. I added some triangle balsa on the stab. This is a lot easier to do and more practical with the stab shape compared to the fin.

[mlachow]

I used a dowel as a pencil guide to scribe some lines approximately the fillet width. Then I applied some tape along the lines. First pass on the fillet is epoxy and West 405 Filleting blend filler. This filler holds it shape pretty well. Eventually I will use some 410 microlight filler over this when the fuselage is glassed. The 410 sands really well. I used a palette knife to apply it and scrape away excess epoxy from the tape. Once it was down to just the filler I needed, I used a dowel wet with alcohol to do a final smoothing pass before removing the tape and letting it cure.



The fuselage nose and tail skid now need to be glued and a small fillet added. I'm going to hold off gluing on the nose block and all that ballast until I get most of the fuselage glassed.

[mlachow]

I picked up the instrument panel set that OldGliders sells. Basically a printout of the instruments, a sheet of plastic, and some 3d printed instrument inserts to put into the plywood instrument panel. They also give you some screws for the panel mounts. Nothing extremely scale, but then with the cockpit and where it mounts, you can't really see it that well either.

I was a little disappointed in the routing of the panel from lite ply. There were several chip outs on the front of the panel so there was no way you could stain it a light wood color. I just used some dark mahogany varnish on the wood after filling the surface with wood filler. I'll spray the screw heads with black before putting those into the panel.