So, I have been designing and using 3D-Printed props for my aircraft for two or three years now. To be clear, this work is all based on Archie Adamisin’s prop work that he has shared with me. While I can do some designing stuff, laying out airfoils at specific radius stations is still beyond my personal capability and I am thankful for Archie’s help and generosity.
But I have taken his designs into a different direction. For at least two years, I’ve been making “breakaway” props for my scale models. These fold on contact with the ground and are perfectly legal for FAC flying as they do NOT fold before landing, a critical point for FAC acceptance. The entire purpose of this was to avoid prop and model breakage on landing.
These utilize a rectangular hub, usually enclosed in a 3D-Printed spinner, and the plastic filament for printing used an hinge pins. In addition, I use Larrabee profiles for the blade shapes. These shapes are generated within a spreadsheet that has been online for years that calculates blade shapes based on Diameter, Pitch, and Blade Width for a quite efficient propeller design.
the breakaway prop, spinner and noseblock for my Aubert PA-20
the breakaway prop on my Comper Swift
one of my first successful breakaway props for a scale model on my Caudron C.460
I have been quite happy with these props on my scale ships and started to put some on my Old Timers. Again, the FAC does not allow the props that fold before landing, so I never worried about “how” the props folded back. I’ve used printed props on my HepCat, Holy Ike, and Miss Production and – again – I’ve been very pleased with their performance.
the breakaway prop on my Miss Production Old Time Stick – not meant to fold against the fuselage
This brings us up to date with development. During the slow winter months (when I should be building) I tend to think back over the possible projects that I have thought about over the season or whenever. With the delay (not cessation!) of the Superior Prop Factory, I know that people are not being served with folders for their Old Timers and I have thought about “could I print folders for non-FAC users, where they want and need folding action?”
The quick answer is “of course you can – you already do that!” But in reality, there is a feature that is not part of my “breakaway” props and that is the blades do not fold back flat against the fuselage. This has two reasons: 1) I have left the hinge action stiff since FAC models do not need to fold and 2) my hinges do not utilize the compound angle that is built-in to most Old Timer props to get them to fold flat against the fuselage.
the Miss Production breakaway – not meant for folding against the fuselage
showing how the Miss Production breakaway does NOT fold flat to the sides of the fuselage
Old Timer folders generally use a 15 degree and 15 degree compound offset for the hinge pin. This sounds strange, but it has nothing to do with the power portion of the flight – the blades are held in their original pitch settings during power but when the power comes off, and the prop locks, the blades fold back against the fuselage for a more aerodynamic model (the spinning prop is no longer acting as a brake and forward rudder). It is the integration of this 15-15 compound angle into the Hub design AND into the Prop design that is the tricky part. A friend told me “you are thinking too much”. As any designer or dreamer will tell you, the thinking is the easy part. I can “see” the finished product, but converting those thoughts, ideas, and scenes into the drafting program and then into a physical product is the hard part.
Again, there are two parts – the prop blade and the prop hub. I worked on the hub first. It took some time, but I translated the vision into a functional prop hub. The compound angle was integrated and it appeared to be functional. but I had no prop blade to test it with. It took some time, but I got it integrated into the blade template that Archie had supplied a couple years ago (the same template I have used to make all my “breakaway” props).
I refined both the hub and the blades and – with a little post-printing clean-up and tweaking, I printed a fully functional Gollywock folder: 13″ diameter, 17″ pitch, 1.875″ blade width, with a proper Larrabee profile and a 1.5″ hub. The blades flop easily and will lay flat against the fuselage side. The assembly weighs in about 17 grams. It could probably lose a little bit through scraping of the blades, but it will always be a little heavier than balsa.
the Gollywock Folder with a good, flat fold
the finished Gollywock Folder
This process took a full week of designing (well, in the downtime between all the holiday obligations!) That week included plenty of moments of frustration as I wrestled with the 3D design program, trying to get it to do what I wanted. The result is very satisfying, personally.
I will probably never use this prop and I don’t know if it is at all viable for production/sales. It takes a bit of time to get it printed and assembled, but this amount of time is certainly less than a cut, assembled, and finished balsa prop. And it is arguably more accurate, since the plastic holds the specifications as designed while the balsa prop needs to be hand carved and the hinges can go out of adjustment. But, for me, it is mission accomplished.