Giles G-200 Cowling
I’m very happy to report that I’m starting a project on a Giles G-200 cowling. My friend Aaron Burhoe of Burhoe Machine Works up in Sonora, California is building a Giles G-200. For those unfamiliar with the Giles, it’s a single seat unlimited acrobat, made primarily of carbon fiber. He’s been unhappy with the stock cowl for some time – it’s too blunt, boxy, and really not all that attractive. Here’s a pic he sent me:
Surely, I said, we can do better than that. I’m tentatively scheduled to fly up there next week, once he has the engine hung on the mount. I’ll bring the FARO arm to digitize the firewall, spinner and engine. In the meantime, let’s get started on the surface model. Wait, what? You might be asking “how do you start a surface model when you haven’t even gotten the plane digitized?” Well, this is truly one of the places where T-Splines just blows my mind. Traditional NURBS modeling packages generate surfaces by taking input curves and surfaces and using those as input for a new surface. So if I was using standard Rhino, I would have to wait until after I digitized the plane to start. T-Splines on the other hand is topology based – meaning the way I define the entire surface and how the faces of that surface relate to each other is what allows me to generate the final surface. T-Splines surfaces are unified, so I can edit them at any time without affecting other parts of the model. So this means I can figure out HOW I want to model the spinner ring and inlets, without having to concern myself with the part of the cowl that mates to the fuselage. Once I get on site and have the relevant sections digitized, I can simply match my existing T-Spline surface to that geometry, and then style and tweak the model.
The biggest part of the learning curve for me in switching to T-Splines was to get to the point where I could visualize the topology that would work for the particular project I was working on BEFORE I started the model. What it comes down to is figuring out where your t points and star points should be. The best short explanation of them, that I can offer, can be found here on the T-Splines forum. So while I was driving around town the other day, I started thinking about what my topology will look like for this cowling. At the most basic level, a cowling resembles a box with one side removed. Okay, so let’s start with that – T-Splines can generate “primitives” which can be used a nice a starting point for models. So I generated a box primitive, deleted one side and then used tsLayout to look at the star point locations:
No surprise to those of us who use T-Splines, you’ve got four star points, one on each corner. The next step is to subdivide the faces, we obviously want more control than this. So I next go something like this:
This should be enough topology to make a very sleek and sexy cowl – really! The next thing to figure out is what the holes for the spinner ring and inlets should look like. I deleted the center four faces, extruded it inwards, and then deleted the two more faces on each side and extruded that edge loop down. I also enabled symmetry. So the topology now looks like this:
You can see the four original star points around the edge. The eight additional points are for the inlets and spinner ring. Here’s what this looks like in smooth mode:
Well that doesn’t look attractive, but believe it or not, all it takes from here is some point editing. I used tsMatch to force the spinner ring into a circle, and put a crease between the spinner ring and the area inside of it. Other than that, all it took was 45 minutes of point editing to get here:
Well that certainly looks promising! I know that I can use this as a starting point for my design. Note the location of the t-points:
They are all in areas of relatively low curvature, which helps greatly in keeping the model smooth. If I need to add more points to match the firewall, I can do that easily – in fact I can use the tsMatch command and it will insert extra points automatically if I need it to. Basically, the hard part of this job is done, and I haven’t even shown up yet to do the work!