The power of 3D scanning & printing is best illustrated by example, so in this mini-series I will walk you through the process of designing a simple reinforcement bracket for a Rans S7S tail.
Randy and the Rans Aircraft team designed an amazingly capable mini-bushplane when they developed the S7S. One specific area reinforced by many that fly off-airport is the tailpost to longeron weldment, so that is what we are going to fabricate before this fuselage gets paint.
During a hard landing the tailpost is being forced vertically with a moment arm at the tailspring bolt putting the entire lower longeron in significant tension. We are going to design a simple bracket which is intended to support and distribute the load to keep the lower longeron from breaking at the tailwheel attach bolt weldment.
The first step is to 3D scan the area to allow my final measurements a greater degree of accuracy. 3D scanning with hobby grade scanners can be tricky with large or complicated assemblies such as this, however this scan does not need to be perfect. Since I can measure the tubes directly I am simply attempting to get accurate relative positions of the tubes.
The white residue in the above photo is a spray powder utilized to allow the 3D scanner to "see" reflective surfaces better and simply wipes off after scanning is complete.
Above is my rough mesh imported from the 3D scanner into Fusion 360. I did very little cleanup and even though the overall scan is poor it is good enough to pick up measurements and position natively drawn tubes in the correct positions.
Since we are designing reinforcement brackets here, the important surfaces to have to be correct are the relative angles between the tubes. These angles are important because I am going to TIG weld these brackets to the thin-wall tubes and a good fit is required to have the strong and more attractive final weld.
Rather than attempt to design brackets to a poor quality digital mesh, I quickly drew correctly dimensioned tubes and positioned them inline with the mesh. Removing the 3D scanned mesh I now have a native Fusion 360 model to allow measurements and comparison to the real thing in the workshop.
Small adjustments were required to get the 3D model positioned accurately however that process is quick and painless. Armed with an accurate 3D model, drawing the reinforcement brackets is now quite simple.
Now I have a digital model I believe to be correct and some brackets which may or may not fit in real life. The good news is that I have not yet cut anything in metal which is often where things start getting expensive...
How do we test fit the digital model against the physical fuselage?
With the help of a 3D printer, twenty minutes after version 1 is modeled I have a physical plastic part in my hands that I can hold up to the actual fuselage. After the first test fit against the fuselage I adjusted two minor angles on the 3D model and printed version 2. Twenty minutes later, the second print was complete and ready to test fit.
When working with an existing assembly I find it often takes a bit of trial and error to perfectly match the digital part to the unique assembly. In the old days we would use posterboard and scissors to cut out and test fit a shape, but in this brave new world I can simply 3D print a dimensionally accurate part in plastic to test fit against the fuselage. Some builders will still use the paper & scissors technique, then CAD the paper template with good results. I prefer to start with the 3D scanner simply because I will forever have that assembly for future reference where a paper template is of lesser value.
The thickness and stiffness of the plastic part allows me to gauge the success of this brace in ways a piece of paper never could. At the design level I can iterate on tiny details as deeply as I desire and at a tactical level I can quickly see the areas that will be tough to weld or make bolt access difficult. Most importantly, I have a digital file that can produce an unlimited number of identical parts.
In the next part of this mini-series we will CNC plasma cut these brackets out of 4130 then tack weld in place. We will also discuss online services that can produce this bracket for you when provided with a DXF file.
You might be asking yourself how you could leverage this development process on your own build?
The learning curve for all 3D modeling software is very steep, however given enough motivation anyone can learn packages such as Fusion 360 or Solidworks. 3D printing is another steep learning curve, however there are many resources available to anyone genuinely interested.
Over the past 5 years I have entirely transitioned from paper templates to a digital workflow. I have done this because it is faster and easier to reach a more accurate output and it allows me to iterate on ideas endlessly for very little money. I have also found this process easy to do for others:
- We talk about your idea & you provide dimensions
- We iterate on the idea via email until satisfied, then I send you a part in plastic
- If it fits, we produce it in a final form
- If it doesn't fit we iterate until we come out with something that does fit
What would you build for your experimental airplane if you didn't have to spend years learning 3D modeling software & 3D printing?
