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The Big Pitts Design Process.

Updated: Jun 28, 2023

I was finishing up on the smaller lightweight PLA printed Pitts project and decided I wanted to try an experiment with something much larger. So, I doubled the size and am in the process of completely redesigning it for printing as another stick built 3D printed project. This will be a 48" wingspan model now. What I want to accomplish is this:

  • I simply want a big, long lasting, great flying aerobatic airplane.

  • I want a durable model.

  • I want a pretty model! - I'm not in love with the look of a 3D printed planes until I spend all kinds of time finishing them. If you do it right, you are not ahead of the game time wise when you are done. All that prettiness adds considerable weight.

  • I want a repairable model.

  • I want a reasonably fast printing model.

  • I want an easy to build model.

I learned that this is all possible with a 3D printed model back when I designed the Fokker triplane project. The one thing I did not do with the Fokker was cover it with traditional iron on covering. I simply wanted to use the silk covering technique at the time because I just love the way it looks. Honestly I would do it again on this model but It clearly scares a lot of people away so on this one I'm going to use an iron on covering solution. What has worked so far:

  1. Traditional PETG filament for the formers and custom stringers - I design them so that they all interlock to create a jig of sorts to ensure the model stays straight while assembling. I greatly reduced the infill amount to 20 percent and used a gyroid pattern. I remove the top and bottom layers completely so that they do not print at all. I have the walls printing a thickness of two lines. The parts are strong where they need to be, and as you can see from the pics there's a lot of air in them. It reminds me a little bit of the look of a rigid foam. I'm very happy with the weight reduction overall.

  2. Covering the Horizontal Stabilizer with Ultracote. - I had an old roll of translucent red so I gave it a shot and the PETG holds up perfectly fine. I only used a sealing iron, and not a heat gun but that's because I broke my only remaining heat gun. When I get around to replacing it I think with care the heat gun will be fine.

What has not worked: This won't be a problem for the main wings as they will be printed much like the formers, but I didn't use the same process for printing the tail feathers because I wanted the top and bottom layers for added strength. These parts actually print as assembled components. It is not terrible but I'm concerned about the tail weight. I can opt for the exact print in LWPLA, but I wanted something with better temperature resistance so I have a test roll of Colorfabbs new LWPLA - HT (high temp) filament coming. This should cut the weight in half for this part and still give me good heat stability. I'll report how it does. Not everything is 3D printed: Clearly I have also got balsa stringers on this model. They are no stronger, but they are easy because they come ready to go in 36" lengths. Any stringer I print has to be assembled due to print bed limitations. I'm designing the model so a builder could choose to print or purchase and use stock sizes from the hobby shop. Here's the unexpected catch though. I walked out of the hobby shop the other day with just the balsa wood for running the stringers. I also had two or three extra solid sheets for a whopping bill of $40 bucks. The entire roll of PETG filament for this project cost 25 bucks from Amazon and for everything printed so far I have not used even 25 percent of the roll. It's far cheaper to print the airplane parts! What I'm going to try: I'm going to attempt to blow mold a real "clear" canopy - Maybe I'll just vacuum mold it. I'm going to print up the enormous cowling as a mold with regular old PLA then create a vacuum formed fiberglass cowling. If this works out great I'll also do the wheel pants this way. I've done a good bit of glass work and I've always wanted to try mold making with the 3D printer. This is a perfect excuse to do it. That's all for now, I'll post updates as I have them on this little experiment.

Update 12-25-22 - She's up on the gear now. Top wing is covered with Red Ultracote.

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