IS-4 Jastrzab

[mlachow]

IS-4 Jastrzab,

Wingspan 3.95m

Length 2m

Weight 8-10kg

Root chord 470mm

Manufacturer - Oldgliders.com

Web Site - https://oldgliders.com/en/offer/is-4-jastrzab-kit/

Main Wings - Two Piece

Main Wing Materials - Balsa / Ply / Pine / CF Joiner

Airfoil SD3024/SD3021. Or at least something close to them.

Fuselage Single piece, high wing, single wheel of Balsa, Ply, Pine

Tail Removable stab, Balsa, Ply, Pine

Control Surfaces Ailerons(2), Spoilers (2) Rudder, Elevators(2), Tow Release. I am going to modify for dual ailerons on each wing.\






I am starting a other project for the building season. I have an IS-4 Jastrzab kit from OldGliders. I also purchases the balsa sheet pack from them. I cut my own pine strips and spruce strips for the spars and other parts of the model.


The wing planform is interesting. The wing sweeps forward a little at the root from the top. From the front, the center part hasa few degrees dihedral then the rest is flat when not in flight. So it is a bit of a gull wing shape.



I have the spoilers, wheel, and instrument panel they supply. All of it looks decent. The wood was good except for a few sheets. There were three 10mm thick sheets for LE's of the wing and ailerons and elevators. One was really light 6lb balsa, another normal 11lb balsa and the third was rock hard 17lb balsa. I'll swap that with some 3/8 sheet I have. It's nice to have a hard LE but that is a little extreme. I'll save that for laminating up nose blocks. The 2.5mm balsa for the fuselage sheeting ranged from 7 lbs to 12 lbs. Nice to distribute light wood at the back and the heavy stuff up front. The 2mm wood for the wing and tail sheeting includes 32 sheets.10 were in the 6-7 lb range. 15 were in the 8-9lb and the remainder 10-12lb balsa. Pretty good. I just can't figure out how he manages to sheet all of that airplane with that much sheeting. I have a few 5-6 lb 3/32 that I will use for the tail.

The instructions are the standard bunch of pictures on a DVD and a few sketches or notes on hinging, cg, and wood list. The pictures look like two different versions of the kit. It looks like some ribs were redesigned over the years. The wood strip list was useless and it did not match the plans. You have to measure the notches to make sure you have the correct sizes. The photos show what looks like 1mm ply for webs. But looking at the number of 2.5mm balsa sheets, I think there is enough sheet there to use that for webs after sheeting the fuselage.

If you ever get an oldgliders kit and wood pack, make sure you spend plenty of time studying the plans and figuring out where to use the sheeting. They just give you the number of sheets they think they use to build the model. There is no info on how to allocate or cut the sheets. If you cut them the wrong way you might end up with spare pieces that are not the right size to finish the job. Be careful how the tapered tip sheeting gets cut and how the center panel behind the LE is cut up.

I have some spruce that I stocked up from Aircraft Spruce last time I traveled through Georgia. I ripped that for the 15x7 spruce spar stock. I use an 80 tooth 3/32 blade and I always clean it before cutting strips. Strips come off the left side of the blade. not the fence.

[mlachow]

The other fun part is figuring out where all the parts go. All the parts are jumbled up in the box. It seems a strange way to pack them but actually they pack and travel quite well. There are a lot of thin pieces of ply and I only found two that were damaged. I lay out the parts and sort them by wing panel, stab, rudder, fuselage, canopy. Sometimes the photos help identifying where things go.

Since it is a gull wing I had to set up the bent board. The bent part of my board is narrower than the rest. Because of the M shape of the wing, I had to add some extensions to that to make it wider so the panel fits when I build the left and the right panel.

Fuselage



Tail



Wing


Building Board

[Steve P]

Great project! Thanks for sharing.

[mlachow]

Some things are not quite scale.

The spoilers are conventional spoilers. The real spoilers were even with the aileron. They were not as effective so it's a nice compromise for RC flying.

The Stab and elevator shape is fine but the structure is very different. It is fully sheeting while the real one is partly open structure. It would be a lot of work to match the scale rib outlines on the elevators.

It looks like the wing is sort of S3021ish. Some of the ribs actually are pretty close to the S3021 except that is the rib and it doesn't include the 2mm sheeting. The nice part of this airfoil is it makes it easier to build the wing. Basically flat on the building board. Some of the other models from OldGliders are HQ variations and to get the right angles across the wing you need tapered shims for the trailing edge. Those are a joy to cut.

There is a YouTube video that describes the real sailplane.

https://www.youtube.com/watch?v=GmanqQVw9bA

There are some other Jastrzab builds out there.

https://mcmmuenchen.blogspot.com/search/label/IS-4%20Jastrzab

AND

http://forum.retroplane.net/viewtopic.php?t=3962&postdays=0&postorder=asc&start=0

[gunny11]

Cool subject, will be following with interest.

[Larrikin]

While I love a “Glass Slipper”, the more I age the more these classic lines and structures get my attention.
The wing is like a trophy wife, high maintenance but looks fabulous.

[mlachow]

The most important part in starting this kit is a collection of file handles, files and a file card. All the parts are routed. That means you need to file corners to use your nice square strips of wood. It takes some time to go through all the parts and take care of corners and the little bump on the outside of the part where the router started and ended the cut. It is not a laser cut kit. I like the screw on Grobet file handles. 6 inch or 8 inch work well. I usually use a hand file but sometimes I use a knife file.

[Soarntz]

Gonna watch your progress

[mlachow]

Well this thing has an M shaped wing as well as gull shaped dihedral. So the wing spar is a bent every which way. There is a long splice mid panel. Real fun to cut a taper that long. It is longer than you can normally sand on a 12 inch disk sander. I initially sanded the parts to the taper. Then clamped all four spars together and used a plane to get a nice straight taper with square corners.

The root of the wing tapers forward so the wing joiner is diagonal to the spar. The solution is to make the spar triple wide at the root for the joiner box. All the parts are glued and clamped together to make a spar. I used https://www.systemthree.com/products...epoxy-adhesiveT-88 structural epoxy to glue them up..



The table saw will get a workout cutting tapered blocks to fill around the joiner tube.

[ARUP]

While I love a “Glass Slipper”, the more I age the more these classic lines and structures get my attention.
The wing is like a trophy wife, high maintenance but looks fabulous.

Agreed! This will be a great looking sailplane!

[mlachow]

The plan is to do it in white and dark red klasskote. The colors of different airframes were generally white and dark blue or white and dark red, I figure it should thermal OK with the S3021 / S3024 airfoils on the wing.

[mlachow]

Started building the first wing panel. Everyone seems to build tail parts first and I'm starting with the wing.

The root layout is nice. Plenty of ribs in the center where the joiner box is located. I can measure square to the end of the panel and the board is bent down for the center. So I can use root as well as a level to align the joiner in each panel. The top spar is not glued on yet. I'll also wait on the front spars so that it is easier to wrap the joiner box.


I have to decide what to do with the ailerons. The plan is simple. It is just an aileron from the wing bend out to the tip. This is not the scale outline. The scale outline starts the aileron out a little bit and it ends before the tip It is also in two pieces. The wing is thick enough there I could split the aileron and put in two servos. I did that on the Bocian. The real horns are on top. It should not be too hard to match the linkage positions.

[mlachow]

If you notice the ribs are really cut out a lot to reduce weight. The wing is sheeted, not cap stripped. So the areas in the center are still a bit squishy. If you really wanted to make maximize the wing strength, some diagonal bracing should be added to all the big areas. Both behind the spar in the center panel as well as the openings in front of the wing. I am going to add some diagonals to the ribs in the center panel behind the spars.

[mlachow]

Some table saw and disk sander time to get ready to assemble the outer panel. I used the table saw to straighten one edge of the 10mm balsa for the aileron LE. Taper was cut with a knife. The top part gets planed and sanded down to match the ribs after gluing up. The 12" disk sander is handy for sanding the scarf joint for the LE. It is longer than the 1m balsa sheet.

I also sanded the back of the tip ribs and front of the aileron ribs on the belt sander. I set up a guide with the 9 degree angle to sand the edges to match the 10mm sheet. It's nice having a S3021 / S3024 wing since the back of the rib is pretty close to flat.

[mlachow]

I decided to split the aileron into two pieces like the real aircraft. It adds some extra parts and I need to add another servo, Building long wood ailerons can be a pain. They don't always stay straight. At least with it split the two ailerons are short enough to not worry about and only 3 hinges so those line up easier.

I predrilled the aileron LE and spar for the hinges Added ribs at the middle and moved the inner end of the aileron to match the 3-views and pictures. It also needs some extra ribs in the wing to move the aileron servos. Common sailplane servos should have no problem handling the individual ailerons.

[mlachow]

More wing building. The next step was to take care of the joiner box,

Lots of filler pieces are needed to block in the joiner boxes. The joiner box is blocked all the way around. Nothing like this shows on the plans or build pictures. It's definitely an overkill but I should not have any joiner problems. There is a little play in the holes where the joiner box is installed. It was much more than I had with the Orlik and Bocian which use similar rectangular joiners. The joiner is held at 90 to the rib and I use a level on the joiner to align it horizontally. I'll have to leave open the bottom sheeting to block up the rear alignment pin once I have both panels done and the basic fuselage frame done. Only the root rib has a hole for the rear rod. It would have been nice if there was something in the next rib or two. The second panel I'll also measure the vertical height to try and match both sides.

Parts were cut from a strip of pine on the table saw. I used a guide on the rib angle to sand the final length of the pieces for each rib on the 12" disk sander. Then some pieces were split on the tube angle with the bandsaw before sanding that edge on the disk sander. The same was done for the top and bottom pieces where the tibe runs.

So far I only found two minor routing issues. One of the wing tip pieces had an extra tab on it that wasn't needed. I really didn't like the position of the spoiler servo access frame. It is really large and it fits right against the spar. It should have at least allowed space for the shear webs to be fully glued to the spar.

​ ​

[mlachow]

I fitted some wing webs between the spars around the wing break. These needed to be glued in with a clamping caul across the workbench to keep the spars flat. Remember it bends instead of being cut with a joiner. I also glued up the joiner box spacers and the joiner box. The few high spots I found after gluing up everything were trimmed with a small finger plane and a rasp. To glue the top spar, I used some MGS resin thickened with a bit of silica. This should be enough to fill any small gaps. This was one of those last thing to do for the day glue jobs..



I only had a few minor blobs of glue to clean up that I missed when the spar was clamped. So now the wing is ready for the remainder of the webs. Ply in the middle transitioning to thinner ply and finally balsa out to the tip.



I guess you can never have enough clamps when building a wooden sailplane. I had to glue things in stages.
The webs were rough cut on the table saw and then fitted with a plane and some sanding balsa ones. Fortunately the table saw blade is a little thinner than the ribs. So I cut ply strips close to the correct height. Marked the rib locations on the strip and then cut the pieces on a sliding table. I cut double the strips and pieces and set aside the extras for the second panel.

I also managed to sheet the top of the aileron.