Category Archives: Hog Bipe

Hog Bipe–Cowl Design

I haven’t posted any progress on the Hog Bipe in quite some time, but I have been picking away at it and I am almost ready to start final assembly. I’ll try to catch up and post some of the key build steps.

The biggest deviation from the stock kit was the creation of a custom cowl. A while back, I posted a Sketchup model of the Cobra 4130 motor to be used in this plane. I then went ahead and built a Sktechup model of the nose of the Bipe including my new cowl.

I started by scanning front section of the plans to use as a guide to the fuselage shape. I then modeled the fuselage for the first few inches back from the firewall and placed the model of the motor in place with the prop plane at the stock location.

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Next step was to try different section shapes and use the Curviloft Sketchup extension to generate the final form of the cowl. This was a bit of a trial and error process until I came up with a pleasing shape that blended to the fuselage and cleared the motor.

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Once again, I ran into the basic limitations of Sketchup where tiny computational rounding errors would add up and cause small gaps and imperfections in the generated surfaces. I had to blow up the cowl to 10x size to work with the design, then shrink it back down once I was done. While this helped quite a bit, I still had to do a lot of fussing and tweaking to stich surfaces together and close up gaps.

Here is the final shape.

Cowl Step 4

Next step was to design the structure of the cowl itself. I decided to use standard formers and sheeting for the back half of the cowl, but with all of the compound curves, I decided to carve the front half from balsa rings stacked together like a wedding cake.

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To generate the “layers” I created sections in Sketchup and then projected them forward. In order to minimize end grain in the final cowl, instead of cutting each layer out of a single sheet, the base form of the layer would be made by gluing  together square strips arranged with as much of the grain running lengthwise around the perimeter as possible. A horizontal and vertical line is used to align the layers.

Cowl Step 6

I then exported the final design to DXF and used a free DXF CAD editor to create a “blueprint” for each layer.

On to actual assembly. Each layer was built up from balsa stock. The outer edge was cut to the final shape using a scroll saw using the printed out layers.

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Here are the layers ready for assembly. Note that I did not yet cut out the center of the last layer, which is cut from a a single sheet  of 1/8” thick balsa, as I was afraid it would be a bit too flimsy. It will be cut out as the last step in shaping the cowl.

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The rear section was then built and sheeted using stringers and formers.

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Then each layer of the “wedding cake” is assembled using the horizontal and vertical tick marks for alignment.

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Then with a bit of carving and sanding, the final shape emerges.

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Using a Dremel with a sanding drum, I smoothed out the interior. I was afraid to get too aggressive with this step and sand through the wall. Note, I forgot to snap a picture of the cowl inside until right before I was ready to assemble it to the back section of the cowl.

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Here is the cowl assembled to the fuselage, ready for final sanding. I use three 1/4” dia rare earth magnets to hold the cowl to the fuselage along with a pair of 1/8” dowels as locating pins.

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Hog Bipe – Custom Battery Tray

Next up is to put together a custom battery tray. When performing a glow to electric conversion, often you can use the fuel tank floor for the battery tray. In this case, the floor needed to be lowered about an inch to have enough room for a 6S lipo pack. The stock floor is aligned by sitting in a notch formed by the fuselage doubler. Since I intended to lower the floor, I started by adding some balsa tri-stock to create an upper alignment point. I wanted an upper alignment so that I could easily add some cross-braces (see below) to reinforce the tray from beneath.

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I cut a new tray from 1/8″ lite plywood. Unlike the stock tank floor, I cut this to fit the full length between F1 and F3. This will give me plenty of room to adjust the pack fore and aft when balancing. I also wanted to add slots for a Velcro battery strap on either side of the middle of the tray. To cut these slots, I clamped a piece of straight scrap-wood to my drill press table, then drilled a series of holes along the length of the slot.  This arrangement allows you to quickly drill a series of holes along a line. The slots are then cleaned up with a popsicle stick with some sandpaper attached.

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The tray is then glued in against the tri-stock. Given that a 6S battery can weigh in at over 1-1/2 lbs, I wanted to reinforce the battery tray. I added some 1/4″ x 1/8″ basswood cross braces beneath the tray. This creates a light but rigid platform for the battery. I may add some balsa tri-stock to the corner formed by the fuselage side and the tray floor before is close up the bottom of the fuselage.

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Hog Bipe – Fuselage Top Sheeting and Battery Hatch

I finally got a few days to spend working on the Hog Bipe. I am deviating from the original kit a bit more now. I plan to add a custom scratch-made cowl (more on that coming in a future post), so I cut the fuselage cheeks off at this point.

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Another significant modification is to add a custom battery access hatch. I started by cutting the hatch floor from some 1/8 lite ply along with a new F1A and two sets of F2 formers. I used lite ply for the formers in stead of balsa since I felt balsa would end up getting damaged with routine handling of the hatch. One of the F2 formers was glued to the fuselage top at a slight rearward angle, allowing for easy installation and removal of the hatch with no interference. The new F1A was installed square at the front of the new hatch floor. I then positioned the hatch on the fuselage and glued the other F2 to the hatch floor at an angle to match the F2 previously installed to the fuselage top. Stringers were then added.

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To secure the hatch, I added a short 1/8″ hardwood dowel to the front and a pair of 1/4″ dia x 1/16″ thick rare earth magnets in the back. A strip of 1/8″ ply was added in front of F2 on the fuselage top for the mating magnets. The magnets were epoxied into 1/4″ dia by 1/16″ deep counter bores made with a forstner bit.

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Sheeting proceeded more or less per the kit instructions (sheeting the hatch and the fuselage front separately). I sprayed down the sheeting with a bit of ammonia based window cleaner to soften the wood, then  formed it around the fuselage using tape to hold it in place. After letting it dry overnight, I then trimmed it to final shape.

I added some scrap 3/16″ square stringer material to provide something to attach the sheeting bottom to.  Instead of CA, I installed each piece with Titebond, holding it in place with pins and tape. I let the glue dry for each piece before moving onto the next. A bit time consuming, but for me, this approach yields the best results.

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Sharp eyes will also note I also added a bit of sheeting to the F1 firewall bulkhead so that it would match the battery hatch. I am happy with how the sheeting came out. The hatch fits nice and snug, and all of the seems are nice and tight and will only require a small amount of filler.

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Next up is the battery tray, motor mounts, and then the custom cowl.

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.

Hog Bipe – Wing Construction Complete

I have been picking away at the wing construction on the Hog Bipe on and off. Sheeting the wing tips went smoothly and I am happy with the end result.

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The lower wing is built in two pieces and glued together to obtain the proper dihedral. One little trick I used was to cut a block of scrap wood with the dihedral angle for the inner wing rib. This allowed me to position and clamp the rib in place before gluing to the spar. The first photo below shows the block against the kit supplied dihedral angle gage. The second photo shows it in use.

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The rest of the lower wing construction was uneventful and came out straight and true with the right dihedral.

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I bought my kit off e-bay, so I am not really sure when it was made, but I was fortunate enough to have one that included the parts for the optional dual aileron servo setup in place of the single servo in the original kit. The dual servo setup is much stiffer and is less likely to have flutter problems. My only deviation from a stock build so far has been to add a couple of paper tubes for the aileron wires and to use 2 inch wide fiberglass tape t reinforce the lower wing joint instead of the 1 tape supplied with the kit.

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The plans call for using medium or thick CA to saturate the tape weave. I chose to use epoxy instead. This was my first experience using fiberglass cloth and epoxy, and I considered it a small scale trial run for glassing a balsa plane. I sprayed a light coat of 3M-77 adhesive on the glass and laid it down between two strips of masking tape. I chose BSI Finish Cure epoxy. This is similar to laminating resin and was available at the my LHS. To ensure that the epoxy would flow smoothly and to get a good cure, I pre-heated the bottles for 10 seconds in the microwave before mixing. I applied the epoxy with and acid brush and spread it smooth with an old gift card. I rough sanded with 80 grit and applied a second coat after an overnight cure, sanding out with 80 and 150 grit after full cure.

Overall, I am happy with the result. Structurally, I am sure this will be plenty strong. Cosmetically, I am satisfied for this application. It sanded out quite smooth, and would be a good surface for painting (if I was going that direction). I did end up with a few dry spots where the epoxy did not wet through the fiberglass fully, but I know what to look for now and should be able to avoid that in the future.

The last major step was to shape the ailerons to the tip. This also went smoothly with no real surprises or major problems. One minor problem was that one of the supplied aileron blanks did not have a well shaped rounded leading edge – a few minutes with a long sanding bar took care of that.

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Next up is to start in on the fuselage, but I will need to order my motor so that I can properly drill the firewall first.

Hog Bipe – Top Wing Construction

Made some progress over the weekend on the Hog Bipe. The top wing is close to being completed. So far, building everything stock and according to plans.

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No major issues so far and only one minor issue. The kit only comes with a single piece of 1/6″ x 3″ x 48′ sheeting to be used to sheet the front portion of the top wing. The piece was badly bowed such that I needed to trim it down to just a bit over 2-3/4″ to get a straight and true edges front and back. Fortunately, this was just enough to give me a good glue joint to the top spar.

The top sheeting was also a bit tricky to glue down properly. I started at the left edge and glued the front of the sheeting to the leading edger, one bay at a time, using medium CA and kicker. Once the front of the sheeting was attached, I wet it down with Windex to soften and help it to curve, and then using a toothpick spread TightBond wood glue on top of the ribs and along the spar. I like using wood glue rather than CA for attaching the sheeting to the ribs, especially considering the number of ribs involved in this long wing panel, as it has a longer work time and allows you to fuss and adjust a bit.

I then start at one end and press the sheeting down into the glue on the rib and on then the spar, pinning it as I go. I work one bay at a time and am careful not to let the sheeting buckle as I go. After removing the wing from the building board, I added a fillet of wood glue between the ribs, the spar, the leading edger and the sheeting on the inside.

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After letting the glue dry thoroughly, I pulled the wing off the board and after adding the maple mounting blocks, added the bottom sheeting. I used a slightly different technique this time. Here, I preformed the curved shape by wetting with Windex and gently bending the sheeting around the rounded corner of my workbench. It is not critical that the shape of the sheeting be perfect, just reasonably close.

I then applied wood glue to all ribs, the inside of the leading edge and spar and then dropped the preformed sheeting onto the glue. I then pinned the sheeting to the leading edge, and worked the sheeting back by pressing to the rib and spar just like top sheeting. On one side, I worked the leading edge to spar one bay at a time. Although this worked well, I ended up with a slight gap (about 1/32″) between the front of the sheeting and the leading edge on the last two bays on the outboard edge. I ended up filling it in with a bit scrap. On the other side, I pinned the sheeting to the leading edge first, then went back to pin down to the ribs and spar. This worked much better.

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That’s all for now.

New Build – Hog Bipe

With my SSE project nearing completion, I cleared off some bench space and kicked off my next build, a Sig Hog Bipe. I will build it as a “fantasy scale” early Army Air Corps plane with the blue fuselage and yellow wings color scheme. I am also thinking about some sort of custom cowl instead of the traditional Sig style open nose.

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Of course I am planning an electric conversion. Sig calls for 6.5 – 7.5 lb all up weight and a 0.60 sized glow motor. For electric motor sizing, I’ll round up to an even 8 lbs and plan on 150 watts per lb, which will put me right at 1200 watts. I want to keep with 6S batteries (22.2v), which will then need to draw about 54 amps.

I like to have the motor on had when I start the build, so I can use the motor mount as a template to drill mounting holes in the firewall before assembling the firewall into the fuselage. I am leaning towards a Cobra C-4130-12 motor. Their site includes a prop chart which lists about 1120 watts with a 13×6.5 prop and just a tick under 1200 watts with a 13×8. The 13×8 prop (drawing 54 amps), is just a bit over the 52 amp continuous limit for this motor, so as long as I am not flying max throttle all the time, I should be in a safe zone.

Battery capacity may be an issue though. I am hoping to use the same batteries here as my SSE. For the SSE, I want to keep to 4000 mAh maximum in order to keep to a reasonable weight. But 4000 mAh may not give me enough flying time for a heavier plane drawing higher current. As always, there are trade-offs with electric set ups, and I need to digest on this a bit before I make my final selection.

Since I have not finalized motor selection, I will get started on the wing panels now. Given my slow build rate, by the time I am done there, I will have settled on a motor and have it in hand. I will be back traveling again this week, so I won’t even be able to start until the week after, though.