OK here's my entry; the P-61. I've done 2 smaller P-61s in the past and they were great flyers. The P-61 has a lot going for it; a large broad wing and elevator plus twin rudders in great proportions for a model. It's also a trike with large wheels which makes it a good candidate for grass runways. Unlike a P-38 and many other twins, it has a long nose which makes it easy to get the CG right without adding lead. Like most twin boom aircraft, it makes sense to build the wing center and twin booms as one unit. This will be 1/12 scale or 66" wingspan so I'll make the outer wing panels removable.
The P-61 has a big downside and that is all that glass! I spent more time making the vacuform plugs than I did on the rest of the model for my smaller one. There is however, the last P-61 variant: The RF-15 Reporter which was an unarmed photographic reconnaissance aircraft. The prototype was modified from the last fighter version, the P-61e. The Reporter had a single tandem cockpit with a bubble canopy.
I've tried to interest some other members of the club to build one and share some of the cost of getting all the plugs made and the canopies pulled by offering copies of the fuselage shells at cost. If I can get a couple takers, I'll go ahead and do the fighter version. If not, I'll buy a canopy from Parkflyerplastics.com and do the Reporter version.
The first thing I do is find a good 3-view and import it into CorelDraw. I use an old version that runs on windows XP but it does everything I need. I resized the 3 view to 1/12 scale. On a new layer, I trace the outlines of the nacelle. I'll trace a second outline 1/4" inside the first and build the forms to that line. On a another new layer I start drawing in structure. In this case, I'll need some support for the retracts. This is where I work out the geometry and the support structures. For the nacelles I'll use a simple 3 former unit for the firewall and retract supports. Finally I derive the shapes of the formers using the top and side views along with cross sections from the 3-view. That will usually get me close. I'll tweek the shapes if needed when I make the forms.
I'm working on the nacelle molds here. I printed out the mold outline and glued it to two 1/2 plywood backer boards. After cutting them out and sanding to the lines with a disk sander, I rough cut foam blocks with a hotwire and glued them to the backer boards. I left a gap between the blocks so I could slide in cardboard templates to check my progress while I shaped the forms. Once the forms are close to the final shape, I'll worry more about getting the right and left halves the same than matching my drawing. You never know if the 3-view is correct or if the drawings are accurate.
Shaping the rudder/fuselage junction was tricky but photographs and a 1/48 plastic model helped get that area at least close to the scale shape. You have to compensate for the thickness the FFF skin will add.
Drywall sanding screen is my weapon of choice for shaping the forms.
(The process for heat formed FFF is covered in more detail here)
First shells off the molds. I was a little worried about the area where the rudder meets the fuselage as it's slightly concave and the molding process doesn't really work very well on concave curves (like fillets). Right as I took the mold out of the oven I pushed down in the concave areas and let them cool. It worked pretty well.
The photos show the pair and the inside of one shell. It's hard to compare the shapes when they are covered in tape with my plastic model, but it seems good. I'll go ahead and mold another set.
Joining the nacelle halves. I want to make the wing cut outs with the hotwire and a jig so the plywood formers can't be glued in yet. The rearmost formers and rudder rib can be glued in now so they were first glued into one half. After marking the seam with a dry erase marker to make it easier to see, I glued the halves together.
Once dry, I pushed in the front three formers (cut from foam) and they are held in place by friction. The end former is 3/4" and tapered like a cork so it fits tightly.
I'll fish the servo wires in later, we'll see how well that goes!
cotton ball, string and a vacuum cleaner. Tie string to the ball (or part of a ball) Stuff the ball in the hole... turn on vacuum and it will pull the string.
The vacuum and string method works great, especially when you are pulling wires in a wing with paper wiring tubes installed. In this case, I'll probably just use lamp pull chain and gravity. There's only two formers to go through and I'll have the large wing cutout to work from.
Originally Posted by thepiper92
Interesting. So feed string through, tie to servo wire, and other end to cotton ball?
Tie a string to a small cotton ball and hold the ball near one end of where you want to pull wires. Hold a vacuum cleaner nozzle near the other end and let the vacuum suck the cotton ball (and string) through to the other side.
Here's a shot of the rudder setup. The P-61 has a distinctive curved LE on the rudder and I wanted to reproduce that. I took 2 layers of 1/16" balsa, wet them, and pre-bent them by rolling over a can. Then I glued them together and taped them to a foam mold cut to the proper curve and let them dry. I made the balsa wide enough to get both sides from.
After the piece was dry, I cut it in half and used it to trace the cut out on the foam. The balsa was then glued to the foam. The balsa will give a nice strong surface to glue the hinges to and acts as a vertical rib to strengthen the rudders.
I built up a pair of new rudders from balsa. I found out on the Tigercat that it's just as easy to build new rudders than to try to use the foam cut offs. The full scale P-61 has fabric covered control surfaces and that's easier to reproduce in balsa. I made the LE over-sized to I could cut it to match the curve of balsa glued to the fuselage.
I'll use a pair of micro servos to operate the rudders. They'll be partially hidden by the elevator and that's much simpler (and probably lighter) than linkage to tie the rudders together and 1 servo. I'll use pin hinges for the rudders in more or less scale locations. I'll need to use a long, 3/16" pin hinge at the center to reach!
I was cleaning up an old computer to get it ready to recycle and found a photo of the first P-61 I did. The main fuse was monobloc style foam covered in glass and epoxy. The nacelles were just profiles in 1/2" foam board but they had full shrunken pop bottle cowls. As I recall, it looked like a full fuselage model in the air. Solid foam wings covered with paper. WS around 48". Hand launched belly lander.
This thing flew great! It was my first multi electric motor model and I've been hooked on multi's ever since. It died a spectacular death when I attempted an inverted outside loop with just a few feet too little altitude. Pretty much had to pick up the pieces with a tweezers!
A little more work on the nacelles. The LE of the fin was reinforced with two layers of 1/16" balsa. The balsa LE will add a lot of dent resistance. The balsa was wet and pre-bent with heat to the approximate shape and glued onto a flat sanded into the foam. The fins were sanded to shape and given a coat of WBPU to harden the foam a bit. That way, any filler can be sanded flush with the foam without removing more foam.
I'll make the wing center section so I can trace it for the templates to hot wire cut the wing saddles in the nacelles.
I'm using the so called "laser method'. Rib blanks are glued between two boards and the ribs are cut with a hot wire. It combines the best attributes of a solid foam wing and a built up wing. The two boards ensure a straight wing every time. It looks complicated, but each step is easy and fast.
Structurally, I'll use a pair of blade spars in the center section. The formers that carry the main gear mounts will bear on the spars as well. The outer sections will be removable and I'll use carbon arrow shafts and epoxy soaked paper wing tubes. I'm going to attempt to make the entire center section straight, in one piece, and then cut it two and sand in the dihedral angles. The spars will be one piece so I'll build in balsa boxes to receive the spars.
1. Cut out the rib blanks and hot melt glue them to a board. The layout was transferred from the plan and drawn directly on the board. Either FFF or 3/4" foam depending on their location.
2. Glue on the top board with dots of Gorilla glue. (Takes too much time to use hot glue.)
3. Affix the hot wire templates.
4. Make the first cut.
5. Make the slots for the spar boxes.
6. Make the holes for the wing tubes. Each side has to be done separately to get the angles right.
Still more to do before I add the top skin but that's enough for tonight.
7. Make holes for servo wires.
8. Glue on top skin w/ white Gorilla glue taking care to keep it out of the spar box slots.
9. Use double sided tape to (accurately) stick the top buck back onto the top skin. When the bottom cut is make, the wing will now be attached to the top buck so it stays straight.
10. Add templates for bottom and make the hotwire cut.
11. Glue in the 1/16" balsa spar box sides (with carpenters glue so no expansion into the slots) and wing tube (with Gorilla glue to fill any gaps).
12. Glue on the bottom skin. Again, keep the glue out of the spar boxes.
13. Before removing the wing from the buck, affix a couple vertical templates and make the cut for the balsa LE with the hotwire. The TE can also be cut this way but I usually cut it with a ruler and Exacto knife.
I make a list of the steps and it really goes very quickly. I'll add a 1/4" balsa LE and a wood or bamboo TE after I cut and sand in the dihedral angles.
The last picture is of a fellow aviator who stopped by the shop to see what was going on.
Now that the center section is done, I cut it in half and sanded in the dihedral angles. I cut the wing in half on the band saw and rough sanded in the angles with a disk sander. I used the wing bucks to hold the wing level and set the angle on the disk sander's table. 4 degrees for the center and 1 degree on the outboards ends.
The blade spars are 0.1" Aspen. I resawed and planed larger boards to the correct thickness. It's characteristics are similar to Basswood but it's available around here much cheaper than Basswood. The spars slide into the spar boxes and will be glued in eventually.
I was able to test assemble the wing and check the alignment of the wing tubes. Within the limits of my ability to measure, everything looked spot on.
The whole point of doing the wing center section now was to trace it to make the wing saddle cutouts in the nacelles. The airfoil was traced and transferred to 1/8" luan plywood. The cut was lined with aluminum tape to make the hotwire slide over the template. The incidence angle and height above the nacelle centerline were built into the template.
The nacelle is held above the table with the fixture shown. It's the same holder I use when using Styrospray. The nacelle is rotated in the jig to compensate for wing dihedral, one rotated right and one left. Tape is used to keep the nacelle stable during the cut and works surprisingly well. The height of the jig is critical for a cut like this. The cut would have obviously been too deep with the jig as built so I slipped a piece of 1/8" plywood under the jig to raise it up. The cuts look pretty good. The depth is close and the angles are OK. The fit is good too, not so tight that a lot of sanding is needed and not so loose that it can't maintain alignment.
Outstanding......as I always do while following your threads, learn new tricks every day.
Firewall and retract mounts.
Because the nacelle is tapered at the front, I can build the firewall, gear mount assembly without actually glueing it in yet. This made it much easier to cut and fit everything into place.
First, I marked the locations of the wing spars on the nacelles (the formers bear on the wing spar) and checked the diameters of the formers at these locations to make sure they fit. Then I started cutting square holes for the 1/2" maple bearers for the retracts. Once that was done and I verified that everything fit, I could cut out and drill all the openings for the gear leg, door servo, wiring holes, and the dihedral angle.
Finally, all the pieces were inserted into the shells and taped in place. The formers were then glued to the maple bearers. Each assembly will eventually be glued into the nacelle shell.
I still have to sort out how and when I'm going to get all the wiring in place. I've found it pays to get that figured out before you get too far ahead with assembly.