Parks and Rec - A Foamboard Piper Pawnee

Introduction

Ever since Josh Bixler designed his foamboard Dusty from the Planes movie I have wanted to do a Pawnee.  A fellow flyer brought a along a petrol powered Hanger 9 Pawnee to one of our flying sessions and watching that was all the stimulus I needed to start my own design. I did a little research (okay I googled for about 2 minutes) and discovered there were two main variants: The original Pawnee and the Pawnee Brave. I decided to go for the earlier original Pawnee because I liked the rounded wing tips and the shape of the tailplane and rudder better. My daughter and I love the TV show Parks and Recreation so, of course, I named it Parks and Rec. I wanted the resulting model to be as scale as I could get it given the limitations of foamboard. I also wanted to further test how 3D printed parts can be used as undercarriage. Finally, and I openly admit this may never happen, I wanted the model to be able to be modified in the future so that I could install a water tank and be able to "spray crops" with it. The result has quite a number of 3D parts but most can easilly replaced with more traditional alternatives. I am very happy with the way it flies. It has a good speed range and is quite manouverable.

The Design

The design, as usual, is stock standard flitetest style other than totally abandoning the flight pod concept. The main reason for this is to save weight, allow a scale width to the fuselage and finally to ensure there is room for a water tank located at the CG point. I was really keen on the water tank idea but in the time since the original concept to now my enthusiasm as faded due to having other projects on the fly.  I gave the wing a reasonablely thick section and once again made use of Peter Sripol's foam box spar concept (as used in his Bushwacker design).

I have been experimenting with using 3D printed parts as undercarriage. There has been quite a bit of trial and error but the results have been quite successful and I will be publishing a Flitetest article on that subject at a later date. I started by replacing the undercarriage on my PT-19 with a fully 3D printed undercarriage. By itself an ABS 3D printed part does not have the strength to stand up to the landing forces of a model plane (especially the way I land and the rough terrain I land on). The art is in allowing the forces to be transmitted and absorbed elsewhere. After quite a bit of trial and error the PT-19 experiment was successful. The next step was the Pawnee. What I did here was to use a combination of piano wire, 3D printed parts and some elastic. The result is roughly scale and has stood up so far. More on this topic when we get to the build guide.

Finally because of the scale fuselage width there is plenty of room for placing the internal parts.

Flying the Pawnee

Because I intended to load water onto the Pawnee I intentionally used a larger than necesary motor (see specifications). The result is that my Pawnee has quite a torque swing on takeoff. It can be clearly seen on the flying video because, on that takeoff, my brain just didn't process the swing so I did absolutely nothing to correct it. Other than the swing, the Pawnee doesn't challenge one's flying abilities. It's speed range is quite wide. in the video I am mostly flying at just over half throttle. At full throttle it speeds up a lot for the type of plane. However on landing it slows down to a sedate rate before settling down to the ground. The roll rate is not 3D model fast but is quite respectable. The elevator is a little more sensitive than the ailerons and the rudder is about the same as the ailerons. It does drop a wing on stall but gently. I suspect if I laterally balanced the model it would not do that. Anyway check out the video for yourself.

I have had half a dozen flights so far. The plane has performed well  but I have had a lot of trouble matching the motor to the right propellor. Mostly due to Emax's recommendation being way, way off. I started with a the recommended 12 x 6 which pulled 45 amps and only gave me three and a half minutes flight time. I gradually moved down in size until, on the second last flight, I used a 10 x 4.7 propellor which resulted in an amp draw of 34 amps and a flight time of just under 4 minutes at full throttle. Seeing as I still had stacks of power in the air I went down to a 9 x 4.5 which pulled 24 amps and gave me almost 6 minutes at full throttle. Unfortunately I never got to test that configuration in the air.

Crashing the Pawnee

I put it down to encroaching old age. On takeoff I slowly advanced the throttle and the Pawnee began its takeoff run. For some reason, as soon as I start waggling the rudder to counter the torque swing I stopped advancing the throttle. The poor old Pawnee rose up about five feet but didn't have enough power to continue flying and so she stalled and crashed nose down. Ironically the 3D printed undercarriage was undamaged but the nose was a different matter. On the same day I did exactly the same with my Hurricane (with its new 3D printed retractable undercarriage) but in that case I realised at the last second and slammed the throttle wide open and avoided the same fate. Repairs were not difficult. For those interested a detailed description of the repairs is included after the build guide. The Pawnee is now back to flying condition and awaiting her first post crash flight. I do not foresee any problems at all with the plane. If the pilot manages to not forget to advance the throttle properly then all will be well.

Specifications

Wingspan: 122cm

Length: 80 cm

Flying Weight: 1180 grams

Motor: Emax BL2815/09

ESC: Emax 40 amp

Receiver: Lemonrc dsm2

Servos: 4 x HXT900 9 gram servos

Battery: 2200 mAh, 3 cell Turnigy Bolt

Emax recomend a 12 x 6 which is too big. Various propellors were tested: 

          11 x 5.5 APC, 380 watts, 35 amps, 1650 grams thrust, 16C, 3.75 minutes flight time at full throttle

          10 x 6 APC, 334 watts, 30 amps, 1300 grams thrust, 14C, 4.40 minutes flight time at full throttle

          10 x 4.7 Slow Fly, 385 watts, 34 amps, 16000 grams thrust, 16C, 3.8 minutes flight time at full throttle

          9 x 4.5 APC, 275 watts, 1200 grams thrust, 11C, 5.5 minutes flight time at full throttle

I am settling on the 9 x 4.5 as this seems the most efficient and should give me almost six minutes at full throttle. The pawnee seems to be pretty quick so I think the power to weight ratio will be fine. If I ever get around to the water tank then I'll revert to to 10 x 4.7 slow fly prop to get the extra thrust.

3D Printed Parts

The major printed part is the cockpit canopy. I designed a solid canopy that I printed, painted black and then added the cockpit framing to using posterboard(see later).  Much later, after the plane had been test flown I designed a second version with open windows. I was very happy with the solid canopy so I haven't even printed this version but it should be fine. In both cases the cockpit was divided into front and rear sections. As an alternative to 3D printing a posterboard version could be designed as a replacement to the 3D printed versions. However it would take a lot of trial and error.

Image of the cockpit front (solid version).

Image of the rear cockpit section (solid version).

Image of the front cockpit section (open and untested design).

Image of the front cockpit section (open and untested design).

The motor mount was designed specifically for the motor I used (Emax BL2815/09). A smaller motor could be used in which case you would need to move the holes in the sketchup model to suit your motor. If you wanted to be crude you could simply ignore the printed holes and drill new ones instead of modifiying the sketchup model. The mount has 2 degrees of right thrust and 2 degrees of down thrust built into it. I also included side panels to give the fuselage a bit more area to grip onto. The three printed parts were glued together using epoxy before fitting the motor and gluing the mount into the fuselage. Liteply could be used instead of a 3D printed part.

Image of the front part of the motor mount.

Image of the side plates for the motor mount.

I wanted to have a scale looking nose so I designed a cowl front using sketchup. Its not overly accurate because I have limited 3D modelling skills and also because it had to blend back into the square foamboard fuselage. However I am happy with the result.  The crude alternative would be to simply replace it with a piece of foam board. A more elegant replacement would be to sand and carve some soft balsa to match the shape.

Image of the nose section

The tail skid is taken from previous designs. It should really be a tail wheel but I got lazy. Also I did a battery tray as well. The tail skid could be replaced by a barbeque skewer (Flitetest style) and the battery tray could be made from foam board or balsa or left out.

Image of the tail skid parts.

Image of the battery tray.

Two wing joiners are required and both could be replaced by liteply parts.

Image of the wing joiner

The Undercarriage

The undercarriage of an actual Pawnee looks like this:

I designed a 3D belly section that is glued to the bottom of the fuselage. I then designed and printed left and right undercarriage legs consisting of the front and rear struts. Finally elastic is used to represent the inner strut and act as a suspension system. The front undercarriage leg is hollow so that piano wire can be threaded through it. The rear section has a hole where it meets the fuselage. This hole matches up to a hole in the fuselage belly plate. A section of bamboo skewer is glued into the hole joining the rear strut to the belly plate. The idea being that the skewer will break before the undercarriage and can be replaced. How the undercarriage all goes together is explained in more detail in the build guide at the end of the article. This could all be replaced withg a simple piano wire under carriage or a flat aluminium plate version from somewhere like Hobbyking. I have used these 

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

on a couple of models and found them very robust.

Image of the fuselage belly plate.

Image of the left undercarriage struts

Image of the right undercarriage struts

The hole for the bamboo skewer can be seen in the image of the left leg and the rectangular bit seen on the right leg is where the elastic attaches.

The Build

The above image shows the fuselage parts ready for assembly. The motor can also be seen. The red at the rear of the fuselage sides is the ends of the wing tape used for the trim. I always paint and decorate as much as possible before assembly as its much easier to do that on a flat piece of foam rather than the complete airframe. Note the side mounting plates for the motor mount have been glued to the front plate of the motor mount before starting on the fuselage.

In true Flitetest fashion we start by folding and gluing the fuselage sides. Note only glue the centre part at this point. There are four folds in the fuselage side and we glue between the rear and second rear most folds. As always make sure our side is 90 degrees to the bottom. These are B folds.

Next we glue the other fuselage side in exactly the same manner, i.e. B folds and 90 degrees to the fuselage bottom. The next step (sorry no photo) is to glue the front fuselage sides and bottom together, one side at a time. There are two folds on each side but you need to do both folds together (but one side at a time). Use the table to ensure a good flat join on the bottom by sliding the bottom back and forth after gluing.

The motor mount is glued in next. You need to ensure that the rear side of the motor mount is vertical (not the front side because that has the down thrust built in) and that the motor mount is positioned so that the motor will be the right distance from the front of the plane and the motor shaft lined up with the centre hole in the nose section once it is glued on. I trial fitted until I was happy and then ran a pencil line along the back of the motor mount. I glued the motor mount with hot glue ensuring it was lined up with the pencil lines.

I next fitted the motor as I had concerns about the large motor fitting and also wanted to do this while access to the nuts behind was as large as possible.

I then trial fitted the top plate to make sure it cleared the motor and the motor mount. It did not so I had to trim the foam a bit. Note lowering the motor is not an option because the motor shaft has to line up with the hole in the nose section.

The above image shows the section of the top fuselage plate that had to be removed. This should not be necessary if using a smaller motor. However its not a bad idea anyway as it gives more access to the motor when the model is completed and makes no difference structurally.

After trial fitting the modified top plate to ensure it still fits okay I then glued it on. Apply the glue put the plate on and then turn the fuselage upside down and slide it back and forth on the table to ensure a nice, solid flat fit. Note that the rear end of the plate folds down a bit you also have to see saw the fuselage as you are moving it back and forth. It sounds a lot more clumsy than it really is. As you can see even a fumble fingers like me can get a good join.

Next (again I forgot to take a photo..sorry) glue the rear sides of the fuselage to the fuselage bottom. Again slide the fuselage bottom back and forth while holding down on your build table to get a good, flat join.

Add the rear formers in the places marked on the plans. Note the hole I cut into the former because I forgot to mark the hole in the plans. The hole is needed so we can feed the rudder and elevator servo leads forward to the receiver.

Now the nose hatch is assembled. First glue the formers onto the bottom.

Next glue the top onto the formers. It is important to ensure that the hatch bottom is absolutely flat when gluing the top on. I didn't and so my hatch had a slight bow in it which has caused me grief since.

The above image shows a test fit of the cockpit canopy and hatch. It is important to ensure they all line up nicely. I found I had the rear former a little crooked so a but of trimming was needed to get a good fit. I also needed to sand the canopy a tad to get everything nice, neat and lined up.

The opening in the bottom of the fuselage as shown in the above image is marked on the plans. In the same way as we don't cut the wing openings in the fuselage completely free, we do the same with this section. Now that we have the fuselage together we can cut it completely free by completing the partial cuts we made when we marked and cut the fuselage from the foamboard sheet. If you did cut it free when cutting the fuselage from the foam board then you would have had a little trouble when gluing the sides to the bottom but now you have nothing to cut free so you are still good to go.

Speaking of the holes for the wings we now remove them by completing the cuts and popping the foam free.

Next glue the belly plate for the landing gear into the fuselage. Line it up so the top of it is level with the bottom of the hole for the wing. Also make sure you have it the right way around. The two holes at the rear and the slot at the front.

As you can see from the above image I screwed up a bit as the plate is a bit high. Fortunately it didn't matter as I managed to squeeze the wing through. Speaking of the wing...

We commence the wing by folding and gluing the wing spars. Be very careful to wipe any excess glue away from the first 10 cm of the inside of the wing spar. The wing joiners will slide in here and they have to be able to lie flat against the side of the wing spar. If there is any excess glue in there then they won't be able to do that. Once again these are B folds. We then crease and fold the wing panels ready for final assembly.

Next we glue the spar to the wing situating it in the normal place between the rear and second most rear wing folds. Note the foam spacer glued to the bottom rear edge of the wing panel. also the small spacer along where the ailerons are going to go. These turned out to be a mistake. I have deleted them from the plans so, in your build, all that is need is to glue a couple of pop sticks end to end along where the aileron is going to be. No spacer is required for the inner portion of the wing. I cannot begin to tell you how much fun it was trying to make that modification AFTER the wing was glued together. Sigh...

Hopefully the above photo more clearly shows the change required. Basically I misjudged the vertical distance between the top of the wing and the bottom of the wing at the rear of the wing bottom.

The next step is to glue the wing joiners into one half of the wing. If you made sure there was no excess glue inside the wing spar then they will slide in easily and end up parallel to each other.

Now glue and fold each wing in the normal way. I glue the leading edge and spar, fold, hold the wing top down and then wait at least a couple of minutes before letting go. I then glue the trailing edge and hold for another few minutes before letting go. Finally the two wing halves are glued together. The wing joiners ensuring the correct amount of dihedral. However you need to ensure that the two wing halves are lined up and not twisted by eyeing the leading and trailing edges at the wing join. After the glue dries run some glue across the wing join top and bottom and then a layer of tape in the normal Flitetest way. Don't forget to remove the tape from the hole for the servo leads. Next slide the wings into the fuselage wing slots until it is centred. Run hot glue top and bottom on each side. Next glue the vertical stabiliser to the horizontal stabiliser and then glue to the fuselage in the normal Flitetest manner. Check to ensure the vertical stabiliser is vertical and the horizontal stabiliser lines up with the wings. Again apologies for no images.

Next we paint the window parts of the canopy with black paint. I wanted to try using poster board for the canopy frame and it worked a treat. The straight bits are easy, just cut strips of poster board and glue them on before trimming them to size. The curved bits involve a lot of trial and error but you can get to a good fit quicker than you might think.

Cut and bend the piano wire to match the template provided in the data section below. DO NOT make the final bends to form the wheel axles yet.

Slide the 3D printed undercarriage legs onto the piano wire. Be careful to get them on the right way. Use the above photo as a guide.

Now make the final bend. I used 2mm piano wire which would normally not be thick enough for a plane this size. However in combination with the 3D printed parts it has proven fine.

Next test fit the undercarriage. Note the two small sections of barbeque skewer used to attach the rear undercarriage struts to the belly plate.

Now glue the undercarriage to the fuselage belly plate. Run glue along the slot over the piano wire, glue the barbeque skewer bits in and glue then ends of the 3D printed parts to the belly plate.

Then run a piece of elastic from the attachment point on one leg, through the slot in the belly plate and out to the attachment point on the other leg. Keep a bit of tension in the elastic when you tie it off. Can you see how it works? The elastic ensures there is a small amount of give sideways and the 3D Printed front struts stop the piano wire from bowing. The idea being that in a hard landing the undercart can spring sidways but will not bow or bend totally flat. The barbeque skewers will break if the landing is really hard. In this way we can transmit the energy from the plane hitting the ground up through the undercarriage and absorb some energy into the elastic and then pass the rest onto the fuselage via the belly plate which ensures the energy is dispersed a bit before moving onto the fuselage. The two bits of barbeque skewers acting like a fuse in an electronic circuit and breaking of the force gets too excessive. Until the crash this was theory but it worked like a charm in the crash. The only difference in result being the barbeque skewers didn't break they simply popped out.

Finish the undercarriage off by attaching the wheels of your choice. I used lumps of hot glue to hold the wheels on and in one flight the hot glue on one of them broke off and one wheel fell off. I then printed some small plastic disks and glued them onto the wheel axles which seems to have solved the problem.

Okay. Its always good to pause and see what we have done so far. So out with the coffee and biscuits while we gaze satisfyingly at what we have created so far. Happy sigh...

This is the time to thread your servos. I always forget to install the aileron servos during wing construction so I have become quite adept at dropping a piece of string with a weight on the end down the wing and out of the wing servo open in the wing centre. The rudder and aileron servos are much more easily installed before gluing the rear fuselage coaming on the plane.

Glue the postorboard hatch piece onto the centre of the hatch top. Mark the centre point on the back with a pencil and do the same with the hatch top to ensure a good alignment. The apply glue to one side and roll the hatch over to get the foam board to curve onto the hatch side. repeat for the other side of the hatch, trim the excess posterboard and you are done. Note ensure the hatch remains flat during the entire process so you do not end up putting a bow in the hatch.

Repeat the same process for the rear fuselage coaming. Glue the posterboard in the centre and then do each side one at a time. Finally trim away the excess posterboard.

Given the large size of the canopy I thought just gluing it might not be enough so I drilled two holes in the rear of the canopy and then glued to ends of two barbeque skerwers into the holes ensuring the pointy bits are pointing towards the rear of the plane.

I then pushed the canopy back so that the skewers poked holes in the former.

Then I glued the canopy into place. In the crash this popped free but all I had to do was glue it back in place.

The next step was to figure out how to attach the top hatch. In the end I used barbeque skewers in the front and...

I used a barbeque skewer with a clear plastic strap to hold the rear end down. Its a bit clumbsy and I am going to come up with something better.

Now all that is left is to balance the plane by deciding upon your battery location and also to stick the receiver somewhere. There is tons of room inside so its no big issue. The CG is on the middle of the wing spar.

Data Files

The tiled plans can be found here.

The full plans can be found here.

The template for the piano wire part of the undercarriage can be found here.

The sketchup files for the 3D printed parts are in two groups.

The first group is here and the second group is here.

The STL files can be found here.

A pdf file for the markings I used on my model can be found here.

The sketchup and STL files for the hatch catch discussed in the Crash Repairs section below can be found here.

Crash Repairs

They always look rather sad and forlorn when awaiitng a major repair. The image above shows the crushed nose section removed.  Sigh...

The above image shows the replacement nose pieces. Note the motor mount. I tried designing the motor mount so it was all one piece instead of three. Basically I turned the 3D model upside down so the front of the motor mount was positioned as the "bottom" in the 3D model. Given there is two degrees of down and right thrust that means everything else was at a funny angle. Visualising which way things had to move kept doing my head in and I kept making mistakes, printing the mount and then trying to correct the latest error. The last iteration had the holes in the wrong place but I made it work anyway and gave up on fixing the design. If you build a Pawnee you are better off with the first version.

The nose is built just liked the fulle fuselage. B-folds, making sure the bottom and sides are perpendicular. First one side and then...

Then the other side and finally glue the top on.

Just as with the original build I cut away the front section of the top to all for motor clearance.

The motor mount is tiral fitted and then glued in place.

The new hatch went togethere the same as the old one but with one exception. Note the vertical line in the posterboard. This time I glued the posterboard onto the top section  but before I glued and rolled each side I cut a thin dart where the hatch bottom changes angle. This made the finish a bit neater than the old one.

I came up with as much better way of holding the hatch down. I designed and 3D printed the U shaped part you can see glued to the bottom of the hatch in the above image. Once the hatch is in place I used a barbeque skewer to hold the hatch down. The sketchup and STL files are in the data section. Note I also had to cut a small rebate into the rear bottom of the hatch so that it could still sit flush with the fuselage when in place.

All the bits ready for final assembly.

I glued some doublers around the join between the old fuselage and the new nose.

The new nose glued onto the fuselage. I needed to be careful that the alignment was correct. I will eventually glue some paper strips over the join to make it look a bit nicer.

All ready to go fly. It never ceases to amaze me as to just how much punishment a foamboard model can take and also how much damage can be inflicted and you can still get the thing back to looking good as new.

Conclusion

I am pleased with the result. Parks and Rec flies nicely, has a wide speed envelope and no nasty habits. The 3D printed gear withstood the unintended destruction test. In point of fact I was surprised that the overall damage was not much worse given the way it went in. It looks cool zooming past me. It was a challenging design and the couple of  mistakes I made, especially with the wing did cause me a bit of trauma but that's the way it goes with a new design. Overall I enjoyed the process of designing and building Parks and Rec and the flying has been fun so far as well (except for the crash). Will I try the water tank? I don't know. There  is a large space in the fuselage for the tank so its very viable. However I have ideas for some other projects that I want to do first. However you never know. Often these ideas keep surfacing in my mind until I can no longer resist. I guess we will have to wait and see. Meanwhile why don't you try your own Parks and Rec?