Bubbles Construction Guidelines
Bubbles is easy to build and can take just a couple of weeks to construct, with perhaps a third week to cover/finish off (assuming you have a day job and free evenings!). I believe you will find it easiest to start by assembling just the basic wing structure and then moving on to construct the basic fuselage. Double back and finish off the wing and then fuz/tail. Please pay close attention to the plans; these instructions do not mention every piece of wood or step in the process.
Wing ribs are all cut from one template, despite the fact that the centre of the wing is sheeted. You can get away with this because the sheeting is only 1/32" thick and it’s not obvious that the centre section is slightly thicker than the rest. Cut the rib template from cardboard (easier than ply) complete with the cut-outs for the leading edge and spars. Use two ‘T’ pins to hold in place, and cut the ribs individually from 3/32" soft balsa. I used 3mm Depron in the prototype. This is slightly lighter but requires compression spars or cap strips to stop chord-wide compression by the covering (not needed with balsa ribs). Use the pin holes to stack the ribs together and sand them lightly. The leading edge half ribs are made in a similar way.
In order to save weight, the spars are built up from a combination of spruce and balsa. There are two spars (top and bottom) which are about 50" long (1260mm) and 1/4" square. I have used two 1/8 x 1/4" x 36" lengths of spruce in total, one for each spar. The rest of the spar is medium balsa. So, start with one length of 1/8" x 1/4" x 36" spruce and butt join 1/8" x 1/4" medium balsa in order to match the length of the wing (50" excluding tips). Double this up with more 1/8" x 1/4" balsa to create a spar which is 1/4" square the full length of the wing. Make sure that you overlap/stagger any joins you may need. Assemble the second spar in the same way. Each spar should have balsa and spruce starting at one end and ending up with only balsa at the other end. Swap one over so that the spruce is evenly distributed across the wing. In other words, if the spruce is biased towards the left on the top spar it should be biased towards the right on the bottom spar. The centre will have double the spruce content which is where the strength is needed. The drawings may show twice the amount of spruce that I have described (this would be a 'safe' error), and naturally solid 1/4" x 1/4" hard wood / spruce could be used for the full length. Both these approaches would be stronger but in three years and a number of major crashes which have destroyed the fuz, my wing has not failed.
The wing has no leading edge sheeting. It is both easier and lighter to install false spars/turbulator strips. The covering probably provides much of the torsional strength, and you will need to visually inspect the wing for warps after covering (it’s easy to reheat Solarfilm and correct with a little manual persuasion). The wing has no washout. The centre of the wing is sheeted, mainly to hide the servos but also to add some rigidity and make it easier to handle. You will probably need to assemble and sheet the bottom of the wing before attaching to the fuz, but it might be easier to add the top sheeting later once the wing has been attached to the fuz.
The trailing edge of the wing is made from light 1/4" balsa. The simplest approach is to make the trailing edge balsa the same height as the back of the ribs at that point and to hinge it in the middle. However, I added 1/2" cap strips made from 1/32" balsa top and bottom (length-wise). I did this because I like to hinge ailerons at the top, immediately beneath the top cap strip. It also attaches the trailing edge to the ribs better. However, I do not believe this is need for structural integrity and it is a little harder because you have to notch the ribs to accommodate the thickness of the cap strips. Note that regardless of the approach, the trailing edge will probably have a join in the middle of the wing. Add more balsa ahead of it to reinforce this section as this is where the rear retaining screw will go. I designed the plane for separate aileron servos to experiment with mixing. However, this did not do much and I don’t use any mixing now (except ‘exponential’ to soften central control throws). One servo would be adequate using torque arms to the ailerons but you would need to reassess the rear mounting screw position.
The ailerons could be made from solid balsa/tapered aileron stock, but the built up ones are lighter and very rigid. They are fully sheeted, top and bottom, with light 1/32" balsa. It is all assembled with medium Cyano, except the ply control horn which is glued with epoxy. A 1/16" drill bit is usually the perfect size for the snap link hole, and this area should be reinforced with a small piece of 1/64" ply. Commercial control horns could be used instead.
The fuz is made from two standard 4" x 36" lengths of 3/32" light to medium balsa. The front is doubled with 1/64" ply (PVA glue) and the rear has 1/8" balsa doublers on the edges and other places to stiffen it (Cyano). This approach is easy to build and is only 0.5oz heavier than the original 'stick' fuz (partly because the latest fuz design is smaller).
'Plug' the two sides onto the battery tray and formers B and C. White glue (PVA) will allow time to tape everything together. Allow to set. Pull the front in on former A (Epoxy) and join the two sides at the rear (Cyano). If your formers and slots in the fuz are accurate you should have a straight fuz. Cut the rear cross-pieces to maintain smooth curves, and add the diagonals, undercarriage ply and various other bits until the fuz is complete.
Mount the wing and measure the distances between wing tips and the rear of the fuz to ensure the two are perpendicular.
Tail surfaces are made mainly from 1/8" balsa. This is doubled up to create a 1/4" overall thickness. The advantage of laminating like this is that you can overlap corners. This makes construction easier and the finished product is stronger. It also allows you to cut shallow depressions in one layer before adding the other to made hinge 'slotting' easier (this is how I hinge the ailerons as well). Cut the diagonals to fit and Cyano in place.
You can epoxy the stab to the fuz before or after covering provided you always glue balsa to balsa. Make sure the stab is parallel with the wing. Attach the fin in a similar way, except add a thin wooden skewer through the stab to the bottom of the fuz to reinforce this joint. Glue the elevators and rudder to the airframe after covering. I use Mylar hinges held with thin Cyano.
I wish I could find a lighter ‘formula’ for undercarriages. They are always heavier than I would like. I have used a single piece of 1/8" piano wire which works well and is easy to bend to shape. I found 3/32" wire to be too flexible. The main wheels can be normal commercial versions, but it is also easy to cut two disks from firm foam (eg: as used in camping mattresses and kneeling pads). Spin these in a drill against a sanding block and round the edges. Use Cyano to attach 1/32" ply disks to each side with a suitable thin plastic axle bearing. My wheels weigh just 4.3g (0.2oz) for both and are very cheap!
The plans show an additional rear mounting point for floats. All ‘40’ sized floats should attach to this and the front undercarriage mounting position with ease. The electric motor gives you much better control in taxiing manoeuvres and take offs are easy into wind. Landings are even better as you simply throttle back and let it descend gradually and settle on the water when the speed bleeds off. Don’t forget to waterproof exposed wood (eg: the control horns are all 1/16" ply reinforced with 1/64" at the hole).
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Art of the Possible ¦
Absence of Matter ¦