Tracing Aircraft Skin Details: Panels on the Outer Wings

I always start drawing the image of the aircraft skin by tracing the lines of the main panel seams. They will form a kind of reference “grid”, which later I will fill with other details: rivet seams, inspection doors, etc.

I will draw all these technical details in Inkscape, because it is much easier to modify such shapes in this vector-based program than in GIMP, which is mainly intended for the raster images. What’s more, I can export this scalable vector graphic from Inkscape to a raster image of any resolution.

Initially I prepared in Inkscape an empty drawing, set up its layer structure, and placed the appropriate links to reference drawings on the UV and Reference layers (Figure 64‑1):

Figure 64-1 Preparing the “canvas” for the texture drawing (Inkscape)

I duplicated here the basic structure for the detailed bump map, which I worked out during my P-40B project. It is explained in all details in the “Virtual Airplane” guide (chapters 3 and 4 in Vol III, or chapters 6 and 7 in the complete edition). In this case I just used the hierarchical layers feature for grouping the related layers (in Panels, Fabric) together. (This feature was introduced in the latest Inkscape 0.9x, while the guide was written earlier, using older versions of this software).

Although I placed my scale drawings in the background, as the reference material, I will not treat them as the “ultimate truth”. Everybody makes errors, so do I. The only method to eliminate most of them is to check every detail as many times, as you are able. For example ­– see the bent sheet metal strip that runs around the wing tip edge (Figure 64‑2):

Figure 64-2 Panel seams on the photo and reference drawing

When I sketched it on the scale plans, it was a minor detail. Its width was not much larger than the width of the thicker line that I used to trace the outer silhouette of the aircraft. Thus I did not studied the photos carefully enough in that time, and drew this strip too thin. Now I have an occasion to look on the source photos with a “fresh eye”, and correct the width of this strip. However, I cannot just offset the original contour from the scale plans. To match the UV layout of the wing, I have to give this curve somewhat different shape that follows the unwrapped area around the wing leading edge (as in Figure 64‑2).

Well, there is no any “magic” way to do it: I have to keep open Inkscape and Blender side-by-side. In Blender I mapped as the texture the initial image exported from Inkscape (and turned on the option that displays it in the Object/Edit mode). Once I modified this wing tip curve in Inkscape, I had to export the whole drawing to a raster file, and then to reload it in Blender. Fortunately, such a transfer takes no longer than 2-3s. Such an arrangement allowed me to make quickly several iterations, resulting in the proper shape of the curve on the 3D model (Figure 64‑3):

Figure 64-3 Drawing the basic panel seams (bottom wing surface)

To see better the lines on the model, I drew them in red. Fortunately, the rest of the panel seams runs across relatively flat areas, so they match the scale plans.

Of course, I also matched their locations against the reference photos (I set up them some months ago, and described it in this and subsequent posts) (Figure 64‑4):

Figure 64-4 Checking the panel lines against the reference photos

Fortunately, there were only slight differences, which I quickly introduced to my Inkscape drawing. Such a “double-check” ensures, that the lines are in the proper places, and I can safely fill this image with minor details. However, the common sense tells me, that I should map the panel seam lines on the whole aircraft, first. There is always a chance that I will encounter something unexpected during this process.

Dauntless had large wing flaps, and one of their prominent features were the rounded holes, that perforated their surface. Distribution of these holes determines the location of the internal reinforcements of these flaps, and the corresponding rivet seams. Thus these holes are as important as the panel seams. I started to draw their first row using a special, dotted line (Figure 64‑5):

Figure 64-5 Tracing the first row of holes in the bottom flaps

Although Inkscape does not offer any UI for user-defined dotted lines, I used its XML Editor feature to create a dotted line pattern that matches the holes in the Dauntless flaps. I used here the same method that I worked out for the rivet seams. (See “Virtual Airplane” guide, Figure 3.1.11 in Vol III, or Figure. 6.1.11 in the complete edition, and the further pages referenced there).

Once I drew the first row, I matched it against the reference photos (Figure 64‑5). After a few iterations I received a satisfactory approximation. (Due to various unknown second-order photo distortions, there location of these holes is a kind of “compromise” between various photos and the known location of the flap ribs. The latter were explicitly dimensioned on the Dauntless stations diagram, as you can see Figure 8-3 in this post).

When I matched the first row of the holes, I copied them into another two rows, which I matched against the photo. The final results differ from my scale plans (Figure 64‑6):

Figure 64-6 Perforation of the bottom flaps – improved version (in red)

It looks that on my scale plans I made a kind of systematic error in calculating ribs stations from inches to drawing pixels. (Since that time, I already made numerous adjustments in this area – see Figure 15-8 in this post, Figure 17-5 in this post, and Figure 31-5 in this post).

The general panel layout on the wing top surface is similar to the panels on the bottom. Thus I copied (and mirrored) their lines from the bottom surface. It required just a few minor adjustments to match their drawing to the photos of the wing top surface (Figure 64‑7):

Figure 64-7 Tracing the panels of the wing top surface

(I was really happy that I did not have to match again the wing tip strip against the photo. The curve copied from the bottom surface fits the top surface quite well).

For the further test, I created a copy of the texture image with a semi-transparent background. It makes the model surfaces transparent (Figure 64‑8a):

Figure 64-8 Using transparent texture to match the spar lines on the wing lower and upper surfaces

I used this effect to check if the panel seams that runs along the wing spars on the top surface match their counterparts on the bottom surface (Figure 64‑8b). (It will be useful, when I start to recreate the wing internal structures).

During further checking of the results, I noticed a minor error on the leading edge (Figure 64‑9):

Figure 64-9 Problem with the seam corner on the bent leading edge

This is a side effect of the corner in the mesh seam, which does not run along a “sharp” (Crease = 1) mesh edge. Unfortunately, I have to keep this edge smooth, because it controls the proper shape of the wing leading edge, especially in the top view. There are two solutions: 1. add two additional “ribs” on both sides of this wing tip rib, to remodel this mesh fragment, 2. create the strip along the wing tip as a separate object, and placed it on the main mesh. I still have to decide, which solution is better.

Figure 64‑10 shows the results of this week: the panel lines of the wing and the image of the flap perforation:

Figure 64-10 The traced panels on the outer wings

In this source *.blend file you can evaluate yourself the current version of the model, and here is the Inkscape file.

Next week I will map at least the wing center panels and its flaps perforation. (Maybe I will do more – but I am still short of time due to a certain project in my daily job).