SBD Dauntless, from scratch

[Witold Jaworski]

I am preparing data from the original Douglas blueprints to verify my model. For the beginning I chosen the wing. This is a well-documented assembly, because I found a master diagram in the NASM microfilm that describes SBD wing geometry (ordinals). Below you can see the first sheet of this diagram (dwg no 5090185):

Here you can download its high-resolution version (5MB). As you can see, it contains the ordinal tables of the wing bulkheads (ribs) and webs (spars). In the sketch on its right side Douglas engineers depicted various other dimensions of the wing center section. In the picture above I marked in red its key wing stations. Their names correspond to spanwise distance in inches from the aircraft centerline: "STA 10" is 10" from the centerline, while "STA 66" is 66" from the centerline.

Another part of this diagram contains a sketch of the larger, outboard wing section:

Here you can download its high-resolution version (7MB).

Wing station names of the outboard section starts with "STA 66" and continue to the tip, which is named "STA 251.06". This distance – 251.06" – is measured spanwise, along the reference planes of the outboard and center wing sections.

The first thing that I noticed comparing these blueprints to my model, are the different reference planes:

For my model I chosen the airfoil chord as the reference plane, with the origin point at the tip of the airfoil nose. (Airfoil coordinates are specified in such a reference system). The SBD wing was inclined at 2.5°, thus I rotated my wing object by the corresponding angle. However, from the photos I learned that SBD webs (spar) planes were vertical, so I adjusted their directions in my model. (If they were also inclined at 2.5°, connecting webs to the fuselage bulkheads would be much more complex, i.e. heavyweight).

In the original Douglas diagram shown above you can see that its engineers took different approach: they used re-calculated airfoil ordinals for given inclination angle. In this way their reference plane crosses the wing trailing edge and remains parallel to the fuselage centerline and perpendicular to the web planes. It seems that it was a standard approach in that era: I have found similar solution in the original Curtiss P-40 blueprints.

Building my model, I used simplified stations diagram from the SBD Maintenance Manual. From that diagram I knew that the wing tip station was at 251.06". I also correctly assumed that the station distances are measured along the corresponding reference planes of the center and outboard wing sections.

In principle I was right, but I assumed wrong reference plane for the outboard wing! Below you can see the plane that I assumed for my model, and the real reference plane from the Douglas master diagram:

Usually designers choose reference planes along an easily distinguishable element, which you can use as the base for physical measurements. Their choice is extremely important for the technology used in the manufacturing (i.e. ultimate product cost), because these reference bases are reflected in the tooling geometry. In the case of the aircraft wing the most obvious candidate for such a base is the trailing edge. However, in this wing Douglas engineers chose a strange reference plane, which fits to nothing! Look at the fragment of Douglas master diagram above: every spanwise line of the wing is oblique: trailing edge, leading edge, upper wing contour (1° downward), lower wing contour… It does not even fit any spanwise contours of its webs (all of them are trapezoids). There was no chance to discover such a thing without this master diagram blueprint!

However, I knew that there is something wrong with the geometry of my wing. To obtain the dihedral angle specified in the SBD Maintenance Manual (in the front view: 7.5° along the wing upper contour) in my model I had to raise the outer wing section by 10.2°. For such an angle, I obtained wing span of 496.4", which is 1.73" short of the documented value. I checked again station locations, to make sure that I did not made an error. I found nothing wrong, thus all what I could do was just to compensate this difference by additional 0.86" offset to each wing. (In picture above I marked it at station 66). Now it is clear, that this difference was caused by the wrong choice of the reference plane. In the master diagram this wing is already "pre-rotated", and you have to raise it by only 8.5° to obtain the proper dihedral angle.

I also made another error. Building the wing model I assumed that declared, 15% thick airfoil NACA2415 at station 66 (joint of the center and outboard wing section) was perpendicular to the reference plane of the outboard wing (i.e. to the wing airfoils chords plane – see figure above). In the effect, I obtained the oblique rib (10.2°), adjacent to the wing center section, as 15.24% thick. Now from the STA 66 ordinals of I learned that they used the 15% NACA2415 for the oblique rib, thus the real wing was somewhat thinner than in my model.

I recreated the ordinals from the master diagram as polygons in 3D space of my model. These are all five webs and some key ribs. You can see their arrangement in figure below:

In the picture above the ribs seem to be smooth, but look at the enlarged nose of STA 138: their vertices (ordinals) are connected with straight edges.

Note also see fragments of the master diagram image placed on the reference plane. To easily place rib polygons at their stations, I used single reference plane for both wing sections (i.e. the outer wing has no 8.5° dihedral). Thus at this moment the outer section is minimally raised above the reference plane, as it was according the ordinals. You can see this arrangement better in the picture below, taken from another viewpoint:

For the outer wing section I also recreated the ordinals of theoretical (i.e. in this arrangement - vertical) STA 66 contour. However, this is just a wire (a "theoretical entity") – because the real rib at outer STA 66 was oblique.

Figure below shows the wing tip contour geometry specified in the master diagram:

The rear part of this contour (from point A to B) is shaped by an arc, which radius is 33.62". The forward part (from B to C) is a free-form curve, described by a few key points, dimensioned in this drawing.

In this drawing you can see an auxiliary reference base, placed at 38.705" from the basic reference plane (located at STA 66 nose tip). This additional reference was introduced to facilitate dimensioning various wing tip details. The master diagram describes this line as "common percent line", perpendicular to aircraft centerline. In trapezoidal part of the wing it connects points located at 33% of their airfoil chord lengths.

For initial verification of wing tip contour, I also placed over this fragment the assembly drawing of the wing tip. You can see the result below (lines from the master diagram are blue, from the assembly drawing – black):

In general, both contours match each other, within the tolerance of the manual sketching and eventual later deformations caused by microfilm camera lenses. However, the shape of aileron cutout significantly differs between these two drawings. What is interesting, photos of the restored SBDs confirm the variant depicted in the assembly drawing.

The width and height of the basic grid "square", visible in this and further pictures in this post, correspond to 1 in.

In this assembly drawing I also identified additional rib (bulkhead) at STA 229.45. It did not occur on the stations diagram. However, the SBD wing tips were demountable, so its presence just at the joint seam is quite obvious. (The rib at STA 228 was its counterpart from the other side of this seam).

I placed on this drawing the free-form curve key points, following the explicit dimensions specified in the master diagram, and connected them with a curve. I was surprised, discovering that the central part of this contour does not fit both drawings:

The difference between these contours is quite significant and reaches about 0.4" near the auxiliary reference plane. All what I could do in such a case was checking this detail in the available photos (especially the archival photos). Unfortunately, it is impossible to precisely compare this shape with the perspective images of real wings. That's why I focused on the contour of the running light base. In the assembly drawing its forward part elevates a little above the win tip contour, while according the master diagram curve there would be no such elevation. I can say that you can observe such an elevation in most of the restored aircraft and in all of the archival photos. They confirm the wing tip shape depicted in the assembly blueprint.

The same applies to the small deviation from the master diagram contour at the aileron tip. It would be difficult to bend the end of the tip edge ring precisely around the master diagram "mathematical" arc contour, because of the sudden change in the curve radius at point B. (In that era aircraft designers cared only about the tangent continuity of their theoretical contours).

What's more, the master diagram does not specify many other wing tip geometry details, for example – it misses the cross-section radius of the wing tip edge ring. To determine it, I had to use the detailed drawings of the wing tip webs. Ultimately I verified the wing tip shape using all available images of its parts:

I have found some further differences between the wing tip webs and ordinals of STA 246. (This is the last wing station, specified in the master diagram. Most of its shape is purely theoretical, because only its central part corresponds to a real partial rib).

I also found some other, less significant differences in the center wing section (for example – in the middle of STA 10).

Finally I concluded that the master diagram describes an initial concept of the wing shape, which was later (in the prototype workshop?) slightly modified, especially at the wing tip. Thus in all these cases I decided to rely on the assembly drawings.

Below you can see the complete "reference structure" which I prepared for the SBD wing:

Around the leading edge I placed a long, bent cone, which reflects its varying radius. Similarly, I signalized the radius of the trailing edge by marking it with two thin "tubes". According the flap assembly drawing (dwg. no. 5066078) the outer radius of the flaps trailing edge cross-section was 5/32" (which means that they were about 0.3" thick). Aileron trailing edge was somewhat thinner: according the master diagram and assembly drawing the radius of its inner wedge was 0.09". Because the SBD ordinals do not include the eventual skin thickness, I had to assume that sheet metal used as the overlay in the aileron structure was 0.05" thick. In the result I obtained the ultimate thickness of aileron trailing edge as 0.28" (2*0.09 + 2*0.05). I also added other details, as the landing gear wheel bay and aileron hinges axis (both dimensioned in the master diagram).

Before matching my model to this "3D reference frame", I decided to check at least one of its overall dimensions – just in case. The most obvious candidate was the wing span, which I could read from the general arrangement drawing: 41' 6⅛":

Note that it was measured precisely between the tips of the curved wing contour: the widths of the running lights (1.5" for each side) were not included. (The width of these running lights was specified in the front view of this drawing). I also noted that the SBD maintenance manual specifies slightly different wing span: 41' 6.3". More precisely: according dwg 5120284 the distance from the center plane to the wing tip was 498.125"/2 = 249.063", while the maintenance manual shows it as 249.187". The difference: 0.124" on each side of the aircraft was minimal, but this wing was identical in all Dauntless versions. Which of these dimensions is wrong?

I made a quick calculation using the stations diagram: if the distance to the wing tip measured along the reference planes was 251.06", then the half of the wing span is: 66 + (251.06-66)*cos(8.5°):

The result is 249.03". However, the SBD ordinals describe "skeleton geometry" and did not take into account the skin thickness. Thus I have to add to this result the typical thickness of the wing skin: 0.03". Finally it gives distance of 249.06", which perfectly agrees with arrangement drawing 5120284.

Reassured by these calculations, I raised the object representing the outboard wing in my 3D "reference frame" by 8.5°, and measured the distance from its tip to the center plane. I was really surprised by the result:

Well, the reason for obtained dimension – 247.60", instead of 249.03" – is simple: in my calculations I did not take into account the initial elevation of the wing tip over its reference plane:

In the result of the rotation by 8.5°, the wing tip is 1.43" closer to the aircraft center than STA 251.06, placed on the reference plane.

As you can see, the location of the of the SBD outboard wing reference plane is so unusual that even Douglas engineer, who provided dimensions for the arrangement drawing, made the same error as mine.

Thus the true wing span of the Douglas SBD was:

• 41' 3.2" between the wing tips (i.e. between bases of the running lights);
• 41' 6.2" between tips of the running lights;

Note that without the arrangement drawings you can easily make another error, and compare the wing span provided in the most of publications (41' 7") with the distance between the tips of the running lights. The result you will obtain in such a case will be quite close to the real value, because these two errors compensate each other.

Wing span from the SBD maintenance manual (41' 6.37") is also wrong. Was this slightly higher value (249.187" instead of 249.063" from dwg 5120284) obtained by adding the sheet metal thickness of the tip lights bases?

On the other hand, in the practice the precise overall wing span was seldom or never used. During production, the most important thing was the proper fitting of the wing segments at STA 66, and compliance of the basic assemblies to their own overall dimensions. To fit an aircraft in a hangar, you always have to provide an additional space of at least few inches on each side. During the operational use of these SBDs, board crews probably did not even notice that they occupied minimally less space than specified. For the long-term stowage or the transport the outboard wing sections were detached, and in this case the overall aircraft dimensions precisely matched the documentation. It seems that the aircraft length and span are most useful for the modelers, who use them for checking their scale plans or model kits.

However, this is one of the most unexpected errors that I encountered so far. Previously I have identified significant wing span mistake in the case of the Fokker D.V drawings from 1916, but I thought that such a thing could only happen in the era of the sketches made with chalk on the workshop floor. I would never expect such a mistake in a dimension from the original blueprints made in 1940!

 

[Wurger]

 

[Lucky13]

 

[Gnomey]

Lovely work so far!

 

[36feet10inches]

This is next level stuff.

 

[Witold Jaworski]

Wurger, Lucky13, Gnomey, conkerking - thank you for following this thread!
_____________________________________________________________
In this posts I will analyze differences between my 3D model (built from 2015 to 2019, as reported in this thread) and the SBD geometry data obtained from the original documentation. Actually, I can perform such a ultimate comparison for the wing, because I found its original geometry diagram in the NASM microfilm. In previous post I used it for preparing a "reference frame" for such a verification. Results of this comparison will allow me to determine the real error range of my previous methods described in this blog, in particular – the photo-matching method.

Unfortunately, the incomplete microfilm set from NASM does not contain any other geometry diagram, so I will not be able to prepare such a precise reference frame for the SBD fuselage or empennage.

At the beginning, I identified an error in the wing location. It was determined by the position of leading edge tip of STA 66, marked as point A in the picture below:

In this post from 2015 I determined this location using the general arrangement diagram that I found in the SBD maintenance manual. As you can see above, there were issues in deciphering some of its dimensions. One of them was the distance from the thrust line to point A. I identified it as 20.38", which means that in my model this distance from the fuselage ref line is 26.38" (6" + 20.38").

A high-resolution scan of another arrangement diagram from Douglas microfilm (dwg no. 5120284) shows that this distance was 26.52". (You can see this dimension in the picture above). Thus – this is the first identified error in my model, caused by a mistake in reading available drawings: 0.12".

After re-adjusting wing position in my model, let's examine the center wing section:

In previous post I already mentioned my wrong assumption about the 15% thickness of the outer wing section at STA 66 (i.e. at the section joints – see Figure 109-4 in that post). Now I can see the effects of this error: upper wing surface of my model is located about 0.35" above the red reference rib (i.e. above the rib created according the master diagram ordinals). As you can see, I also made minor mistakes in the leading edge shape. There is an offset of 0.15" in the top view contour, and a difference in the shape of the forward part of the wing root profile. The latter does not exceed 0.3". Because of these differences, you can see that the original wheel bay protrudes a little from the wing leading edge of my model. In general, wing webs (spars) locations are quite precise – just the Web #2 is shifted by 0.3".

Let's look now at the bottom side of this wing section. Surprisingly, the Web #5 location and the radius of the trailing edge precisely match the master diagram data. However, the lower wing surface is placed 0.05" above the wing ordinates (this is a minimal difference, but still visible in this model):

I also properly identified location of the Web #1. In my model I placed the most forward ("nose") web in a slightly different location – about 0.15" to the rear. I made the biggest error in the shape of the lower, forward part of the wing root profile: it reaches about 0.5" near the leading edge. (As I remember, I assumed this airfoil shape basing partially on fitting the main wheel, and partially on the photos). The wheel bay location and size are surprisingly close to the master diagram data: the overall error does not exceed 0.15".

In general, the center wing section of my model matches quite well the master diagram ordinals. Of course, I will fix all these differences, but I will do it later (I describe it in another post).

Now - what about the shape of the outer wing section? I found there much greater differences:

While the aileron and flap seem to match the master diagram pretty well, there is a significant difference at the wing tip. Also, the fixed slats are shifted outward by about 1". However, we are comparing here my original wing model (dihedral angle along trailing edge: 10.2°) with the master diagram drawing placed on the original reference plane (dihedral angle: 8.5°). In Figure 109-15 from previous post I demonstrated that the real wing tip was 1.43" closer to the aircraft center plane than the tip depicted in this reference image. (This is because of the strange location of the reference plane in the outward wing). To fully assess these differences, I switched the current view to the projection aligned to the original, oblique reference plane of the outward wing:

Here we can see that the wingtip of my model exceeds the master diagram shape by 2"! On the other side, this is the result of two coupled errors. As you can see in this post and this post, I used the arrangement diagram from SBD maintenance manual as the primary source of the basic geometry.

In this diagram the basic trapezoid of the outer wing is described by five dimensions. I could not know that three of them are wrong! In the picture below I placed these dimensions in blue and red boxes (the wrong values are in red boxes):

In practice, this means that I unconsciously "stretched" the outer wing in my model, matching the wrong wing span. (For example – I shifted entire outboard wing section outside STA 66 by 0.8". Such a modification did not agree with the stations diagram from the maintenance manual, but it was the only way to place the wing tip according the dimension from this arrangement drawing).

By the way: now I can see another error in the drawing above. Note the partial span dimensions, which I marked as L1 (66) and L2 (183.05). As you can see, their sum should give the L dimension (249.187). Does it? Certainly not! (66+183.05 = 249.05). While L1 is the location of STA 66, confirmed by many reference dimensions on various blueprints, L2 value fits to nothing. This could be the span of the outboard wing measured along the reference plane (185.05 – the third digit in the picture above can be both: 3 or 5). If this assumption is true, such a dimension should never appear in this top view, in particular in this closed dimension chain.

As I mentioned in at the beginning of this post, in this comparison I would like to determine the error range of my photo-matching method. It bases on the relative comparison: to determine absolute dimensions of the analyzed object, you previously must know at least one or two of its original dimensions. As you can suppose, I treated the wrong basic trapezoid of the outer wing from the arrangement diagram as the confirmed contour, and appropriately arranged the reference photos. You can see the results in this post about verification of the wing shape.

To estimate the error range related to the photo-matching method alone, I compensated the effect of the assumption errors (improper wing span) by adjusting my model dimensions. I scaled spanwise the outboard wing, decreasing its size by about 1%. After this update, the basic wing trapezoid of my model fits the real "reference frame". Below you can see the result of a new comparison:

There is still difference in the wingtip contour, but now it does not exceed 1". What's more – now the wing slats match the blueprint quite well (the error does not exceed 0.12"). There is still a difference of 0.8" in the location of the gap between the flap and the aileron. However, I placed this gap in my model according the stations diagram from the maintenance manual, so it is shifted now. (I remember that I always noticed small difference in location of this gap when I matched my model to the photos).

You can see the originally matched photos in this post about wing shape verification. In particular, look there at Figure 31-8 and my description below that picture – it looks that initially I made a proper assumption about the contour arc, just its radius was somewhat different! A few months later I made another verification, described in this post (in particular, see Figure 42-8). Now I think that some problems with matching the wing tips of my model and the photo (as in Figure 42-9 in that post) were caused not by the dynamic wing deformation, but by assuming wrong basic dimensions.

CONCLUSION: it seems that despite the few available photos of the wing the errors of my photo-matching method did not exceed 1" (0.4%) in this SBD model. I think that this is an acceptable result, especially in the cases of aircraft which original blueprints are not available.

(Last year I wrote a detailed two-part tutorial on the photo-matching method: here you will find its part 1/2 and part 2/2).

However, in my SBD model these photo-matching errors coupled with other errors, caused by assuming wrong overall wing dimensions. I read them from the only available documentation, which I had in that time: the general arrangement drawing from the SBD maintenance manual. I think that such a situation is a special case. I will still trust the dimensions from manufacturer's blueprints. This is the ultimate source, and – despite issues like the one described in this post – they build the real aircraft using these drawings! Of course, if you find two different blueprints of the same assembly, it is a good idea to compare a couple of their key dimensions. Just in case.

 

[Wurger]

 

[36feet10inches]

This is mind boggling!

 

[Gnomey]

Great work so far!

 

[Maty12]

This has been one of my favorite threads on this website, it's fascinating seeing you work on the SBD! I also relate heavily to a lot to the issues you've mentioned regarding listed dimensions being in odd reference planes, or not adding up to the sum of their parts, as for me these have by far been the most frustrating things about decoding blueprints.

 

[Witold Jaworski]

Wurger, conkerking, Gnomey, Maty12: thank you for watching!

(...) I also relate heavily to a lot to the issues you've mentioned regarding listed dimensions being in odd reference planes, or not adding up to the sum of their parts, as for me these have by far been the most frustrating things about decoding blueprints. (...)

Indeed! Below my last post about such an issue (in the further ones I describe building my "reference frame" for the fuselage)

 

[Witold Jaworski]

In some aircraft it is difficult to provide the precise value of overall length. One of them is the SBD Dauntless, because of its easily demountable spinner used in the first three variants (SBD-1…-3). Also the length of the Hamilton Standard Hydromatic spinner hub, used in the later SBD variants, can vary – especially in the restored aircraft. Thus, for verification of model kits or similar purposes I would suggest checking the distance between two easily distinguishable points: from the firewall to the tip of the tail cone. This dimension remains the same in all SBD variants. Preparing the fuselage blueprints for my model, I could determine this distance using the tail cone assembly drawing:

The key information is provided by the stations marked in this drawing: their names describe distances from the firewall. (You can read them yourself from the high-resolution version of this drawing). I could read that the fuselage tip cone starts at station 271.6875 (11/16) and ends after station 308. However, this drawing does not provide the closing dimension (from station 308 to the tip), thus I had to measure this distance. Assuming a quite wide tolerance (1%), I estimated it as 6.7". This means that the distance from the firewall to this fuselage tip was about 114.7" (+/- 0.05"). The light bulb protruding from the tip is not documented in this drawing. From the photos I could estimate its length as 1". However, the information provided in this blueprint forced me to follow the "Douglas convention" of skipping the light bulbs dimensions. I already noticed this convention in the case of the SBD wing span: it was distance between the bases of the running lights, located on the opposite wing tips.

Later I discovered that I could verify this estimated distance using certain dimensions from the general arrangement drawing (dwg no 5120284):

I marked these three partial dimensions in the figure above. I subtracted from the total length (399.25") the distance from the spinner tip to the wing leading edge (at STA 66: 91.56"), then added the distance from this point to the firewall (9"). I was surprised when I obtained a quite different result: 316.69". This is 2" more than I measured in the previous blueprint – I could not explain such a significant difference!

However, looking at this arrangement drawing, I found another dimensioning chain:

First two dimensions (9" + 274.812") describe position of the rudder hinge axis, while the third dimension describes distance from this axis to the rudder tip (29.875"). (This rudder chord length is confirmed by corresponding dimension in its assembly drawing no 5156459). Obtained result: 313.688" is 1" short of my measurement, but it does not include the small part of the fuselage tip behind the rudder:

Fortunately, another source confirmed this result: the arrangement diagram from the SBD-6 maintenance manual:

Although the aircraft total length here is slightly different from dwg 5120284 (33' 0.1" vs 33' 0.25"), all three dimensions related to the rudder size and location are identical. (Just their fractional parts are expressed using decimals). This arrangement drawing is more detailed: at the tail tip you can see 1" dimension related to the remaining distance from the rudder tip to the running light base. Thus, the distance from the firewall to the running light base at the tail tip (314.687") agrees with my initial measurement (314.7").

Comparing these two versions of the arrangement diagram I found some other simplifications in dwg 5120284. It seems that it was drawn by a draftsman who skipped many detailed dimensions, and truncated the less important fractional parts from the remaining ones:

Fortunately, the main dimensions like the overall span of the stabilizer are identical in both drawings.

Frankly speaking, after finding these errors I decided that I cannot trust dimensions from drawing 5120284, if they are not confirmed in other blueprints. This conclusion seems to be quite paranoid, but it looks that somebody in Douglas accepted this blueprint without proper verification.

Ultimately, for the SBD scale plans and model kits I propose following length checks:

Both distances are identical in all Dauntless versions.

I do not suggest checking the overall wing span, because the value used in all general drawings (including the manufacturer arrangement diagram): 41' 6.37" is wrong due to conceptual mistake. (I described this error at the end of in this post). The real SBD wing span was 41' 3.2", but I doubt that any drawing/model kit fits this dimension. Thus I suggest measurement of two other spans, which are confirmed in all assembly blueprints:

I suppose that you can trust more the model kit/scale plan that fits these four dimensions. The other distances can vary, due to wrong wing span, provided by Douglas itself, or the wrong fuselage length specified in the BuAer SBD-1, SBD-2 and SBD-3/-4 performance reports. (These lengths were repeated in all publications about his aircraft. In the result – for the SBD-3 both: the length and the span were wrong in all sources that I saw. See this and this post for more details).

 

[Wurger]

 

[36feet10inches]

I look in on this thread from time to time just to make myself feel inadequate!!

 

[Gnomey]

Lovely work so far!

 

[Maty12]

Comparing these two versions of the arrangement diagram I found some other simplifications in dwg 5120284. It seems that it was drawn by a draftsman who skipped many detailed dimensions, and truncated the less important fractional parts from the remaining ones.
(...)
Frankly speaking, after finding these errors I decided that I cannot trust dimensions from drawing 5120284, if they are not confirmed in other blueprints. This conclusion seems to be quite paranoid, but it looks that somebody in Douglas accepted this blueprint without proper verification.

I've had the same problem quite a few times with the Marauder, though sometimes the rounding was at least understandable. My other plague has been mistaken or missing decimal points, like the obviously incorrect fuselage length of "58 ft 25 in" that is repeated across multiple manuals.

The Glenn L. Martin company has also listed multiple times that the wing chord at the root is 154.501", which comes from an approximation of a horizontal projection of the chord many inches away from the root. It would appear both companies, at least during wartime, allowed blueprints to be produced without double-checking all measurements first, so we unfortunately have to keep cross-examining measurements across multiple documents.

 

[Witold Jaworski]

Wurger, conkerking, Gnomey, Maty12: thank you for following this thread!
Maty12: it is always good to know that such errors also occurred in other WW2 blueprints...

Today I am reporting part 1 (of 2) of my struggle with the missing SBD fuselage geometry:
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As I already mentioned in this post, my microfilm set does not contain the fuselage geometry diagram. (I suppose that it was included in the missing roll C). Thus, this part of my work will be much more difficult, because I even do not have complete set of the bulkhead drawings! Just found a structure assembly drawing (i.e. side and vertical views), skin panels assembly drawing, mid-fuselage bulkheads, and some bulkheads of the tail. In the picture below I marked these undocumented areas of the fuselage in transparent red:

Douglas blueprints refer to the fuselage bulkheads as "frames". They are numbered from 1 (the firewall) to 17 (the mounting base for the tail wheel and horizontal stabilizer). Of course, the fuselage assembly drawings provide their positions, measured from the firewall. (You can find them in this assembly drawing of the skin panels). In this post I will refer to fuselage bulkheads using their ordinal numbers, shown in the picture above (for example: "frame #05").

However, even the identified blueprints of the fuselage frames are usually assembly drawings, which means that they do not contain any useful dimensions (as the frame #05 in figure "a", below):

All you can do witch such a drawing is to use it as a reference image. This is not the best option, because these microfilm pictures are always slightly deformed, due to paper folds and distortion of the camera lens. In the case of detail drawings you can find some useful dimensions, like the overall fuselage width (measured from the center plane), or the height. (As in drawings of frame #07 in figures "b", "c", above). Note that these frames are split along the fuselage reference plane into the upper and lower part. While the upper part (figure "b", above) at least resembles a bulkhead, in the case of the lower part (figure "c", above) I had to think for a while before determining its inner and outer side.

Because I have only a few vertical photos of the fuselage, one of the most important information are bulkhead widths. I have collected them just for the 7 frames:

Except frame #03, these fuselage width are measured at the reference plane (longeron #06). In the case of the firewall, this is not the point of the maximum width. (Firewall has an elliptic shape, which origin lies 6" lower, on the propeller thrust line – see figure below). Note that I also found two widths of the cockpit frame. These two points, combined with the height from figure "b", above, allowed me to determine precise location of this very important longeron. (It forms a base for many other dimensions). Unfortunately, I did not find explicit width dimension for the last frame (#17).

I also verified the top view of the fuselage (the third illustration in this post), checking its symmetry. Unfortunately, I found that this contour is asymmetric (i.e. the drawing is deformed) in the area without dimensions (between frames #8 and #11). This deformation could occur during microfilm production. (This blueprint was originally split between four film frames, and frame 3 and 4 overlap in this area). However, these drawings were manually traced in ink, and this can also be a human (draftsman) error.

If you wonder about the error range of the lines traced on the original blueprints, look at the picture below. It shows two drawings of the firewall. One of them is in red, while the other in black:

As you can see, contours of these two firewall "instances" can vary even by 0.3" (0.4% of the overall size). Which one is closer to the real contour? I decided that I will use the red drawing, because it is symmetric, and its contour mostly agrees with the left side of the black blueprint. I suppose that at least part of these differences are non-critical draughtsman errors.

The most important elements of the technical drawings are their explicit dimensions. Depicted object has just to resemble the real thing, making the drawing readable. All curves are described by datapoints (ordinals), grouped in the geometry diagrams. That's why for a manufacturer/workshop both of the compared firewall assembly drawings are valid, in spite of these differences.

Unfortunately, without the fuselage geometry diagram I have to rely on these "not-so-precise" object contours. This means that in my "3D reference frame" that I am building in Blender the possible error range will be much wider in the fuselage than in the wing. (Because the wing reference objects were mostly based on the original geometry diagram). In the second part of this post I will try to improve this situation a little, checking some doubtful/undocumented areas with the reference photos.
I placed the identified drawings of the fuselage frames in the 3D space, using some of their largest horizontal and vertical dimensions for determining the proper scaling. However, in the effect there were so many overlapping half-transparent pictures (as in figure "a", below) that in the practice the whole thing could not be used as a reference. It forced me to recreate each frame as a 3D planar contour, and hide their source drawings:

For the beginning, I focused on the better documented mid-fuselage. Once the bulkheads were created, I connected them with the cockpit frame and other longerons:

The longest longeron #06 lies on the fuselage reference plane, thus its shape from frame #05 to frame #17 determines the fuselage contour in the top view. For the convenience, these longerons run along the original longeron lines, depicted in the fuselage assembly drawing.

These longerons helped me to find and fix minor differences in the shape between subsequent bulkheads:

(When you look along the longeron at high angle, as in the picture above, you can quickly find all differences in the local width of the subsequent bulkheads.)

That was the easier part. Now I have to recreate the tail bulkheads. As you can see in the first illustration in this post, I found only the upper parts of frames #13..#16, and complete closing frame (#17). They contain just a few usable dimensions. Paraphrasing well-known Goya's title, "the lack of dimensions produces assumptions", and assumptions are the main source of eventual errors. However, there is no other way. In this case I have to make several assumptions about the shape of tail bulkheads.

Of course, first I reviewed all the blueprints for any hint about their shape. I have found some fragmented contours of tail bulkheads placed as additional information in other drawings. For example – figure below shows all what I found about frame #09:

The continuous red line marks the frame #09 contour, identified in two drawings. The left picture comes from cockpit enclosure assembly drawing, while the right picture is a fragment of an internal diaphragm assembly. The drawing of the cockpit enclosure (left) contains upper part of frame #09 contour. It looks as it was formed from an arc and straight segments (this is my first assumption). In the diaphragm assembly (right) you can see a complete frame #09 contour, up to the reference plane. It seems to be formed by a 3 arcs and a straight segment (this is my another assumption).

Design of this fuselage tail comes from the historical Northrop Gamma/A-17 aircraft line. First Gamma was built in 1932. In that time aircraft designers seldom incorporated advanced curves in their shapes, because they were much more difficult to recreate in the workshop than ordinary arcs. That's why I stick to identifying circular sections in these contours.

I placed these two images in the 3D space, then created auxiliary circles fitting the assumed arcs. Using their contours, I created the model of frame #09:

As you can see, there is still a completely undocumented segment above the fuselage reference line. I based its shape on the corresponding segment in the last documented frame (#07).

I could identify similar arcs in the tail bulkheads. They fitted the available drawings of frame #13, #14, and #15. To ensure smooth shape transition between subsequent bulkhead contours, I organized these auxiliary circles into "cones". Below you can see pictures of the first two cones, which form the main part of the tail:

For easier recognition, I marked the first documented tail frame (#13) in blue. Its width and height are determined by explicit dimensions, while the shape of previous frames is based mainly on assumptions. In picture above, left, you can see the upper cone. It starts with the frame #09 contour, which should fit the cockpit enclosure (arc diameter: 22.5"). Then it remains nearly constant up to the frame #13. (In fact, it seems to slightly enlarge its diameter, reaching maximum – 24" – at frame #13, then quickly decreasing in the further frames. For frames #16 and #17 this contour is meaningless, because their upper parts are completely hidden inside the horizontal stabilizer fairing).

In the picture above, right, you can see the central cone. Its axis lies on the fuselage reference axis, and its side contour is the side contour of the fuselage. I did not find any drawings of the lower part of frame #13. Basing on the side and top fuselage contours I decided that it was a simple circular segment, precisely matching the central cone. I assumed that the lower contours of all further frames (#14, #15, #16, and #17) were elliptical, because frame #17 seems to fit into an ellipse (as you can see in figure "a", below):

This frame had relatively large, tapered side walls (1" wide), thus I placed in this drawing two ellipses: one for the inner, and one for outer contour (both marked in the source blueprint). As you can see, these contours fit them pretty well. Note also that the upper part of this bulkhead is 2° oblique, to fit the attached rear stabilizer spar (web).

I also verified this elliptical contour in another drawing, of the first bulkhead in removable tail cone section (you can see it in Figure "b", above, marked in red). The last ellipse fits both blueprints. However, I noted differences between these drawings in the empennage fillet shape. (Compare its black and red contours in the marked areas of Figure "b", above).

In the next post I will continue my work over the lower part of the fuselage. I still have to prepare references for the large wing fillet, which spanned along half of the SBD fuselage length.

 

[Wurger]

 

[Maty12]

Excellent work, mate! Your progress updates are teaching me quite a few helpful strategies for when I finally get back into modeling, especially the use of auxiliary circles to properly match the curves at the bottom of the frame.

 

[Gnomey]

Great work so far!