Initial Approximation of the Engine Cowling (1)

SBD Dauntless had a radial engine hidden under typical NACA cowling. The Douglas designers placed its carburetor air intake on the top of this cowling, and the two Browning M2 guns behind it. In the result, the upper part of the SBD fuselage, up to the pilot’s windscreen, had a quite complex shape (Figure 40‑1):

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Figure 40-1 Complex shape of the upper cowlings

I am sure that I will tweak this shape multiple times before I reach the most probable compromise between all the reference photos I have. It will be much easier to do it by modifying a simple mesh instead of the complex topologies of the final cowling. Thus I decided to create first a simpler version of this fuselage section and adjust it to the all of the available photos. I will describe this process in this and the next post. Once this shape “stabilizes”, I will use it as the 3D reference in forming the ultimate cowling. Because I am going to recreate all the internal details of the engine compartment, I will create each cowling panel as a separate object.

I started by creating the three key contours of the NACA cowling (Figure 40‑2):

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Figure 40-2 The key sections of the NACA cowling

Section 1 (see Figure 40‑2b) was a perfect circle, while section 2 was a little bit higher than wider (see Figure 40‑2a). Ultimately section 3 was a regular ellipse. Note that all these sections have the same number of the vertices (32).

Once I created these three edges, I connected them using three arrays of faces. Them I added in between (using the Loop Cut command) three additional edge loops (Figure 40‑3a). I needed them to form the curved forward part of this cowling (Figure 40‑3b):

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Figure 40-3 Forming the general shape of the NACA cowling (SBD-5)

As you can see above, I fit the silhouette of this NACA cowling to the reference photos. This is a photo of the SBD-5, so I moved this cowling forward by 3.5” (see in this post about differences between SBD-3 and SBD-5 cowlings Figure 4-6, for the explanation).

When I finished the NACA cowling, I formed the basic shape of the next fuselage section (Figure 40‑4):

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Figure 40-4 Another section of the engine cowling

I created this part in the same way as the previous one. First I copied and shrunk the last NACA cowling edge, creating the gap for the outgoing air. Then I copied the firewall edge, and joined these two edges by an array of new faces.

In the next step I extruded the upper part of this surface, creating the section below the windscreen (Figure 40‑5):

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Figure 40-5 Another section of the engine cowling

It is a “linear” continuation of the previous fuselage segment. I fitted its sides to the mid-fuselage, which I formed some months ago.

The next elements are the “bulges” that covered breeches of the Browning M2 guns. In this ‘quick and dirty’ approximation I formed them from a separate mesh patch (Figure 40‑6c):

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Figure 40-6 ‘Bulges’ over the machine guns breeches

Of course, I verified their shape on the available photos, as you can see in Figure 40‑6c.

This comparison revealed, that the intersection lines between these ‘bulges’ and the main fuselage require some improvements: they have to resemble straight lines (Figure 40‑7a):

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Figure 40-7 Adjustments of the main mesh shape

To obtain such an effect, I had to decrease the upper radius of the last bulkhead (Figure 40‑7b). In such a simple mesh it required just to move a few vertices. If I had to perform such an operation on the final panels, it would be much more difficult!

In the front of the gun barrels there were long recesses in the NACA cowling. The outer edges of such a feature are always a tough test for the model, because they depend on the proper shape of the both intersecting elements. First of these objects is the NACA cowling, the second is the shape of this recess — a cylinder in this case (Figure 40‑8):

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Figure 40-8 Cutting out the openings in the front of the guns

I cut these openings using a Boolean modifier. Figure 40‑9a) shows the result:

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Figure 40-9 Resulting openings in the NACA cowling

In Figure 40‑9b) and Figure 40‑9c) you can see the evaluation of the obtained contours. They seem to fit the borders of the recesses on the reference photos. (There are minor differences, but I suppose that they are results of the rounded edges of these features.

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

In the next post I will continue shaping this first approximation of the engine cowling.

 

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7 thoughts on “Initial Approximation of the Engine Cowling (1)

  1. Grate job!, The leading edge is made by using 2 different radius. A small one leading edge and a big radius that connect to a flat surface. Try to make the second big leading edge as industrial (radial curve) as you can so it loos the soft feeling of the intersection interpolations. The tangent transition in the Figure 40-9 “b” is hard, passing from a big radius to a flat surface in only a point. And the flat surface after the radius is so flat that looks negative in the real airplane. This industrial style feeling is repetid in the engine cone Figure 40-9 c. Grate job!

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  2. Thx 4 the explanation. I think you need to push forward the edge loop to terminate all curves contours more Forward. You can make a fast rendering with the same directional light reflection to doble check your surface with the picture. Or maybe you need to add 2 edge loop at the end of the curve. in any case looks great!

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    1. Thank you, I also received your e-mail with the picture. I think that you are right. I will re-examine the photos to be sure!

      P.S. Technical note: I practiced that if you put in such a reply a full internet address of an image file, WordPress shows it here as an embedded image, as in my reply.

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