I have wanted to design a Turbo Porter for a long time. However the undercarriage was always the stumbling block. I could never figure out how to emulate the undercarriage. Part of the art in cartoon scale designs is figuring out which bits to leave out because they are too hard to model and which bits have to stay to preserve the character of the model. I always felt the undercarriage on the Porter was a significant component in its character. If I left it out then I just have a boxy, highwing model. If you have read any of my previous articles then you know I have been playing with using 3D printed parts in undercarriage design. The last attempt being the undercarriage for my Piper Pawnee. My experience with 3D printed parts in undercarriage design led me to believe it was possible to design a reasonable replica of the Turbo Porter's undercarriage. It would even have suspension built in just like the real thing. The Porter is a STOL plane so I also wanted flaps and reasonable STOL performance. The problem with flaps is they use up wing area that could be used for ailerons so there is a compromise between the area used on the flaps and the ailerons. I also needed a wing section that was going to give me high lift at low airspeed but not too much drag.
The goals I wanted ot achieve were to end up with a plane that is instantly recognisable as a Turbo Porter while still being constructed from foamboard and 3D printed parts. I wanted the all up weight to be 1kg or less but would be happy if it ended up around 1100 or 1200 grams. In designing the Piper Pawnee I used 3D printed parts for most of the character parts. The ones that really defined it as a Pawnee. In the case of the Pawnee that was the nose, cockpit canopy and the undercarriage. I would do the same here. Initially I went one step further and tried 3D printing the ailerons and flaps but had to discard them as they weighed too much.
As always I started by downloading three view drawings of the real thing. I also found some three view drawings of scale models of the Turbo Porter. These are extremely useful because you can compare them to the drawings of the full size and note the differences. These differences have often been made to ensure the model flies well despite its reduced size. Straight away I noticed that the chord on the models had been increased and was not scale. Similarly the tailplane area was also increased. I also increased the wing chord. I was aiming to keep the wing area about the same as the Bushwacker. That way I knew I would end up with a light wing loading. The next bit of research was to look at the aileron/flap sizes on various models. I found the best contrast was to examine the comparitive areas for aileron/flaps on the FT-Bushwacker, the Multiplex Fun Cub and the Durafly Tundra. I have built a Bushwacker and a Fun Cub so I knew how they flew. The full sized Porter had the same area for the flaps and the ailerons. However I wanted to make sure that I had good aileron response despite having flaps. Keep in mind the response required of a full sized aircraft is typically a lot less than with a model aircraft. If full sized aircraft were flown the way we fly our models then the pilots and passengers would not be alive when the plane landed! They would have been crushed to death by the excessive G forces. In the end I made the ailerons 50mm longer than the flaps which turned out to be a good compromise.
I followed the same principles as with the Pawnee. The "character" parts were going to be the same. i.e. the nose, the cockpit canopy and the undercarriage. Everything else could be built from foamboard in the typical Flitetest manner. Examining the three views I identified where the 3D printed nose would start and the foamboard fuselage would end. I repeated that process for the canopy but ignored the undercarriage completely for the moment. I thought the easiest way forward with the undercarriage was to design the plane without it and then see what needed to be modified in order to incorporate it. There was always the risk in that approach of a ending up with a design which could not be modified for the undercarriage. To be honest I couldn't fit it all into my head at the same time so I had to sub-divide the project. Fortunately it all turned out fine.
I would like to say that a lot of science and calculation went into the choice of wing section. However the truth is I drew out the wing section and played with it until I felt I had a good combination of wing thickness versus wing chord and basically felt it looked right. As things tuned out it seems to work okay.
I designed the foamboard parts first and then the 3D printed nose and canopy. After that I did the undercarriage which turned out to be a real head scratcher. More on that later on.
Flying the Porter
When designing a model like this you make lots and lots of choices along the way. You try to think them through as best as you can. However when the thing is finished and waiting for its maiden you start to question all your choices. Are the ailerons big enough? Is the tailplane large enough? Is there enough area where the wing is glued to the fuselage to keep it in place? Is the CG right? On and on until you finally resolve them all with the maiden flight. In the case of the Porter the maiden flight was a pleasure. She rolled forward with a slight swing to the left which was easilly countered with right rudder. She left the ground fairly quickly all by herself and a tad of up elevator put her in a steep climb and demonstrated plenty of elevator authority. That she could maintain such a steep climb told me there was plenty of power. In fact she will comfortably loop on just over half throttle. The Pilatus Porter is boxy in shape but I guess that long tapered nose cuts down on the drag because she feels quite streamlined to fly. A bit of trimming followed and then she was hands free. Some aileron rolls demonstrated there was enough aileron response to keep me happy. Not a 3D model but plenty for rolls and for keeping the wings level in loops etc. The landing was a complete non event. She slowed down quite nicely even without flaps. I have 20% down elevator mixed in for full flaps and 10% for half flaps. The link below shows how she flies.
- Wingspan: 1225mm
- Length: 835mm
- Flying Weight: 1100gms
- Empty Weight: 888gms
- Motor: Emax GT2215/09
- ESC: Turnigy Plush 30 amp
- Prop: 10 x 4.7 slow fly prop
- Battery: 2200 mAh, 3S Turnigy Graphene
- Max Watts: 212 watts
- Max Amps: 21amps
- Max Thrust: 1056 grams
- Max C: 10
- Flight Time at Full Throttle: 6.5 minutes
3D Printed Parts
For all of my previous designs I have either suggested or provided alternatives which do not require 3D printing. Unfortunately this is not the case with the Porter. It just isn't possible to build this model without a 3D printer (or a friend who has a 3D printer). In most cases I have included a brim to aid those with lesser able printers. The undercarriage parts are not listed here. I felt the undercarriage needed a section of its own so these parts are listed there (see later).
Printed in two halves and glued together. After gluing and possibly painting (depending on what colour filament you used) a strip of poster board is glued down the middle of the completed canopy to emulate the canopy framing on the full sized plane.
Engine Exhaust Pipe
Just a cosmetic piece for scale purposes. I printed mine in black but it would have been more realistic to print in white and then paint in a bronze colour.
The holes are spaced for the Emax motor used (Flitetest power pack C). Holes could be repositioned for other motors or deleted and holes drilled after printing. Motor mount is glued onto the front of the fuselage and then the nose piece is glued on top of it. I have 2 degrees of down and 2 degrees of right thrust built in which seems about right in flight.
Glued on top of motor mount. You will need to add supports when printing. These are needed under the circular part that forms the top of the cooling inlet. I have included STL files with and without support added.
Many thanks to Pintokitkat who published an article which included the STL files for a 52mm spinner. I took his design and scaled it down to 40mm for the Porter. It looks and works a treat! I had concerns about vibration from balance issues with a 3D printed spinner but Pintokitkat put my mind at ease and he was absolutely correct. The spinner has performed flawlessly and looks cool.
Just a rectangular piece the right size. Slots into foam box spar on each wing (see build guide further down).
I decided to go away from the standard Flitetest cambered wingtips. I did this purely for looks. A more traditional Flitetest style wing could be made by adding 50mm to the length of the upper wing surfaces and not using these tips.
Wing Control Horns
I used standard (other than being 3D printed) Flitetest style control horns for the rudder and elevator. I designed the ones above for the ailerons and flaps. I wanted to make sure I could get as much throw as possible for the ailerons and the flaps. I wanted to be able to get the flaps down to around 40 or 50 degrees which is the maximum you should use for flaps. Anything more and you have airbrakes and not flaps. Having said that if you wanted to get carried away with larger flap throws you would need to hinge them differently (like the Tundra or the Fun Cub). The top image is of the aileron control horns and the bottom image is of the flap control horns.
The first attempt seemed fine but just wouldn't go together the way I wanted it. I then remembered that Multiplex had brought out a PC6 last year so I looked at their undercarriage. Mine is based on the way theirs worked. It isn't exactly the same and it still took quite a while to sort out but persistence is a virtue and did win out in the end. The second attempt works fine but is a tad difficult to explain. I'll start with a description of each of the 3D printed parts and then move onto how they all fit together.
The Axle Hubs
The above picture is of the left hand axle hub. The upper hole has a section of 3mm x 10 mm piano wire glued into it. The main leg (see below) is then glued onto that. The short piece of piano wire ensures a strong joint between the two plastic parts. The two lower holes receive 2mm piano wire which form the lower part of the landing gear.
The Main Leg
This is simply a cylinder with a hole through the length of it. As described above, one end is joined to the axle hub. A section of 3mm piano wire slides inside the hole at the other end and a spring sits on top of the leg. The piano wire is free to move up and down inside the hole in the main leg with the spring giving suspension.
The Lower Fuselage Mount Point
Two pieces of 2mm piano wire slot into this piece. Each piece of piano wire is bent so that it runs from each wheel axle hub through the front or rear slots in the mount point which is glued to the bottom of the fuselage. Each piece of piano wire is pushed into its slot in the mount point and then a section of barbeque skewer is pushed in and glued in front of it to hold it in place. Templates are provided in the plans for the piano wire and the location of the lower mount point on the fuselage bottom is marked on the plans.
Upper Fuselage Mount Point
This piece is glued into the fuselage as indicated on the plans. basically the rectangular bit lies inside the fuselage crossing from one side to the other. The round bits stick out each side and are the mounting points for the 3mm piano wire used for the undercarriage struts. The 3mm piano wire is glued into the holes on each side.
Confused yet? Now that you are familiar with the parts, hopefully the diagram below will allow you to understand how it all fits together.
This gets a tad fiddly so take your time. You might need to read the following description a few times before you get the idea. Two pieces of 2mm diameter piano wire are bent to form the lower two supports. As previously mentioned templates are provided in the plans. The front one feeds through the two hubs on either side and the protruding bits of wire form the wheel axles. The middle part then slides into the front of the lower fuselage mount point which has been glued to the bottom of the fuselage. The rear piece slots onto the rear hole on the axle hubs on each side and then slips into the the lower fuselage mount point. As previously mentioned a section of barbeque skewer is glued in front of the piano wire where it slots into the lower fuselage mount point to keep it in place.
3mm piano wire is glued into the holes in the upper undercarriage support. A short piece of 3mm piano wire is glued to the hole in the axle hub and then the main leg is glued onto that. The piano wire is to give the joint between the two plastic parts the strength they require. A longer piece 3mm piano wire is then slotted into the hole in the main leg and the spring placed over the piano wire and on top of the main leg. It is then glued into the hole in the upper fuselage mount point. The 2mm piano wire pieces are then slotted into the hubs and the mounting point on the fuselage.
Below is a picture of the completed undercarriage from a different view point.
I did say it was a tad fiddly. However the effort is well rewarded in a realistic looking undercarriage that works just like the real thing. I can testify to its sturdiness and the suspension as I have tested it a number of times when the ground snuck up on me.
Its hard to see clearly but this is taken from the video at the moment I touched down too hard and moving too fast. If you do a frame by frame view on the video you can see the undercarriage compress like its supposed to.
Finally when you have that lot put together you can slide the main wheels on and use a collet to hold them in place. I printed my tyres using TPU filament with 5% infill. The resulting tyre is a bit soft and provides a bit of compression of its own. I decided on a diameter of 80mm for the wheels. This is larger than scale but helps with lumpy flying fields.
The two halves of the tail wheel are glued together and glue into the rear of the fuselage. Basically its a tail skid that looks like a tail wheel. I wanted to keep the rear end light so I didn't want to do a proper tail wheel. It does tend to dig in a bit so turning around when taxying requires a bit of room but you can still turn(check out the end of the flight video and you will see what I mean).
Those not intending to build their own PC6 Turbo Porter can skip to the conclusion. The rest of you buckle up and read on.
We start with the wings. Typical Flitetest style. The strip of foam is for the trailing edge. It acts as a spacer between the wing top and bottom. Also note the servo cutouts. I got them completely wrong initially but they have been corrected on the plans. The flap servos are arranged so that a servo reverser is not required.
The first step is to do the B folds for the box spar. Make sure you get the sides down flat with the bottom and at right angles. Also wipe away any excess glue from inside. When you slide the spar joiner in later any excess glue is going to get in the way. There are two, one for each wing. Do both.
Next glue the trailing edge spacer onto the inside of the wing bottom. Line it up with the rear edge of the wing bottom.
The spacer needs to be about 3mm high at its rear edge. Use a barbeque skewer to flatten the rear edge down a bit.
Glue the box spar onto the top surface of the wing. Lining it up with the two rear most folds. Then glue and fold and glue and wait in the typical Flitetest wing manner. Do both wing halves.
Install your flap and aileron servos now. Hook them up to your receiver and get them moving in the right direction before you finish the wing. Also label the leads so you don't have to figure out which is which later on.
Mark the centre position on the wing joiner so you can make sure it slides an equal distance into each wing.
Glue the joiner in on one side. It doesn't matter which side.
Glue the wings together. Test fit before you glue to ensure the wing will slide all the way and is a good fit. If you look really closely you can see my left wing is slightly forward of my right wing. Fortunately little differences don't affect the way it flies and is not noticed when finished. After wrap some 2 inch wide tape around the join.
Now glue the 3D printed wing tips onto the wing. I tend to use UHU or epoxy when gluing large printed parts as you get a stronger bond than with hot glue. That completes the wing. Let's move onto the fuselage.
Examination of the fuselage parts reveals a typical Flitetest style build. The pdf files for the side doors and side cockpit windows are included with the plans.
Print the pdf file of the side windows and doors on ordinary paper. Cut around the doors and windows and glue them to the fuselage sides. I use a glue stick for this. Note the misalignment on the fuselage side in the bottom of the image. I lined up the front edge of ther side cockpit window with the edge of the fuselage side. Don't do that because if you were a little out on your foamboard cutting you are going to end up with what you see here. A better move would be to line up using the bottom of the side doors. Sigh...
Glue the B-Fold for one side of the fuselage. Do it in stages. Glue the middle horizontal bit first then the front and then the back.
Repeat the process for the other side.
Glue the front bulkhead in place.
Next glue the rear bulkhead in place.
Glue the rear top deck in place. Use the table top to get a good flat join by turning the fuselage upside down once you have the top deck in place and gently but firmly pressing down. Slide the fuselage back and forth a bit to avoid any excess glue sticking it to the table top.
Now glue the front top deck in place followed by the front fuselage formers. Later the nose top will be formed using poster board. I did think about designing and printing the top section but I worried about the weight and decided not to. If you are feeling adventurous and want a slightly more finished look then give it a try.
Now for the undercarriage. In the above image all the parts are layed out in their rough positions in relation to each other. I made a mess of bending the piano wire on this attempt mostly because my initial templates were completely off. I replaced them and modified the templates so they reflect the correct angles.
In the above image we can see that the main legs have been joined to the axle hubs as described in the undercarriage section. The 3mm piano wire that forms the leg struts has been glued to the upper fuselage mount points and slid into the plastic main legs with the spring set in place. Lastly the front lower undercarriage wire has been slid into place. I used epoxy glue for all of the joins. I did wonder if it would crack away from the piano wire after a while but so far it has held perfectly. Do wipe down the ends of the piano wire to be gued with alcohol or acetone and then roughen the metal up a bit with a file. That is what I did and it seems to have done the trick.
The same point in construction but from a different view point.
Finally glue the rear lower piano wire in place and the fiddly tricky bits for the undercarriage are done!
Slide the wheels on and the main gear is finished. Initially I hot glued some wheel stoppers on the end of the axle to keep the wheels from sliding off. They kept coming off so I have since replaced them with collets.
Next bolt the motor to the motor mount and glue the motor mount to the front of the fuselage. Don't use hot glue for this join. I used epoxy.
Next glue the poster board on in the usual way. i.e. glue the rear edge to the rear edge of the fuselage top panel and then tack glue the postor board to the centre of the front former. Finally fold down and glue each side one at a time. Use a metal ruler or something similar to press the posterboard flat against the fuselage side.
Trim the excess posterboard off.
Now the elevator and rudder servos are installed and hooked up to the receiver.
The tail feathers are next. Once again standard Flitetest construction. Glue the vertical stabiliser to the horizontal stabiliser ensure they are at right angles to each other and then glue them to the rear fuselage. Check you have a right angle join between the horizontal stabilser and the fuselage side or your loops are going to be a tad skewed.
Glue the nose piece onto the motor mount and the glue the dummy exhausts onto the nose piece. Notice that I got a bit lazy with the number of polygons in my design so my nose looks a bit flat sided. Easily remedied with a bit of sanding to remove the straight lines visible in this photo. Do that before you glue the nose section on. I didn't bother and then decided I should have and sanded the nose section while it was in place. Trust me when I say that made the job so much more painful. This shot also demsontrates the fact that if you build one of these you HAVE to use a spinner or the looks are spoiled.
This is my battery hatch. I was surprised to find my battery ended up needing to be placed pretty much on the CG. I thought with that long nose it might have to go back a bit. Yours will end up more forward if you are using Flitetest foamboard. The cost of getting their foamboard to Perth, Australia is more than the foamboard itself so I use foamboard from foamboards.com.au. There just isn't an economical way to ship a small amount of foamboard halfway around the world.
I glued two pairs of doublers (triplers?) to each side of the fuselage where the wing is to be glued on. I wanted to have plenty of surface area to ensure a strong joint. A parting of the ways between the wing and the fuselage is a sure fire way to ruin your day.
Glue the wing onto the fuselage and then glue the cockpit canopy onto the fuselage. I used epoxy for both joins. Finally sort out your electronics, do your balancing (CG is marked on the plans) and you are good to go. I tend to fly with the maximum throws I can get. I start with 42% expo (because my hands shake and because, well, it is the meaning of life, the universe and everything so it has to be a good starting point right?) and the adjust to suit.
Finally go fly!
The plans can be found here.
The STL files for the 3D printed parts can be found here.
Designing and building the undercarriage got a bit real but on the whole I really enoyed designing and building the Porter. I admit to be surprised at how well it flies. Josh Bixler would call it a happy accident and he would be correct in doing so. Since the maiden I have flown this plane almost exclusively. The other models I bring to the field sit on the grass and glare at me for ignoring them. It looks cool and flies well. I am glad I took the project on and I am very happy with the result. If you build one you will not be disappointed. Enjoy.