Giles G-200 – Wheel Pants, 3D PDF Exports
I figured I would start with something easy on the Giles surface modeling, since I haven’t done any really intensive CAD work in about a month. The wheel pants seemed like a great place to start, since they are a stand alone surface that does not have to directly integrate with the cowling. I started by modeling the wheel, tire, brake caliper and bottom section of the landing gear leg. As I said in my post on digitization, I try to model just the things that I need to be concerned with and not get too involved in making a fully detailed model. After I got the “homework” section of this model out of the way, I made myself some coffee and sat down with my copy of Mike Arnold’s Moldless, low drag wheel pants video and my copy of Hoerner’s Fluid Dynamic Drag. If you’re interested in aviation and drag reduction and you don’t own both Hoerner’s book and all of Mike Arnold’s videos, I would highly encourage you to get both. Arnold’s wheel pant video does a fantastic job – in all of about 10 minutes – in showing the ins and outs of designing a low drag wheel pant. He references Hoerner’s book in all of his videos, so it’s best to have your copy of the book while you watch Mike’s videos. As Arnold states via Hoerner, the ratio of the length to the width of a streamlined two dimensional section should be 3.7:1, with the maximum thickness around 35-37% of the chord. For a three dimensional body of rotation, the ratio should be 2.7:1. I decided to split the difference and start with a ratio of 3.2:1 for both the top and side view, and have my widest point at 37% of chord. The problem, as Arnold describes it, is that since the wheel is taller than it is wide, some compromises must be made or else things just won’t match up. Since the ideal length of the top view is shorter than the ideal length of the side view, his solution is to truncate the side profile. Another interesting thing that I noticed is that since the top and side views are of different lengths, their points of maximum thickness do not coincide. The maximum thickness of the top view is a few inches ahead of the maximum thickness of the side view. I think I’m happy with my overall geometry and the thinking behind it, but I’m going to let it sink in a bit before proceeding further. Here is a screen shot of where I am right now:
I have to say that T-Splines is an absolute joy to work with in this fashion – the green lines you see on the above screenshot are the lines of symmetry – for now it’s symmetrical in both the vertical and horizontal axis. I was able to easily match the wheel pant surface to my reference geometry, and then just push and pull on the points to get the blends between the top and side views to look nice. Once I go to blend in the gear leg fairing, I can isolate just the region where the gear leg fairing blends, and maintain the symmetry of the rest of the model. Just so cool!
I’ve been looking for a quick and easy way to share my work with customers in 3D, one which would not force me to send my master files. I decided to purchase Simlab’s 3D PDF Exporter for Rhino and give it a shot. Overall, I’m impressed, although there have been a few quirks. First, I thought that since you could output surfaces, surely you would be able to output points and lines. Wrong! If it’s not a surface, it doesn’t show up in the exported pdf. I’m told by Simlab that this may be added in future versions, we’ll have to wait and see. The other thing I notice is that the layer hierarchy is not right. This is probably due to the fact that I have lots and lots of sub-layers with the same name in my file (ie points, curves, surfaces) and this may be throwing it for a loop (no pun intended). Anyway, these are fairly minor quibbles, overall I’m very happy with it and I think it will be a great help in allowing customers to review work before we commit to milling and the costs associated with that.
If you’d like to, you can download the 3D pdf of the landing gear and basic mockup of the wheelpant. You can zoom and rotate to your heart’s content. This is just a first pass at this, so yes, I know, the brake caliper is sticking out of the wheel pant.