Using Panel Lines to Verify the Fuselage Shape

In every creative process, after each “big step forward” you have to stop and carefully examine the results. Usually you have to make various corrections (sometimes minor, sometimes major), before taking the next step. This post describes such minor corrections that I had to make after mapping the key texture of the panel lines.

In my first post published in October, I drew the panel lines on the model, then compared them with the photos. Sometimes a minor difference between their layouts can lead to a discovery of an error in the fuselage shape. I in that post already found and fixed an issue in the shape of the tailplane fillet.

I also mentioned (see Figure 65‑9 in previous post) that I can see a difference in the bottom part of the wing fillet. Now I would like to resume my analysis at this point (Figure 67‑1):

Figure 67-1 Difference in the shape of the wing fillet bottom seam

As you can see in the photo (Figure 67‑1a) the shape of one of the seams on the bottom of the trailing edge (in red) differs from the photo (yellow dashed line). In my model this seam contains two segments (Figure 67‑1b): a straight line, corresponding to the flat, bottom surface of the fuselage, and a curved segment, resulting from the cross-section of the rounded trailing edge. From the geometrical point of view, such a shape is absolutely correct. However, it differs from the real airplane. Why?

Well, we should never forget about the way in which such an aircraft structure was built: there were fixed bulkheads of a fixed, determined shape, and the stringers (stiffeners) between them. It was possible to bend a little such a stringer between two subsequent bulkheads. However, the resulting curve always had a shape similar to a uniform, gentle arc – as you can see in the photo (Figure 67‑1a). The combination of the straight segment and a curved segment (as in the model from Figure 67‑1b) would require at least an additional bulkhead between these two segments. All in all, the real shape of the aircraft was not as ideal as you can see in my model. I had to modify its shape in this area.

Figure 67‑2 shows the fuselage mesh before and after my modifications:

Figure 67-2 Modifications that I made in the area behind the wing

As you can see, in the final version I split the bottom of this fuselage into much more faces. It was one of these cases, when you try to change a single detail, then it occurs that this modification causes a “network effect”. Initially I rearranged faces on the fillet trailing edge, creating two additional n-gons. It improved the shape of the seam line. However, this removed small crease edge that was “fixing” the deformation around seam corner. Thus I had to find another place for the seam… Well, the resulting mesh does not look especially elegant, but it finally creates the desired effect.

Figure 67‑3a) shows details of my new concept for unwrapping this area in the UV space. I had to reduce the low-distortion area behind the wing. Actually it is just large enough to contain the identification lights. (It would be extremely difficult to obtain their circular frames on the highly distorted faces “glued” to the main part of the fuselage):

Figure 67-3 Modifications that I made in the area behind the wing

Figure 67‑3b) shows the modified UV layout of the fuselage mesh. This time I was able to not break any of the panel seam lines in the middle. Actually the new UV seam crosses just a single rivet line. (It does not create as many further complications as in the case of the crossed panel line).

Figure 67‑4 shows the panel lines on the updated model:

Figure 67-4 Panel lines on the updated model

After so many modifications applied to the fuselage mesh, it is a good idea to check if they did not spoil something in the alternate UV layout. (This is second UV layout in this model. As you can find in the previous posts, I created the first UV layout, named UVMap, for the other textures, for example – for the camouflage).

Indeed, when I switched the current UV layout from UVTech to UVMap, I saw that I have some troubles here (Figure 67‑5):

Figure 67-5 Results of the modified UV seams in the alternate color texture mapping (UVMap)

The primary reasons of these troubles are:

• Substantial modification of the mesh topology in this area (some of the original faces that were mapped in this layout have disappeared);
• Alteration of the seam line: seam lines are shared between UV layouts. I altered the original seam to another edge loop, while working on the UVTech layout;

In the effect, now I have now some highly distorted and stretched faces in the UVMap layout (as you can see in Figure 67‑5).

To fix this flaw, I modified the UVMap layout. I had to accept that there will be some distortion of the texture image on the bottom wing fillet areas, as you can see in Figure 67‑6a) and Figure 67‑6c). I decided that such a distortion is passable for the color textures (for the technical details I will use another, UVTech layout).

An important element of the UV layout for color textures is the location of the seam lines. (The unavoidable color differences between separate parts of the texture image always occur along the UV layout seams). Usually I try to hide them, marking as the UV seams the mesh edges that run along a panel seam (see Figure 60-2, in this post). That’s why I cannot use here the seams from the UVTech texture: they run across a “blank” area of the aircraft skin. However, there are no appropriate panel seams in this area. Thus I when I decided to create an additional seam, I placed it along one of the rivet seams (Figure 67‑6a):

Figure 67-6 Modifications in the color texture mapping (UVMap)

Then I had to modify the layout of the mesh faces in the UV space (Figure 67‑6b). (I used a little Blender trick to quickly obtain such an effect. First I temporarily removed the seams from the alternate UVTech map. I also removed all the “pins” from the vertices around this seam. Then I invoked the “Unwrap” command, and all the mesh faces “reorganized” themselves around the new seam. Finally I had just to pin them again, and restore the removed seams from the other mappings).

However, it seems that I went in my modifications too far, when I “improved” the upper part of the wing fillet:

Figure 67-7 Wrong shape of the wing upper fillet

I deformed its original, conic shape, unconsciously reducing the cross-section radii of this surface over the wing upper area. It seems that I had forgotten to look on the photos. Now I have to fix this error.

To ensure that the shape of the panel lines in my model will match the photo, I placed in the model space some auxiliary “stiffeners” (Figure 67‑8a):

Figure 67-8 Auxiliary objects, prepared for shaping of the wing fillet

The reference photos were a great help here: some of them depicted these stiffeners in the side view, the others – in the top view (Figure 67‑8b). I used these pictures to precisely determine locations of these seams in the 3D space.

Using my auxiliary objects, I was able to recreate the wing fillet with greater precision (Figure 67‑9):

Figure 67-9 Modification of the wing fillet shape

As you can see in the figure above, I also created two auxiliary conical segments. They provide me a kind of “indicator” of the differences between the “ideal shape” and the fuselage surface.

Figure 67‑10 shows the results. Because there are no panel seams along the inner stiffeners of the wing fillet, I drew their rivet seams on the model texture. As you can see, they match both reference photos:

Figure 67-10 Using the reference photos for mapping the key rivet seams

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

Within two-three weeks I should prepare the first texture. It will be the bump map.