Hog Bipe – Fuselage Construction Continues

Spent a bit of time working on the fuselage this weekend. After installing the T-Nuts for the Sullivan tail wheel in the fuselage bottom, I spent some time fussing about with the formers and the fuselage top and bottom pieces. I followed the basic approach that SeamusG used in his very well done Hog Bipe Build Thread on RCU. I used some 1/4″ balsa blocks to pin the forward section of the fuselage to the building board, I then dry fit the formers and other parts together with clamps, tape, and rubber bands, fussing about until everything was square.

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To ensure that I had everything straight, I set up my laser level with a vertical beam, and lined it up with the fuselage, checking that it hit the top center of each former. I also dry fit the rudder to ensure that it would install square. When happy with this, I went ahead and started to apply medium CA working from the bottom up as described in the instructions. Hopefully, everything stayed straight and square while gluing.

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Just like SeamusG described in his build thread, I also ended up with a bit of a gap between the in the fuselage bottom plate and sides near the front of the plate. I decided, the fill the gap in with a bit of aliphatic resin (Tite Bond). As a bit of extra insurance, I used some balsa triangle stock to reinforce the joint between the bottom plate and fuselage sides and F5. I probably would not have done this if I had gotten a good wood-to-wood bond joint between the sides and the bottom near F5, but the area right behind the wings tends to be a weak spot in many planes, and for a gram or two of weight, this will add a lot strength.

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I also started planning out the battery hatch. Although SeamusG built a glow plane, his build included a mod to add hatch to access the fuel tank. Following his lead, I will add a hatch between F1 and F2. With this in mind, I realized that I should not have glued F1A to the back side of the firewall F1. This double really is not there for strength, but as a place to attach the stringers used to help shape the forward section sheeting. Instead I want to use this piece as the front section of the hatch.

With a little care and a long X-Acto blade, I sliced F1A off from the firewall. The back side is a bit damages and the part will be too week to use for the hatch, but it is in good enough shape to be used as a pattern to cut a new piece. A little sanding and the back of the firewall looks like F1A was never there!

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Next up will be the creation of the hatch floor and bulkheads in order to prepare for sheeting. I also will need to modify  and reinforce the fuel tank floor to serve as a battery tray.

Hog Bipe – Tail and Fuselage Started

Now that the wings are done, I started work on the tail feathers. I wanted to give the plane more of a “vintage” look with a more rounded fin and rudder. I started by tracing the original outline from the plans. I then used my old flexible curve (an old drafting tool that you can bend to an arbitrary curve and it will hold its shape) to create a new outline. No science involved, just going by look. You can see the dotted line for the original and the darker line for the new shape.

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I then cut new pieces from 1/4″ x 4″ wide balsa sheet and assembled to the original kit supplied tail piece. I sanded a taper in the rudder from 1/4″ to 1/8″, like the original kit supplied part, but the rudder seems a bit too flexible to me now, so I may cut another and leave it un-tapered (or less tapered). I also may cut some lightening holes in the rudder (and elevators), but will wait for a trial balance first.

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The stabilizer has been built according to the plans, but I will probably give the elevators a similar rounding off treatment to the rudder. I ended up with a slight bow to the stab after sheeting (about 1/16″ total). Not perfect, but not enough to worry about either.

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My motor arrived earlier this week too. I settled on a Cobra C-4130/14 instead of the C-4130/12. The “14” had a slightly lower Kv (450 vs 540) which will let me run slightly larger prop. Equipped with the motor mount, I marked and drilled the firewall. I also added a few holes for air flow (one directly behind the motor and set along the bottom).

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I glued up the fuselage doublers using Titebond weighted down with some paint cans to hold them flat while the glue cured (no pic). Unfortunately, there are no positive features in the design of the doublers to guarantee alignment. and one of the doublers slipped forward. The notches in the doublers will set the firewall position, and if they are not perfectly aligend with each other, the firewall will not sit square. To compensate, I slightly widened the back of the notch to compensate for the slip, this will allow the firewall to sit square against the back of both notches and keep the fuselage sides in line with each other. If I were to build this kit again, I would drill a couple of 1/8″ holes and use a couple of dowels to pin the doublers before gluing. The dowels could then be cut flush and sand smooth and you would never know they were there.

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The cabane mounting blocks were next up. Thes must be positioned properly in order to ensure proper wing alignment and incidence. Here again, there is no positive alignment feature in the kit, so to ensure proper alignment, I used the following technique. First, I drew a line marking the proper position for the top of the block (1/8″ below the top of the fuselage side. Next I cut a short piece of 1/8″ dowel, and  inserted it into the laser cut hole in the fuselage side. I then slipped the aluminum cabane strut over the dowel (the dowel was cut so that it was flush with the cabane strut). I next positioned the mounting block over the cabane strut with the top of the block on the alignment mark (not seen in the picture, but the mounting blocks have a rabbet groove that the aluminum strut fits tightly into. This ensure that that the block is in the proper position and that the hole in the cabane strut will be aligned with the hole in the fuselage. Finally, I position the laser cut “C” shaped plywood positioning plate around the block (basically in the reverse order that the instructions call for). With everything lined up snugly, I tack the plate in place with a few drops of CA, then remove the block, the cabane strut, and the alignment dowel. I use CA to secure the mounting plate and a little Titebond to glue down the mounting blocks (being careful to keep the groove clear of glue). This is then repeated for the other three mounting blocks.

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The instructions call for then drilling a 9/64″ hole through the fuselage and into the mounting blocks. This allows you to install a 4-40 blind nut in the back of the mounting block, but a 9/64″ hole is way to sloppy for a 4-40 bolt. I start by drilling a 1/8″ dia hole from the outside through the mounting block, using the existing hole in the fuselage for alignment. I then drill a 9/64″ dia hole through the back side of the mounting block (not going through to the fuselage side) to accept the blind nut. Last step is installation of the blind nuts themselves. I did not want to take a chance at messing up my perfectly aligned assembly by hammering them in place, so I simply drew them in by tightening a screw and washer.

Finally, I epoxied in the firewall and F3 former, using my 1-2-3 blocks to ensure everything was square. This was done in two steps, first gluing the firewall and F3 former to one side, letting the epoxy cure overnight, then flipping this assembly over and gluing to the other fuselage side.

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Next up is to assemble the rest of the formers into the fuselage. Since I plan to use a Sullivan tail wheel assembly, I will need to drill and install blind nuts in the fuselage bottom. This will be much easier to do before everything is assembled, so I guess it’s off to the LHS before I can continue.