Replicating Cowl Shape with a hybrid low and high tech method

Replicating the lower cowl shape is moving right along. In the last post we discussed how it was going to be more difficult than the top was to duplicate and mirror since it is more complex. One of the benefits of blogging and posting all about this project is the constructive suggestions I receive. Thomas suggested that I use some technology to make the duplication easier.

Turns out, there is an iPhone app that can help. Scandy Pro is a low cost app that uses the facial recognition camera and technology to be able to do a 3D scan. Thats right, some of us have a 3D scanner in our pockets. I have found it to be very useful for multiple projects already. So far, I have found the app can be finicky, difficult to use, and routinely failed to actually generate a useful file. I haven’t learned enough about why yet, but I am guessing if I learn better technique I will have more success with it. That being said, when you are successful with a scan and get a file, it is amazingly accurate.

In preparation for the scan, I dropped plumb bobs and marked the centerline of the airplane. I then placed a box on this line that I would also scan so we would have reference planes to work from. Again kudos to Thomas for the idea. Using the Scandy Pro app, I then scanned the lower left cowl shape and my box.

This process was actually infuriating because of how often a scan would fail. It took dozens of attempts to get a few useable files. I tried doing small sections of the cowl when doing the full thing didn’t work. No matter what I tried, it was more miss than hit. I am sure it is part of the learning curve, but it seems that the app gets lost often. That is to say that it often doesn’t know where in space to place what it is seeing through the camera. It could have even been that as I was moving around trying to scan my shadows and reflections may have been affecting the scan. Much more to be learned here to make it a routinely useable tool.

All that being said, for the low cost, it did eventually turn out a useable scan. I then took the solid model from the scan and made templates to check the accuracy. I was pretty impressed. Now, admittedly a few of the stations didn’t fit perfectly, but they were better than is necessary, and easier to do than making templates manually.

From there, I took these templates and mirrored the exterior curve so I would have the profile of the right side of the cowl. I turned them into drawings, added some thickness, and printed them. These would become patterns to jig into place on the right side. Of course I don’t have a large format printer, so I had to print them in pieces and tape them together. From there I traced them onto foam core board and cut them out. If I would have been really slick, I could have had them laser cut. But I am too impatient to wait for that.

Now, turns out that fitting these patterns in place every 2 inches is tedious and riddled with approximations when there are conflicts. Part of the center tunnel already exists and we have to match into the existing wing roots and pieces of Brian’s original cowl. Each of these conflicts require template modifications that took significant time to cut/fit/cut more/fit/cut more and so on. I decided to take a hybrid approach.

The tried and true method you’ve already seen using foam splines is pretty simple when the geometry is already defined by the adjacent areas. So I will use that for the simple areas, and use patterns for the lesser defined areas where I had to make choices. That would mean the shape above the wing root, forward of the trailing edge, and below the top cowl is simple to spline. It is well defined by the top cowl and fuselage shape. Another area that is easy to spline is forward of the trailing edge of the wing on the bottom surface. Again it is defined by the fuselage shape and the centerline tunnel. There is no difficulty in either of those places.

The place where there is difficulty is blending aft of the trialing edge of the wing to my augmenter boat tail shape and the top cowl. There is a lot of shape change going on over a short distance. So I made 4 patterns along that length. These were glued into place. That way when I pour foam around them I can sand until I see the full pattern and I am a match to the other side.

With that plan in place, I got to work splining and foaming. By lunch time I had the foam in place. Then it is the usual contour sanding to reveal a nice shape. You can see by the end of the day today, I am 75% there. There are some places that need attention yet. Most notably two low spots where the splines transition to the patterns. One on the bottom and one on the side. The splines didn’t transition as well as I would have liked. The good news is I have my templates which will guide me on how much filling I need to do. Not a big deal.

I think it is already looking very good. Some improvements to the symmetry and filling of low spots and we should be ready to do drywall mud very soon.