SBD Dauntless

Witold_Jaworski · September 1, 2018, 5:37am

PFJN, thank you for following!

In this post I will finish my model of the R-1820-52 “Cyclone”. (This is the continuation of the subproject that I started reporting in the previous post). Figure below shows the oil sump, used in this engine:

Oil sump shape vary even within the same G100 family: I observed different proportions of the front “barrel” and its forward pipe in the early and the later of these “Cyclone” models. This particular oil sump (Figure “a”, above) was used in the later G100s engines, like the R-1820-52. Apart from the forward pipe, it was also attached to the front crankcase via a “chin” (Figure “c”, above).

To avoid eventual confusion, I would like to clarify the multiple naming conventions of the same engine: In the further text I will also use the internal Wright name for this “Cyclone” family: R-1820G100, or simply “G100”. The R-1820-52 engine is one of its members. (A month ago I finished a model of one of the later “Cyclone” versions: R-1820-60, which represents another, the “G200” family). I explained details of these nomenclature in this post.

While I have a few photos of the forward part of the oil sump, I have not any evidence of the shape of its rear part. (Because of the different shape of the intake pipes, I do not think that it forms the “Y-shaped” fork, like in the G200 series). All what I had found is a single, poor quality photo of the damaged engine recovered from Lake Michigan (Figure “a”, below):

There is “something” at the bottom of this crankcase: it has a trapezoidal shape and (probably) two inner (oil?) ducts inside. I decided that this is the rear base of the oil sump (Figure “b”, above). It is quite thin (no more than 1 in), fitted between the crankcase main section (cylinder bases) and the intake pipe (Figure “c”, above).

As you can see, I have made lot of various assumptions about the rear part of this oil sump. Well, in every model you can always find some elements that have such a “hypothetical shape”. However, this is the last resort, when all my photo queries brought nothing.

As I described in the post about “Cyclone” versions, the R-1820G100 and R-1820G series used the same deflectors. Thus I recreated the upper deflector (the “rectangular” version) using photos of a restored R-1820G engine, from the F3F-2:

I recreated the sheet metal frame and the flexible (rubber?) tip (Figure “a”, “c”, above). The photos from the recovered SBD-1 show, that there were some variations in the shape of the deflector rear part, around the spark plug. In the R-1820-32 from the SBD-1 I can see a kind of additional cut-out for the ignition cable, which is missing in this F3F-2. (F3F-2 had a different ignition harness – compare the deflectors in Figure “b” and “d”, above).

The top cylinder in the SBD-2…-4 had the elastic tip removed. (Because of the fitting the engine to the “Duntless” NACA cowling – I will show it later n this post). Thus I defined this deflector as another group instance, named F.G11.Deflector. (In the R-1820-60 model the top deflector is the part of the cylinder group).

In similar way I modeled the side deflector:

This deflector also has a flexible tip. As you can see, I skipped here some details (Figure “a”, above) that do not appear on every object instance. Note the characteristic “bat-like” fitting in the front of this deflector (Figure “b”, above). (The R-1820-60 deflectors had different fittings).

The last remaining details are the spark plug harness and the oil scavenge pipe:

As in the previous case, I am leaving the invisible, rear part of this engine in the simplified, “block” form.

Finally, I imported the NACA cowling from the main model and placed the engine inside. Fortunately, it fits very well:

In the R-1820-52 (and -32 in the SBD-2) the deflector on the cylinder 1 top was mounted without the flexible tip, to fit below the air intake duct of the upper cowling (Figure “b”, above). Both of the cylinder 1 side deflectors also had their flexible tips removed, to fit below the gun troughs.

In the R-1820-60 and -66 (used in the SBD-5 and 6) the cylinder 1 featured the full top deflector. (It was possible, because, as I explained in this post, SBD-5 and -6 had two filtered air intakes, used for takeoffs. For the higher airspeeds there was enough “fresh” air for the air intake hidden behind the cylinder row). The R-1820-60 had different fittings on the cylinder 1 side deflectors that fit the gun troughs (Figure “c”, above).

The R-1820-52 is now complete, for the assumed level of details. You can download the model presented in this post from this source *.blend file. I think that it can be also useful for the models of the other aircraft that featured the geared R-1820G or R-1820G100 engines. (Like Brewster “Buffalo”, DC-2 and some versions of the DC-3, or Curtiss “Hawk” 75). The exhaust stacks are not included, because this is an aircraft-specific detail (as the eventual air intake filters in the SBD-5 and SBD-6). I will recreate these details in the next post, for both of my ‘Cyclone” models.

PFJN · September 5, 2018, 1:50pm

Hi,

I can’t remeber if you may have already answered this question, but are you planing to put all this into a book or anything. There is so much useful info here not just on the SBD but also on detailing a Wright Cyclone that I’d really like to buy a copy of anything that you publish for future reference.

Can’t wait to see more.

Pat

Witold_Jaworski · September 7, 2018, 2:55am

Pat, I am really happy that you have found these articles useful!

Unfortunately, there is no “hardcopy”. (Without the elaborate attempt to the page formatting, it would be a quite thick book - over 500 pages, at this moment. The version formatted “for printing” would have about 400-450 pages).

These posts are also available in a more “structured” form of this wordpress blog.

_{In fact, I treat this blog as an “live” appendix to the my e-book guide, which describes all this workflow to the smallest detail. (This guide describes creation of another aircraft model: the P-40B).}

PFJN · September 7, 2018, 9:31am

Cool, thanks

Pat

Witold_Jaworski · October 27, 2018, 1:05pm

In my previous post I have finished the second variant of the R-1820-52 “Cyclone” engine, which was used in the SBD-3 and -4. (It looks like the earlier R-1820-32 model, mounted in the SBD-1 and -2). In the resulting Blender file linked at the end of that post you will find two “Cyclone” versions: the R-1820-52 (for the earlier SBD versions, up to SBD-4) and the R-1820-60 (for the SBD-5 and -6). Each of these engines has its own “scene”.

To “mount” these engines into my SBD models, I imported both scenes to the main Blender file. I defined each engine variant as a group, to facilitate placing them in the aircraft models as the group instances. I also added the firewall bulkhead and updated the shape of the cowling behind the cylinder row. (I will refer to this piece as the “inner cowling”). So far I did not especially care for the shape of its central part, hidden below the NACA ring. Now I updated it for the real size and shape of the engine mounting ring (as in figure “a”, below):

On the photos I noticed a kind of bulges, extruding from the both sides of the inner cowling (figure “b”, above). I assumed that they are shaped around small triangle plates welded on the sides of the mounting ring. (I have no photo to proof this assumption). Anyway, I modified the shape of the inner cowling in the SBD-5 to match this feature. I assumed that the inner cowling in the earlier SBD versions (SBD-4, SBD-3,…) also had such “bulges”.

In Figure “b”, above, you can see three openings for the intake air in the SBD-5 and SBD-6: a rectangular one in the middle and two round holes on the sides. These side openings are for the air filters, intended mainly for the takeoff and landing:

The idea is that the during the dusty conditions on the airfield the direct intake door (1) is closed, while the doors for the filtered air: (2) and (3) are open. When the aircraft climbs higher, its pilot flips positions of these doors, closing the filtered air input (2), (3) and opening the direct input (1).

There was no such a thing in the earlier SBD versions. It seems that the alternate filtered air input was introduced to many US aircraft in the same time: between 1942 and 1943. (You can also see the filter intakes in the P-40 starting from the M version, and in the P-51, starting from the B version). Maybe it was a general suggestion from the Army, after several months of the airfield war experience?

I added these two filters and their intakes to the R-1820-60 engine:

As you can see, these intakes are tightly fitted between cylinders 2-3 and 8-9 (Figure “a”, above), so they have a quite complex shape (Figure “b”, above). I do not have a photo for such an obscure detail, but the location of the filter determines, that the mixture intake pipes of Cylinder 3 and Cylinder 9 went through the corresponding intake body. (In principle, it is technically possible). I did not make holes in the deflectors between cylinders 2-3 and 8-9. (They would not be visible anyway, because both elements: the deflector and the intake are covered with black enamel).

The next aircraft-specific element is the exhaust collector. In the SBD-4 and earlier versions its outer contour had a circular shape (Figure “a”, below). However, in the SBD-5 (and -6) it went around the air filters, so it had a slightly different shape around this area (Figure “b”, below):

I built these collectors from simple tubular segments. Each of these segments is first tapered by the Simple Deform modifier (1), then bent along its shaping curve by the Curve modifier (2):

The offset of the original tube object from the curve object determines the origin of the resulting shape on the curve. Small gaps between subsequent tubular segments are hidden under the joining rings (as in the real collector). 95% of this collector is closed inside the NACA ring, so I decided to not recreate the fillets along the edges of the individual outlet pipes. (Joining all these tubular meshes would be a time-consuming task).

I used some photos to compare proportions of the exhaust collector, circular reinforcement and the cross section behind the NACA cowling in the SBD-3. The findings led me to the conclusion that I should modify the bottom part of the engine cowling:

Many months ago I found that the cross-section of the lower inner cowling in the SBD-5/SBD-6 had a non-elliptic shape (shown in Figure “b”, below). I also assumed, that such a cross-section also occurs in the earlier SBD versions. Now I can see that I was wrong: the photo above shows that in the SBD-1… SBD-4 it was a regular ellipse (as in Figure “a”, below):

It seems now that Douglas designers modified a little the bottom part of the engine cowling in the SBD-5, shaping its circular “chin” (Figure “b”, above). Maybe they did it because of the larger oil cooler used in this version? (It was required by the more powerful engine). If in the SBD-5 they shifted whole engine 3.5” forward, such an additional modification is also possible. (There was no any bulkhead at this station, and this cowling piece was already shaped anew).

To determine the exact location of the engine along the fuselage centerline, I used the high-resolution reference photo of the SBD-5 (Figure “a”, below):

I shifted the engine along the fuselage centerline, until its crankcase matched the crankcase visible on the photo. Then I measured the f distance (Figure “a”, abve) and applied it to the SBD-3 and SBD-1 models. (In the SBD-5 the engine together with the NACA cowling was shifted forward, thus in the SBD-1 and SBD-3 I could not simply apply the absolute location of the engine origin).

Finally, I applied the materials to the engine models, copied the environments from the SBDs to the R-1820-52 and R-1820-60 scenes, and made test renders:

On these renders I placed the engines “in the middle of the air” just to be able to evaluate all their materials in the full light conditions. Due to relatively small size of most of the engine elements, I used here only the procedural textures. I did not apply to this engine models any oils stains or other dirt. The historical photos show that the blue enamel on the crankcase was kept surprisingly clear, even in the worn-out aircraft. The other parts of the engine are obscured under the NACA cowling, so there is no need for additional “dirt” textures. You can see it in the test render of the R-1820-60 “Cyclone” inside the SBD-5 NACA cowling:

You can download the model presented in this post from this source *.blend file.

In the next two posts I will work on the details of the cowling behind the cylinder row.

PFJN · October 27, 2018, 7:23pm

Hi,

Thanks for the new post. I continue to learn alot from them, not only about 3D modeling, but also alot of details about planes and how they were put together. [:D]

PF

Witold_Jaworski · November 10, 2018, 10:45am

PFJN - thank you for following!

This time just about some minor details:

After “mounting” the R-1820 engines into my SBD models, I decided to recreate some details of the inner cowling (the cowling panels placed behind the cylinder row). In this post I will form the missing parts of the carburetor air ducts, hidden under the NACA ring. There are significant differences in this area between various SBD versions, which never appeared in any scale plans, or in any popular monograph of this aircraft. I think that the pictures presented below highlight these differences. They can be useful for all those scale modelers who are going to build the SBD “Dauntless” models with the engine cowlings opened. (Sometimes you can encounter such advanced pieces of work on the various scale model contests).

Let’s start with the SBD-5s (and -6s), which are better documented (because they were produced in much larger quantities). They had a dual intake system, of the filtered/non-filtered air, which I discussed it in the previous post. I already recreated the two intakes of the filtered air, placed between the engine cylinders. Now I have to create the central, direct air duct and its opening at the top of the internal cowling.

Figure below shows the initial state of my SBD-5 model:

As you can see in Figure “b” and “c” above, initially the carburetor protruded from the simplified shape of the internal cowling. On this stage of work I was sure that the engine is at the proper location (I matched it against the reference photo in the previous post). Thus, I concluded that the shape of the internal cowling requires an update. To determine its real form, I reviewed the available photos. Unfortunately, I have only few pictures of this obscured area:

Figure “a” above shows an archival photo, taken at the Douglas factory. I can see there that edges of the cowling around the central air intake are shifted forward. Unfortunately, the closing, top element of this cowling is not attached here. I can see it in Figure “b”, above, taken from the front (which makes it less usable). On this picture I noted that the edges of the air intake are elevated above the cowling (by less than inch). It was confirmed by the pictures of the restored SBD-5 from the Pacific Aviation Museum Pearl Harbor (Figure “c”, “d”, below):

In Figure “c” above you can see the side contour of the inner cowling. I can see that the central area is shifted forward (marked in blue in Figure “b”, above), and side segments around air filters are shifted back (marked in brown in Figure “b”, above).

Figure “a” below shows these faces on the updated mesh of the inner cowling:

You can also see there the top of the air duct (this is a separate object). Figure “b”, above, shows its simple, “box-like” mesh. When both elements were in place, I used a Boolean modifier to cut out the central opening in the cowling (Figure “c”, above).

Let’s look at the corresponding area in the earlier SBD versions. Figure below shows the rear side of the SBD-3 engine cowling:

This case is quite different. It seems that the upper part of the inner cowling in the SBD-3 forms a “box” around the carburetor air duct. It was quite difficult to find any photo of this area taken from above (you know, in 99% cases the photographer stays below, on the ground). All what I have are the photos of the SBD-3 wreck, salvaged from Lake Michigan:

I learned from them that this “niche” had flanges around its edges, and the air duct stood inside it like a “statue”. (There was a lot of space around this duct). The designers even formed a kind of “pedestal” at the base of this “niche” (I marked it on the right photo).

Using all this information, I reproduced this “box”/“niche” in my SBD-3 model:

I started with a simple box, then I fitted it into the elliptical contour of the inner cowling. Finally I joined these two meshes, and rounded their edges using the Bevel (Weight) modifier. I also extruded the flange around its rear edge.

I also tried to determine the shape of the air duct. In this case the only available references were the photos of the SBD wrecks:

It seems that this part of the air duct had a “jug-like” shape. Its upper edges fitted the horizontal air duct mounted in the NACA cowling. (That’s why the forward edge of this intake is lowered a little – just as the bottom of the air duct in the NACA ring).

Figure below shows my attempt to recreate this part in the SBD-3 model:

I had some doubts about the forward edge of the upper cowling that overlaps the flange behind the air intake. Finally, I wrapped it around the topmost edge of the “niche” (see Figure “b”, above). I also improved the shape of the “pedestal” in the inner cowling. (It covers the front section of the carburetor – see Figure “a”, above). I assumed that the air intake looked like that in the SBD-2, -3 and -4, because they share the same air duct design.

I have not any reference materials about the internal air duct in the SBD-1. I assumed identical shape of the inner cowling as in the SBD-2, -3, and -4. Figure below shows other assumptions:

I assumed that the general shape of the internal, vertical air duct segment was as in the later versions. The only difference are the simpler upper edges, fitting the opening in the NACA cowling. (There was no “lower” part of the external air duct under the NACA cowling, which you can see in the SBD-2, -3 and -4).

In the next post I will add the last details to the inner cowling, finishing my work on the engine compartment.

You can download the model presented in this post from this source *.blend file. To reduce its size, it is stripped from the texture images. (During the last year the size of this source file has significantly increased, reaching 40 MB in the compressed form. More than 35MB of this amount is used by the texture images. Thus, I will preserve the texture images in the source file only when they are relevant to the topic of the post)

Witold_Jaworski · December 29, 2018, 12:41pm

This winter I am busy with my daily business project, so you will see the further progress in this model in the spring 2019. However, I have just found a little unpublished tutorial that I made in November, so I decided to publish during this break:

This post is dedicated to a minor feature, which I have found surprisingly demanding: modeling the grooves pressed in the curved surfaces of the aircraft panels. In the SBD you can see some of such reinforcements on the inner cowling, behind the cylinder row:

They are 0.7-1.0” wide (Figure “a”, above) and span over the inner cowling along its radial directions (Figure “b”, “c”, above). In the SBD-5 and -6 these reinforcing grooves occur only on the lower part of the cowling (Figure “b”, above), while in the earlier versions (SBD-1, -2, -3, and -4) they are also present on the upper part (Figure “c”, above).

Even when the flaps on the NACA cowling are closed, you can still see rounded endings of these grooves around the cowling rear edge:

In the earlier versions (SBD-1…SBD-4) they appear on the narrow strip behind the NACA cowling (Figure “a”, above). You can see more of the upper grooves when the NACA cowling flaps are set wide open. In the SBD-5 and -6 the engine and the NACA cowling were shifted forward by 3.5”, and the gap between the NACA ring and the inner cowling is wider. Thus, in these versions you can see even longer fragments of the grooves behind the NACA cowling (Figure “b”, above).

Such grooves appear on many sheet metal elements, so I decided to write this post as a small tutorial that teaches how to recreate these elements. Thus, do not be surprised when I list the detailed Blender commands in the text below.

Usually I would recreate such a feature using bump map. However, this is a special case: it may happen that in the future I will also recreate most of the inner details inside this NACA cowling (for a cutaway picture). This means that in the future I will have to render some close pictures of this area. That’s why I decided to model these groves in the mesh geometry. (It seemed that this addition did not require any substantial retopology, due to the radial direction of these groves. Their layout matched the general “spider web” mesh layout of this inner cowling).

There are two methods to reshape the mesh in Blender:

Classic, manual modification of the faces, edges and vertices;
Displace modifier, which uses a texture image (a kind of bump map) to shift (displace) the mesh faces along their normal directions;

The Displace modifier is a great tool for the cloth wrinkles and similar effects. However, for the relatively sharp edges as in these grooves, it would require an extremely dense mesh (subdivided 4 or 5 times). Because the Displace modifier required significant increase of the polygon count in my model, I decided to recreate this feature using the basic modeling techniques.

Figure below shows the initial stage of this work. For each groove I created an auxiliary “plate” (Figure “a”, below) and adjusted their thickness and locations to the reference photo (Figure “b”, below):

Each of these plates is a simple, four-vertex plane. Its thickness is created by the Solidify modifier, and the rounded edges – by a multi-segment Bevel modifier. As you can see in Figure “b”, above, I set their locations and rotations, so each of their cross-sections with the cowling fits the edge of a groove visible on the reference photo. It occurs that the distances between the grooves are not uniform – as you can see, comparing the distances (1) and (2) in Figure “b”, above. (Note that there is a panel seam within range (2) – I think that this is the reason of this additional “spacing”).

I also thought about the mesh topology. In the optimal case:

each plate should cross only the perpendicular edges of the cowling panel. Eventual single radial edge along the middle of the plate is also acceptable;
there should be at least single “radial” edge on the cowling panel between two subsequent plates;

To fulfill requirement 2, I had to make the initial mesh of the cowling panel denser (subdividing each face once, by applying the Subdivision Surface modifier). You can see the resulting mesh in Figure “a”, above. I also marked there the potential “trouble area”, where the plate is crossed by the skew edges. It is always better to know such a thing in advance.

(Before applying the Subdivision Surface modifier, I also had to apply the original Bevel modifier, which rounded the gun throughs edges. Thus, at this moment this cowling object has just a Mirror modifier in its modifier stack).

In the next step I modified the cowling mesh, creating some space for the grooves:

I redirected the edges in the “trouble area” marked on the picture. I also rotated by a fraction of degree many of the other radial edges, so that they go straight in the middle of the plate, or run along its side, far enough for the rounded edges of the groove. It is hard to notice these results at first glance, but this is the key work which determines the quality of the final effect.

Once the mesh of the cowling was updated, I removed from the plates their Bevel modifiers and applied the Solid modifiers, converting their meshes to “boxes” of fixed width (0.7”). (The only purpose of the Bevel modifiers was to round the plate edges for comparison with the grooves on the reference photo).

Before I started “chiseling the grooves”, I added to this cowling panel a new Bevel (weight) modifier, and a Subdivision Surface (1) modifier. Figure below shows the current state of the modifier stack of this model part:

These two new modifiers will round the edges of the grooves. The size of the Bevel (0.2”) is appropriate for the width of these grooves (0.7”). Of course, in your case you have scale these dimensions proportionally for your groove width.

To cut out a groove contour, I joined (Object:Join, or [Ctrl]-[J]) the plate object with the cowling, and selected all of its six faces (Figure “a”, below):

Then I invoked the Mesh:Faces:Intersect (Knife) command, obtaining the initial edges of the groove (Figure “b”, above). The Intersect (Knife) command produces some overlapping vertices, which I quickly fixed, selecting all of them and invoking the Mesh:Vertices:Remove Doubles command. I also removed the “cutting box” (I do not need it anymore). In the last step I adjusted the ends of this “strip”. I created four additional vertices (two of them at each end), to add four additional edges (Figure “c”, above). (I created these new vertices by selecting the corresponding edges and invoking the Mesh:Edge:Subdivide command). Finally I shifted the corner vertices of this strip inside, using Mesh:Vertices:Slide command ([Shift]-[V]).

Then I use the Mesh:Transform:Shrink/Fatten command ([Alt]-[S]) to move the central vertices of this groove down, each along its original normal direction:

This groove has width of 0.7”, so I shifted its vertices downward by 0.4” (I found this proportion optimal). Then I assigned the Bevel weights to round the edges of this groove. I set the weight = 1.0 to the central edge, and weight = 0.5 to the side edges (Figure “a”, below):

Figure “b”, above, shows the resulting surface.

I repeated this sequence of operations for each groove. When the intersection produced a contour without the central edge, I used the Mesh:Edges:Subdivide command to create a new one:

Figure below shows the finished grooves on the SBD-5 and SBD-3 cowlings:

(The SBD-1 uses the same inner cowling as the SBD-3).

After this work, I had to refresh the UV maps of the modified elements. Figure below shows my test render of the SBD-5 cowling:

As you can see, I removed the NACA cowling from this model, so that you can evaluate the ultimate result of my work.

Conclusion: as long as you can, use bump maps to recreate such grooves. Recreating this feature by pure modeling requires much more work (by an order of magnitude).

You can download the model presented in this post from this source *.blend file. To reduce its size, it is stripped from the texture images.

PFJN · December 29, 2018, 7:39pm

Hi,

Thanks for posting the tutorial. I’ll stand by til later this year to see how things turn out.

Hope you have a Happy New Years.

PF

Witold_Jaworski · April 27, 2019, 4:05pm

Thank you, PFJN, for following!

I will be back at this SBD in May.

Just a small off-topic note: this winter I was busy with my daily business and took a break from the SBD model. However, in February and March I spent few Sundays helping in another project: recreating the Fokker D.V biplane, used in 1917 as an “advanced trainer” by German Air Corps:

My part was recreating the geometry of this aircraft, especially its fuselage frame made of steel tubes. All what we had was a dozen of various archival photos, a poor general drawing, and the landing gear dimensions. In this case I had to turn the available photos into the precise reference, as I did for the SBD, then use them to determine the required geometry details:

Doing it, I also made a “discovery” about wing geometry of this airplane. See details in this post.

PFJN · April 29, 2019, 3:33pm

Hi,

That’s an interesting plane too. Thanks for sharing the details of how you go about making these 3D models

Pat

Witold_Jaworski · August 4, 2019, 11:35am

Important update about this SBD project: thanks to C West help, I identified a microfilm roll set of the original Douglas documentation for the SBD/A-24. In June I ordered its copy from NASM and now I am waiting for these materials. When I got them, they will be scanned by a local service company which scans various museum archives. This is not cheap, because the only possibility is to scan all microfilm frames (and pay for each frame, of course - I estimate that this set of seven microfilm rolls contain about 5500-6000 frames). Then I will organize these scans for quick use as the reference materials.

In the meantime, I am going to update my P-40 model, also using original blueprints. I already bought their scans. See another thread in this formu about my experiences on this subject.

Witold_Jaworski · November 11, 2019, 7:37am

Finally (after 5 months) I received the Douglas SBD microfilms from NSAM (7 rolls).
I have already contacted a local service provider, who scans microfilms for museums. They promised me to scan them in the beginning of December.
(I have no any microfilm viewer, and do not want to spoil these films in a slide projector. At this moment I just checked that the title page of these microfilms says that this is the Douglas SBD/ A-24, and that it contains blueprint pictures).
I will keep you informed on the progress.

PFJN · November 11, 2019, 11:38am

Cool,

I’m looking forward to seeing more of your work.

Pat

Nino · November 21, 2019, 4:26pm

I have been following along for the last year and a half. I wish I could post something of a helpful nature other than saying what a fantastic effort you are making and doing such excellent work, well beyond anything I could have imagined.

Thanks for bringing us along on your Dauntless project,

Nino

Witold_Jaworski · March 27, 2020, 5:16am

Nino - thank you for following!

Below more on the original SBD blueprints:

In June 2019 I followed C. West suggestion and ordered a set of Douglas SBD original technical documentation from U.S. National Air and Space Museum. Technically these blueprints are stored on several microfilm rolls. In that time all what I knew about this package (NASM id: “Mcfilm-000000408”) was the information printed on the order form:

As you can see, this set has no index, which I could order earlier to examine its contents. When I finally received these microfilms in November 2019, I also discovered the meaning of enigmatic “(roll C” in the item description: it was truncated phrase “(roll C missing)”!

Well, this set was incomplete, but anyway I ordered its high-resolution scans from a local company that provides professional microfilm scanning services to museums. In January I received these data (4700 high-res, grayscale images in LZW-packed TIFF format – in total, about 300 GB). Finally I was able to scroll these blueprints. Frankly speaking, I was afraid that the most important drawings were lost with the missing roll C. Fortunately, during the initial review I noticed many detailed assembly blueprints among the scanned images. I even found a complete inboard profile of the SBD-5:

Here you can download the high-resolution version of this inboard profile (about 70MB).

The scanning company (Digital-Center, located near Poznan) did its job well, adapting the scanner resolution to the size of the depicted drawings. Scans of frames that contain large assemblies are usually 11 296 px wide and 7 874px high, which ensures that even the smallest references are readable. For example – see these four subsequent frames of the fuselage assembly drawing: 1, 2, 3, 4 (beware: size of each of these linked images is about 70MB).

There are even larger images in this result set: the biggest one is about 20 000px wide. The smaller drawings of various details are scanned at 7 672x5 682px.

Conclusions from the first review of this microfilm set are as follows:

Roll A: blueprints of various small elements. Many of them (various angles, straps, bolts, pins, etc.) are standard Douglas parts, shared between many aircraft types. (As the angle from Figure 107‑5 – it was traced in 1936, before the SBD appeared on the designer desks);
Roll B: blueprints of various SBD details - special bolts, pins, screws, various brackets, supports, forged parts, some minor assemblies (for example – arresting hook);
Roll C: missing
Roll D: assembly drawings
Roll E: more detailed assemblies, some larger details
Roll F: remaining elements (this is the shortest roll)
Roll XA, XB: updated drawings, published a few months later. Most of the drawings are duplicates of those depicted in rolls A-F, but can differ in minor details. However, some of these drawings are new – most probably they are updates of the drawings from the missing roll C;

These blueprints describe SBD-4 and SBD-5 (which is OK – the SBD-4 is similar to SBD-3, and SBD-2, while SBD-5 is nearly identical to SBD-6). It seems that rolls A-F were made from September to October 1943, while their updates – rolls XA, XB – in January 1944. Because of the missing roll C, this documentation is not complete. In general I could not find the fuselage ordinates (I only found the wing ordinates). My first impression is that the missing roll C contains most drawings of wing ribs and at least half of the fuselage bulkheads. Fortunately, there are drawings of the tail bulkheads and the firewall on the other rolls.

For my project I need to organize these blueprints into a tree-like structure, with the largest assemblies in the root (as described in this post). However, after this initial review I could not determine the rules of the drawing numbering system used by Douglas. (I wanted to use them for quick grouping all parts belonging to the same subassembly). It could happen that in this Douglas factory the drawing numbers were assigned sequentially, just as the subsequent blueprints were ordered! It also seems that some of these drawings use original Northrop numbers, dating from the SBD predecessor: the BT-1 dive bomber.

To organize this documentation I have to start from the general assembly drawing and then step down to its subassemblies. For this purpose I need an index of these blueprints. Recreating such a thing is a monotonous task that will take some time, but I cannot see better option to fully explore contents of these microfilms. What’s more, I think that once such a list is created, it can be also useful for the others. While NASM forbids publishing technical drawings from their microfilms by any means (except so-called “fair use”, which I am stretching a little in this article), I still can publish such an index. Of course, I will also donate its copy to NASM. I hope that in this way the eventual future buyers of these SBD/A-24 microfilms will benefit from my work.

First obstacle in creating such a drawing list is quite unusual: while most manufactures traced the digits of drawing numbers in ink, as the all remaining drawing lines, Douglas stamped them in the title block. The ink often spilled over the edges of the stamped digits, and now it is hard to read them from the microfilms photos. For example – look at the title block of this sample drawing:

When I altered the gray shades of this drawing, I was able to identify the first four digits (5063):

Fortunately, after this adjustment I discovered that they also stamped (most probably) the same number on the right margin. So here it is: 5063493. (You can also find a mirror image of this number on the right below: most probably it was stamped on the other side of this drawing). Of course, sometimes even these additional stamps on the margins are hardly readable: I frequently wondered whether a particular “splash” in place of a digit represents “4” or “1”. In other cases it was difficult to tell if there is a “3” or “8”, or “5”, or even “0”. Some help came from the observation that the numbers of these microfilm drawings are always in ascending sequence. Thus, if the unreadable digit in the drawing number “519456x” can be “3”, “5”, or “8”, but the previous drawing is “5194560”, and the next drawing is “5194565”, then this last digit must be “3” (giving drawing no: “5194563”).

Still there are cases in which I was unable to decipher the drawing number even after adjusting the grayscale, and there was a wide gap between the previous and the next drawing number:

The standard parts, like the one depicted in figure above, often occur in several variants (you could cut the depicted standard angle in a variety of lengths). In such a cases Douglas placed in the drawing an example of the full part number:

The prefix of this part number is the drawing number!

In case of unreadable dedicated part/assembly drawing numbers, Ester A. from AirCorps Library suggested the “last resort”, indirect method. Usually the blueprint of a non-standard part contains a table named “NEXT ASSEMBLY” on the side of the main title block. It provides drawing numbers of the assemblies that use this part:

In the case from figure above this is a single assembly drawing: 3063922, which was used in the SBD-1, -2, and -3 models. Using this drawing number I found the corresponding blueprint:

Examining this assembly drawing, I found the sought detail and read its number. Of course, this method requires searchable list of the available drawing numbers, from which I would read that drawing 3063922 is in roll B, frame 1014. That’s why in the first step I needed to create the drawing index.

t took me about 100 hours of work, but here it is:

Click here to download the index (*.xlsx file, 303kB).

Frankly speaking, for my purposes I do not need the details from roll A, which took about 25% of the total work time. But I decided to index all the rolls, just to provide a complete list for eventual other users.

Below you can see how this list looks like:

For each microfilm frame I note in this table not only the drawing number, but also the name copied interim from the drawing title block. (I preserved in these texts original caps and the grammar errors – for example: “PILOTS CHAIR”). Eventual unreadable digits in drawing numbers are marked as “?”, and unreadable text fragments as “<…>”.

It seems that in Douglas numbering system drawing numbers of standard parts should have “S-“ prefixes. However, I found that these prefixes were often missing, or even manually written in drawings of certain parts that do not seem to be standard. Thus I decided to skip “S-“ prefixes in the numbers placed in this index, because they could be misleading and make the eventual searches more difficult. Instead I marked each drawing of a standard part in the Comments column, basing on the presence of the “STANDARD PART” statement in its title block.

When a single drawing spans over two or more subsequent microfilm frames, I described it in the index table in following way:

Each line in the table represents single microfilm frame, thus drawing 5196835 is described by two subsequent lines, which differ by the drawing frame number in the Partial frame column (1, 2). In the first of these lines I placed the roll id of this drawing (“E128”). I did this just in case, because I do not think that I will use these ids. I placed drawing description (title) in the line that corresponds to the frame containing its title block. In this way the table contents is more readable. (You can instantly recognize for each drawing where it starts and ends).

In this list you can use Excel filtering feature, searching for drawings that contain certain phrase. When you click the auto-filter button in the right corner of Description column, and search for “BONDING” phrase, you will get following result:

If you did this search because you wanted to find the drawing of the fuselage bonding, in the result set you will see the last line of drawing 5196835 (the one that contains the description from its title block). You can read the roll symbol and the frame number from Roll and Frame # columns (roll: E, frame: 193). From the Dwg frm column you will learn that this is frame 2/2, thus you will also know that previous frame (192) from roll E contains the first part of this drawing.

Finally, figure below shows one of the most complex examples of a multi-part assembly drawing:

A very long drawing 5094762 (E138) was originally traced in two parts (1/2, 2/2). Each of these parts spans over 3 frames and has its own title block, thus I placed their labels in the corresponding index lines. What’s more, this assembly is accompanied by additional BOM tables, depicted in the subsequent microfilm frames (6, 7, 8).

Witold_Jaworski · April 20, 2020, 10:46am

In general, the set of 7 SBD/A-24 reels from NASM contains 3308 unique microfilm frames, belonging to 3022 drawings. On reels “XA” and “XB” you can usually find updated copies of the previous reels (“A”, “B”,… “F”). However, 350 frames from “XA” and “XB” are unique – most probably this is a part of the missing roll “C”. Duplicates from these “X*” reels are also useful, when a drawing from one of the previous reels is unreadable.

I chose about 1000 frames (mostly assembly drawings) from this microfilm set, and organized them into a tree-like structure as in figure below:

To preserve disk space, I placed in these folders shortcuts to files located in the original directories (These original directories correspond to microfilm reels: “A”, “B”, …, “XB”). I practiced that when I click such a link, it opens the image in Photo Viewer, as if it was the original file.

I think that Douglas did not use any sophistical drawing numeration (at least in this project). The SBD/A-24 drawing numbers seem to be assigned as they were ordered: for example, drawing numbers of subsequent wing bulkheads belong to number series that begins with: 206*, 209*, 212*, 406*, 409*, without any visible order. Maybe this is due the fact that part of these drawings came directly from the Northrop Co, without any renumbering? (You can still find “Northrop Aviation Division” name in the title blocks of some standard parts from this microfilm set).

The documentation from NASM microfilm is missing many important details – I suppose that they were on the lost reel “C”. For example, figure below shows the identified and missing wing bulkheads:

Fortunately, reel “E” contains also wing geometry master diagram (ordinals), so I can use it for recreating shapes of these missing elements.

Fuselage structure also misses many bulkheads:

However, there is no master diagram for the fuselage. I will have to recreate its shape basing on the few reference dimensions placed in the identified assembly drawings. I have also found some contours of these missing bulkheads, drawn as additional information in various installation drawings. However, these blueprints are not as precise as you think – due to barrel distortions of the photo lens and draughtsman mistakes, I estimate their tolerance to 2-3% of the overall size. Unfortunately, there are no data about the wing fillet shape, especially its outer edge.

Several years ago I analyzed the SBD photos and concluded that SBD-5 (and -6) engines were mounted a few inches forward than in the previous Dauntless versions (SBD-1…-4). In this post I estimated this difference in length as 4 inches. Now I found the proof of this observation in the SBD-4 and SBD-5 engine mount dimensions (drawings 5055954 and 5159336):

The explicit dimension of SBD-4 engine mount (dwg no 5055954) specifies its length (distance from the firewall to the back faces of the engine mounting lugs) as 34.1875 (I switched original fractional dimensions to decimals). Similar dimension of the SBD-5 engine mount (dwg no 5159336) declares its length as 38.1875. Thus we have the 4” difference!

However, to determine the ultimate difference in the fuselage length (marked in the figure above with the question mark) I also have to determine the overall lengths of the elements in the front of the engine mount. Both versions (SBD-4 and SBD-5) used the same propeller (length: 21.75”). From the drawing of the SBD-5 NACA ring I can read the overall length of the entire engine cowling: 60.8125” (see figure above). Unfortunately, there is no such information in the SBD-4 drawings. I have to determine this dimension in an indirect way. Let’s try it. The blueprints show that the length of the NACA cowling was identical in all versions (31.5”), as well as the distance from the R-1820 cylinders plane to the front of the NACA ring (14.3125”). (The NACA ring was attached to the mounting points on the R-1820 cylinder heads. These points were placed on the engine cylinders plane). Thus the potential source of the eventual further fuselage length differences is the distance from the back faces of the engine mounting lugs to the cylinders plane. From the SBD-5 blueprint I can calculate that it was 8.3125”. This result is close to the dimension specified in the R-1820 installation drawing (8.24”):

However, this installation drawing describes the R-1820 G200 (also known as C9GC), which was used in the SBD-5 under military designation R-1820-60. (For more information on the Wright “Cyclone” engine variants see this post). The R-1820-52, used in the SBD-4 and earlier Dauntless versions, belonged to the earlier R-1820 G100 series (also known as C9GB). Crankcase of the G100 family significantly differs from the crankcase used in G200. This could also mean differences in the coaxial location of the engine mounting lugs.

The B dimension marked in this drawing is the piston bore. This is well-known parameter of this engine, specified in the manufacturer documentation as 6.125”. I can estimate the sought distance A by comparing it to known B. The A/B ratio that I measured in a high-resolution copy of this drawing is 1.286 (+/- 0.8%). This means that A = 7.875” (+/- 0.063”) – i.e. the same distance as in the R-1820 F. I assume this value for the R-1820-52as as the most probable distance from the mounting lugs to the cylinders plane.

Thus the overall difference in the SBD-5 and SBD-4 horizontal lengths comes from the difference in their engine mount lengths (4”) and the difference in the distance from the mounting lugs to the cylinders plane between the R-1820-60 and -52 engines (8.3125– 7.875) = 0.4375”. This result can be rounded to 4.44” (or expressed precisely as 4 and 7/16”). According Douglas general arrangement drawing, the overall length of the SBD-5 was 33’ 1/4” (396.25”). Thus the overall length of the SBD-4 could be 32’ 7 13/16” (391.81”).

What about the earlier Dauntless variants: SBD-1, -2 and -3s? The eventual difference in their lengths and the SBD-4 length comes from the different propellers. (They used “Hamilton Standard Constant Speed”, while the SBD-4 used newer “Hamilton Standard Hydromatic” propeller). In the NASM documentation I found a powerplant diagram (dwg no 5094793), which shows this older propeller variant and the contour of its spinner. Although the spinner shape a little bit different than in the archival photos, all other drawing elements seem quite precise. Using this picture I could make a more precise estimation of the few key dimensions:

In this blueprint (above) I identified vertical lines that mark the firewall and the engine mounting lugs planes, as well as the cylinders plane. I scaled the distance between firewall and engine mounting plane to the corresponding dimension (34.19”), then I read the B, C distances, and – just as additional check – the A distance. I received: B = 37.66”, C = 42.38”, A = 7.88” (which confirms the previous estimation: 7.875”). Thus the overall length of the SBD-1,-2, and -3 can be estimated as:

when the spinner was mounted: 32’ 6.3” (+/- 0.3”);
without the spinner (the case often observed in the archival photos): 32’ 1.5” (+/- 0.3”);

It is interesting that p. 2 agrees quite well with the SBD-1 and SBD-2 length (32’ 2”) listed in the BuAer performance data sheets from 30^th November 1942 (an repeated in many other later publications).

The aircraft dimensions listed in these BuAer data sheets are rounded (up?) to full inches. For example: the wing span is listed as 41’ 7”, while the exact value in Douglas arrangement drawing is 41’ 6.25” (41’ 6 1/8”).

Similar (single) BuAer sheet from 6^th August 1942 examined the SBD-3 and SBD-4. The horizontal length specified in this document (32’ 8”) agrees quite well with the length of the SBD-4. We can assume that in the sheet which examined both: the SBD-3 and SBD-4, BuAer engineers simply put the length of the latter (evidently they treated these two aircraft as a single variant).

Many publications cites SBD-3 length as 32’ 8”. This is wrong value, coming from overinterpretation of the BuAer data sheet from August 1942.

OzzyDog · April 20, 2020, 1:06pm

What an amazing coincidence! Just purchased the Hasegawa 1:48 SBD-3 as my next build. This thread could not have popped-up at a more opportune time. I have only glanced over the first page before jumping to the last, but I am certain that this will be an invaluable source of information. Considering my meager skills, I’m probably not worthy of such painstakingly detailed information. But thank you for pulling this together. John

PFJN · April 20, 2020, 8:12pm

Wow,

The level of detail that you have gone to in collecting and using all these drawings is amazing.

Pat

Witold_Jaworski · May 16, 2020, 12:52pm

Pat, OzzyDog - thank you for watching this thread!

Actually 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” (20.09 + 20.05). 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!