Building that Pesky “Round” fuselage… by Chris Cox
I don’t think there was a conscious decision for several of us West Coast flyers that we should all bird Warbirds. Paul Walker, Alan Resinger and I were more interested in a motor, wing and stabilizer inline setup. The issue with this is that the fuselage profile, when going inline, takes on quite a different appearance than the more conventional setups that we are accustomed to. One solution to get around this is to model the aircraft after one of the many “round fuselage” World War II aircraft that utilized radial engines, which generally sat more in line with the aircraft centreline than the Allison or Rolls-Royce powered aircraft. In deciding what to build for the upcoming season, Paul decided to go with another P-47 to explore a few modifications that he felt were necessary from last year’s model. Alan chose the Bearcat which he would paint in Rare Bear livery. I really liked the looks of the F6F Hellcat, so I decided to go that route.
Mike Haverly was really taken by Alan’s drawings, so Mike began construction on a Bearcat as well and as we later learned, Monty Summach from Saskatoon, Saskatchewan began construction of a Harvard (Canadian version of the AT-6 Texan).
Rather than drawing new plans from scratch, it occurred to me that Kaz Minato had an outstanding Hellcat model that, although not in-line, could potentially be modified if drawings were available. A few quick emails back and forth with Kaz and, in no time at all, I had a set of plans delivered to my home.
It should be noted that the Minato Hellcat was an all composite model, so very high-tech molds had to be manufactured from which the composite lay-ups were produced. Because of this the drawings were somewhat lacking in the required number of bulkheads and even the bulkhead shapes that were not totally scale-like. That said, the drawings did provide me with a great starting point. I had several copies of the provided plans produced, which I then proceeded to cut up in order to move the thrust and stabilizer centreline down and the wing up. I quite literally used the original “cut and paste” method to do this. At the same time, the nose was lengthened to what I felt would be required for the electric setup. I then “Googled” 3 view drawings of the F6F looking for proper bulkhead placement and shapes, from which I reproduced these bulkhead drawings to the appropriate size and fitted them to my “cut and paste” drawings.
It was now time to begin construction! This would be my first “round fuselage” stunt ship, but with many pictures and guidance from Paul and last year’s P-47, I figured I had the necessary skills to do so.
The Molds (Moulds for you Commonwealth folk)
First up was the construction of the “mold”. Using the fuselage profile from the drawings, I cut out two ¼” thick plywood blanks. I glued/screwed on 1” x 2” spines to help ensure the molds remained straight, both over time and during the balsa shell lay-up. Next, I produced cardboard bulkhead templates that were then cut vertically down the centrelines. At the same time, I marked the horizontal centrelines which would be very important later on when jigging the bulkheads. I then reproduced these left and right side bulkheads on ¼” plywood, which were then glued onto the two fuselage blanks in their appropriate locations. I took some time here to ensure they were aligned as accurately as possible and vertical to the fuselage blank.
A trip down to the local hardware store was necessary to purchase foam board (I used Owens Corning Foamular C-200 extruded polystyrene. It was only 1” thick which necessitated layering pieces to get the correct thickness), which in turn would be glued between the plywood bulkheads. Now came the fun part. I initially carved the fuselage sides to shape with a long blade knife, and then followed up with coarse sanding and eventually fine sanding. Once the shaping and sanding was complete, I applied a coat of Zpoxy over the mold to provide added hardness and resilience.
Next up was to find some light “A” grain 3/32” balsa sheet from which the fuselage shells would be molded. A call down to Elliot Scott in California confirmed he had what I was looking for, which he hand delivered to me at the Golden State Championships last October. It required three sheets of 4” wide balsa to produce the overall width required to wrap around one side of a mold. When splicing balsa sheets together, I accurately cut a nice straight edge on the two pieces to be joined, sand the edges lightly with a long straight sanding board, and then with the sheets held tight to one another, I run a line of thin CA down the entire seam. The seam is then sanded to ensure both sheets match in thickness with no irregularities across the glue joint.
The joined sheets are then cut to shape to ensure the fuselage area will be completely covered, but not a lot of extra extending past the mold edges. It should be noted, that because the F6F incorporates a turtledeck aftof the cockpit, a vertical cut needed to be made to allow the balsa to conform to both the front and rear portion of the fuselage. This could be done using two separate pieces and joined later, but I chose instead to only make the cut long enough to allow the balsa to conform to both the front and rear sections. I then fill the bathtub with about 2” of hot water. Some people like to add some ammonia to the water believing the wood will flex better, but I have not found this to be necessary. After about 30 minutes of soaking in the tub, the balsa should be pliable enough to work with. Use a tension bandage available from most pharmacies to wrap the balsa onto the mold. I started from the rear and worked my way forward. A second set of hands comes in quite handy here, although the 1” x 2” spine could be clamped in a vise to hold the mold steady when wrapping. I was a little delusional in hoping that no wedges would need to be cut from the sheeting in the nose area due to the concave/tapering shape of the fuselage, but it quickly became clear that wedges would indeed be necessary. The best way to do so is to make the first cut to the length you feel will be required and then overlap the sheeting so that the fuselage shape is attained. Then make a second cut to remove the excess balsa. It sounds difficult, but in reality, it is not. By the way, Bob Hunt has an excellent instruction manual available on how to do all of this!
Once the sheeting has been all wrapped up, the molds can be set aside to allow the balsa to dry out and the permanent shape captured.
Unlike your conventional stunt model, there is really no way to build this fuselage on a flat building board. (Well actually there is, much like to Guillow/Comet rubber model kits if one were so inclined…) Using the horizontal centrelines on the bulkheads, I was able to determine the height above the building board each bulkhead would need to be placed. Please note that I am building the fuselage upside down! Alan Resinger, as he so often does, came to my rescue and fabricated the necessary jig pieces that the bulkheads could be slipped into and held at the correct height. Perfect alignment of the bulkheads is critical, and Paul Walker developed a rather ingenious method to ensure everything is aligned, both vertically and horizontally! Another trip to the local hardware store was required to purchase a laser levelwith both vertical and horizontal projections! These tools are fairly inexpensive, and you could probably buy a decent one from eBay or Amazon in the $40.00 to $60.00 range. Trust me when I say, this is a purchase you will not regret as it has so many other really cool applications, many of which I will soon explain.
Set up your laser at one end of the work table so that it is shooting the vertical reference line straight down your table and the horizontal reference line at the pre-determined height, parallel to the building surface. Align your jig fixtures in the correct locations and in perfect alignment using the vertical projection from the laser. Starting from the rear bulkhead, (this is the really cool part) insert the rear bulkhead into the rearmost jig and position it with the laser so that both the vertical and horizontal lines on your bulkhead match that of the laser, then pin it securely in place. Move ahead to the next bulkhead and, once again, align the vertical and horizontal reference to the laser. Just keep moving forward until all bulkheads are in place and all in perfect alignment. How easy was that!
Because the horizontal height is pretty much locked in place, I then raised the level so that the vertical line was visible on all bulkheads in order to ensure I was not introducing a warp as the stringers were added.
The Electronics Bay
The next pieces to be added were the battery side rails. I’m sure other electric flyers will undoubtedly have their own favorite way to secure the battery down, but if not or you are looking for something possibly better than what you are currently using, I encourage you to have a close look at this option (yes, yes, it is yet another Walker innovation…). The rails are cut from 3/8 x 1/2 basswood. These rails can be notched to save a bit of weight and make for a cooler looking installation! A bunch of blind nuts need to be installed, or like me, you can insert the inner yellow rod from R/C pushrod tubes called “Golden Rod”. I then run a 2/56 tap down the centre of the yellow rod making for an effective and light attach point for the cross members. Spacing between the upper and lower rails should match or be very slightly less than the thickness of your battery. For vertical CG considerations, it is important to ensure the rails are positioned to match the battery centreline to the aircraft centreline. By placing 4 carbon 3/8 x .063 rails across the lower brackets, one is able to slide the battery back and forth to establish the CG in the desired location. Two additional carbon rails then screw into place on top to secure the battery in place. A piece of foam or other material can be used to cause additional pressure on the battery to ensure it does not shift in flight.
Note: When I talk about upper and lower rails, I am talking about how they are observed when looking down at them through the battery hatch. If the battery hatch (I prefer to call this the electronics bay door – sounds cooler) is on top of the fuselage, the bottom rails are indeed the top rails. If the hatch is on the bottom, the lower rails are actually on top when looking at the actual aircraft orientation. Confused yet?
Another good thing to install now rather than later is a balsa floor under the lower rails. The reason being is so if you drop something into the electronics bay, the floor will hopefully prevent the object from puncturing the fuselage shell. What an utter bummer that would be!
Installing the Bottom Shells
Surprisingly, this turned out to be much simpler than I imagined it might be. The first thing you want to do is trim the excess balsa from the sides of the molds. But don’t do what I did! I carefully trimmed these edges to what I thought would be required edge, but I would have done better to leave between 1/8 and 1/4 inch of material past the mold edge, and then trimmed to the exact size once mounted onto the fuselage skeleton structure. As it turned out, I had to glue in some filler pieces later where there were some gaps between the two shells. With the shells still on the mold, mark the horizontal reference line on the front and rear bulkhead locations. Now shoot your laser down the length of the mold so that the reference line hits both marks. You can now carefully cut the balsa shell down this line which will correspond with the bulkhead horizontal centerlines you drew on the bulkheads and also align very nicely with your stringer, assuming it was installed on the fuselage centreline (of course it was, right?). What you should have when all is done is four fuselage shells. Two left and right upper shells, and two left and right lower shells. Because the fuselage to this point is being built upside down, you now need to attach the left and right lower shells. I used aliphatic glue to attach the shells in order to give me the required time to align the shell(s). I first attached the right side, secured with pins and tape and left to dry. You should be very pleased with how perfectly the shells conform to the fuselage bulkheads. Although in reality, why shouldn’t they fit perfect? After all, they were molded using the exact same bulkhead shapes as found in the frame. Once dry, trim the bottom edge/seam to correspond with the vertical centreline. The laser can help here as well. Then lay on the remaining bottom shell with the centreline edge carefully lined up with the centerline stringer. Now, trim the vertical centreline edge to match that of the opposite side shell. Once satisfied with the fit, glue on the shell.
Once the bottom shells have been attached you can remove the fuselage structure from the jig and flip it right side up. This is where the reference to “the canoe” will become apparent.
I took some leftover foam sheet and made three crutch jig pieces that I laid the fuselage into, right side up, that will hold the fuselage parallel to the assembly board, but also to hold the fuselage at an exact height above the assembly board. This assembly board is a FLAT 4’ x 5’ x 3/4” hardboard. The centreline mark is laid down with corresponding lines exactly 90 degrees to the centreline at the wing and stabilizer training edge locations. With the fuselage set aside, jig pieces are constructed and carefully positioned in relation to the reference lines drawn on the assembly board. Considerable time is spent to ensure the wing and stabilizer training edges are perfectly parallel to one another and the assembly board, and that the height of each is identical and at zero degrees incidence. Do not put any positive incidence into the stabilizer as seems to be a common trend these days. It must be dead level!
In the correct location, carefully cut away the fuselage shell and stringer to match the wing and stabilizer shape. Remove the wing and stabilizer from the assembly board and, using the crutch jigs you built to hold the fuselage, place the fuselage along the drawn centreline. Now the fun part; place the wing and stabilizer back in their jigs. They should integrate perfectly with the fuselage cut-outs. A nice close fit is preferred, but there should be no pressure spots between the wing/stabilizer and fuselage joints as this could result in some twist when everything is removed from the assembly board. Once you are satisfied that the three major components are perfectly aligned, carefully remove the wing and stab from the assembly board. Lay a bead of 2-hour epoxy along the fuselage wing and stabilizer saddles. Then place the wing and stabilizer back into their respective jigs. Check alignment one more time, then walk away and let the epoxy cure.
Like pretty much everything else on this build, I used the Walker Flap Adjustment Device (WFAD). The WFAD allows you to adjust the flaps with no fear of breaking them by exerting pressure to the control horns. Instead, a simpler twist of a 4/40 ball driver provides very accurate and repeatable flap adjustment. The WFAD needs to be added now, in addition to the flap/elevator pushrod. I use a Tom Morris threaded pushrod end to allow pushrod length adjustment, and a Tom Morris slider elevator control horn to provide flap/elevator ratio adjustment. The ability to adjust these components is vital when trimming your model for maximum performance, so do not omit them!
Note: A hatch will need to be built later on allowing access to the elevator control horn…
Top Shells(we’re almost there!)
Any over wing/elevator bulkheads may now be carefully installed, once again using your laser to ensure they are aligned with the fuselage centreline. Any additional gear required in the electronics bay can also be installed now as it is far easier to do now rather than later when the top fuselage shells are attached. Once you are satisfied everything that needs to go into the fuselage structure has been installed, it is time to close the thing up by gluing on the top shells. Attach the appropriate shell from where the 4/40 ball driver alignment tube access the WFAD first! The upper and lower shell seam edges should butt up to the bottom shell pretty much perfectly, seeing as the two pieces were originally one piece, cut in half. Trim the top seam to match the vertical centreline of the model using the laser. Glue and pin this shell in place and let dry.
Once dry you need to add the aluminum alignment tube that will direct the 4/40 ball driver into the adjuster bolt of the WFAD. If you add the remaining shell before you add the alignment tube, it is probably safe to say you never will, and that would be very bad.
Now is the time to review all that has been done and be absolutely certain nothing else requires installation prior to the final top shell being glued on. You may want to sleep on this before gluing. Just saying… Once again ensure a nice tight edge fit between the top and lower shells. Carefully, trim the top seam to match the opposite side shell. It is better to have a slight gap than too tight. If it ends up being too tight it will result in some bulging of the shell to bulkhead fit, which would mean a less than perfect glue joint. Once satisfied, glue and pin the remaining shell on and set aside to dry. Before removing from the jig, I discovered one more nifty excuse to utilize the laser. The vertical fin can be installed now with the laser vertical reference line providing an exact 90 degree angle to the wing/stab. Very cool to see the laser shooting down the fuselage and up the vertical stabilizer. What a wonderful tool!
And that’s it! Well, not really. Once the glue has set, the airframe can be removed from the assembly board. Cooling vents, hatches, canopy, safety plug receptacle, on/off switch, and whatever else you deem necessary can now be added.
The very first thing you will probably notice is how incredibly light the structure is. Likely much lighter than anything you have built in this size of model before. The second thing you will notice is the rigidity of the structure. It is solid with absolutely no flexibility. This rigidity can only be a good thing when it comes to stunt models. Provided you built the structure straight, it will remain straight!
As stated earlier, this has been a fun build. It was more intimidating to think about it than actually doing the work. I am sure many reading this will push it aside and say this is too hard and not worth the added effort. To that I would say three things. 1. It is more complex, but not a lot more than any other precision stunt model. Even conventional fuselage construction requires jigs to build straight and an extra effort when installing the wing and stabilizer. 2. Anytime you can build a straighter, lighter and more rigid airframe, it is worth the added effort. 3. I really did enjoy this build more than anything I have built in quite some time. It is nice to walk out of the workshop with a smile on your face and looking forward to the next step(s).
Finally, should you decide to try a “round fuselage” model, help is always available. Feel free to drop me an email and I will be happy to answer any questions you may have. There are no secrets being kept.