A Foamboard PT-19

Introduction

My first experience in flying model planes was when I was about 10 years old. I was given a Cox control line PT-19. I think it was a Christmas present.

This first venture into flying model planes was not a success.  I had nobody to teach me and nowhere to look for information so it took me a bit of time to figure out how to start the motor and get the thing going.  My first and final few flights consisted of the PT-19 rising off the ground, gracefully cutting an arc over my head before plunging into the ground on the other side. Designed and built by Fairchild, the full size wasn't that successful either. However, in spite of all that, I have always had a soft spot for the PT-19. After my Hurricane was finished the image of a PT-19 kept floating into my head so I decided that would be my next project.

The Design

After my experience with the Hurricane the design of the PT-19 was pretty straight forward (or so I thought). I designed it in the same way and included a power pod. However the fuselage on the final result looked totally wrong. What I had was a 30's looking monoplane but it simply didn't look like a PT-19.  After checking through my plans and looking at lots of pictures of the PT-19 I realised there were two problems. The first was that I had neglected to move the vertical stabiliser rearwards so that the rudder was behind the elevators and not in line with them. The second was caused by using a powerpod. The actual PT-19 has a tapered nose section due to the inline motor used. A parallel nose section (needed for the powerpod) just didn't cut it.

I decided not to worry about fixing the tail feathers however the nose had to change. I scrapped the powerpod and designed a new, tapered nose section. Foamboard is much more expensive down under in Australia so I didn't want to throw the entire fuselage away. Instead I cut the nose off, built a new one according to the new plan and stuck it on the front of the plane. The result was much improved.

Hiding the join where the new nose section was glued to the fuselage took a bit of thought but in the end I painted some paper with the same paint as the fuselage, did a bit of sanding and filling and then glued the paper on top of the join.

The Build

Other than the issues above the design and basic build is exactly the same as with the FT-Spitfire, the only difference being the shape of the parts being glued together.

3D Printed Parts

This model ended up with quite a few 3D printed parts. The files are included at the end of the article.  If you do not have a 3D printer and want to build the plane then there are ways to avoid the 3D printing. I will go through each part one at a time and explain. If you just want to see the final result flying then skip to the videos at the end of this article.

The Windscreens

These are printed flat and then folded to fit the fuselage when gluing them on.  These could be cut from plastic sheet instead. I have included templates for them on the plans.

The Nose Section and Motor Mount

I attempted to compromise between the shape of the nose section of the real thing, fitting the brushless motor and the limitations of both me and Sketchup when it comes to modelling surfaces.

I built down and right thrust into the motor mount. Basically the motor is bolted to the motor mount which is then glued to the front of the model. The nose section is then glued over the top of the motor and onto the motor mount.  The holes in the motor mount do not go all the way through and need drilling out after printing. This is needed so that the 3D printer has a surface to print the holes onto. Also I forgot to include a hole for the motor wires so I just drilled one. In order to use these you must cut the nose section off 50mm from the front edge. If you do not use this then the plans will give you a fuselage and nose which is the right length and taper. However I did not include a non-printable solution for the motor mount or the front of the nose. If it was me I would mount some light ply the right distance in from the front (calculated so that the motor ends up in the right place). I would then cut a flat piece of foamboard to the right shape for the front of the nose and glue it on.

Undercarriage

It took a few goes to get the dimensions correct but the above two blocks fit into the wing in the holes on the plans where the undercarriage should be.  The thin edge of the wedge sits at the leading edge and the wide end goes up against the wing spar.  There is a hole at the wing spar end in the end of the L shaped trench. Piano wire is bent so that it fits into hole and sits in the L shaped grooves and then bends ninety degrees at the leading edge and goes down to the wheels. They stood up very well to normal landing loads but the piano wire did come loose under extreme conditions (like repeatedly stalling in from 4 feet). So, at the suggestion of my friend Paul Brown, I added some washers and screws (see below). The result is bullet proof.  I haven't been flying models with undercarriage for a while and it showed up in some really crappy landings. We have gone from belly landing anywhere we feel like to having to line up and land on a runway so it was a big jump skillwise. However practice makes perfect and I am improving but, in the process, I definitely gave the undercrarriage a good test. To avoid printing the blocks you could either make them from hardwood or simply copy the Flitetest undercarriage as used on their P-51.

Wheels and Tail Skid

I designed and 3D printed some wheels and a tailskid. Obviously Printing these could be avoided by using normal wheels and a pop stick for the tailskid.

Crash Bars

It took me a bit of theorising before I finally figured out what the frame work between the cockpits on the real thing is for. Its protection for the pilots if the plane noses over onto its back.

At any rate to avoid the 3D printing it would not be too hard to knock something up out of dowel and balsa.

The "Boys"

I found the starting point for these two guys on Thingiverse. The guy in front was called Duke Nuke 'em and the guy in the rear was called Kanan. At any rate I imported them into Sketchup and then cut the tops of their heads off and added the flying helmet, goggles and flying jackets. They are a bit crude but look fine in the air. Ironically this took way more time than designing and building the PT-19!

CG and Control Throws

The CG is on the wingspar and the control throws are basically as far as I can get without straining the servos or hitting something. I was a bit surprised that I had to have the battery right up in the nose to get the balance.  These settings have proved pretty much spot on for me.

Flying

I used the same Emax motor that Flitetest call their C powerplant and found it provided plenty of power.  I was pleasantly surprised at how harmonised the controls were. I have no idea why but, despite two open cockpits and other draggy bits, she accelerates and retains energy much better than my either my FT-Spitfire or my Hurricane. This makes the PT-19 an absolute delight to fly but does make landing a tiny bit tricky (although this could be just my rusty ability with landings). To land you need almost all the power off and just trickle in little bits of throttle to keep the height correct. Also because she speeds up so nicely you have to force yourself to keep her slowed down to approach speed. If you do that she will come down nicely. of course if you are not trying to hit a runway threshold then its all much easier. She handles the wind pretty well also. The flying in both videos below are in fairly strong wind. The first one it is a cross wind.

Abuse

On her last flight I pulled her vertical to do a stall turn and then got disoriented at the top of the stall turn. I have no idea what I did but she dropped into a spin. Its been years since I have done spins with a model so my next thought was "How do I get out of a spin?". In these situations its really hard to remember exactly what you did. At the time I thought I cut power, centralised controls and pulled on up elevator. However after a bit of thought and reflection I think I responded by simply applying up elevator. Of course that just flattened the spin and she hit the ground slightly nose down and under power (I think). Oh well, I thought, there goes that model. However as I reached the model I was shocked to find it relatively undamaged. The glue on the left side wing joint had cracked, the prop had broken and the undercarriage had been bent backwards but otherwise she was fine! Half an hour of mucking about and she is back to flying condition.

Data

Click here for a zip file containing the plans both tiled (A4 and US letter) and full.

Click here for the Sketchup files for the Pilots and wheels.

Click here for the rest of the Sketchup files.

Click here for all of the STL files.

Conclusion

The PT-19 is a great looking plane to model and this one flies well. I am really satisfied with this one as my second design. If you have never designed your own model I strongly encourage you to do so. While it can be frustrating at times and you do chew through more foamboard than a normal build, the buzz you get watching your own design take to the skies is wonderful.  I have more on the drawing board so there will be more articles and plans from me. Enjoy.