Original SBD Dauntless Blueprints: Fuselage Geometry (2)

In previous post I started creating 3D reference objects for the SBD fuselage. In this post I will complete this work. I will focus here on the difficult part: the wing fillet. It spanned along more than half of the SBD fuselage length. In this post I am going to prepare reference geometry that describe its shape from bulkhead #4 to bulkhead #13. Unfortunately, in drawings from the NASM microfilms I found just a few contours related to this feature:

Figure 113-1 Identified wing fillet contours

Just in case: the illustration above uses the fuselage bulkhead (frame) ordinal numbers, introduced in the previous post. I am referring them using these ids (for example: “frame #05”). See Figure 113‑2 for their map

Figure 113‑1a) shows horizontal contour of this fillet, which I found in the panels assembly diagram (dwg 5063493 – see its high-resolution version). Figure 113‑1b) shows two contours of the forward wing fillet segment (the part placed over the wing). I found the contour of frame#08 (marked in red) in a carbon dioxide installation assembly drawing. Figure 113‑1c) shows four contours of the rear part of the wing fillet (the part behind the wing trailing edge). In previous post you can find description the source drawing that I used for recreating frame #09 contour.  Contours of frame #10 and #11 are copied from battery and tool compartment drawings, while frame #12 (and #11) – from flares assembly drawings. As you can see, all these contours were placed in their original blueprints just as additional information, thus I do not expect that they precisely depict the real lines.

It seems that the geometry of this wing fillet can be described as a fragment of two bent cones, shown in Figure 113‑2:

Figure 113-2 Fuselage bulkheads and wing fillet cones

The forward cone is adjacent to the wing and fuselage. Rear cone starts at wing trailing edge, and extends up to frame #13. (At frame #13 it finally joins the central cone of this tail, described in previous post). I identified circular cross sections of wing fillet cones using drawings shown in Figure 113‑1b,c. These two cones must be adjacent. To see better what I mean, look at Figure 113‑3a):

Figure 113-3 Checking the wing fillet cones

I built instances of these fillet cones (shown in Figure 113‑3a) around circles from Figure 113‑2. Note their adjacent area between frame #09 and #10 (ideally it should be just a single adjacent edge).

I shaped the horizontal contour of the rear cone (precisely: its topmost segment, between frame #08 and #09) according the fuselage panels diagram (as in Figure 113‑3b). The side contour of this cone simultaneously fits the bottom contour of the fuselage. Then I noticed a serious flaw in this drawing: its wing fillet contour between frame #09 and #10 it does not fit the actual contour of the rear cone (see Figure 113‑3b). What’s more, this blueprint does not show any wing fillet contour behind frame #10.

To better show in the 3D space what this difference means, see Figure 113‑4a):

Figure 113-4 Implications of the contour from the fuselage panels diagram

The contour from fuselage panels diagram suggests that the bottom part of frame #10 was as wide as the upper part. To illustrate this difference, I shifted in Figure 113‑4a) the circular cross section of the rear fillet (black circle) into corresponding location. Because in this blueprint the contour line “sinks” into the fuselage contour at frame #10, one can only speculate about eventual position of the next cone section, at frame #11. Anyway, the photo of the frame #10 from a restored aircraft (Figure 113‑4b) confirms the current shape of this bulkhead (highlighted in the picture above). This means that the contour from the fuselage panels diagram is wrong, at least for the part that spans behind frame #10. If so, what about the remaining part, between frames #08 and #09?

I decided to check this issue on the reference photos, that I matched using the previous version of my SBD model. Surprisingly, the PAM photo of the original SBD-5 (before restoration) fits ideally the reference frame (Figure 113‑5a):

Figure 113-5 Checking the wing fillet contour with the reference photo

Figure 113‑5b shows enlarged fragment of this photo. As you can see, the shape of the real wing fillet contour is completely different from the shape copied from the fuselage panels diagram! The only common point lies at frame #09. The remaining contour of the rear fillet cone (marked by white dashed line) seems to be a perfect continuation of this real curve.

How to explain such an error? In technical drawings, the most important thing are the explicit dimensions or references. (In the case of this panels diagram these are references to skin thickness and riveting seam types). Depicted object has just to resemble the real thing, making the drawing readable. All eventual curves are described by data points (ordinals), grouped in the geometry diagrams. That’s why for a manufacturer/workshop this panel assembly is valid, in spite of these wrong lines. I suppose that it would be less readable with the real fillet contour drawn after frame #09

I modified the shape of the rear cone between frame #08 and #09 according this reference photo. To fit it into the fuselage side contour I had to modify shape of the last two sections of the forward cone (Figure 113‑6a):

Figure 113-6 Final shape of the wing fillet and its verification in a matched photo

I marked the modified area of this cone in purple color. (To make this reference object more convenient, I reduced the forward cone to the most important quarter). As you can see, I flattened a little the outer parts of last two circular sections. Of course, I checked this shape with another reference photo (Figure 113‑6b). Now it differs a little from the ideal circle depicted in the assembly blueprint of the carbon dioxide system (Figure 113‑1b), but I assumed that it was a simplified contour, used for the illustration purposes.

The updated, somewhat shorter outer wing segment fits much better the reference photos (this is another confirmation that I read properly its assembly drawings). Now I used these photos for checking the fuselage side contour:

Figure 113-7 Checking tail contour with another reference photo

It is interesting that in this way I am comparing the real aircraft with its blueprint (in this case – the side view of the structure assembly). As you can see in the picture above, I found some differences along the upper edge of the tail. Figure 113‑9 shows their details:

Figure 113-8 Differences in the fin shape

First, look at the fin contours: the real shape is quite different from the blueprint contour! Of course, I also checked this difference in reference photos of other aircraft (the PAM SBD-5). They confirmed these findings. In the picture above you can also see a difference in the fuselage height behind gunner’s cockpit enclosure. However, this one was not confirmed by photos of other aircraft. In the cockpit enclosure drawing I also found the explicit dimension of the fuselage height at the edge of gunner’s cockpit: 26.65”. Thus, I suppose that this is an individual shape variation, specific to the restored machine depicted in this photo. (For example – caused by the modified gun doors).

However, I used information from the reference photos to make an adjustment of the fuselage height, in the front of the fin:

Figure 113-9 Updated upper contour of the fuselage

As you can see in the picture above, the upper contour of the fuselage is “anchored” by two explicit height dimensions: at the corner of the gun doors (26.65”) and at frame #13 (18.09”). Except these two heights, I also found among the SBD blueprints a drawing of the flares loading door. Unfortunately, it does not contain any useful height dimensions. However, its side shape fits the reference photos pretty well, while it does not fit the fuselage contour in the basic structure assembly drawing. Ultimately, I decided that the in the reality the top of the fuselage was located somewhat higher in this area than in the structure assembly blueprint. (If the fin contour in this blueprint is wrong, the same could happen to the contour of the upper fuselage). The difference was about 0.3”. I adjusted the fuselage shape according the reference photos and the flares loading door drawing.

After shaping this side contour (it corresponds to longeron #01 in the SBD skeleton) I also recreated the 14 remaining longerons. Their locations match the structure of the real aircraft:

Figure 113-10 Final fuselage reference frame (viewed from the rear)

In fact, it was quite slow process: I had to fix minor differences in the bulkhead shapes along each longeron. Sometimes the line of newly added longeron forced me to correct the previous one. The surfaces of front and rear wing fillet cones were a great help: without them I could not properly form the complex shape of longeron #09 or longeron #12.

Finally, I also added a few bulkheads and the root rib of the horizontal stabilizer fairing. Figure 113‑11 shows this reference frame from another side:

Figure 113-11 Final fuselage reference frame (viewed from the front)

Note the three auxiliary circles at the firewall. After comparing three different drawings of this bulkhead, I decided that it was not a regular ellipse, but a similar contour created by three arcs.

The differences between this and the elliptic contour are within eventual draughtsman error range. However, I took into account certain “tendencies” in the firewall shape. It could also happen that SBD designers approximated the elliptic shape in this way, because it is easier to recreate such a combination of three arcs in the workshop.

Figure 113‑12 shows the final bulkhead shapes, as well as the longeron diagram (note that they were set in the radial directions):

Figure 113-12 Fuselage longerons and bulkheads