Project ALTius
Part I: A simpler approach to high-performance glider development using CAD/CAM.
We’re delighted to announce a new contributor to the New RC Soaring Digest: Tiberiu Atudorei who is based in Ploiești, Romania. We’ve been hoping to find a series like the one Tiberiu kicks off this month — one that will walk through, step-by-step, the practical use of the modern tools of RC soaring design and development. Thanks, and welcome Tiberiu, we’re all looking forward to your series. — Ed.
Once Upon a Time
Twenty-to-twenty-five years ago the world of RC gliders was dominated by Mark Drela’s designs. If you have been in the hobby for more than ten years, you’re likely familiar with airfoil series like the AG25 and AG35 and models like Apogee, Allegro-Lite, Bubble Dancer and Supra. Mark Drela’s designs were likely the first ‘open-source’ gliders and in some cases employed XFOIL for their development. See Resources below for this and all relevant links for this article. For both balsa and composite designs Drela published airfoil data, building details, ribs and plans. Most designs used three airfoils — root, at the wing joiner and tips. He sometimes even used four airfoils. The designs were suitable to low-tech building methods: balsa ribs or foam cores cut between templates which, by the way, is maybe the best non-computational method to compute the transition between two airfoils.
Ten years ago there was a new generation of open-source glider designs by Gerald Taylor: Zone, Zone2, Synergy, Synergy2 for F3H/DLG and SynerJ for F3J/F5J are a set of airfoils and wing designs usually published as XFLR5 projects. Airfoils are thinner at 6–7% and the wing designs became more complex consisting of seven, eight or even nine airfoils. Free for both non-commercal and commercial use, they were very popular with high-end manufacturers who could afford the high cost of tooling design and production. There were also popular with the competition-oriented hobbyist who is often ready to pay for expensive models. With the DIY crowd, well, not so much. Why was that, exactly?
Now we have laser cutters, CNCs and 3D printers. Materials are quite cheap or at least affordable. We have free or cheap CAD software. Why is it so damn hard to build a performance glider? Why do we prefer the easy and cheap and convenient foam glider over a DIY low-tech balsa model or high-tech composite model? Why do we take pride in what we buy instead of what we make?
We Have the Tools
The simple answer is that we have tools such as AutoCAD, QCAD, DraftSight, SolidWorks and Fusion 360 — for example — but these are quite hard to use. There is a lot to learning them and in some cases the price is quite high in both time and money. Even if we know how to use them, these glider designs are quite complex projects. For those who say “it’s not rocket science”, I remind you our rocketeer friends don’t have to deal with eight, nine or even ten airfoils! For instance I’ll give you some details of the workflow for a simple balsa project and a more complex composite project:
Balsa/2D Workflow
Let’s consider a simple three airfoil Drela-type project — or even a seven-through-ten airfoil Taylor-type project, as candidly there is not much difference between how we handle them. We have as input a set of airfoil .dat files. These are basically a two column text file with X- and Y-axis normalised coordinates of the upper and lower profiles of the airfoil. In the best case scenario all sets of airfoil data have the same ‘cardinality’ — a fancy word to tell us that all sets of airfoils have the same number of points and in the same position on the X-axis.
How do we compute each intermediate airfoil? Using a spreadsheet, of course — any current one will do. First column: the positions in the X-axis. Second column: Y-axis values for the first airfoil. Third column: Y-axis values for second airfoil and so on. Let’s say that we have a transition for airfoil A to airfoil B over a distance of 50 cm and the balsa ribs are 5 cm spaced. You can compute the first rib as 90%A+10%B AKA as 0.9A+0.1B, second rib as 0.8A+0.2B and so on. I’ll let you figure out how to adapt the formula if the (segment span)/(rib distance) is not 10.
Now the ‘hard work’ of computing intermediate airfoils is done all you have to do is save these numbers as a set of .dat files, use a tool to draw the airfoil (that is create a .dxf file), scale it, rotate it (don’t forget the washout!), add the main spar and maybe a secondary spar, consider control surfaces (spoilers, ailerons and flaps), compensate for D-box material, joiners, place it in the drawing in order to compensate for material consumption and — finally! — review the design for cutting. Just a whole lot of ‘fun’ — and prone to errors. More like ‘boring’ if you ask me but then again, I’ve done it so many times.
Composite/3D Workflow
I’m afraid the situation is not very different. At least you don’t need to compute the intermediate airfoils. You still need to scale the airfoils, rotate them, translate them in 3D XYZ coordinates to exact positions and then create surface or volume out of this set of polygons. I’m told this is called ‘lofting’. Excuse my ignorance but Fusion 360 or SolidWorks is not my cup of tea. I’m more of an OpenSCAD person. And of course you have to deal with the control surfaces and hinges. And servo pockets. And spars and joiners. And plugs and molds. Yes, never ending ‘fun’ time. And after all this hard work it will still look like a hatchet job or — in case we use some fancy curves for shaping the wing — it will have a different aerodynamic behaviour compared to what the designer had in mind. And if you have just a small adjustment in data input—say, the airfoil set or the wingspan or chord or a ‘big flaps’ variant— you can trash all your work and start over.
Making You SAD with a Little HAM on The Side
If we look closer to both these workflows we see that they both involve dealing with a large set of numbers — the .dat files — the changes are similar — scale, translate and/or rotate — and quite repetitive. All these operations can be automated in a program or application. You give as input the wing definition — the set of .dat file and the distribution of airfoils — and some building parameters — wingspan, central chord, position of control surfaces, type of construction, materials used and so on — and the program does the rest. It creates a .dxf file with the set of ribs which are (almost) ready to be laser cut. Or a set of 3D model files (.stl files) you can 3D print or machine on a CNC. If you think of CAD and CAM programs are ‘tools’ then this is like a ‘pre-tool’. It’s definitely like CAD but a little bit different. CAD/CAM are general use tools, while these are very specialized to the task at hand. We can call them, say Program Assisted Design (PAD) or maybe Application Assisted Design (AAD). But I’ll use Software Assisted Design or, you guessed it, SAD.
If you are the buy-and-fly/crash type probably this article and subsequent parts are not for you. But if you are the dream-design-build-fly-crash-fix-repeat type I will try to help you with the design-build part. To give you the software tools described above but also the hardware tools. This is another great acronym: HAM for Hardware Assisted Manufacturing. Don’t worry, it’s just a term for DIY laser cutters and 3D resin printers.
Basic SAD Workflow
It’s actually quite simple in structure. SAD apps read some .dat files — defining the airfoils used in the wing, a wing definition file — often just plain direct exports from the XFLR5 project — and a file for parameters and options. Some number crunching for interpolation, scaling, translation and rotation operations and we have a large set of 3D coordinates describing the wing. The next step is to write the output file(s): both are in ASCII text form: either .dxf or .stl. This looks complicated but it’s not — it’s just drawing a line or circle for .dxf or a vertex for 3D .stl. Don’t expect a fancy user interface: these are very simple, text-based, command line interface (CLI) apps.
As long as you platform has a C compiler you’re fine: you can run them in Windows, MacOS, Linux or even Linux/ARM. And with no big difference except in compute times: you get the same output in a recent Windows laptop — in two-to-three seconds for a drawing or two-to-three minutes for a full set of 3D models. In a Raspberry Pi or similar it will take seven-to-eight minutes for a 3D model file. Actually, all the computation phase is done in a couple of seconds, most time is spent in writing the files.
Some Examples
Exhibits A1 and A2 are an example of SAD 2D: a simple application for drawing the elements, in this case the ribs, of a Drela — type three airfoil wing. Note that for all of the examples shown below you can click on the image to see it in much higher resolution.
Exhibits B and C are examples of SAD 3D: a couple of apps to help you design an F5J fuselage and wing consisting of 3D models for wing segments, surfaces, molds and plugs. It’s a complex wing model, a Taylor-type F3J/F5J SynerJ-like project.
And finally, for this month at least, Exhibit D is an example SAD 2.5D: an app for drawing normal and geodetic balsa ribs for the wing shown in Exhibit C in case you want to have a geodetic balsa construction or, even better, a composite hollow wing with geodetic rib reinforcements.
If you are interested in these SAD little apps and you think you can use some help in your projects: please don’t hesitate add your comments and questions below in the Responses section, which you can find by clicking the little 💬 below. Please let me know if you are interested in balsa or composite or perhaps some unorthodox methods — composite hollow wings reinforced with geodetic balsa ribs or maybe 3D printed wings?
For the moment I’d recommend to get familiar with the ALTius project from the RCGroups build log (link also in Resources) so there is no need to repeat what’s there. I’ll just post some updates in the next issues and focus more on practical parts and other apps.
Thanks for reading and see you next month!
©2023 Tiberiu Atudorei
Personal footnote: I’m dedicating this series in two ways both of which are of great importance to me: to Viktor Frunze of Krasnodar, Russia and noted F3K and F5J designer, builder and competitor who passed away recently and, equally, to the people of Ukraine.
Resources
- AutoCAD — “2D and 3D CAD software trusted by millions to draft, engineer, and automate designs anywhere, anytime…”
- DraftSight — “The Ultimate Editor for DWG and DXF Files From The Makers of SOLIDWORKS…”
- Fusion 360 — “Unified CAD, CAM, and PCB software…”
- LibreCAD — “LibreCAD started as a project to build CAM capabilities into the community version of QCad for use with a Mechmate CNC router…”
- OpenSCAD — “software for creating solid 3D CAD models. It is free software…”
- QCAD — “a free, open source application for computer aided drafting (CAD) in two dimensions (2D)…”
- SolidWorks — “SOLIDWORKS® and the 3DEXPERIENCE® Works portfolio unite your entire ecosystem…”
- XFLR5 — “an analysis tool for airfoils, wings and planes operating at low Reynolds Numbers…”
- XFOIL — “an interactive program for the design and analysis of subsonic isolated airfoils…”
- Project ALTius on RCGroups. — “altius, citius, fortius — sounds familiar? That’s the Olympic motto where ‘altius’ means ‘higher’. But the spelling (ALTius) is related also to my initials — Atudorei Lucian Tiberiu…”
All images by the author unless otherwise noted. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
