Cox PT-19 for indoor RC

[FlyingTyger]

The Cox PT-19 control line model has introduced countless people to the world of aeromodelling. It is a classic design that I have always wanted to pay homage to in RC form. Recently I was looking for a new indoor model to build and thought that this would be a great chance to take on the Cox PT-19 RC project.

But it cannot end up just looking like a small PT-19 model; it needs to represent the Cox product. There are some nuances to the Cox model that are not exactly “scale” that I want to make sure I capture correctly. I also want to make sure it retains the “plastic” look. Obviously, a molded plastic airframe would be too heavy, especially for an indoor flyer, but I did not want it to look like a foam model either. 3D printing would be a viable option but is outside of my skill set. My decision was to use foam skinned with covering film.

Armed with a plan, I took every picture I could find online and fired up CAD. I was able to find a detailed list of dimensions of the original model and tried to hold tight to those. I was able to draw up a simple design that holds true to the original's proportions.

Time to build.

 

[FlyingTyger]

I started with the wing. I had an idea of how to achieve a lightweight “plastic” wing but thought I should do a quick test sample first. I ironed covering film onto the top surface of a piece of foam, then pushed the piece down into a mold to create an airfoiled shape. I then ironed film onto the bottom. Viola, it held its shape perfectly and proved to be surprisingly rigid. Perfect.

The wing panels were cut from 3mm Midwest Cellfoam. The ailerons were separated and beveled. Then all the edges were sanded round. For the covering, I found an old partial roll of yellow Top Flite Econokote in my stash. I covered the bottom of the ailerons and hinged them to the wing using the film, then covered the top. Time for the fun part, molding the final shape.

I cut forms to the airfoil shape and pinned them down to my bench. I preshaped the panels a bit by bending them over a length of PVC pipe then pressed them down into the forms. Slowly and carefully I ironed down the film. All looked well until I looked down the LE. Both panels had warped like crazy. It took a couple of hours (and a Beam & 7 or two) to massage and heat the panels and get them back to acceptable straightness. The panels were then propped up for dihedral and the center joint was glued.

The finished wing did not end up as stiff as I had hoped either. The finished product holds its airfoil shape well but bent too easily spanwise. I attached a small (1mm) carbon rod along the bottom to help stiffen it up. I think it will be good enough now, at least for an indoor flyer.

All-in-all, this assembly technique did work to produce a lightweight “plastic” wing but I am not totally happy with the way it turned out. If it proves to be an issue later on, I have a few ideas of how to build it better using balsa and will give that a try. For now, it’s time to move on to the tail.

 

[cyclone3350]

Oh man, does this bring back memories. I bought this one, & the Miss America P-51. For some reason, I couldn't get either one to fly. I did have fun using the motors on other projects that were more successful. BTW, what film R U using?

 

[Scotto]

I started with the wing. I had an idea of how to achieve a lightweight “plastic” wing but thought I should do a quick test sample first. I ironed covering film onto the top surface of a piece of foam, then pushed the piece down into a mold to create an airfoiled shape. I then ironed film onto the bottom. Viola, it held its shape perfectly and proved to be surprisingly rigid. Perfect.

View attachment 234375
View attachment 234376

The wing panels were cut from 3mm Midwest Cellfoam. The ailerons were separated and beveled. Then all the edges were sanded round. For the covering, I found an old partial roll of yellow Top Flite Econokote in my stash. I covered the bottom of the ailerons and hinged them to the wing using the film, then covered the top. Time for the fun part, molding the final shape.

View attachment 234377
View attachment 234378
View attachment 234379

I cut forms to the airfoil shape and pinned them down to my bench. I preshaped the panels a bit by bending them over a length of PVC pipe then pressed them down into the forms. Slowly and carefully I ironed down the film. All looked well until I looked down the LE. Both panels had warped like crazy. It took a couple of hours (and a Beam & 7 or two) to massage and heat the panels and get them back to acceptable straightness. The panels were then propped up for dihedral and the center joint was glued.

View attachment 234380

The finished wing did not end up as stiff as I had hoped either. The finished product holds its airfoil shape well but bent too easily spanwise. I attached a small (1mm) carbon rod along the bottom to help stiffen it up. I think it will be good enough now, at least for an indoor flyer.

View attachment 234381

All-in-all, this assembly technique did work to produce a lightweight “plastic” wing but I am not totally happy with the way it turned out. If it proves to be an issue later on, I have a few ideas of how to build it better using balsa and will give that a try. For now, it’s time to move on to the tail.

That looks great. I have tried that general technique multiple times with paper, but I havent tried real covering plastic. I guess I assumed the heat to set it would ruin the foam. That should be 100% waterproof, right? I just looked for a few minutes, it appears Midwest Cellfoam is no longer in production.

 

[FlyingTyger]

That looks great. I have tried that general technique multiple times with paper, but I havent tried real covering plastic. I guess I assumed the heat to set it would ruin the foam. That should be 100% waterproof, right? I just looked for a few minutes, it appears Midwest Cellfoam is no longer in production.

Cellfoam has been out of production for several years now. It's similar to depron. I had found some in a back corner of my LHS awhile back and had just enough leftover for this project. If I make a mistake I will have to order some depron and wait 😬

 

[FlyingTyger]

Tail assembly complete. Nothing special here, just 3mm foam cut to shape. I added a short strip of carbon between the elevator halves to strengthen that connection.

The stab and elevator were covered with the yellow Econokote. The fin was covered with sapphire blue Monokote while the rudder got some white Ultracote. The stripes are vinyl.

 

[cyclone3350]

Tail assembly complete. Nothing special here, just 3mm foam cut to shape. I added a short strip of carbon between the elevator halves to strengthen that connection.

View attachment 234502

The stab and elevator were covered with the yellow Econokote. The fin was covered with sapphire blue Monokote while the rudder got some white Ultracote. The stripes are vinyl.

View attachment 234503

What R U using to cover it with. I've tried VH covering once & Parklite Ultrakote, but found the heat required, would distort the foam. Then I came across EconoKote & that worked great but it is no longer available.

 

[FlyingTyger]

What R U using to cover it with. I've tried VH covering once & Parklite Ultrakote, but found the heat required, would distort the foam. Then I came across EconoKote & that worked great but it is no longer available.

The yellow is Econokote. The blue is Monokote. The white is Ultracote.
So far the only place I distorted the foam with heat, was the starboard leading edge on the wing. Trying to remove the warps, I got it too warm.
I am being very careful to get the covering just hot enough to activate the adhesive, not shrink it.

 

[cyclone3350]

The yellow is Econokote. The blue is Monokote. The white is Ultracote.
So far the only place I distorted the foam with heat, was the starboard leading edge on the wing. Trying to remove the warps, I got it too warm.
I am being very careful to get the covering just hot enough to activate the adhesive, not shrink it.

Thanks. Do U find the Ultracote any better than the Monokote or R they about the same.

 

[TooJung2Die]

I'm enjoying watching you build this. Anyone with an interest in Cox airplanes during the 1970's will remember the PT-19. This hits two of my favorite topics; nostalgia and indoor ultra-light. You are doing a good job with the conventional covering films on foam. Getting the iron temperature just right must be tricky. My favorite covering for foam is document laminating film. It is clear film so you have to use something else to give it color. I've also used colored transparent gift wrap from the dollar store. Works good but color choices are limited and you have to use aerosol adhesive.

 

[TooJung2Die]

You can get a dummy Cox motor from Willy Nillies Hobbies. I've used them on a few old timer RC airplanes converted to electric. They're very realistic and cause quite a few double-takes.

 

[FlyingTyger]

Thanks. Do U find the Ultracote any better than the Monokote or R they about the same.

Hard to say. The fin and rudder are the only things I have covered with the Monokote and Ultracote and they are each about 2 square inches. The fuse is larger so that will be my first good test using Monokote.
My next build, a Great Planes Electro Streak, will use Ultracote on the wing and Monokote on the fuse. That one will be a better test.

 

[FlyingTyger]

You can get a dummy Cox motor from Willy Nillies Hobbies. I've used them on a few old timer RC airplanes converted to electric. They're very realistic and cause quite a few double-takes.

View attachment 234519

Getting the temp right has been difficult, but mostly because I lost my last 21st Century iron and am left using a basic Top Flite iron. It's temp varies a lot.

I LOVE the WillyNillies engines. I converted my father-in-law's old 1/2a racer using a Babe Bee mount. I still have one the shelf waiting for another nostalgic project I have planned.

For the 19, I got a cylinder from them. The motor I am using wouldn't work on one of their mounts.

 

[FlyingTyger]

I'm not exactly sure what I was thinking when I designed the fuse. I got it mostly built this weekend and noticed it just didn't look right. It needs to be at least 1/2" taller. So back to the drawing board CAD station, and will try again...

 

[TooJung2Die]

Looks good in the photo, must be the angle of the shot.

 

[danskis]

I need some help with this plan as I'm clueless with drawing programs (yes clueless). I can open it in Inkscape but I only get one page. It would be nice to have it in usable chunks.

 

[luvmy40]

I need some help with this plan as I'm clueless with drawing programs (yes clueless). I can open it in Inkscape but I only get one page. It would be nice to have it in usable chunks.

What you need to do is import the original PDF into a project. Then ungroup the entire file(shift.control,G) multiple times until the entire file is broken down to it's individual line segments. Then you can select the sections you want, group them(control,G) and organize the sections as you like.

Are you planing to make 20" x 30" drawings?

 

[danskis]

@luvmy40 - that is what I tried to do (which is the extent of my skills) and the only thing that was in the project when I opened it was the elevator section. Nothing else imported. I'm limited to 36" wide but I can do unlimited length at my local print shop. 36" by 45" would be good.

 

[danskis]

@FlyingTyger - IDK while holding the control line handle I'm sure it looks just like a PT19.

 

[luvmy40]

OK, I see that now. Let me work on this a bit. There has to be a way to open it in acrobat and "print" it full scale as a new PDF.