Shaping the Gun Troughs

The gun troughs in the aircraft usually are tricky elements. Their edges depends on the shape of two curved surfaces: the fuselage around the recess and the tubular inner surface. When you make mistake in any of these two shapes — you have to remodel the whole thing.

In the SBD there are two symmetric recesses in the upper part of the NACA cowling, in the front of its 0.5″ guns. Figure 44‑1 shows the left one:

Figure 44-1 The gun troughs in the NACA cowling (SBD-3)

As you can see on the photo, these recesses were formed in a separate metal sheet. It was riveted afterwards to the main body of the NACA cowling. I will repeat such an arrangement in my model, because using a separate object for such a feature simplifies its mesh. (I can make this mesh denser than the NACA cowling around it, and still I do not have to worry about the topological implications). The sheet metal around these recesses seems to be relatively thick, which ultimately makes the fitting of this panel to the NACA cowling surface easier. To make some space for this dedicated panel, I created initial openings for the gun recesses in the upper panel of the NACA cowling. They are generated by a Boolean modifier, and are a little bit larger than the final recesses.

The most difficult part of the gun trough in this aircraft is the fillet around its edge. To obtain a high-quality shape, I decided to start this panel as two separate objects. The first of them is the tubular inner surface (copied from the “cutting” object used in the Boolean modifier). The second object is just a small cylinder, which radius is close to the fillet radius. I will deform it along a 3D curve, which follows the border of the gun recess opening (Figure 44‑2):

Figure 44-2 Initial steps in the creation of the recess mesh

When I started to extrude subsequent segments of the “fillet” cylinder, it automatically follows the assigned curve. (The curve allows me to do it without worrying about preserving the circular cross-section along the whole length of the opening border). Technically, this is the effect of a Curve Deform modifier that I assigned to the cylinder object. This is the first modifier in the stack, and it precedes the smoothing (Subdivision Surface) modifier. Such an arrangement allows me freely slide the circular cylinder sections around the opening border, finding the proper locations for these key vertices (Figure 44‑3):

Figure 44-3 Forming fillet sections

Then I shifted this resulting contour down, and adjusted its “spine” curve so that the “fillet” cylinder barely touches the opening edge.

When the basic cylinder was shaped, I removed (applied) the curve modifier, as well as the unnecessary ¾ of the cylinder surface. The result is a regular fillet, formed around the opening (Figure 44‑4a):

Figure 44-4 Preparing the two complementary meshes for joining

Now I started prepare the inner part of this recess for joining with this fillet. I had to add some additional sections. They are placed at the corresponding sections in the fillet mesh (Figure 44‑4b).

When all the edges of the inner recess mesh were verified and adjusted to match the filet, I joined these two objects and removed the unnecessary faces (Figure 44‑5a):

Figure 44-5 Joining the inner surface of the recess with its fillet

Then I created new faces that join these two meshes (Figure 44‑5b).

Once the inner part of the recess panel was completed, I started to form its outer part by extruding its outer edge (Figure 44‑6a):

Figure 44-6 Forming the outer surfaces

I placed its vertices on the outer edges of this panel (Figure 44‑6b). Then I added another edge loop in the middle and started to elevate the ‘sunken’ part of this surface above the cowling panel (Figure 44‑6c).

Figure 44‑7a) shows the outer surface neatly fitted to the cowling. As you can see, it requires not one, but two inner edge loops outside the fillet, to reproduce circular cross section of the NACA cowling around this recess:

Figure 44-7 Fitting the outer surfaces to the NACA cowling

Finally I used the same auxiliary object as for the underlying panel to cut out the space for the topmost cowling flap (Figure 44‑7b). (It is made using the Boolean modifier).

I think that the gun trough in Figure 44‑7b) looks good enough. However, when I looked onto another reference photo, and then onto another, I slowly started to discover that these recesses had different cross sections! I assumed that it was an arc, while the more I study the photos, the more I came to a conclusion that it had narrower, ‘U’-shape cross-section!

Such surprises are common, when you are making a precise model. Thus, do not assume that the progress of your work will go as a ‘waterfall’. It is more similar to a ‘spiral’: you often come back to the completed parts and adjust some of their details. Just keep the objects ready for such situations: they are normal part of the work.

That’s why I still keep as much features as possible implemented as the modifiers applied to relatively simple meshes. Thanks to such an arrangement, the adjustment of the recess shape does not require a lot of work (Figure 44‑8):

Figure 44-8 Modification the recess shape (from circular to ‘U’ – shaped)

First I created a simple auxiliary object as the reference of the correct cross-section shape (white contour in Figure 44‑8a). Then I placed the panel being modified over the reference shape of the NACA cowling (in red). Then I started to shift the complete fillet sections and the near lengthwise edges in the front view, placing them on the new contour. When it was done, I made minor adjustments along the recess edge, shifting the fillet sections until they fit the red surface of the NACA cowling.

The difference in colors helps me to estimate the remaining deviations from the reference surface. I usually shift the modified section downward, until the resulting gray surface around it ‘sinks’ in the red reference surface. Then I move it minimally upward, so that the resulting surface appears just above the reference object.

Figure 44‑9 shows the final result: gun troughs in the upper panel of the NACA cowling:

Figure 44-9 The final result — gun recesses, fitted into the engine cowling

For convenient “handling”, the recessed panels are attached to base cowling panel by the “parent” relation in the internal hierarchy of this model.

In this source *.blend file you can evaluate yourself the model from this post.

In the next post I will form another element of the upper cowling panel: the carburetor scoop.

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