Scale Build-Off – Marcus Thörn 54" FFVS J-22

[willsonman]

Marcus Thörn 54" FFVS J-22

With FTFF over and the popularity of the SE5a build I've had many inquiries here and there about showing additional building techniques. At FTFF I had a built-up Funder and Lightning where I diverged from the "flat out" style and built up airfoiled wings and fuselage and made a great ship. It was fast and slow... it just did not survive a pilot error.:black_eyed: I need another speed demon in my hangar and completing this build fits all those things mentioned above.

The FFVS J-22 was a Swedish fighter designed by Bo Lundberg out of necessity for Sweden needing an advanced fighter designed and built in-house. An embargo was placed on aircraft purchasing due to the outbreak of WWII. I'll share some historical tidbits along the way but needless to say it was a tech marvel for the country whose limited resources came together to make a fantastic machine.

I started this build back in February, where I was out of town on a business trip. With nothing to do in a hotel room and a snow storm upon me I hunkered down to make the bones of the fuselage. Knowing I would again make this plane out of adams foam board I cut out the fuselage formers sans the extra material for supporting stringers. The plans are from Marcus Thörn over at RC Groups. No posting of plans folks. I paid for them. They are originally for 1:13.33 scale. I've enlarged them to about 1/6 or 1/7. I'll do the calculation later to get the exact scale. Its been so long I cannot remember off the top of my head.

At FTFF, here on the forum, and even on my youtube channel, I've been asked many many times to show how to do the foam sheeting technique. If anyone has ever done insert planking on balsa builds its along those lines but I'll outline the job here.

The main thing to consider before building is factoring in the thickness of the foam you will sheet with. Your formers will need to have that thickness removed. Here it was a happy accident that my foam thickness was that of the stringers after the plan was enlarged. I'm using white gorilla glue here. My glue of choice. It fills voids and adheres to the foam very well.

For the Fuselage there is an about of strength that you will need to counter torque. The majority of this strength will come from the final glass work. The other strength will come from the glue holding it all together. Another key point is to sheet in such a way as to prevent twisting of the frame. I employ a checker pattern build. I'll show what I mean. Chose one side to start with. I started on the top left side. I started at the front as the curves are gentler and curving the foam will be easier and add strength as I build. I'm going for one section at a time, only between two formers. Break for the next section and then again. On the top right side I stagger the sections sheeted again, but in the alternate way. I made the side and top longerons extrude past the formers so I would have more for the foam to glue to. Cut the foam super straight on one side. I use my aluminum ruler for this. Over size the piece and I use aluminum ducting tape all around for this. Super sticky and adheres even when there is a bit of foam dust on the surface. I taped the straight side to the side longeron after curling the foam sheet on the edge of the table. Then I carefully mark where the top of the sheet should be cut. I always cut it over-sized the first time. Then do a second cut for final fit. Its ok if the sides go past the formers. I prep several pieces of tape and apply glue sparingly... not gobs. I tape the piece in position and the fun part is that anywhere you do not want the glue to expand to... cover it with tape. Once the glue has cured cut off the overage on the sides along with the glue that has puffed out. Watch your fingers as thicker glue sections will create more cutting resistance and your razor blade may slip as you get to easier spots.

Rinse and repeat for each section. See pictures.

 

[willsonman]

Power for this ship will come from a Tacon Bigfoot 0.32 and a 60A ESC on 4S. I'm going scale with a 3-blade 12x6 or 13x8 prop. The first will be for maiden and the second for real speed. I'm estimating around 90 MPH on my big 4500 packs. I have a nice aluminum tru-turn spinner that will fit the bill well for the front end. THis plane needs nose weight and that hunk of aluminum will do great. I'm shooting for 3 pounds AUW with a span of roughly 54 inches. That is roughly 1/7.3 scale. It was the biggest enlargement I could do on the printer.

 

[willsonman]

I neglected to mention a key ingredient for this technique for those who are new to working with this stuff without paper. When you hold up your boards you will see a series of ridges. This indicates a grain in the foam. You want to cut your pieces so that the grain is favoring the bend. So, when I go to make my curves and curl the foam on the edge of the table its easier and the foam will play nice with what you are trying to do to it.

Along those lines I got to the aft end where some pretty heavy curves start playing in. The object here is NOT speed. Take the time to pinch and squeeze the foam into the curl that you need. dont rust it or the foam will crack off. little surface cracks are ok. They will be filled with spackle later. I'm done the checker board on the top now so its time to move to the bottom.

 

[Craftydan]

Once again, a masterpeice in the making . . .

. . . but even if you stopped right now, you could call this art:


View attachment 29046

would beat most modern art I've seen

Keep it up, sir!

 

[SnowRocker88]

Wow. So basically this is a balsa build with foam? Lol

 

[willsonman]

Basically, yes. Some modifications have to be made due to the change but the overall idea is that you can use this foam as a build medium with minimal divergence from standard balsa plans. The benefits are that you can generally build faster, lighter, and cheaper. Also, if you screw up something its much easier to fix and you are not worried about loss in any of those areas either. I suspect that this will be a 15-20 sheet ship. So if you figure just the foam being $20 you have a lot less $ invested. After glass, carbon fiber, glue, spackle, and other things you are looking at closer to the $50-75 range. Still cheap! The end result is that you can build any subject you want. This one I have NEVER seen at a flying field. Most folks have never heard of it either.

 

[willsonman]

More history...

I'll drop some quotes here from the following link:
http://www.j22-project.com/?page_id=374&lang=en

The Swedish Government oredered the development of a new fighter, without involving the Saab people and facilites already busy with their own aircrafts. The FFVS company was created and Bo Lundberg was designated as manager of the project. He presented his ideas for a small lightweight fighter of rather unconventional design. The J 22 had a mid-wing design and its fuselage and wing structures were made of welded steel tubes, covered by moulded birch plywood panels. Those panels were integrated part of the loadbearing structure and took a considerable amount of the stress and loadforces.

The J 22 was highly appreciated by the pilots for its good manoeuverability. The technicians were also pleased by the simplicity of maintenance of the different systems. The aircraft was the fastest fighter of the Swedish Air Force when introduced, and its firepower and manoeuverability were almost as good as contemporary fighters.
The aircraft was powered by a 1065hp Pratt & Whitney Twin Wasp, which at the end of the war was already poorly powered, and poorly efficient at high altitude, compared to latest engines produced in the USA, UK or Germany.
However, as the aircraft was of very light construction, the aircraft happened to be very good at low altitudes with a fast turning rate and high manoeuverability. In a mock combat with a P-51 Mustang, the J 22 initially climbed faster and at a steeper angle. Below 1.500 metres, the Mustang was outturned horizontally by the Swedish fighter which was able to turn “inside” and get the mustang’s tail in the gunsight.

 

[willsonman]

Had a busy evening last night so I only got two panels on the bottom. Once I get the bottom "checkered" then I'll move to the next step to prep for filling the voids.

 

[willsonman]

Ok, the bottom checks are done and decided not to do the cheeks around the wing saddle are for good reason. I want to make sure that when I do them they are as straight and true as possible so they will be done last after all the voids are filled.

So, moving on, we start with prep work for filling the voids. I cut a bunch of strips that are 1cm wide. Length just needs to be sufficient for what comes next. So, basically you are just using the strips to make a ledge for the sheet to stick to. As you can see you can bed it outright just fine. If you pinch and squeeze the foam you can get even tighter radii. Apply gorilla glue to ONLY the inside radius VERY SPARINGLY. You are trying to prevent the glue from expanding out the side where you will glue your panel. This is not an issue if it does... it just makes for more work later because you have to trim out all the hard expanded glue to prep more for your panel work.

So now all the areas where you have voids need the support for the panels. Just roughly shape them and glue in place. I use my aluminum ducting tape here again because I know they pieces will not move. Taking your time here pays off to save time later. Less trimming of glue and foam later is the idea of prevention here. This also adds rigidity to the frame as you have more torsional stiffness from the glue. I only got about 30 minutes into this so I still have more to do but you get the idea.

Once the fuselage is sheeted I want to show something that most folks freak out about when they try this for the first time. I don't want to forget it so if I do please make sure I do. I call it panel dip.

 

[willsonman]

Folks, you must be interested since the number of views has gone high. I've not really done much yet but I'm glad there is interest, sort of why I post this stuff. If you have questions or feel like I did not explain something right feel free to chime in. Better still if you have a suggestion for improvement, by all means let me know!

 

[LordVader]

Nice work, kinda looks like a Hellcat or Cosair so far. I will be following this one too.

 

[willsonman]

If you look at 3-views it looks closer to a BF-109. This was part of trade studies done prior to the final design by Bo Lundberg.

 

[LordVader]

It does, I really like the BF-109. It's one of my favorites.

 

[willsonman]

One of the great benefits of the airplane over its lifetime of service was that it was designed around minimal drag. Since the only engines available were twin wasp rated at 1050 HP. The plane was designed around the engine. With all the NACA research being done at the time it was one of the first planes to incorporate an airfoiled cowl. The mid-wing design was also done to reduce drag. The landing gear was also taken from the wildcat idea that drag on the wings reduced deficiency. The narrow under carriage posed issues for landing on carriers in the pacific but in the fields of Sweden they were a non-issue.

 

[willsonman]

Over the weekend I finished the support pieces of foam and started sheeting the empty spots. Still more work to be done there but its progress! I'll cover this in more detail as I get to some "trouble" areas.

 

[willsonman]

Ok, I'll address trouble spots here.

First is a picture of my current progress. Then we move on to the other side where an odd-shape hole is to be cut. Just rough cut the piece and allow overage for the odd side. I then traced the back side with a ball point pen and trimmed until it fit. This was only a reference line... not a cut line. Then you can see the resulting fit.

After that panel was in place there was a great big issue with building straight. Herein lies the issue with building with foam. Because it is not very rigid your frame WILL warp during the build. You have two options. 1: use hard balsa points or 2: fix it. I usually fix it as the foam is pliable and if you need to, once the frame is assembled you can use some heat to soften the foam and twist and bend it back into shape as a last resort. I cut a piece of foam the right shape to get the alignment back... as you can see from the subsequent photo.

Finally, you can see the results of my labors last evening. I spent an hour getting 5 panels on bu they all required pretty specific trimming. Considering that I've spent about a total of 5 hours skinning the fuselage we are not talking a big investment of time.

Another tip is that when you are done skinning and you see a major problem spot from warping you can simply cut out a panel and put a new one in to fix the warp. Currently I do have a slight twist in my fuselage around the wing mount area. If I cannot fix it with the sheeting on the bottom end then I can use this method on to to relieve some of the stress.

 

[willsonman]

Finished up the major spots of the fuselage last night. Tonight I'll trim the one on the very front and try to get the sheeting done for the wing saddle area. This will take some time to correct some warping in that area. After that I will examine each panel for "dip" and determine if I need to re-sheet any panels.

 

[willsonman]

Ok, lets talk about Panel Dip. Basically you are now noticing that some panels (particularly the first ones you put on) have a slight dip in them between the formers. So basically what you are observing is the foam's natural tendency to lay flat. Sometimes the curling over the edge of the table relieves this but in tighter radii the foam just wants to go flatter. So the best part about this foam stuff is how easy it is to sand it back. A little 80-100 grit on a block and you will be looking much better. See the obvious before and after pictures.

After addressing the wing saddle sheeting It was time to start filling the panel gaps. Some of the panels will have gaps and others will not. I just fill them all because that way I dont have to think about it and I sand the majority of the filler away anyways. So, This will be curing for awhile. Its cooled off and been rainy. Never good for this stuff to cure quickly. After this cures I'll sand it all back super smooth and coat it all with a bit of WBPU to harden it up. If you don't the working with the fuselage you do next will loosen it and most likely make it crumble and fall out. Do it or prepare yourself for re-work.

Moving on will be to address the extreme ends of the fuselage. I will be doing the aft end first to prep for H-stab installation. Then I will prep the fore end to finish off the cowl. We are switching mediums here. I'm moving to pink foam from the Home Depot. Its a great filler that is about the same consistency of Adams. I can just get it in 2" thick sheets. Big chunks make for less work.

 

[willsonman]

Buttoning up the rear end. First make a plug for the bottom part. Two wedges that are cut 90 degrees in relation to the former and side. Gorilla glue and then carve with a coarse rasping bastard file. then with a fine metal file. Finally with 100 grit sand paper. Then moving to the top portion. I'm creating a platform for the H-stab to sit on here so we measure the distance from the top of the side former to the bottom of where the H-stab sits. This is to make sure incidence is set properly. 0 degrees there so cut to shape and glue. For this pink foam its better to poke lots of tiny holes for the gorilla glue to expand into and hold better. Once the glue is dried I'll do the same to knock off the extra foam and sand to shape. Another goal is to make the elevator linkage internal here so that is the general plan. Just make the foam rigid all around the area and I'll carve out what I need to make the linkage internal. More than likely the rudder will be external.

 

[willsonman]

I worked out some stress yesterday on the build and the fruits of my labors are good. I knocked off the shape of the rear foam and decided to do some wire cuts for the H-stab. In general the H-stab (should you choose to airfoil it) should NOT taper (chord thickness) toward the tip. The outlying principle here is that generally the chord is shorter than at the root. If you use the same airfoil at both ends the tip is likely to stall before the root and exaggerate any propensity for ground looping or tips stalls in tight turns. So I chose to keep the chord constant. After doing some measurements I determined that the root will be NACA 0006 and the tip will be NACA 0012. The chord thickness is about the same considering the chord lengths. I use Profili for my exporting of DXF formatted airfoils. I import them to Illustrator and size from there. Making duplicates I then import them into the Silhouette program to cut them from cardstock. Easy and I do not have to do the cutting! After the are cut I make templates as instructed in my tutorial here. I'm using the outer most parts of the H-stab and then will cut and sand to shape. Cut the big blocks of foam and hotwire cut using the templates. I then taped one set of the beds together and cut out a groove to embed a carbon spar. Glue with gorilla and set in the beds to cure to make sure they stay straight. I'll do my usual tissue and WBPU treatment. Elevators cut from scrap balsa pieces. The elevators are fabric covered on the full-scale so I will show construction and placement of hard points and foam ribs later.