Build a Tribewt - 2

Build a Tribewt 2

Carrying on from article 1.

The Turtle Deck

I actually decided on the turtle deck profile for this version of the Tribewt a little earlier in the build by sketching it out above the fuselage. For ease of construction I’m using the slots provided in the BB plan for the turtle deck formers, plus one extra former partway down the tail.

Here’s how the original Tribewt turtle deck would be sketched out (the paper isn’t quite long enough but that’s not a desperate problem).

Here’s the sketch for this version. I pondered a removable hatch for the back servo, but decided I could fit both servos in the new cockpit.

I sketched in the largest former and worked out its size (width of the fuselage x height in the sketch). Notice that I opted for a semicircle at the top rather than any tapered design – tapers would work but it’s easier to have a regular curve to work with. Remember a locating tab for fitting the former to the fuselage.

Here’s the former in place.

We already have the smallest former in place at the tail, so you can see the line my turtle deck will follow. Here I’m working out the height of the next former. – which will be 70mm wide at this point on the fuselage and 38mm tall from my measurement (yours will most likely be different).

Once the second former is glued in place you can work out the size of the next former using the same technique of holding a rule over the top, but this time the former will not be as wide as it's part way down the tail - you'll need to measure the width at that point. Leave a little extra material on this one and sand or pare it down to size, checking top and sides with a rule to get a good fit.

Shave a little material off the sides of the pod carrier so they don’t interfere with the fitting of the turtle deck.

Check again to make sure you have clearance to get the pod out and measure the width across the front so that you can make the front turtle deck former.

At the front I decided I wanted a flatter and squarer ‘look’, so I cut the formers to this shape..

Here’s a quick test-fit to see how it all looks.

You’ll need to make room for the motor.

... and leave plenty of space for air to get at it.

Before gluing up the nose be absolutely sure you can get the pod in and out.

Now you’ll need some paper templates. To help make the templates I wrapped thin paper around the formers and held the fuselage up to a bright light. This showed me where to draw the outline of the template.

Then I cut out my templates.

Here’s a test fit. Initially I did it in 3 panels, nose, headrest & tapering tail, then I had a think about the cockpit.

I played with a few ideas…

…before deciding on an open cockpit for ease of build.

Finally I transferred the shapes to thin cardboard. Important! Don’t glue/tape these on yet.

Now is a good time to test fit the servos and control rods. I positioned the servos down the centreline in the middle of my cockpit. Then worked out where the control rods would go. It’s much easier to create a series of holes to feed the control rods through without the turtle deck in place.

So here it is with the cardboard turtle deck glued in place. Note that I raised the height of the cockpit sides in this final arrangement.

Let’s look at some added detail now.

Foam wingtips.

I was able to find foam that would add another 4cm each side, increasing the wingspan to 78cm. You could just build the wing to this size from the start, (most likely using more than a single sheet of foamboard) or you could leave the wing clipped at 70cm. A smaller wing would influence the slow speed flying, but I suppose not a great deal – in fact, a shorter wing might allow greater manoeuvrability and top speed...anyway, I like the way my Tribewt flies so I’ll stick with the foam. I like to use a fairly dense polystyrene which doesn’t pick apart too easily.

You only need enough to cover the thicker part of the wing. Glue it on with hot glue – pressing firmly until the glue sets to make sure of a good fit.

Once the glue is set I pare the foam down using a very sharp kitchen knife.

Lots of thin cuts, with a slight sawing action! With the kitchen knife I find I can rest a good portion of the blade on the paper, which helps pick up the shape of the wing and extend it onto the foam for the last few cuts.

Add some shape to the trailing edge of the wing. Take the cut through this point in the wing, just where the foam starts to get fatter (and stronger).

 Sketch-on a curve and extend your design onto the foam wingtip.

Use a craft knife and a sawing action in the foam to trim off the excess. Keep the foamboard offcut in one piece.

Flip the offcut and use it as a template to mark the curve on the other end of the wing.

After some gentle paring with the kitchen knife to remove the sharper corners, the foam can be sanded – use old finer-grit paper that has lost its ‘catchiness’ and go very gently. It might take longer to sand but you’ll avoid tearing off chunks. You can now paint the foam with slightly watered PVA glue to strengthen it. This coating will also protect the foam if you decide to spray paint the model.

The Nose.

This could be left as-is or you can add some detail.

I was lucky to find some foam with a slot in it and a bit of preformed shaping - I cut this to an oversize outline and glued it on with hot glue.

Give yourself a bit of material to work with.

I pared this down with the kitchen knife, following the outline of the nose.

...then sanded it carefully.

Again it is finished with a couple of coats of slightly dilute PVA glue.

Fittings.

All the fittings are pure BB.

Here’s the popsicle sticks on the wing to take the pressure from the wing elastics, centre aileron servo, single attachment for the undercarriage, and skewers in the fuselage for the wing elastics. On this plane the front skewer needs to be removable to fit the pod so don’t glue it in place. Note the use of packing tape on the wing leading edges and other places I think it might be useful. I’ve also added a few ventilation slots behind the skewer mounts to get some airflow through the pod.

Here’s where to put the wing elastic skewers.

If you’re locating the rear swappable pod skewer later in the build, here’s a way to do it - locate the pod in the fuselage and then use a bent wire to poke a hole through the foamboard from the inside.

The top of the wing – zip-tie holding the undercarriage in place with reinforcement. Feed hole cut for the servo wire.

Here’s my steering for the tail. Note the hinge tube trapped in the middle as you bend it up. The bit at the top is for the wheel, the bit at the bottom is a lever that sits against the rudder.

Here it is in place – maybe a little more obvious what it does now.

The trapped tube gets glued in tight against the back edge of the fuselage. Note the second tube lies against the rudder – this tube needs to be a loose fit on the wire to get it to work. Because the hinge point for the rudder and the wheel assembly are slightly different the wire slides in and out of the tube very slightly. Also this little bit of the assembly can't go directly from the top of the hinge tube to the rudder or it would bind and stop the rudder turning the plane right. That little kink is essential. Push the wire up as if the wheel were taking load to take any slack out of the hinge before using glue or tape to fix the little tube to the rudder. That way the weight of the tail is taken by the steering hinge and not by the rudder hinge.

The plastic tube comes from the ink reservoir in ball point pens or from Q-tips. Also you don’t really need the little coil spring, but I liked the idea of a little bounce in the wheel. This is just my solution to tail steering, you’ve probably a system of your own, or a little skewer skid would work too.

Rudder control horn. I like these swing-in fittings for locating the control rod. Not too obvious here, but the control rod has a slight offset bent into it to take it from the back of the fuselage to the end of the horn. This removes any binding as the control rod passes through the fuselage. The wire bends up (slightly) and at the point where it lines-up horizontally with the control horn it is bent down (slightly) to 'straighten' the wire.

Elevator control horn. Again you can just see a little bend in the control rod. I’ve still to fit two small screws through the horn, which will catch on the popsicle stick underneath and make everything super-secure.

Here’s the servos at the front. These aren’t glued-in yet as I’ve still to paint the model. (Before painting stick a little bit of masking tape over the foamboard to keep an area clear for hot gluing.)

Aileron servo and horns. Bent wire adjustment on one side only, as adjusting one side affects both sides. Again nothing glued until after painting. The control horns here are chopped-up credit/store cards.

Finishing.

Final test assembly before painting.

I do like this black and yellow paintjob. (Mostly because that’s what I have sitting around.)

I’m not too sure about the shine on the paint, I might go over it with some satin varnish to dull it down.

Any details I’ve missed in the build sequence that are particular to the Tribewt or questions about the build – please ask/let me know. Anything else is likely available from the BB or BB2 build videos.

Here it is with a windscreen & a fake padded edge around the cockpit. I misted the windscreen with excessive applications of CA - but that would be authentic for flying in Scotland. :)

If I’m completely honest I’m a tiny bit disappointed that the plane looks a bit ‘generic’. To give it a bit more character I might remove the section of turtle deck behind the cockpit and extend the clear canopy back. I’m not putting-down this version, I know plenty of people would be very happy with this result – it’s just a matter of taste - I chose this version to demonstrate the versatility of the build.

The CG of this plane is around 45mm from the leading edge of the wing. I started by setting my throws to the little throw guide that came printed on the FT cruiser plans, though I have since  increased the elevator throws to allow inverted flying. I couldn’t get the plane to climb inverted with the limited throws (which was a bit scary first time I tried it!). This is my first ‘season’ of flying, so I’m still getting the hang of things and I definitely like my flying sedate and predictable!

Finally, a BIG thanks to Josh Bixler and the team for all their inspiration and enthusiasm. Though I've obviously got a few background skills, this is my first time making and flying model planes. Without the clear instruction from Josh & co. this would have been a lot slower learning curve.

Alibopo.