Jump to content

SBD Dauntless (from scratch)


Recommended Posts

  • Replies 208
  • Created
  • Last Reply

Top Posters In This Topic

  • 3 weeks later...

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:

0113-01.jpg

 

Quote

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 below for their map.

 

Figure "a" above shows horizontal contour of this fillet, which I found in the panels assembly diagram (dwg 5063493 – see its high-resolution version). Figure "b" above 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 "c" above 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 below:

0113-02.jpg

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 the previous figures "b" and "c". These two cones must be adjacent. To see better what I mean, look at figure "a", below:

0113-03.jpg

I built instances of these fillet cones (shown in Figure "a", above) around circles shown in the previous figure. 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 "b", above). 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 "b", above). 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 "a", below:

0113-04.jpg

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 "a", above, 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 "b", above) 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 "a", below):

0113-05.jpg

Figure "b" above 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.
 

Quote

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 "a", below):

0113-06.jpg

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 "b", above). Now it differs a little from the ideal circle depicted in the assembly blueprint of the carbon dioxide system (see area "b" in the first figure in this post), but I assumed that it was a simplified contour, used for the illustration purposes.

(Continued in the next post...)

Link to post
Share on other sites
(Continuation of the previous post)

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:

0113-07.jpg

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 below shows their details:

0113-08.jpg

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:

0113-09.jpg

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.
(Continued in the next post...)
 
Edited by Witold Jaworski
Link to post
Share on other sites

 (Continuation of the previous post)

 

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:

0113-10.jpg

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 below shows this reference frame from another side:

0113-11.jpg

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.

 

Quote

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

0113-12.jpg

 

 

Link to post
Share on other sites
  • 5 months later...
  • 1 month later...
Posted (edited)

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:
_________________________________________________________________________________________________

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):

 

ecda2e8b9f26de2925cc145ae8bbed263d2e79fe

 

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) :

 

bfa1859fc6f7e3a4067ba785e546fc96fb156a91

 

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):

68b5f6523b37f469e3fa780cc1931cc1274c45d5

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:

0fe5b3f79d25dfdca38238c68f09cb6873f9c16d

 

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:


b3119956c41bfb8665fbca1ff2947617dda93253

 

 

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:

21e369dee2b06ec077ecf554064dafc0ed37cf93

 

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.

Edited by Witold Jaworski
Link to post
Share on other sites
  • 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.
Quote

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.
 
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...