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Old Dec 08, 2010, 03:43 PM
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Build Log
F7F Tigercat

(This build log was for the $50 challenge on Wattflyer.com. I'm reposting here to keep all my builds in one place. Comments and questions are welcome of course,)

My entry will be the F7F Tigercat. I wanted to do one for a long time, ever since I saw one fly at the EAA fly in several years ago. This thread also got me juiced up. Originally designed as a heavy fighter for the US Navy, it failed Navy tests but was adopted by the Marines for a variety of roles.

As a model it has a lot going for it: Nice broad wing, ample stabilizer and an almost comically large rudder. It has huge propellers so no problem powering it with efficient outrunners. I had begun to draw up plans a few years ago but was stymied by the long spindly nose gear and small front wheel. Flying off of grass looked questionable for the size I was contemplating. The narrow fuselage and nose gear that retracted through about 110 degrees only made things more difficult. But after the success of my He-219, I began to reconsider the Tigercat. The He-219 had a similar problem with the nose gear so I left it off and made a belly lander instead. That plane is a blast to fly and so I'll do the same with the 'Cat. I dug out the uncompleted plans and finished them up.

Specs:
1/12 scale
WS 51.5 in.
Length 45 in.
Area 440 sq in.
Target weight 45-48 oz.
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Old Dec 09, 2010, 01:14 PM
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I got started on the fuselage. I'm making it from heat formed FFF. I like this method because it makes a light, rigid and cheap fuselage. The other big advantage for me it that you can scratch build from a 3-view without CAD. (If you are interested in the details of the method go here and look at my last seven builds.)

I started with a decent 3-view and blew it up to 1/12 scale. Then subtracted a 1/4" from the side view and cross sections to account for the FFF.

To make the molds, I cut out the side view in 1/2" plywood for the backer boards. After that, I used the cross sections printed out and glued to cardboard to rough cut XPS foam blocks and glued them to the backer boards. I leave gaps between the blocks so I can slip in cardboard to trace the cross sections so I can compare the right and left halves.
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Old Dec 13, 2010, 02:56 PM
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The molds are wrapped in FFF and baked to form the fuselage shells. A 1x3 wooden spine was screwed to the back of the mold to keep it flat.

First, the FFF is covered in PVC packing tape overlapped 1/4 to 1/2". The tape is applied to the film side. The film comes off when the tape is removed and actually makes the tape removal easier. The packing tape functions as the female part of the mold.

The taped FFF is then rolled over a pipe to pre-form it into a curved shape to make it easier to tape to the mold. The FFF is then taped to the mold. Finally, it's baked in my oven (foil faced foam box with a modified space heater inside) until it reaches 100C (usually 15 to 20 minutes.)

When it's cooled, The spine is removed and the formed shell is trimmed flush with the top of the backer board. I'll leave the tape on until the halves are joined to protect them from dents and stray blobs of glue. The weight of the first half was 62g. Without the tape and film it should weight about 38g. or about 12g/sqft.
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Old Dec 15, 2010, 12:46 PM
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Getting ready to join the fuselage halves.

One of the characteristics of this kind of construction is that you are "off the plan" almost immediately and are building on the shells themselves. To get an accurate reference line to lay out the wing and stabilizer locations I drew a line on a board (bench was occupied with a different project) parallel to the edge. I used the reference line off the backer board and marked it's location on the ends of the shell then taped the shell to the board. I taped a marker to my square and set it to match the line on the bench. As long as I kept the square parallel to the board, I could accurately mark the fuselage reference line as it rose and fell. Trying to use a long ruler, even a flexible one, just doesn't work. I poke through the foam with a pin to transfer the reference line to the inside of the shell when I need to.

The stabilizer is almost 24" wide but the fuselage where it mounts is only about an inch wide. That's a lot of leverage so I added 1/32" balsa sheet to reinforce where the stabilizer mounts. That area is a compound curve so I picked the best line I could find for the balsa (least compound curve.) I steam bent the balsa and clamped in place to dry with cloth strips. When it was dry I glued it into place.

Foam alone is not strong enough for the wing mount so it too got some reinforcement. I used some very hard 1/16" balsa and ran a piece from the wing saddle all the way to the tail. The fuse is so narrow and the tail so wide that I figured it would be a good idea. I pre-bent this too to roughly match the fuselage and clamped it while the glue dried by replacing the shell back onto the mold and wrapping it with pallet wrap. (I bought a roll of it to ship a piece of equipment 17 years ago and am still using the same roll!)

I slipped fresh cardboard into the gaps in the mold and traced new cross sections. I'll use these to make the formers. I laid out the paths for the elevator and rudder pushrods and marked them on the former templates. I'll make the formers from foam.
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Old Dec 16, 2010, 03:36 PM
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The first photo shows the internal structure. I cut out the formers using the newly traced templates. Holes were cut for the carbon fiber rudder pushrod and the flexible cable for the elevator. The elevator pushrod sheath is installed. I'll add the end support for the elevator pushrod sheath after I cut out for the stabilizer. I'll have to make a small door to access the elevator horn to connect the pushrod. The formers are glued into one side first and checked to make sure the seam will close up when the other shell is glued on. Only a couple formers needed some sanding.

The balsa sides and bottom for the battery box were added by first cutting slots in the formers, gluing the formers into place and then sliding the balsa into the slots and gluing. Only after the balsa is glued in place are the centers of the formers removed. That way the smaller formers aren't distorted or fall apart when gluing. I plan to use a 2200mAh 3s battery but made the battery box just big enough to fit a 4s battery or slightly larger 3s.

I marked the edges of the shells with a green highlighter marker. This will make the vertical center line easy to see. Often the glue joint is good enough to vanish after sanding. I use white Gorilla glue to join the shells. It foams up to fill any gaps between the shell and formers. It is a pain to sand so I'm careful to not let much foam up on the outside. Leaving the tape on also keeps the glue from getting on the outside foam. Any glue on the outside will be carefully excised with an Exacto knife. You really can't sand dried Gorilla glue flush with the bare foam without sanding the foam around it away too.
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Old Dec 20, 2010, 11:18 AM
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I added a 2.5mm bamboo reinforcement to the rudder TE and balsa to the fin LE.

The bamboo comes from a roll up blind I bought a few years ago from the bargain bin at the local builders supply. It has a lifetime supply. I planed a flat on the bamboo and then heat bent it to the shape of the TE. I glued it on using carpenters glue. The LE got a tapered 1/4" balsa piece.

I don't know if I'll be able to use the foam rudder once it's cut off or if it will make more sense to build a new one but the wooden TE and LE will help when I shape the fin. The green marker on the seam really helped at this step to keep things centered.
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Old Jan 03, 2011, 11:46 AM
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Got to work on the rudder. After removing the tape and film from the back part of the fuselage (I'll left it on the rest for dent resistance), I shaped the fin and rudder. I realized that I should have included some ribs for the rudder to keep it's shape once it was cut out so I added them now. I cut slots and glued in more foam and sanded them flush. The foam is now about 1/8" thick in this area after shaping.

I cut out the rudder, added 1/16" balsa and more foam to the LE and sanded it to shape. The rudder is really thick so I added shrouds to cover the gap. I sanded in a recess (the tool was a piece of Formica with a square of sand paper glued to it) and glued in some maple veneer I had laying round. After that, I sanded the recess to fit the rudder. I'll use a pivot on the bottom and point hinges for the rest. I'll have to glue in mounting pads in the recess to mount the rudder to the fin.
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Old Jun 11, 2011, 04:22 PM
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I made the stabilizer from hot wire cut foam core skinned with 1/32” balsa. I feel that a “real” airfoil (NACA 0009) on the stabilizer makes the plane fly better than a flat stabilizer. This also makes a very stiff, strong stabilizer and really doesn't take that much longer to make than a flat foam stabilizer with balsa reinforcement. It does weigh a little more but the benefits are worth it. The foam came from some packing material and is very light, core weighed only 4g (but very soft too). I have a thickness sander and made my own 1/32” by sanding down 1/16” balsa. I bought bulk seconds from Balsa USA for only $1.50/ sheet for 4”x48”x1/16” (at an air show so no shipping!)

I cut both halves from the same block of 2” foam. This guarantees that both sides end up the same size and any errors in the templates end up on the same side of the stab so no net rolling force due to inaccurate templates.

Tips are made from 1/32” balsa sandwiched with FFF. The balsa sticks out a bit so it can fit into a slot cut into the tip of the stab. This increases the glue area and makes sure it stays stuck during shaping. After shaping the tips, the halves were joined and the elevator cut free. A 1/8” balsa TE was added to the stab and a Ό” balsa LE added to the elevators.
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Old Nov 10, 2013, 03:44 PM
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The fuselage is pretty much ready to cover but first I need to make the canopy mold. I was very happy that the cockpit area formed well enough, and the foam was thick enough to shape the canopy. Otherwise I would have had to section in a block of foam and shape that.

I used full sized excerpts of the 3 view and photos of the real plane to shape the canopy. Once that was done, I gave the whole fuselage a good coat of WBPU to give it some dent resistance.

To make the plug to vacuform the canopy, I need to make a fiberglass cast of the fuselage in order to pour the plaster vacuform plug.

I start by covering the area to be cast with window heat shrink film to form a barrier between the glass and the fuselage. I taped the film in place as best I could then started to shrink it down. I had to wrap a piece of tape at the canopy nose to pull the film down. It's not perfect so eventually I'll have to shape the plaster cast to get a nice sharp corner at the front of the canopy.

Then I applied the fiberglass. First a layer of 1 oz cloth followed by several layers of 6 oz. cloth. The glass and resin are left overs from a kayak I helped a friend build. I got the leftover resin and scraps of cloth for free. When the glass is cured I'll remove it from the fuselage, trim it to size, and add a suport frame and end dams to get it ready to pour the plaster. The cloth isn't thick enough to be self supporting. Besides, I wouldn't want the heat from a thick lay up to damage the foam.

More work on the canopy.

Once the resin had mostly cured but was still a little green, I removed it from the plane. The window film leaves a nice smooth surface on the glass. I trimmed it to size and taped it back on the fuselage to cure fully. The next day, I glued a FFF frame around the glass mold. The glass shell is too flimsy to support the plaster. I used the former templates with an extra 5/16" around the edge to account for the fuselage and glass and added a few cross braces. When that was set, I clamped a couple sticks to the top of the frames and hot wired them flat and parallel so the mold would sit flat. Dams were added to the ends.

Also shown is a foam insert roughly the shape of the canopy but smaller. When inserted into the mold, the plaster walls will be about 1/2" to 3/4" thick. The mold will use less plaster and dry faster this way. On larger molds with thick and thin sections, something like this would help prevent cracking.

I clamped the insert in place and filled the mold with water not only to check for leaks but tell me how much plaster to mix. I dumped out the water, mixed up some plaster and poured it into the mold. I removed the plaster plug after an hour.

I'll let it dry for a few days and then reshape the area where the windshield meets the fuselage. Then it will be ready to use to vacuform the canopy.
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Old Nov 10, 2013, 03:58 PM
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The first photo shows the rudder ready to coat. The bottom has a pin to pivot in a hole in the bottom of the rudder recess. Two pin hinges were also added. The rudder LE is so thick that the pivot point is buried; I had to use a 3/16" pivot hinge in the middle so it would be long enough. Also shown are the spacers that will be glued to the last former down the middle of the vertical stabilizer. It will be tricky to get everything in place and aligned properly in the deep rudder recess. Should have coated the rudder, mounted it and then done the shrouds.

I'll finish the fuselage and tail with Styrospray 1000. It's a 2 part polyurethane coating that adds a hard, structural shell to foam. I've used it for a couple years and its great for areas that would be hard to cover with paper or glass. I estimate its weight, dent resistance, and strength is comparable to 3/4 oz. glass and epoxy or WBPU. It's sensitive to environmental conditions: at least 70F and 50% R.H. (See the He-219 link for more on it's application.) Correct application takes some practice but when done right it leaves a smooth finish with no brush strokes or runs. I apply it with an old Gorilla glue bottle and move it around with a foam brush. I applied three coats, one every hour or so.

The fuselage was suspended above the bench on pivots glued into the fuselage and supports clamped to the bench. That makes it much easier to apply the Styrospray. I gave the fuselage 3 coats and the stabilizer only two. Styrospray also works on balsa. It soaks in and hardens it. It raises the grain so I'll have to sand it. The grain won't be completely filled so I'll either give it another coat or fill it with high build primer.
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Old Nov 10, 2013, 04:03 PM
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Wing

I'm using whats called the "Laser Method". Not sure why, but it works great. It gives you the advantages of both a built up wing and a hot wire cut, foam cored wing.

The foam ribs are glued between two building boards. That holds them in position while they are hot wired and creates bucks (like a foam cored wing) that are useful for clamping. Also, like a foam cored wing, you only need the tip and root airfoils. Washout (and dihedral if its not too great) can be built into the templates. The ribs are lighter than a solid core and its much easier to add internal structure (like a built up wing). You can add ribs of the thickness and location you want without having to derive their size and shape.

I'm using $Tree foam for the skins with FFF and some 3/4" XPS for the ribs.

The photos show the steps. It looks complicated, but each step is straight forward. You must however, do them in the proper sequence so a checklist is handy.
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Old Nov 10, 2013, 04:06 PM
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Now that the top skin is on, I'll add the spar box and wiring holes.

The blade spar will be a one piece, 0.1" thick piece of aspen (which is very similar to basswood.) It's sold locally as a secondary wood for furniture. I buy it rough sawn at the sawmill and its cheap. I selected a board that was quarter sawn and milled that for spar stock. Since the spar is one long piece, I'll use a 1/16" balsa box in the wing so I can add the spar when I join the wing halves.

The holes for wiring are cut with templates; a circle template for the ESC wiring with paper tube and a half moon template for the servo wiring. It would have been nice to use a tube for that too but the wing is too thin out by the aileron servo. Note too, a block of EPS foam was added to the servo mounting area to give me something to recess the servo into.
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Old Nov 10, 2013, 04:07 PM
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Now that the wing is built, I used the hot wire to make vertical cuts at the LE and TE to remove excess foam.

I added a strip of bamboo to the TE like I did with the rudder and a balsa LE. The bucks are useful for holding the wing while shaping the LE. I put down a strip of packing tape on the foam next to the balsa and planed the LE to shape. The plane will skate across the tape when you get close so you can plane right down to the foam. I used a sanding block to finish up.

The same building boards are used again for the other wing. The foam ribs pop off and any glue blobs easily come off the melamine coated shelving I use with a putty knife. You can use the same layout lines for both wings so they match. It just depends how you put the templates on as to whether you make a right or left wing.
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Old Nov 10, 2013, 04:09 PM
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Working on the fuselage again.

I cut out for the wing saddle. I added the servo tray and wing bolt mounts. The elevator pushrod is a flex cable and the rudder will get a CF pushrod with wire ends.

I had to reshape the plaster vacuform plug to fix the front edge. The shrink film release layer left a radius at the front edge of the front pane. Plaster is easy to sand if you need to fix problems. I'm pleased with the way it came out. I got a pretty good edge around the flat front pane. I got a decent canopy on the first pull too, which is rare for me.

Mounting it presents a problem in that the canopy will be part of the battery hatch and there's no fuselage between the back of the canopy and the turtle deck which will be attached to the wing. We'll see how well I can get the two lined up.

I recessed the fuselage to accept the canopy so it will sit flush. I'll give this area a few coats of Styrospray, masking the edge of the windows to get a ridge to simulate the frame. I still have to figure out how long to make the battery hatch. Then I'll add the cockpit floor, pilot, and instrument panel.
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Old Nov 10, 2013, 04:12 PM
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Working on the cockpit. I added a floor, back, and instrument panel. The instrument panel is just a jpeg that has been resized and altered to fit. It will be glued in place later. I'll add a seat back too. I'm not going to get carried away doing a detailed cockpit though it turns out I would have had the depth.

I calculated the moments of all the parts and determined where the battery will have to go to get the CG right. It looks like the battery will have to go way up into the nose. That has prompted me to reconsider the battery hatch. Now I'm thinking about molding a removable fiberglass nose. I wanted to redo the nose anyway (the nose isn't pointy enough) so it won't be a total waste of time.

The pilot figure was slosh cast with Easy Flo 60 casting resin. It weighs only 6g. The mold was made for my He-219 project from silicone caulk and corn starch as a catalyst. It's a great way to do silicone casting on the cheap.
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