The carburetor scoop passed significant evolution in the subsequent Dauntless versions. In the SBD-1 there was a rather large air duct placed on the top of the NACA cowling (Figure 45‑1a):
However, it was quickly discovered that it obscures one of the most important spots in the pilot’s field of view: straight ahead and slightly below the flight path. That’s why it was somewhat corrected in the next version (SBD-2). In this aircraft the designers lowered the scoop, increasing the field of view from the cockpit. Such a solution persisted in the SBD-3 and -4. In the SBD-5 they completely redesigned it, placing the carburetor scoops inside the NACA cowling (more about this — see in this post the paragraphs around Figure 11-6).
Close examination of the various reference photos led me to the conclusion that in the SBD-1 the air duct ran between the inner surfaces of the scoop and the top of the NACA cowling (Figure 45‑2a):
There was a rectangular opening in the rear part of the cowling, located just above the Bendix-Stromberg carburetor of the R-1820 engine. (There was a short, vertical duct inside the NACA cowling from this opening to the carburetor intake. I will model it later, together with the engine).
The later scoop version (from the SBD-2, through SBD-3, up to SBD-4) was a typical “quick and dirty” solution for the identified problem. The designers could not split the upper panel to place the lowered air duct there, because it would hinder the stiffness of the whole NACA cowling. Instead, they cut out another rectangular opening in its leading edge (Figure 45‑2b). In this way a half of the incoming air went to the engine as before, over the NACA cowling. However, the bottom part of the air stream was directed below the cowling surface. Both streams were joining inside the rear opening, before they went into the carburetor.
I created both openings using Boolean modifiers (Figure 45‑3):
Then I started by forming the lower part of the air intake. I started with a single strip fitted to the side edges of the frontal opening (Figure 45‑4a):
Then I extruded this edge and flatten the subsequent segments, forming the characteristic shape of the inner inlet, as in the reference photos (Figure 45‑4b).
When this first part of the bottom air duct was ready, I extruded its subsequent segments, forming the rear part (Figure 45‑5a):
Finally I reduced the roundings along the duct side edges by adding there a multi-segment Bevel modifier. It not only diminished their size, but also made its cross section more circular (Figure 45‑5b).
When the bottom part of the scoop was ready, I started the upper part. It begins in the same way: from a single strip, fitted to the cowling surface (Figure 45‑6a):
Then I extruded the vertical faces (Figure 45‑6b).
In the next step I extruded their upper edge into the horizontal surface (Figure 45‑7a):
Finally I extruded the subsequent segments of the rear part of this mesh (Figure 45‑7b).
Initially I kept the lengthwise edges of this object sharp, because I intended to create their fillets using the Bevel modifiers. However, a careful study of the reference photos revealed that the radii of the upper and bottom edge vary along the length of the scoop. Thus I created them by adding two additional lengthwise edgeloops to this mesh (Figure 45‑8):
Figure 45‑9 shows the real scoop (on the left) and the final version of the same scoop my model (on the right):
Although I did not managed to set up the picture on the right precisely as in the left photo, the carburetor scoop looks quite similar on both images. I can leave it “as it is” and start the work on the next cowling element. I can always fix its shape during the next stages of this project.
In this source *.blend file you can evaluate yourself the model from this post.
Airplanes in 3D 