Capturing “Clean” Planar Sections for the FARO arm
With a mechanical FARO arm, one of the biggest challenges when scanning aircraft structures is in getting just the right data. What you ideally want is the simplest, most orderly points which will get the job done. On some projects, that’s a relatively easy task. On the Giles G-200 cowling project, digitizing the back of the cowl where it meets up with the front of the fuselage is a relatively easy task – the joggle in the fuselage skin provides a very nice reference. As you can see in this photo, I typically put a strip of painters tape just behind the joggle, and capture a set of points along each edge of the tape:
Depending on how much curvature is in the part, I will do more or less points, but I always try to keep them evenly spaced. For this section, I used a 1″ spacing between the points. By capturing a “band” of points like this, I can create a nice and clean surface that includes both the positional and tangential data for the cowling to mate up with:
But what about when you need to scan something that has no joggle or reference from which to align your scanning? The most troublesome example of this for me has been wings – especially composite wings that are tapered. What we want to capture is a very clean planar section of a wing – we want the data points to more or less reside in one single plane. I spent a great deal of time trying to come up with a solution to this problem, and I’m happy to report that I finally come up with a very nice trick. I started by looking for a laser level/plumb that was self leveling and wouldn’t cost a fortune. I found this nice unit on eBay for about $250:
It’s a PLS180 made by Pacific Laser Systems. It shoots out both a plumb and level line, and can be mounted to a camera tripod. Now, a plumb line will work great for a wing – but only for one side! So, I set about trying to come up with a way to reflect the plumb line that I shoot onto the top of the wing onto the bottom surface as well. I had all these wild ideas about mounting the unit to some sort of mirror holding fixture, but it seemed like an awful lot of work for something that should be simple. Finally I came up with what I felt was a simple and elegant solution – and so I present to you the best idea I’ve had in quite some time. A mirror. Taped to some sheet foam. Floating in a bin filled with a few inches of water.
Yes indeed, in the bottom of the above pictures you can see my grand invention. Since the PLS180 is self leveling, and the water in the bin naturally settles to level, by floating a mirror in the bin I can shoot the laser line onto the bottom of the wing in exactly the same plane as the top. Here is the laser line on the top of the wing:
And here it is along the bottom:
Since the laser level and the mirror reflect the line onto the leading edge of the wing, you can watch the bottom line synch up with the top line as the water settles down. Here it is after the water has completely settled, as you can see there is barely any discernible overlap, which tells us it’s working perfectly:
There was one instance when I was setting up where the top and bottom line did not line up, and I sat there for a few minutes trying to figure out what I had done wrong. I pushed one corner of the mirror down, and an air bubble “burped” out from under the foam on the other side! After that, the line settled perfectly into position. Now I simply slide the mirror into the tub at a slight angle so that the air can escape. The result of this was some of the cleanest data I’ve ever gotten from the FARO arm:
Here’s a view of the outboard section scan – as you can see all the points line up very nicely in a plane:
Starting with nice orderly data like this just makes EVERYTHING easier downstream during the design process. Although not everyone out there has a FARO arm, I’m sure some of you will find this trick useful whenever you need to project a section line on both sides of an object.