Mustunt IV, Part 5, Basic Building Techniques

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Finally, work begins on the fuselage. The fuselage sides are laser cut in two pieces because of size limitations of the laser cutting machine. Strength isn't an issue as the joint is located over the wing and is reinforced with the fuselage plywood laminations. The two pieces have to be carefully aligned and joined. First, I laid the two pieces on a bit of parchment paper for gluing. Nothing sticks to parchment paper. It will protect the building surface. Next, I align the two pieces. The fuselage top, aft of the cockpit, is straight and parallel to the thrust line of the airplane. The fuselage top forward of the cockpit isn't straight but there is a removable projection on the top at the very front which is there only for aligning the fuselage. To obtain the very best alignment, I use a 4' level for a straight edge. Its edges are machined to a perfect straight edge. The two pieces of the fuselage sides are aligned against the level and glued with CA.

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The one piece fuselage sides are then laminated to a 1/32" plywood reinforcement. While I'm at it, the two forward fuselage bulkheads are also laminated to laser cut plywood matching parts. The adhesive I use for laminating large surfaces is Zap finishing resin mixed one part resin, one part hardener and one part acetone. This makes a very thin adhesive applied with a disposable acid brush. I buy these brushes in packages of 36 from Harbor Freight. I avoid using TightBond to laminate because it is water based and the moisture will warp the laminated pieces, first one way, then the other as the glue dries. Small pieces, like bulkheads, can be laminated with medium or slow CA.

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After applying the epoxy, the fuselage sides are carefully aligned and held down tightly with a number of steel weights purchased from Tom Morris. The laminated parts will mostly cure in about three hours, but I leave them overnight.

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Curiosity compels me to check the fuselage side fits and examine the way the bellcrank projection will fit against the inboard fuselage side. Obviously, every thing is fine. The laminated fuselage side fits the wing light tight with no need for fitting.

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When the sides have hardened, the engine mount assembly can then be installed using either CA or the same thin epoxy. The engine mount has a bit of engine offset so it isn't obvious where the mounts should be aligned horizontally. The engine may be offset but the rear of the plywood engine mounting plate is square to the fuselage. The top of the plywood engine mounting plate aligns with the top of the forward fuselage with the rear of the mounting plate just touching the fuselage at the location of the engine compartment bulkhead. This is where the fuselage side changes direction from along the engine mount to straight up. The wing leading edge bulkhead is also installed at this time and its top will help with the accurate alignment of the engine mount assembly.

Al

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Fantastic series. I'm learning stuff all along the way.

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This is one of the more critical operations in the construction of the fuselage. The aft ends of the fuselage sides are brought together matching the tops. The sides generally match in length but not accurately enough to trust. The sides can be jigged, but I find that sufficient accuracy can be obtained by sighting down the fuselage from the front. Add a bit of 1/8" strip between the fuselage sides protruding to later form a mount for the vertical fin spar. Check the last photo on this post to see the fin mount between the fuselage sides. Slip the sides against each other until the fuselage is visually straight. My sides mismatched less than 1/16" in length but the misalignment was noticeable when the ends matched. After the ends are glued, add the cockpit instrument panel bulkhead. Its width is the same as the forward bulkheads at 2 1/4". to this point the sides are parallel. Two 1/4' x 1/8" pieces of strip wood 2 3/16" are added at the top and bottom rear of the plywood laminations. This begins a very slight narrowing of the fuselage, but the sides are still square. At this time add 1/8" square strips to the bottom of both fuselage sides from the wing cutout rearward to at least the stab leading edge strip. Call these rounders. These strips add bulk to the fuselage bottom to allow rounding the fuselage sides to the 1/16" sheet bottom. I failed to use these strips, didn't like the shape of the fuselage, and had to remove the fuselage bottom to bring my airplane to this configuration.

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The fuselage, back to the ends of the plywood laminations, has plenty of strength and rigidity at this point. The fuselage aft of the laminations must be built up in stages to maintain alignment. From the laminations rearward the fuselage narrows faster at the top than at the bottom. There are only two bulkheads aft of the laminations. One is at midfuselage and the other at the stab leading edge. The midfuselage bulkhead is made up of strip wood cut to plans lengths and vertical pieces to complete the bulkhead box. Bulkhead strip wood is 1/4" x 1/8". The stab bulkhead is a piece of 1/8" x 3/4" x 1/2" wide. There is another piece of strip wood 15/16" wide between the fuselage bottoms at the stab leading edge location. This also locates the front of the tail wheel mount. These two bottom cross pieces can be cut around the 1/8" square rounders. This narrowing of the fuselage top is tricky but doable and, I think, worth the effort. If the fuselage is still straight at this point it can now be firmed up by beginning to sheet the bottom from the wing rearward to the stab leading edge strip bulkhead. At this point I actually install the tail wheel using a bit of scrap wood on each side of the mount at the front and rear of the mount. The rest of the bottom can be sheeted anytime. In this photo, the stab is simply sitting in place.

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Now the stab is inserted into the stab slots and aligning begins. The ends of the stab are "trammeled", that is, adjusted to an equal distance from a point on the fuselage center line at the wing leading edge bulkhead using a yardstick. This makes the stab hinge line perpendicular to the fuselage center line.

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The stab ends are checked for equal height above the building surface, with a ruler, to insure the stab isn't tilted. The laser cut fuselage sides will probably have the stab level but, if not exactly so, then trim one or both slots slightly to level the stab. To complicate matters a bit, at this point I actually add a small cross piece under the stab trailing edge to slightly widen the fuselage at that point to make a more stable bed for the stab. This doesn't interfere with the elevator pushrod as it exits the fuselage at about the stab leading edge. Incidentally, the stab slots locate the stab with one degree of incidence. I use two degrees on all of my semi-scale airplanes and one degree is a bit of a compromise.

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With the stab in place it can be glued. To give additional resistance to stress cracking at the stab leading edge, the vertical fin has a downward projection on its leading edge, between the fuselage sides, which fits tight to the top of the stab unifying the two assemblies. The fin can be installed now or after the fuselage and wing are joined. I waited to insure the stab was level and parallel with the wing in case adjustments were necessary. None were.

Al

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This shot shows most of the information used to assemble the fuselage to the wing. Slip the fuselage onto the wing and confirm that the inboard and outboard wing panels are equal in length when a 3/32" ply "hard spot" is inserted between the triangular projection of the BBQB bellcrank mount and the inboard side of the fuselage. I goofed in this and wound up with an outboard wing 1/8" longer than the inboard wing panel. No problem. I fixed this later by adjusting the wing panel lengths and asymmetry in the carved wing tips. I picked up the problem when I didn't allow for the difference in fuselage widths between the fox 35 and OS LA 46. In any case, now is the time to make sure that this isn't necessary. Add shims or trim the triangular projection as necessary. Properly, the wing tips will be 2" plus 1/4" inboard and 2" minus 1/4" outboard for 1/2" of asymmetry. I initially glue the wing in place with CA. Later I come back and add pieces of glass cloth inside and outside of the fuselage/wing joint, full length of the chord, to reinforce the joint. Note again the fit of the wing and fuselage intersection. It is light tight without adjustment. This fit is complete insurance that the wing is installed with no unwanted incidence.

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The underwing strips of fuselage sides can be added at this time also. Make sure that they follow the lines of the fuselage nose tapering only 1/32" each side to the laminated aft fuselage. There isn't much point to adding glass cloth and epoxy here, though make sure that there is plenty layers of glass and epoxy around the wing leading and trailing edges to resist stress cracking. This is a good time to add the 1/8" square rounders to the wing area. The nose bottom block will come to nearly mid chord of the wing and won't need rounders.

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With the fuselage and wing joined, the vertical tail surfaces can be installed. Make sure that they are vertical. The sheet sides of the fin bottom glue to the 1/8" fin mount in the very end of the fuselage. The downward projection of the fin leading edge should be glued to the top of the stab.

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This photo shows the general arrangement of the top of the narrowing fuselage. If the fuselage doesn't narrow, the turtledeck moldings won't fit the fuselage sides.

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The elevator pushrod can be made now. It starts with a 90 degree bend at the flap horn and runs to an elevator horn clevis at the aft end. The clevis should be located so that there are plenty of adjusting threads on each side of the clevis when the clevis engages the elevator horn. The plans are useful in deciding where the only bend, at the flap horn, is made. The clevis I'm using here is a plastic clevis with a metal pin to resist wear. The pushrod will exit the fuselage about an inch ahead of the stab leading edge. Only one shot, coming up, shows just a bit of the opening. Ultimately, the pushrod opening will be covered with a fairing to make its appearance less noticeable. An exterior elevator horn is a lot easier to build, install and adjust. Its use follows the Mustunt concept and functionally, is equivalent to an internal horn.

Al

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This is how I add strips of glass to reinforce the wing fuselage joint using thinned Zap epoxy.

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Here the elevator pushrod has been installed and soldered at the flap horn through the top of the fuselage. Pushrods made from music wire are notorious for rattling noticeably against structure inside the fuselage. Remember the noisy old Noblers before we started using glass pushrods? It is virtually impossible to get a straight shot between the flap and elevator horns. Practically, there will need to be a very slight bow in the pushrod to exit at the desired location and with enough clearance on the aft fuselage to be installed and adjusted on the external elevator horn. The bow is slight and to minimize noise effects it is installed through a fairlead mounted on the midfuselage bulkhead. I used plastic tubing. Maybe a smaller i.d. would have been preferable. In any case, some kind of fairlead will be necessary. It will take a bit of finagling to get every thing situated.

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Making turtledeck molded shells is much easier than you might expect. Kyle cuts mold parts consisting of a plywood top view, side view and several bulkheads. these are assembled and filled with foam. Blue foam works best, but any foam or scrap balsa will do. The foam is sanded to shape and the mold glassed. It shouldn't take more than an hour to make the mold and another hour to mix resin and glass it. Making shells is pretty much the same as the explanation for molding wing leading edges in an earlier post. Installing the shell is also easier than you might expect. The shell is held against the turtledeck bulkheads and marked, from the inside, at the top of the fuselage side, for a cut line. Cut the shell here and trim the top to fit the center of the turtledeck top longeron. The first half can be glued from the inside.

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Once the first half of the turtledeck is installed, the process for trimming the second half is obvious. In fact, both sides can be trimmed before either half is glued on. When CAing the inside of the first shell, hold pressure with your hand to press the shell against the formers, fuselage side and longeron. The second side will need to be installed with a slower drying adhesive like TightBond. It is held in place with strips of masking tape until dry. Parts of this installation can also be reached from the outside with CA like the cockpit bulkhead. If I have a really good fit, I just TightBond the bulkheads and Ca the rest from the outside and clean up the exterior joints later with nitro.

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Ahhh yes! It is starting to look like an airplane and I've only posted a hundred and fifty photos and explanations so far including the update and dihedral posts.

Al

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Work begins on the nose with adding a 1/16" balsa floor to the tank compartment between the engine bearers. Making the floor flush with the top of the bearers saves room for tank adjustment.

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With the tank floor in place, the tank compartment, except for the missing roof, is totally fuel proofed with epoxy and glass cloth over every seam and corner.

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The top of the tank compartment is now added. It is painted with epoxy on the bottom and installed parallel to the bottom of the tank compartment. The fuselage sides are not parallel with the engine mounts in this area. The rear of the tank compartment top is spaced down about 1/8" and a spacer added which will later be the base of a forward fuselage bulkhead.

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With the tank compartment top on, the airplane is inverted, and the seams between the tank compartment top, back and fuselage sides are painted heavily with epoxy to completely seal the compartment. The tank compartment will be open at the front to allow access for installing, removing, and shimming removable tanks. There will still be bulkheads above to complete the shape of the forward fuselage.

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Here are the original forward fuselage top bulkheads. The configuration was changed later to respace the bulkheads and allow modification to the airplane's profile. This will also be covered later as a modification which greatly improved the appearance of the airplane.

Al

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Hi Al....would it be possible for you to show how you do your pushrod bushing and connection? I can sort-of-see in the one pick....but a miny tutorial would be wonderful.

It also looks like you put those glass strips on the inside of the fuse to wing joint....am I correct in this assumption

Glen,

Check out the "Design and Construction of the Mustunt IV, Part 2" which is actually going to be renamed "Mustunt IV, part 2, Basic building Techniques". I think that will answer your questions.

And yes, glass was used both inside and outside for an exceptionally strong joint between the fuselage and wing.

Al

Thanks Al....appreciate the advice and the time your spending to do this. I am currently downloading all of it to my computer so I won't loose any of it. Thanks again.

Sorry, just copying some information.

Al