A 1.4m Foamboard Decathlon.

Introduction

Previously I designed an Ugly Stik for a flying buddy of mine. We came up with the idea of using the same wing section is the Flitetest Bloody Baron. I never flew it but I could see by the way it flew that it had some nice flight characteristics. It was agile and had a wide speed range and my flying buddy always wore a wide grin when he flew it. The logical thing to do was to build one for myself but I had already built and flown two Stiks and, lets face it, they are a bit on the crude side in the looks department.

I had also used the Stik wing on an FT-Explorer that I built for another flying buddy (my intrepid camera man, Paul Brown) and that also flew great. Then the idea hit me. Why not use the same wing on a scale model of a nicer looking plane. A Decathlon was the perfect choice because the wingspan and chord matched perfectly. All I had to do was ensure the tail feathers had a similar area and keep the weight down as much as possible and it should be a winner. It would inevitably be heavier than the Stik but it would also be more streamlined so I hoped one would balance the other out. As things turned it it did.

Design Goals

The idea was to keep the design as simple as possible to keep the weight down. The wingspan on the Stik (and therefore the Decathlon) was 1.4m. Most of my previous designs have been around 1.2m wingspan. Twenty cm (8 inches) doesn't sound like much but I was surprised at how much bigger everything had to be. I started with the wing because all I had to do was take the Stik wing, remove the trailing edge scallops and modify the wing tips to make them Decathlon like and I was done. As with the Stik I decided to add a wooden spar to augment the foam one (see build guide). I am not positive this is required as I didn't do this on the FT-Explorer wing and that has been fine. However I wanted peace of mind so a 10mm square piece of Tasmanian Oak was glued in front of the foam spar for the entire length of the wing. Dowel or pretty much any type of wood could be used. I simply went to the local hardware store and picked what I thought was the best balance of weight versus stiffness. The tail feathers were next because I knew they would be fairly straight forward and so they turned out to be. I increased their size from scale to ensure I had the same area as the Stik tail feathers. The fuselage was next and, other than the nose and the fuselage bottom was pretty straight forward.

The saga of the nose section is described in the section below on the 3D printed parts. The picture above shows how the bottom of the engine cowl flows into the fuselage bottom and results in a 3 sided flat profile that extends beyond the cockpit area and is then smoothed back into a flat section all the way to the rudder.

The above image shows the shape of the fuselage bottom. I used foam board formers and poster board to achieve the desired shape. It was a tad fiddly to get it to look nice and it only did because I then covered it with wing tape. There are other ways to achieve the same shape but I was trying to keep the weight down and this method involves the lightest materials. I thought I was never going to get the nose cowl finished but finally it all came together and so all of a sudden I had a plane ready to maiden. I was about to find out if I had achieved my goal or not.

Flying the Decathlon

After the PC6 and the Stik I was reasonably confident this one was going to fly okay. The only thing that was a bit of a guess was the cg which I put exactly on the foam spar. Turns out that was a good guess. Maiden day was perfect. Not a breath of wind and clear sunny skies. She surprised me with a small swing to the left as she started forward. Subsequent takeoffs have shown the swing not to be consistent. I suspect I have a left wheel that occasionally doesn't want to move but I haven't been bothered about it enough to check it out properly. The swing is slight and easily countered with a nudge of right rudder. The Turnigy motor supplied plenty of power and so up she went. Basically she flew like the Stik and I was grinning from ear to ear. She will happily cruise along at half throttle but you need at least three quarter throttle to loop. Rolls are fast but not snappy. At a guess I would say about 1.5 seconds to do a 360 degree roll. For the maiden I was using a 10 x 6 prop which sucked most of the amps out of a 2200 mAh graphine in 5 minutes flying. On subsequent flights I have propped down to a 10 x 5 prop and my batteries are much happier with that. The only noticeable difference in power was the distance she would go pointing straight up was a bit shorter but still plenty. Landing was gentle and straight forward although she is a bit of a floater. Unless you are aiming for a specific touch down point throttle isn't required. Dead stick the Decathlon floats gently down. You can see for yourself how she flies yourself at:

Specs

• Wingspan: 1460mm
• Length: 980mm
• Flying Weight: 1421gms
• Empty Weight: 1200gm
• Motor: Turnigy Air 3730-1000kv
  
• ESC: Turnigy Plush 40 amp
• Prop: 10 x 5 APC
• Battery: 2200 mAh, 3S Turnigy Graphene
• Max Watts: 245 watts
• Max Amps: 24 amps
• Max Thrust: 1150 grams
• Max C: 11
• Flight Time at Full Throttle: 5 minutes

3D Printed Parts

To build your very own Decathlon you need to be able to print the following parts. The more complex parts have been cut into sub-parts so they can be more easily printed.This means they will need to be glued together after printing.

Cockpit Canopy/Battery Hatch

The image above shows the three parts to the canopy/battery hatch. Glue them together after printing then sand, fill and paint for a nice finish.

Motor Mount

The holes are spaced for the Turnigy motor I used. I found this motor to be a good power plant and intend to use it on future builds. It produces as good amount of power, doesn't weigh too much and, with a 10 x 5 prop, gives you 5 minutes of full throttle flight time. It can also take a 4S battery. The motor mount is glued onto the front of the fuselage such that the side cheeks line up with the front of the foam board sides of the fuselage. I have 2 degrees of down and 2 degrees of right thrust built in which seems about right in flight. The original motor mount did not have the re-enforcing running from left to right. I found it could flex a bit due to the fuselage being so wide so I added the re-re-enforcing to prevent the flexing. I also made the vertical wall thicker and printed it with 100% infill. The result is a rock solid motor mount. Replacing mine in the already completed model was an exercise in pain and torture but it had to be done.

I found that the motor mount left the motor 3mm to far back for the propellor to clear so I printed and used the spacer shown in the above image to fix that problem.

Nose

The nose turned into a bit of a nightmare. I had thought that building the 3D model in Sketchup would be the hard part because of tis complex shape. However it turned out the hard part was cutting it up into pieces so that it could be printed. If printed as one piece it would take about 10 hours and what would happen is that after 3 or 4 hours the model would start to bow and eventually come off the print bed. I also decided to save a bit of weight by extending to foam board fuselage sides all the way to the front and removing the flat triangular side pieces from the nose model. In the end it took quite a bit of mucking about before I had a set of parts that would print and glue together to form the nose.

The image above shows the six parts that combine to form the nose. You will need to sand and fill to get a smooth finish.

Undercarriage Mount

I decided to use a standard Hobbyking pair of undercarriage legs. They are cheap, light and really strong.

 Above image is courtesy of Hobbyking.  The legs can be found here:

https://hobbyking.com/en_us/alloy-struts-12cm-high-19g.html

I never worked out what the blue plastic bits are for but they slide off and can be discarded. You could use other legs but the holes in the undercarriage mount are spaced for these. The undercarriage mount simply slots into a rectangular opening in the bottom of the fuselage and the legs are bolted to it.

The vertical uprights on each side are to spread the load and help disperse the force of a heavy landing. The underside of a Decathlon is not flat and so I have used poster board to recreate the belly shape of the Decathlon. However there is a gap where the landing legs go so a piece is needed to fill the gap. The image below shows the piece required.

The piece is glued onto the aluminium landing gear after that has been bolted in place.

Wheels

I designed bigger than scale wheels and did away with the wheel pants used on the actual Decathlon. The flying fields I use don't have nice flat surfaces so I need big wheels. The tyres were printed using flexible filament(TPU purchased from sainsmart.com) with 5% infill. The result is tyres which are a bit squishy and have some flex for landing. The hub was printed using ABS and 15% infill.

Tail Skid

I used my standard tail skid. I don't like using tail wheels because I down't want to add weight at the rear end. With my PC6 I made the tail skid look like a wheel. This looked great but I did notice that getting the Porter to turn while taxying on rough grass was a challenge at times. Hence I decided to go for the plain tail skid which has served me well on the Bushwacker, and my PT-19.

The Build

Those not intending to build their own Decathlon can skip to the conclusion. However if you envisage a Decathlon is in your future then read on.

Construction starts with the most complicated bit, the fuselage. I was lazy to paint so I used wing tape. It should have been red but I didn't have red so I used orange. The lines for the door were achieved py prining them on plain paper, cutting them out and gluing them to the fuselage sides before applying the wing tape. They show through the wing tape quite clearly so I was happy with the result. The windows are printed on plain paper, cut out and then glued to the fueslage after the wing tape has been applied. Note the two curved formers in the bottom left of the image are not required. My mistake.

Fairly predictable beginning. The fuselage folds are B folds and we start with the middle of one side.

And then the middle of the other side. Its a big, wide fuselage so its really important that these sides are at right angles with the bottom of the fuselage.

Due to the large size of the fuselage I decided to add a couple of fuselage bulkheads for strength. The next step is to glue the front bulkhead in place.

Followed by the rear bulkhead. The bulkhead locations are shown on the plans.

Next glue the front fuselage bottom to the fuselage sides.

Followed by the fuselage top piece. Make sure the join for this is strong because later on the wing is glued to this part.

The rear bottom of the fuselage is glued to the sides next. Apply the glue as quickly as possible and then turn the fuselage over and press the bottom against the table. This will ensure a nice, flat join. Slide the bottom up and down while you press down to prevent any excess hot glue from sticking the fuselage to your work surface.

Now we do the same for the top. This part has a slight curve to it so run it over the edge of the table while pressing down to get the same effect.

I ran wing tape over the joins to make a nice neat finish. If you are not using wing tape then obviously no need for this step.

In the original plans I hadn't quite worked out the battery access. When I did I realised that part of this front piece needed to be cut away as shown in the above image. I have modified the plans to reflect this change.

The front piece is now glue down. This completes the main part of the fuselage structure.

The above image shows where the undercarriage mounting point slides into the fuselage bottom.

The undercarriage mounting plate is glued into the fuselage. I used UHU Por glue for this join. It allows a bit of time to slide the plate around and get it in the right position plus it remains a bit flexible even when dry and (hopefully) helps absorb some of the landing forces. Hot glue isn't very good with plastic printed parts so I tend to use either epoxy or UHU Por.

Once the undercarriage mounting plate is glued in the formers for the fuselage belly can be glued in place. It is important to make sure the formers are at right angles to the fuselage bottom.

All the formers except the rear most one should be vertical. The rear most one needs to be at a slight angle.

Next the poster board is glued on to form the shape of the Decathlon fuselage bottom. The piece forward of the undercarriage is easy because its nice and flat.

The rear part is a bit tricky. Glue the front, flat piece first. It should be aligned so that the forward most darts line up with the 2^nd rear most former. Test fit then apply glue to the front two formers and position the poster board. Once dry glue the sides down. Repeat the process for the piece between the rear most and second rear most formers and finally the end.

I then applied wing tape to the edges to cover any slight cracks or white bits. Also trimmed the poster board at the undercarriage mount end. Like I said, a tad fiddly but looks fine in the end and, most importantly, its light.

Bolting the undercarriage legs on is straight forward.

I used Nyloc nuts as I didn't like the idea of trying to get in there to tighten nuts after the wing goes on so I wanted to make sure they would not come loose.

Similarly the motor is much more easily bolted onto the motor mount before installation. Again I used Nyloc nuts.

The above image shows the front of the fuselage ready to accept the motor mount.

I used epoxy to glue the motor mount in. The side cheeks of the motor mount line up with the sides of the fuselage. The result should mean 2 degrees of right thrust and 2 degrees of down thrust. Other than the tail feathers and the electronics, that completes the fuselage. Onto the wings.

A very standard Flitetest style wing.

The two wing spars are folded over and glued together.

Then the spars are glued to the wing bottoms.

The wing bottoms are then lined up and taped together.

Then the join between the to wings is glued. This is achieved by folding one wing back over the other, running some hot glue along the join and then laying the wing on a flat surface inside facing upwards. Finally any excess hot glue is wiped away before it cools.

Before final assembly of the wing, don't forget to cut the tape away from the opening in the centre of the wing.

I wanted to be sure I had enough strength as these wings are larger than I usually use in my designs so I grabbed a piece of 10mm square Tasmanian Oak. The type of wood doesn't matter as long as its light and reasonably stiff. I got mine from the hardware store. I suspect it might not be needed but it made me feel better. I used UHU Por to glue the spar in.

I used clamps and weights to ensure the spar was sitting flat agains the wing surface and the rear of the foam spar. I had to be careful with the clamps because I wanted them to hold the wood in place but not deform the foam. I also needed to allow enough room at each end for the servos so the wood is about 25mm shorter on each end than the wing bottom.

The top of the wing is then folded over and glued onto the spar and bottom of the wing in the usual Flitetest manner. I apologise for forgetting to take pictures. The aileron servos are then glued in place. You will need some servo lead extensions and a Y lead. Again apologies for no pictures. No excuse I simply forgot.

I returned to the fuselage to glue the nose pieces in place. I used epoxy glue which meant holding each piece in place while it dried as I could not figure out any other way of keeping things in place until the glue dried. I used 30 minute epoxy and watched television. I did need to test fit each piece, glue it and then fill and sand to get a reasonable finish. In the end I was happy with the result. Note that when gluing the top nose section in place the canopy/battery hatch has to sit in its place to ensure the nose top lines up with it properly.

I originally intended to use a barbecue skewer to hold the battery hatch down. Lining the skewer up was an exercise in patience, pain and torture so I switched over to using pins instead (see later photo).

The above picture shows how the battery hatch fits onto the fuselage. If you look carefully you can see the mounting points that the barbecue skewer slide through. I also had to cut slots in the foam fuselage to allow room for these mounting points to slide forward. This was because the hatch sits partially under the wing and must be slid forward for removal.

The above image shows the underside of the battery hatch. Again the mounting points are visible. You can also see that the inside of the fuselage is cavernous.

Tail feathers are glued on next. Vertical stabiliser is glued onto horizontal stabiliser ensure they are at 90 degrees to each other. When dry they are slid into place on the fuselage. The horizontal stabilisers is checked to ensure it is 90 degrees to the fuselage sides and then it is glued in place. All very standard Flitetest.

I used epoxy to glue the tail skid onto the fuselage. Experience on previous models has shown me that any other glue will result in the tail skid eventually parting ways with the fuselage. I also glued barbecue skewer supports struts onto the horizontal stabiliser. Again past experience has revealed that large foam board tailplanes will warp with time. If you got the horizontal stabiliser a bit crooked these struts can also be used to level it up again. The elevator and rudder servos are glued in place. You will need servo extension leads.

Next the wing is glued in place. I scratched the paper surface of the foam board where the wing attaches to the top of the fueslage and used epoxy to ensure a strong joint. I used a pencil to mark the mid point of the rear of the fuselage and the join on the battery hatch at the front to ensure the wing was on straight.

The above image shows the wing in place while the glue is drying. Again I couldn't figure out how to clamp it so I just pushed down on the wing while the glue dried.

The above image shows the pins I used to replace the barbecue skewer. The advantage of this method is I do not have to line up the barbecue skewer onto a hole I cannot see. Each pin is simply a small 3D printed disk with a short section of barbecue skewer glued into it. Works a treat and is quick and simple.

The CG needs to be directly on the foam wing spar. Turns out my battery needed to be just forward of the cg to get it to balance properly. Battery tray is just a piece of foam board with velcro which was glued to the front former. Being the cautious type I wrap a second piece of velcro round the battery just to be on the safe side.

All finished and ready to commit aviation. I didn't build the struts to keep the weight down. I really should as it adds to the appearance. Well there you have it. All done. Why don't you build one I cannot stress how much of a joy it is to fly.

Data Files

The full sized, non-tiled plans can be found here.

The A4 tiled plans can be found here.

The Sketchup files for the 3D printed parts can be found here and here.

The STL files for the 3D printed parts can be found here.

Conclusion

There is something almost mystical about the wing section created by Dan Sponholz. It seems to have a very wide speed range. All I know is the Decathlon is the fourth time I have used it and the result is always something that flies brilliantly. I was concerned that the Decathlon would end up too heavy thus negating the properties that make the wing work so well. Happily that didn't happen. I will admit that the trials and tribulations with getting the nose section to print and go together nicely was unexpected but ultimately the result made the effort worthwhile. I love flying this thing and you will too. Why not build one?