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SBD Dauntless (from scratch)


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This time a technical post about the overall dimensions of the subsequent Dauntless versions. We are using these values for scaling the reference drawings. If they are wrong - the whole model you are building is also wrong. That’s why they are so important:
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Since 2015 I have tried to determine the true length of the early SBD Dauntless versions (the SBD-1, -2, and -3). There was something wrong with the source of this information: the original BuAer performance data sheets. You can find there a different length of the SBD-2 (32’ 2”) and the SBD-3 (32’ 8”), while the differences between these variants cannot explain the reason of such a longer fuselage in the SBD-3. The other sources repeat these figures without any reflection. Fortunately, last month I found in the SDASM resources two interesting drawings of the SBD-1. One of them is a general arrangement diagram, which clearly specifies its overall length (and how it was measured):

0116-01.jpg

 

As you can see, the overall length the SBD-1, without the spinner, was 32’ 1 ¼”. This agrees with the BuAer data sheet for the SBD-2 from November 1942, since they rounded each dimension up to the nearest inch. (For example: this BuAer sheet specifies the wing span as 41’ 7”, citing the general arrangement diagram which provides a more accurate dimension: 41’ 6 1/8”.) According the general convention in these drawings, the small transparent blisters of the running lights are excluded from these overall dimensions (see this post, Figure 111-5, and this post, Figure 109-12).

The BuAer data sheet from August 1942 treats the SBD-3 and the SBD-4 as a single variant, thus I assume that it provides the overall length of the SBD-4. Using the available blueprints, I concluded that it was 32’ 7 13/16”, which BuAer rounded up to 32’ 8” (see this post, Figures from 108-4 to 108-6). The sole reason of this difference is the length of the Hamilton Standard Hydromatic propeller, used in the SBD-4. Its central “hub” was longer than in the Hamilton Standard Constant Speed propellers, used in the SBD-1, -2, and -3. Basing on these facts, we can safely conclude that overall length of the SBD-2 and -3 without the propeller spinner was the same as the SBD-1: 32’ 1 ¼”.

 

What about the length with this spinner mounted? I did not find any explicit dimension, so I still have to rely on my estimations from the previous year (figure below corresponds to Figure 108-7 from that post) :

 

0108-07.jpg

 

Now, thanks to the SBD-1 arrangement diagram, we know the overall dimensions up to the B baseline (compare figure above with the first picture in this post). In this post you can see that I approximated this length as 32’ 1.5”, +/- 0.3”, so the true value 32’ 1.25” lies within declared error range. According to the data from the same post, the difference between B and C dimensions can be estimated as 42.38” – 37.66” = 4.72”. Let’s round this distance to 4.75”. (Although I suppose that the overall error range for this value is smaller than the error range of the estimated overall length, this 4.75” still lies safely within these limits.) This gives the overall length of the SBD-1, -2, and -3 with the spinner = 32’ 6”.

 

Below I am providing the length of each Dauntless version, according to their general arrangement diagrams:

  • SBD-1:  32’ 1 ¼ ” / 32’ 6”;
  • SBD-2:  32’ 1 ¼” / 32’ 6”;
  • SBD-3:  32’ 1 ¼” / 32’ 6”;
  • SBD-4:  32’ 7 13/16”;
  • SBD-5: 33’ ¼”;
  • SBD-6: 33’ 1/8”;

All these dimensions do not take into account the transparent covers of the running lights. Lengths in italic are the estimated lengths with the propeller spinner. Note the minor difference in the lengths between the SBD-5 and the SBD-6 (0.15”). I copied this dimension from the SBD-6 general arrangement diagram attached to the BuAer performance data sheet from 1944. It is repeated (as 33’ 0.1”) in the SBD-6 “Erection and Maintenance Manual”. What is interesting, minimally differ from the Douglas blueprints. One of them is the overall length. I cannot explain these variations.

 

Everything would be fine, unless I checked the alternative dimension chain in this SBD-1 drawing (below it is marked in red):

0116-03.jpg

When you sum up these three red dimensions, you will obtain 386 3/16”. This does not agree with the blue overall length drawn above (385 ¼”)! The difference is close to 1 inch (precisely: 15/16”). One of these two lengths is wrong. Which one?

 

Let’s check similar arrangement diagram of the SBD-5:

0116-04.jpg

 

In this case the sum of the red dimensions matches the blue overall length (396 ¼”). The redesigned engine compartment in the SBD-5 was 11” longer than the SBD-1, so you can see this difference in the overall length and in the red dimension on the left (91 9/16” in the SBD-5 vs. 80 9/16” in the SBD-1). The middle dimensions (22’ 10 13/16”) of the red chain are identical in both variants. But there is an interesting difference in the third red dimension. In the SBD-5 this is 29 14/16”, wile in the SBD-1 it was 30 13/16”. The difference is 15/16” – precisely as the difference between  the alternate SBD-1 lengths!

 

In the rudder assembly I found that the 29 7/8”, listed in this SBD-5 arrangement diagram, is the chord length of the rudder:

 

0116-05.jpg

I suppose that the SBD-5 and SBD-1 used the same rudder. (Behind the firewall, the geometry of both variants was identical). However, behind the lower tip of the rudder trailing edge there was additional 1” of the tail cone:

0116-06.jpg

 

I signalized this detail in one of my previous posts. However, it was not dimensioned in this assembly drawing, so in that time I could only estimate its length to about 1”.

 

Now it seems that the partial dimension from the SBD-1 general assembly diagram provides the accurate distance from the rudder hinge to the running light base, so this additional length span is 15/16”. For unknown reasons, it was not included in the overall length, specified in the general arrangement drawings!

 

In fact, these general arrangement diagrams are also misleading in other dimensions. There was an error in the overall wing span specified in the Douglas drawings (see this post, figures from 109-12 to 109-15).

Conclusion: because of these errors in the original Douglas blueprints, none of the widely published SBD overall dimensions is true. Below I am providing the updated values for each variant of this aircraft. Although the wing span was the same in all Dauntless versions, I am repeating it just for the reader convenience:

 

  • SBD-1:  wing span: 41’ 3.2”, overall length: 32’ 2.19” / 32’ 6.9”;
  • SBD-2:  wing span: 41’ 3.2”, overall length: 32’ 2.19” / 32’ 6.9”;
  • SBD-3:  wing span: 41’ 3.2”, overall length: 32’ 2.19” / 32’ 6.9”;
  • SBD-4:  wing span: 41’ 3.2”, overall length: 32’ 8.75”;
  • SBD-5: wing span: 41’ 3.2”, overall length: 33’ 1.19”;
  • SBD-6: wing span: 41’ 3.2”, overall length: 33’ 1.19”;

The wing span is measured between the running lights bases on the wing tips. Fuselage lengths are measured between the spinner tip and the running light base on the tail cone.

 

If you want to check accuracy of any existing scale drawing or plastic kit, use the well-documented partial dimensions, shown in Figure 111-7 and 111-8 in this post. I suppose that the overall dimensions will be always wrong, due to confusing Douglas general arrangement diagrams.

 

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  • 3 weeks later...

This February I found among the SDASM resources a diagram (dwg no 5060837), which describes the geometry of the SBD fuselage. This is the key piece of the information that was missing in the NASM microfilms I used before. Below you can see these lines:

 

0117-01.jpg

 

The original drawing is slightly distorted. I was able to stretch its upper and lower portions, so in the central part its rectangular “grid” fits the blue guide lines drawn in Inkscape. However, this is a non-linear deformation, so it still occurs along the edges of this image. (In the illustration above, I marked these distorted areas in pink.)    

 

The subsequent fuselage frames are placed at following stations:

 

0117-02.jpg

 

Fortunately, fuselage diagram contains not only these distorted lines, but also tables of their numerical ordinates. They are provided for equally spaced horizontal and vertical “grid lines”, as in the illustration below:

 

0117-03.jpg

 

The diagram provides two tables. One of them lists at each frame the fuselage widths along the horizontal lines (“waterlines”). The other provides heights of the upper and lower contour, measured along the vertical lines (“buttocks lines”). For some frames, like Frame 9, the table provides more than two heights, as show in the illustration above.

 

I used these numerical data for building corresponding “contour planes” in Blender 3D space:

 

0117-04.jpg

 

Each of these planes is a polygon. Each vertex of these polygons corresponds to a single ordinate. These vertices are connected with straight edges. (On this stage, I did not want to interpolate them with curves.)

Then I used the same data points for creating section contours:

 

0117-05.jpg

 

They are also simple polygons: vertices connected by straight edges. Because I generated them from the cross-sections of the vertical and horizontal planes, you can see on each of them the characteristic “grid” pattern.

 

Building these shapes, I found some obviously wrong points in the waterlines. In the table below I marked them in red:

 

0117-06.jpg

 

Fortunately, the table of the buttocks ordinates is less erroneous. Just some data points are shifted to a wrong column. (In the figure below, I marked these values in yellow):

 

0117-07.jpg

There are also others, less visible inaccuracies. In that times all these ordinates were measured from large drawings (some of them were in the 1:1 scale). Still, you cannot avoid minor measurement errors in such a manual drawing.

 

Once I placed these values in the 3D space, I examined resulting lines, looking for irregularities. For example, I found a suspicious point at station 7, on the cockpit frame:

 

0117-08.jpg

 

The vertices from the previous frames (1..6) formed around this cockpit edge a polyline which you could extrapolate with a gentle curve. These data points were somewhat dispersed, but no more than by 0.02”. However, the vertex at frame 7 lies about 0.1” from this extrapolated curve. Was it a measurement error, or a real feature of this shape? To determine this, I checked the nearest waterlines (at +16”) and buttocks lines (at 16”). I did not find similar deviation there, thus concluded that this is just an error, and adjusted this outstanding vertex.

 

However, when I noticed a recession which repeats in the three subsequent waterlines – I concluded that this is a real feature:

 

0117-09.jpg

 

I suspect that this is a “side-effect” of the large fillet between the wing and the fuselage.

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In general, I assumed that the error range for these ordinates was about 0.05”. There are just a few larger deviations, as the one at the cockpit edge.

There are also differences between the data points plotted according to the numerical ordinates and the fuselage lines depicted near these tables. In the illustration below the plotted lines are in black, while the reference polygons (created according to the numerical data) are in orange:

 

0117-10.jpg

 

I suppose that these inaccuracies are mainly caused by the irregular distortions of the scanned blueprint. On the other hand, drawings in this diagram are just illustrations for the numerical ordinates. Thus, you should not treat these black lines as an accurate reference.

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  • 2 months later...

Currently I am working on a new edition of my book. I just saved some hours to discuss updates in the SBD Dauntless fuselage geometry, which I made using the newly obtained SDASM data.
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In the previous post I used ordinals from the newly found fuselage geometry diagram for creating a set of the 3D reference planes:

 

0118-01.jpg

 

In this post I will span a smooth subdivision surface between these points. I think that such an interpolation will provide a more accurate reference than the longerons (stiffeners), which I previously shaped in this post (see there Figure 112-07).

 

I compared my previous approximation of the fuselage shape, based on the partial data from the NASM microfilm, with these ordinates. In general, it seems that it was quite accurate:

 

0118-02.jpg

 

The wing fillet fits well these ordinates – its shape requires just some minor adjustments. On the other hand, I can see that the radius of the upper parts of the tail bulkheads was somewhat smaller. At least I was right, assuming that this radius was constant along the rear gun doors. In this way these doors could be formed as a part of the cylindrical surface, which simplified their production.

 

Ordinates from this SDASM blueprint confirmed many of other assumptions that I made basing on the partial NASM blueprints (see my first two posts on the fuselage geometry) :

 

0118-03.jpg

 

Note the flattened cross-section of the wing root fillet. This diagram confirms my hypothesis about this shape, based on the shape of its trailing edge in the top view (see this post). It also confirms another assumption: that in the rear view all the fuselage stiffeners (I called them “longerons”) run along straight lines, spanning radially from the fuselage center. In the tail area, these lines are equally spaced: 15° from each other. In the mid-fuselage some of them are bent upward.

 

When I looked at the forward part of the fuselage described by the Douglas geometry diagram, I realized that there is something wrong with the upper contours of frames 1 and 2:

 

0118-04.jpg

 

While most of the frame 1 data points perfectly fitted the firewall assembly drawing, they missed the “bulged” covers around the fuselage guns. Fortunately, I was able to recreate this cowling using dimensions from the windscreen and firewall assembly drawings.

 

I think that this diagram was based on the original Northrop XBT-2 prototype drawings. As you can see below, the upper cowling between frame 1 and frame 2 looks like in the geometry diagram:

 

0118-05.jpg

 

XBT-2 was equipped with a single forward-firing gun, mounted on the right side, in the front of the pilot. Thus, left contours of its frame 1 and frame 2 could match the elliptic shape, depicted in this diagram. I suppose that the geometry of all other XBT-2 fuselage frames (3 … 17) match their counterparts in the serial SBDs.

 

Illustration below shows the smooth surfaces, spanned over the reference polygons. In this case, I corrected the shape of the wing fillet surfaces (blue and red), extending them up to frame 13. Then I added new surface (gray) which covers the main portions of the fuselage. Behind the cockpit, I fitted its shape to the cylinder. Upper parts of this cylinder cross sections fit the corresponding ordinal points of the fuselage frames:

 

0118-06.jpg

 

To make sure that this “skin” passes through the original geometry, I placed it little below the ordinal points. In the effect, the vertices of the reference polygons minimally protrude from this surface – by about 0.01”. This is well within the range of eventual errors in locations of these ordinal points, and below the thickness of the real fuselage skin (0.03”). In this way I was able to visually check if the modeled surface fits all ordinal points.

Quote

On the other hand, the geometry of the WW2 aircraft was always given “as for the skeleton”, i.e. did not take into account the skin thickness.


When I compared the resulted shape with the fuselage assembly blueprint, I saw that its upper contour precisely follows the ordinates. There were some minor differences along the bottom contour plotted on this drawing:

 

0118-07.jpg

 

These minor differences are OK, since these lines on such an assembly drawing are of least importance. In this blueprint, the key information are the referenced part numbers.

 

However, some months ago, when I did not have these explicit ordinates, I concluded that the upper fuselage contour was 0.3” higher than on this blueprint (see this post, figures 113-7 to 113-9). It looks like that on this high-resolution photo, which I matched  with my model:

 

0118-08.jpg

 

Because the explicit dimensions did not confirm these findings, I verified this hypothesis using matched photos of another restored SBD-5:

 

0118-09.jpg


In both aircraft we can see identical difference in the dorsal fillet shape, but the fuselage, shaped according to the ordinals, perfectly fits the second picture. There is no visual difference, especially as significant as 0.3”. Thus – this higher upper contour is an individual feature of the restored SBD-5 from the first photo. Most probably they inaccurately rebuilt its tail section.

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When you are looking into something for too long (as I did in the case of this fuselage) you can sometimes “find” a non-existing feature! In such a case, the explicit dimensions, as these ordinals, are invaluable.


On the other hand, I suppose that these photos depict the true shape of the dorsal fillet. (Unfortunately, its ordinals were provided in a separate blueprint, which is still missing.)

Edited by Witold Jaworski
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  • 8 months later...

I decided to upload the Blender file in which I reproduced in the 3D space the original ordinates of the SBD fuselage and wing. (I described creation of this 3D reference in my previous posts). I think that in this form they can be useful for other modelers, who would like to recreate the geometry of this aircraft. Here is the link to the *.blend file (102MB) that contains the model presented below:

 

0119-01.jpg

 

The fuselage ordinates are organized into horizontal “water lines” (blue), vertical “buttock lines” (green) and resulting sections (red). Each vertex of these polygons corresponds to an original ordinate (data point). For simplicity, I connected these vertices using straight edges. (You can find more details about these “reference polygons” in this post.

As you can see, there are also original blueprints in this scene. In fact, they are the only reason of the large size of the uploaded *.blend file. In the initial view most of them is hidden because they would obscure all other objects. For example: I clipped from various assembly drawings silhouettes of the assembly frames. Each of these images is assigned to the corresponding section.

To manage this complex structure, I organized it into two basic collections named Wing and Fuselage:

 

0119-02.jpg

Each of these collections contains a sub-collection named Blueprints and a sub-collection named Ordinates. Blueprints contains clips (raster images) of the original Douglas drawings. Ordinates contains the reference meshes (planes) recreated from the numerical ordinates provided in the Douglas blueprints.

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Note the alphanumerical prefixes in the collection names (like “#5.A2a..”). I added them just to ensure that each name is unique. (This is a requirement in Blender.)

    
You can turn on/off visibility of these collections, as well as the individual visibility of their objects. For example: I manually turned off visibility of most of the reference images. I am turning them on when I need them.
 
The internal structures of the Blueprints and Ordinates collections differ from each other. In the case of the wing, both are split into three sections: center wing, outer wing, and wing tip. In the case of the fuselage, Blueprints contains just a sub-collection for the bulkhead blueprints (Frames), because there were so many of them. Fuselage ordinates (i.e. polygons) are organized into separate collections for the Buttock lines and the Water lines. There is another collection: Stiffeners, but its data are less reliable, because they were provided as single values per each fuselage station. For the stiffeners #0, #1, #2, #12, #13, #14, #15, which are closer to the fuselage centerline, ordinate tables provided their widths. For the other stiffeners (#3 … #11) ordinate tables provided their heights from the fuselage centerline. It seems that Douglas engineers “traced” them by projecting onto the surface described by the buttock lines and the water lines.
 
In the Sections collection I placed cross-sections of the fuselage buttock- and water- lines. The only additional information there are the arcs between these data points. (For example – in the fillets that span between the fuselage and the wing, or between the fin and the stabilizer.) I recreated them using the radii provided by Douglas (in the blueprint with the fuselage ordinates). These radii were not complete, but they are better than nothing. It seems that the SBD designers used a fixed 3” fillet radius where they could.
 
You can easily identify these assumed (non-confirmed) data points of the fuselage sections, because they do not belong to any horizontal or vertical line:
 
0119-03.jpg

 

These horizontal and vertical lines are the traces of the corresponding buttock planes and water planes. I left them in the resulting mesh as additional, disconnected edges.
 
In some water- and buttock- planes I also added a few additional vertices, to match better the eventual fuselage surface. (This is a purely aesthetic purpose.) They are non-confirmed by any numerical ordinate. For easy identification, I colored the additional faces created by such a vertex in brown:
 
0119-04.jpg

 

The last Fuselage sub-collection, named Interpolation, holds my approximation of these ordinates. First of its sub-collections, named Surfaces, contains  smooth surfaces that I spanned over the buttocks- and water- lines:

 

0119-05.jpg

 

I described details of these surfaces in the previous post. They are something between a pure reference object and an initial attempt to forming the fuselage with smooth subdivision surfaces. (Shaping these contours, I learned about the minimum number of the control polygons that are needed to fit all available data points). You can also see there a windscreen “wireframe”. I built it using the dimensions from the cockpit assembly drawings. I needed these lines for reconstructing the shape of the guns cowling, which was not described by the original ordinates.

 

Two other Interpolation sub-collections, named Frames and Stiffeners, contain smooth interpolation of the fuselage bulkheads and longerons:

 

0119-06.jpg

 

In addition, I also modeled the oblique parts of the bulkheads at station #4 (object: R1.Frame#04o), #5 (R1.Frame#05u) and #7 (R1.Frame#07b):

 

0119-07.jpg

 

In the uploaded file their visibility is initially turned off.

 

Ultimately, this file also contains some reference photos. Each of them is assigned to an auxiliary camera which projects this model onto this photo. To easily switch between these projections, download this add-on and install it in Blender. It adds additional Cameras tab to the 3D View property pane (the one which you open using the [N] key). Use its contents to switch between available photos:

 

0119-08.jpg

 

You can find more details about this add-on at the end of my tutorial on photo-matching (see the description around its Figure 104-26).

 

Playing with these photos, on three of them I observed a difference in the upper part of the windscreen contour:

 

0119-09.jpg

 

While the bulkhead and stiffener lines (thin black in the picture above) perfectly match the photo, there is a difference in the windscreen heights. This requires further investigation, because I formed this 3D shape according to the explicit dimensions from the original cockpit canopy blueprints. Of course, I could make an error while creating these lines.

 

I observed similar (but not identical!) differences in the photos of another SBD-5, from the Pacific Aviation Museum Pearl Harbor:
   
0119-10.jpg

 

The resolution of this photo is lower than the previous one. However, it is still enough to reveal this “offset”. At this moment I cannot exclude the possibility that these minor differences were created by the renovation teams. (It seems the least probable explanation, especially in the case of the Pacific Aviation Museum).

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