SBD Dauntless

_{(Double post - removed)}

In this post I will finish all the remaining details on the front of the R-1820 engine. (As I mentioned in earlier posts, this model is intended for the outdoor scenes, with closed cowlings. That’s why I recreated the more complex rear part in a simplified form, just to check if it fits properly to the airframe).

One of the most exposed “Cyclone” details is the variable-pitch propeller governor:

This is an additional unit that controls the pitch of the Hamilton-Standard propeller. (It controls the oil pressure, which determines the actual pitch of the propeller blades). You can find it in every aircraft, but it is often dismounted from the “standalone” engines, presented in the museums. The large wheel at its top is used as an actuator attachment. The actuator can be a pushrod or a cable from the cockpit. In the case of the SBD (and many other WWII aircraft) it was a control cable (Figure “b”, above). The engine depicted in Figure “a”, above is a standalone museum exposition, thus it lacks such a cable.

Analyzing the photos, I slowly recognized that this governor was mounted differently in various Dauntless versions. In the later SBDs (SBD-5, -6) it is placed in the front of the topmost cylinder, and its actuator wheel is on the left side (as in Figure “a”, above). In the earlier versions (SBD-1, -2, -3, -4) it is mounted between cylinder 1 and 2 (as in Figure “b”, above). Let’s focus on the later versions first, because I have more its photos. I could even find the propeller governor base on one of the original installation drawings (Figure “a”, below):

This drawing shows, that the governor was rotated by 18⁰. The reason for this unusual arrangement became obvious when I fit into the model the first, simplified version of this object. This rotation directs the control cables between cylinders 1 and 2 (Figure “b”, above). Do you remember the two small holes in the deflector depicted on the second-last photo from the previous post? They were made just for this cable.

However, I could not determine the ultimate shape of the governor unit. Most of the photos that I had looked like those that I show in the first picture of this post. They were taken at unusual angles, or the object was in black, which obscured its details. I was only able to determine, that there are several versions of this part, which differ in important features (for example, they have different number of outlets). It seems that this is a third-party component, delivered by independent vendors. In desperation, I looked for it on the e-bay, where I ultimately found a decent photos:

The version on the pictures above was widely used in the aircraft from the post-war period. It has two additional outlets, which did not exist in the propeller governors used during WWII (at least not on the photos that I have). Anyway, it still resembles the governors that you can find on the historical pictures. Using it, I was able to build a more detailed model:

First I recreated the governor shape using a group of simple “blocks”: cylinders and boxes with cylindrical sections. Then I adjusted their proportions and positions, so that they resemble the original object. Finally I started to join these objects (using Boolean (Union) operator), and rounding their intersection edges using a multi-segment Bevel (Weight) modifier (Figure "a", above). I set up a large “nominal” radius of this Bevel modifier (1.3”). Then I controlled the radii of individual fillets by assigned fractional bevel weights to their intersection edges.

I practiced that you can set these fractional values in the Mean Bevel Weight field of the Edge Data section, at the top of the View>Properties region. (The region at the right edge of the 3D View window that Blender shows/hides when you press the N key).

 You can see the final result in Figure “b”, above. Note that I had to check the control cable clearance behind the deflector (it has to pass by the intake pipe of the cylinder 2 – as in Figure “c”, above). However, the fillets in Blender are far from the ideal: I gave up with the edge of the rear outlet (Figure “d”, above). To not spoil the previously rounded edges, I had to leave this cylinder as a separate object, just attached to the main body by the “parent” relation. (Fortunately, this is a less visible detail).

To round this edge, I should sculpt it in a mesh that is “dense” enough (i.e. has enough faces in this area). Such a labor-intensive solution does not match the level of detail assumed for this model.

The next detail is the elevated edge around the valve timing inspection hole. You can see it on the front crankcase section (Figure “a”, below):

As usual, I started with a simplified, conceptual object (Figure “b”, above). It allowed me to adjust the proportions and size of this feature, as well as the mesh topology. Then I joined it with the corresponding crankcase segment, and rounded the newly created intersection edge with a multi-segment Bevel (Weight) modifier (Figure “c”, above). You can see the final result in (Figure “d”, above).

Finally, it is time to populate this engine with all nine cylinders. I delayed this operation to the end, because I was going to duplicate these objects as the clones (i.e. new objects that share the same mesh). After such a multiplication, if I discovered that these cylinders lack a certain detail, I would have to copy it nine times. That’s why it was better to wait with such a multiplication until it seemed that none of such modifications is needed. However, in May I received an invaluable suggestion from Jeff (pzzs7f, in this post) that I should try the so-called group instances. I did it in following way:

First I placed all the cylinder elements on layers 13, 14, 15, then declared them as an object group (Figure “a”, above). (I did it using the Object>Group>Create New Group command. Blender highlights the objects that belong to the same group with a green outline, which you can see on this picture). I named this group G.G05.Cylinder. Note that it also contains the crankcase segments located around the cylinder (elements from layer 11). Beware that Blender assumes the center point ([0, 0, 0] in the global coordinate system) is the eventual origin point of an object group. Thus place your source objects accordingly in the space around this point.

When this “group definition” was ready, I turned its source layers off, set the 3D cursor to the engine center point, and in the top view I inserted the first instance of this group (Add>Group Instance). You can see it in Figure “b”, above. (Accidentally, it is located in the same place in the space as the source objects, but you could insert it anywhere). When you examine this cylinder, you will discover that this is an Empty object, which contains reference to the G.G05.Cylinder object group.

To “populate” this engine, just create clones of this first instance, and rotate them around the center point by 40⁰, 80⁰, 120⁰, and further angles. You are placing in this way the subsequent cylinders in their locations (and rebuilding the mid- and rear-crankcase, as well). Figure “c”, above, shows how it looks like. Note that I have placed all these cylinder instances on a different layer: 3.

Such instances of an object group are a great tool in dealing with repeatable machine parts. When you add an additional object to this group, it immediately appears in all cylinders. When you remove an object from this group – it disappears from all instances (although it still exists on one of the source layers). This means that I could use these instances earlier, without worrying about adding the remaining details! Well, in my next model I will recreate the cylinders at least in the middle of the project. It is always better to see the whole engine.

What’s more, Blender optimizes the way it displays and renders such instances: note that its Faces/Verts/Tris counters do not take into account their meshes!

Figure “a”, below, shows all nine cylinders in place. Note, that each of them contains also the spark plugs. After this “multiplication”, I carefully examined each of these group instances, looking for eventual intersections with other objects:

As you can see in Figures “b”, “c”, above, there are just few of such collisions, caused by the clamps on the pushrod seals. I have tried to rotate these clamps, hoping to find a universal “neutral” position that does not collide with anything. Finally I gave up: I excluded clamps from the object group and copied their clones around the engine. Then I could rotate each of them separately around the pushrod, fixing every collision that I had found.

I also recreated the side deflector as another instance group:

I named this group G.G10.Deflector. Its source objects are located on layer 16, while the group instances – on layer 6. At this moment all the deflectors are identical (for example, they were mounted in this way in the “Cyclones” used in the B-17s). For such an effect I could simply add the side deflector into the cylinder group (as I did for the cylinder top deflector). However, in the SBDs there were two gun troughs in the cowling, on both sides of the topmost cylinder. Thus I decided to define this deflector as another group, because in the future I will have to replace the two topmost deflector instances with modified clones. For the same reasons I “extracted” the side beam from the rocker cover into a separate object (Figure “b”, above).

Looking at Figure “a”, above, you can find some additional parts: a few dozens of new bolts, as well as the scavenge oil pipe. (This pipe connects the bottom of the oil slump with the pump in the rear).

Finally I added the last remaining detail: spark plug cables:

As you can see, the cables occur in pairs. In each pair there is a longer and a shorter cable. The longer one connects the rear spark plug (Figure “c”, above). (I know that this is the “invisible” area, but I could not resist the temptation to recreate this detail). The shorter one connects the front spark plug (Figure “b”, above). All cables of the same length (short or long) and their terminating nuts share the same mesh (they are clones). However, each of them has its own shape, because their Curve Deform modifiers refer to their individual curves (Figure “b”, above). I copied these curves around the crankcase, and then introduced minor modifications to their shapes. Also the clamps that attach these cables to the pushrods are individual clones. I introduced some random variations to these shaping curves and the positions of the cable clamps that attaches them to the pushrods. In this way they resemble the original, manually connected cables. The only missing element in this model are the engine data plates. I will recreate them later, together with the cockpit details. (They require a dedicated, high-resolution texture).

The engine seems to be complete. (Of course, for the assumed level of details: the rear crankcase sections and their equipment are recreated in the form of simplified blocks). I will fit it into the cowling, then cut out the deflectors below the gun troughs.

I zoomed the data plate on one of my reference photos, and found that this is the R-1820-60 (the version used in the SBD-5: 1200hp for takeoff). All the manuals and blueprints that I have collected describe this or one of the later “Cyclone” versions. Thus I can conclude that the R-1820-66 (the version used in the SBD-6: 1350hp for takeoff) seems to be identical (at least as viewed from the front).

I also expected just minor differences between this one and the earlier R-1820 versions, used in the SBD-1, -2, -3, -4. The first difference that I have found was in the propeller governor positions (at the beginning of this article). Then I started to analyze the other older photos:

I quickly found another one: in the R-1820-52 the ignition manifold forms a full circle, while in the R-1820-60 it is a 300⁰, “U-shaped” arc). I decided to look closer at the differences between the R-1820-52 and -60. I will report my findings in the next post.

You can download the model presented in this post (as in second-last figure in this post) from this source *.blend file. It is available under CC-BY license and can be useful for other aircraft, for example the B-17 or the F4F-4.

Hi,

Your work is amazing, and it also really helps you understand the different parts of the plane

PFJN - thank you!

__________________________________________

I decided to write a post about the first decade of the R-1820 “Cyclone” development (up to the R-1820-60 version, i.e. 1940). This engine was used in many designs from 1930s, and you can find the references to its various models in many technical specifications. However, sometimes it is difficult to determine how such a referenced version looked like! The early models of the “Cyclone” were produced in small batches, so there is less historical photos. Sometimes even the specialists from the museums are misguided: in one of them, you can find a SBD-3 fitted with the engine and the propeller from the SBD-5. My query, which resulted in this article, started with comparison of the R-1820-60 (used in the SBD-5) and the R-1820-52 (used in the SBD-3 and -4). I have found so many differences, that I started to wonder about the engine used in the pre-war SBD-1 and SBD-2. (They used the earlier “Cyclone” version: R-1820-32). The results presented below may be interesting to the modelers who recreate aircraft from this period (for example – the Curtiss “Hawk” biplanes, or the Grumman F3F-2 “Flying Barrel”).

Let’s start from the beginning: below you can see the first model of the R-1820 family, designed in 1931:

Frankly speaking, there is only a general resemblance to the later “Cyclone” versions. Note the small crankcase front section and the “archaic” cylinder heads. (They have different shape, and their fins are much shorter and widely spaced: these are indicators of a simpler casting technology). Another strange feature is the exhaust, which could be also mounted in the reversed (i.e. forward) direction. (Some of the aircraft from this era used front exhaust collectors). This engine used large spark plugs, mounted horizontally (in parallel to the centerline). It was rated at 575hp on takeoff, and used in some contemporary designs, like the Curtiss “Hawk” biplane.

Before introducing the next “Cyclone” version, let’s try to decode its symbol. It seems that there are two parallel conventions: one used by the engine vendor (Wright Aeronautical), and another used by the US Air Corps and the Navy.

Wright designated this engine as R-1820E. The “R” stands for “radial”, “1820” is the displacement (in cubic inch), and the “E” denotes the model. There were also other, 7-cylinder “Cyclones” produced by Wright during 1920s, as well as smaller 9-cylinder “Cyclone” (R-1750) produced before 1931. About 100 of these “Cyclones” were sold for the Navy flying boats. Technically, this “E” model, featuring 1820 in³, was an advancement.

For the US military purposes, there was a similar convention, in which this engine was referred as the R-1820-1. The “R” stands for “radial”, “1820” is the displacement (in cubic inch), and the “-1” is the sequential version number. (I suppose, that this “sequential” suffix applies to the purchasing chronology, not to the engine development).

Wright offered simultaneously two variants of the same engine: direct drive and geared. (“Direct drive” means that there was no reduction gear). The “geared” models had the “G” prefix: for example GR-1820E. According the “military” convention, each of these “parallel” versions could have a different sequential number (depending on the date of the first purchase?).

The next development stage was the R-1820F. It featured larger cylinder heads (because of the deeper and closer cooling fins), simple supercharger, forged (and then machined) main crankcase, and many other important improvements. It was difficult to find a decent photos of this model. Finally I identified one (military designation: R-1820-19) in National Museum of the USAF. It was mounted in the only preserved Martin B-10:

This particular airplane was built in 1938 for Argentina, as the export model Martin 139W. (In 1970 this only survived B-10 in the world was given to the US as a donation from the Government of Argentina. It was later restored by the National Museum of the United States Air Force). It seems that Martin used in this aircraft the R-1820-19 engines (rated at 665hp). It was the same “Cyclone” model as in the original US Army versions (YB-10 and B-10, delivered in 1934).

Looking for the decent photos of the “F” model, I finally found a few pictures of the Soviet M-25 engine. (In 1933 the Soviet Union bought a license for one of the R-1820F variants. First Soviet “Cyclones” were built in 1934 from kits delivered by Wright Aeronautical. After conversion to the metric system, from 1935 they were produced in thousands by a dedicated factory in Perm). Below you can see photos of this engine:

Wright licensed to the USSR the direct drive version named R-1820F3.

The last digit in this symbol (“3”) could indicate the blower (i.e. supercharger) gear ratio. Wright offered several variants of this engine, optimized for various flight altitudes. Each of these variants had different supercharger gear ratio. The suffix “2” means ratio of 7:1, “3” – 8.31:1, “4” – 10:1, “6” – 8.83:1. Similar engine was used in the DC-1, but its symbol had an additional “SG” prefix: SGR-1820F3. This “S” probably stands for an external supercharger, and “G” for the reduction gear.

The direct drive versions of the “Cyclone” had much shorter crankcase front section than the geared models (compare the figure above and the B-10 picture). Their oil slump also lacked the “L”-shaped forward pipe. (I suppose that it was not needed in the much shorter crankcase, without the reduction gear inside). The deflectors, attached to the cylinder heads on the photo above, have circular shape. (They differ from the rectangular deflectors mounted in R-1820-19. I will come back to this issue in a moment, discussing the “G” model). It seems that they removed corresponding side deflectors from this museum exhibit. On the crankcase front section there is a small base for the governor of a variable-pitch propeller. The rocker covers differ from the “E” model, and Wright engineers added small attachment points at their ends. (These points were useful for mounting the large NACA cowlings). Note that most of the cooling fins concentrate around the exhaust valve. There are only few of them around the intake valve.

The “F” model of the “Cyclone” was a commercial success, powering many aircraft in the first half of the 1930s (for example – the Douglas DC-2 airliner). The later versions of this engine had two-digit numerical suffixes, like “F52” or “F62”. (It seems that these middle “5” or “6” indicate an improved version). They were rated at about 745 – 785hp. The GR-1820-F52 reached 890hp for takeoff, but it was the upper limit of this design. (The F52 model had the lowest blower ratio: 7:1, and it was rated at 725hp at sea level and 775hp at 5800 ft.

The next “Cyclone” model was the R-1820G. It had larger cylinder heads than the “F” version (as explained in this article, to get higher power from a cylinder, you need the larger area of its cooling fins). I have found some detailed pictures of an early, direct-drive “G” versions in the F3F-2s restored at Chino Planes of Fame. Comparing to the original photos, it seems that this is not the R-1820G5 (R-1820-22), but another “Cyclone” version. However, it seems to be nearly identical with the original engine:

It is not clearly visible on these photos, but the intake duct in the heads is set at about 45⁰ to the centerline, as in the blueprints of my R-1820-60 version. (In the “E” and “F” models it was parallel to the centerline, as the exhaust duct). The intake valve is finally covered by short fins. The spark plugs are thinner, and placed in a less asymmetric way than in the “F” model (in fact, this head looks similar to the version that I recreated in the previous post). The top rocker covers received the new shape and four bolts around their rims. On the left photo you can see the attachment points on the rocker covers, introduced in the “F” version (they are more visible here than on the previous picture). The higher pressure produced in the combustion chambers of this engine increased the number of attaching bolts to 16 per cylinder (there were 12 in the “E” and “F” versions). On the left photo you can see the details of the “rectangular” deflectors. Note their elastic tips – I think that this brown material is the rubber (or leather?). The original R-1820-22 (GR-1820G5) engine, used in the F3F-2, was rated at 950hp for takeoff. (The same engine was used in the F2A-1 Buffalo, and the export version of this fighter, Brewster 239, delivered to Finland).

Figure below shows the later, geared version of the “Cyclone” (it was rated at 930hp at 2200 rpm for takeoff):

Note that its front crankcase section is larger than in the geared “F” model (compare it with the B-10 picture). Wright referred to these more powerful series as “Cyclone” GR-1820-G100. I studied many historical pictures of these “G” engines. It seems that in certain versions Wright placed the ignition harness in the front of the valve pushrods, while in the other versions - behind these pushrods. The semi-circular deflectors occur together with the latter variant of the ignition harness. In such a configuration every second cable goes between the cylinders to the rear spark plugs.

The “rectangular” deflectors usually occurs in the engines withe ignition harness placed in the front of the pushrods:

In such a configuration the cables go to the rear spark plug around the cylinder head (and across its deflector – as you can see in figure above). The picture above shows the later “Cyclone” G100 model, most probably one of the R-1820-5x series (I am not able to precisely determine this version). It is nearly identical with the R-1820-52 (used in the SBD-3 and -4). This photo also show us detailed fragment of the angular main crankcase section. (It was built from two symmetric, forged and machined aluminum parts). Similar crankcases appear in the “F” family. In this later model the base for the propeller governor was even more elevated than in the R-1820-45 (compare this photo with the previous illustration). I do not think that the engine from figure above uses a different version of the deflectors. I rather suppose that this particular museum exhibit uses their standard “rectangular” model with the flexible tips removed.

I have just a few pictures of the engines used in the earlier Dauntless versions (SBD-1 … -4). Below you can see two of them (unfortunately, both are in low resolution):

The engine depicted in figure “a”, above belongs to a restored SBD-1 (BuNo 1612). (When you can see the original wreck, you can be at least sure that this is the original piece). The engine in figure “b”, above, was attached to the SBD-4, restored in Chino. It seems that there are no external differences between these two engines (at least as viewed from the front). The “-32” is missing the ignition harness, but you can see in the attached miniature that the restored version of this harness is identical to the “-52” on the right. Both engines have the standard, “rectangular” deflectors with flexible tips. Note how Douglass engineers make use of these auxiliary attachment points on the valve covers (figure “b”, above). The cowling flaps bow was supported by the rear row of these points, while the supports of the NACA cowling use the forward attachments.

Why the R-1820-45 differs a little from the R-1820-52, while the R-1820-32 seems to be identical? There are two possibilities: 1. the military symbols do not correspond to the development chronology; 2. Wright run in parallel several development lines of this engine;

Both engines – R-1820-32 and R-1820-52 – were rated at 1000hp for takeoff. I have no information if there were any differences in their blower or gear ratio. The most powerful “Cyclones” G were rated at 1200hp for takeoff (the geared R-1820G5E). 

The next “Cyclone” generation was a result of significant reengineering. Wright referred them as the GR-1820-G200 series (or, skipping the “G” suffix, as the R-1820-G200, because there were no direct drive versions in this family). It seems that the first of these models had military symbol R-1820-56. One of them is the R-1820-60 (the version that I already have recreated) and the R-1820-66 (version used in the SBD-6, presented in figure below):

Frankly speaking, Wright has redesigned most of this engine, so it is easier to point out the elements that did not change between the G100 and G200 series: spark plugs and propeller governor (these two items were delivered by the third party vendors). While preserving the overall dimensions and mounting points, all of their external details are different. In the G200 the enlarged front section of the crankcase housed even bigger reduction gear and more efficient auxiliary shafts for the propeller governor. The pushrods became slightly shorter, thanks to the enlarged diameter of the front crankshaft. The cylinder heads grew bigger, because of the longer fins. (It was another increase of the cooling area - thus the shape of these heads is slightly different than in the G100 series). The deflectors are all-metal, without the flexible tips, and have different mounts among the cylinders. The cylinders are prepared for the high-octane fuel which means higher pressures, thus their bases feature 20 bolts (instead of 16 bolts in the G100 series). What is not visible on this photo, the main crankshaft section (under the cylinders) is a steel cast of gently curved shape.

In this engine you can see different auxiliary attachment points: double bolts on each intake rocker cover (the exhaust rocker covers have none). (This “bolted” mount already appeared in the late G100s models, but not in the engines used in the SBD-1 …-4).

The R-1820-60 was rated at 1200hp for takeoff. It was also used in the B-17C, D, E, and F. The R-1820-66 was rated ever higher: at 1350hp for takeoff. (Similar R-1820 version was used in the B-17G).

Finally, to increase the overall confusion (hopefully of the Axis spies ), at the beginning of 1940s Wright Aeronautical altered its naming convention. Since this time:

• The R-1820G and GR-1820G series are referred as “Cyclone” 9GA (shortcut: C9GA);
• The R-1820-G100 series are referred as “Cyclone” 9GB (shortcut: C9GB);
• The R-1820-G200 series are referred as “Cyclone” 9GC (shortcut: C9GC);

Thus you can find in various source documents and the books three different symbols for the same engine. For example – the full title of my Wright service manual from 1943 is: “Overhaul Manual/Wright Aircraft Engines Cyclone 9 GC”. This means that it applies to the R-1820-G200 series. In particular, this group includes the models named in the U.S. Army and Navy documents as the R-1820-60 and the R-1820-66.

Well, all in all this means that I have to recreate another engine: R-1820-52 (late G100 series) for my earlier Dauntless models: SBD-1, SBD-2, SBD-3 and SBD-4. There are too many differences to adapt the R-1820-60 model that I have already created. I describe shortly this “subproject” in the two next posts.

Bibliography:

1. “Parts Catalog for Wright Cyclone Aircraft Engines Series GR-1820G-200”. Wright Aeronautical Corporation, 1940;
2. “Wright Cyclone 9 Aircraft Engine, series C9-GC: Installation, Operation, and Service Maintenance”. Wright Aeronautical Corporation, 1942;
3. “Overhaul Manual Wright Aircraft Engines Cyclone 9 GC”, Third Edition. Wright Aeronautical Corporation, 1943;
4. “Operation and Service Manual: Wright Cyclone 9 Aircraft Engines Series C9GA, C9GB, C9GC”, First Edition. Wright Aeronautical Corporation, 1943;
5. Francis H. Dean “America’s Hundred Thousand: The US Production Fighter Aircraft of World War II”, Schiffer Publishing, 1997 (ISBN: 0-7643-0072-5);
6. Francis H. Dean, Dan Hagedron “Curtiss Fighter Aircraft – a Photographic History 1917-1948”, Schiffer Publishing, 2007 (ISBN: 978-0-7643-2580-9);
7. Wawrzyniec Markowski “Boeing B-17 Flying Fortress”, parts 1 and 2, AJ-Press 2004, (ISBNs: 83-7237-143-1 and 83-7237-152-0);
8. Barret Tillman “The Dauntless Dive Bomber of World War Two”, Naval Institute Press, 2006 (ISBN: 1-59114-867-7);
9. Bert Kinzey “SBD Dauntless”, Detail & Scale, 2016 (ISBN: 978-0-9860677-5-4)
10. Robert Pęczkowski “Douglas SBD Dauntless”, Stratus, 2007, (ISBN: 978-8389450-39-5);
11. Kimble D. McCutcheon “Wright R-1820 ‘Cyclone’”. Aircraft Engine Historical Society, www.enginehistory.org, 1999, revised: 2014;
12. Поршневой авиационный двигатель М-25 (Wright «Cyclone» R-1820 F3) (in Russian). Accessed 2018-07-10;
13. The Wright R-1820 “Cyclone” Engine. Acessed 2018-07-02;
14. Martin B-10. Accessed 2018-07-12;

Photo collections of:

1. National Museum of the USAF, Riverside;
2. Muzey V. P. Chkalova (Музей В.П.Чкалова), Chkalovsk;
3. Planes of Fame, Chino;
4. National Naval Aviation Museum, Pensacola;
5. Yanks Air Museum, Chino;
6. Jimmy Doolittle Air & Space Museum, Travis AFB;
7. “Life” magazine

Hi,

Thanks for the great info.  I have an old Wright Cyclone manual that I bought off eBay once while researching the Brewster F2A/B239/B339 but never really knew much about how the different models of the engines varied/changed over time.

I still can't get over how detailed your 3D models are.   

Thanks again for sharing your info.

Pat

I am really happy that you have found this comparison interesting

- indeed, it took me a couple of months of looking at the photos of this engine, to become aware of these differences...

Following the conclusion from my previous post, I have to recreate yet another “Cyclone” version: the R-1820-52, used in the SBD-3 and SBD-4. Fortunately, the R-1820-32, used in the SBD-1 and SBD-2, seems to be identical (at least – as viewed from the front), thus I do not need to recreate this “Cyclone” variant. I will describe the modeling process of the R-1820-52 in the “fast forward” mode, compressing the whole thing to two posts: this and the next one.

Initially I identified just two differences: the shape of the front crankcase section and the different ignition harness. I assumed that I will be able to reuse most of the R-1820-60 components. I had discovered most of the issues described in my previous post while working on this R-1820-52 version. In fact, it occurs that such an attempt to create a 3D model of such an engine is like an scientific experiment: it verifies the initial hypothesis and reveals the new facts that otherwise would be overlooked.

I started by renaming in the source Blender file the scene that contains the previously finished engine as “R-1820-60” (the “military” symbol of an engine belonging to the “Cyclone” G200 family). Then I created a new scene, named “R-1820-52” (the G100 family). This is my new “working place”. I copied there (precisely speaking: “linked”) some of the “R-1820-60” parts that were common for the G100 and G200 family. In this “*-52” version I followed the same “building path” which I used for the previous one. So I began with the crankcase and the basic cylinder elements:



I assumed that all the key dimensions and bases are identical in both versions, just the details are different. This assumption allowed me to determine the shape of the forged, “angular” main section of the G100 engine crankcase using just a few photos of its fragment (as in Figure "a", above). (This element is quite obscured on all the photos that I had). The nine side faces of this section had to fit the corresponding cylinder bases. The adjacent, oblique faces between the cylinders had to fit the space between cylinder bases and the front / rear plane of this central crankcase section. However, while fitting the crankcase and the cylinders, I also had found that the 16-bolt cylinder base used in the R-1820-52 had a longer straight side segment (Figure "b", above) than the 20-bolt base used in the R-1820-60. Because most of the cylinder parts were assigned to the E.100.Cylinder Base object (the “bare” cylinder), I decided to split it into the upper and lower part. The mesh of the upper part is assigned to this original E.100.Cylinder Base object, and used in both engine versions (Blender scenes). Each of these engines has its own lower part of the cylinder (marked in red in Figure "b", above). The 20-bolt version is used in the “R-1820-60” scene and named G.100.Cylinder Base, while the 16-bolt version is used in the R-1820-52 scene and named F.100.Cylinder Base.

The crankcase front section in the R-1820-52 had smaller diameter than in the R-1820-60, and different side silhouette. Thus I had to model this fragment anew. I split it into 9 identical segments, as I did in the R-1820-60. However, after some measurements, I decided that the disc that closes this crankcase from the front is identical in both versions (Figure "b", above). To avoid eventual “orphaned” objects in my further work, I used this disc the root object in the “parent-child” hierarchy of both models.

In Figure "b", above you can also see the initial versions of the pushrod bases, which I placed around the front crankcase section. They had characteristic “diamond” shapes. I recreated the “pattern” of these pushrod bases around the crankcase. This work led me to another small discovery: the G100s and the earlier “Cyclone” versions used different valve pushrod arrangement than in the G200s:



Compare the a and b distances in Figures "a" and "b", above. As you can see, there is wider space between the intake and exhaust valve pushrod (the b distance) in the earlier “Cyclone” G100 series (incudes the R-1820-52) than in the later G200 (includes the R-1820-60) series. The reverse proportion occurs between the pushrods of the adjacent cylinders (the a distance in Figure above). It seems that the pushrods in these earlier “Cyclones” were set along the radial directions, while in the later (G200) models they were set at a different angle.

There is also another difference: Wright engineers reversed in the G200s the order of the pushrod cams. In the G100s and earlier engines the base of the intake valve pushrod was shifted forward (Figure "c", above). In the G200s they set the exhaust valve pushrod first (Figure "d", above).

To match the rear rim of the front crankcase with the photos, I prepared its simplified, “block” version (Figure "a", below):



This “block” version is built of several simple elements, like the pushrod bases, the rear, flat elements, and so on. Once their shape matched the reference pictures, I joined these elements into the single, more complex object using temporary Boolean (Union) modifiers. Finally I joined it with the basic front crankcase segment (Figure "b", above). I also rounded the new intersection edges with a multi-segment Bevel (Weight) modifier.

In the G100s (incl. R-1820-52) and earlier “Cyclone” models the propeller governor was mounted at an oblique angle on a quite complex “shelf” extending from the crankcase:



I applied here the same “approximation first” method, using intermediate simplified parts (Figure "b", above). (As you probably observed, it became my usual approach to such complexities like this one). After the “fitting” phase I joined the bottom part of this “shelf” with the crankcase, and rounded the resulting edges (Figure "c", above). On top of the “shelf” there was an additional, “stacked” part (I think that it was a kind of a cover). In the pictures above I marked it in red. In the final version I left it as a separate part, attached to the crankcase by the “parent” relation.

In the background of figure above you can also see the first versions of the cylinder instances (I will modify them in the next steps), and the ignition harness manifold. (I preferred to fit it on this early stage of this the model, to avoid unwanted surprises later).

When the “shelf” was ready, I put the propeller governor in place:



I copied this governor from the R-1820-60 scene, then modified it a little (rotating the head with actuator wheel by 180⁰). Unlike in the R-1820-60, this object is set in the position parallel to the engine centerline. Looking from the front, it is mounted in an oblique position just to pass the control cable in the gap between Cylinder 1 and Cylinder 2. However, looking along this cable, I stumbled upon a new problem: it collided with the intake pipe! (Figure "b", above).

I quickly found a photo that explained me this puzzle (Figure "a", below):



As you can see in the picture above, the intake pipes in the G100s models formed large arcs, leaving the gap between the Cylinder 1 and Cylinder 2 open for the control cable. This means that I have to modify these pipes in my R-1820-52 model.

Thinking about the altered angle of the valve pushrods (see the second figure in this post) I checked the clearance between them and the cylinder head. In the R-1820-60 they were placed in deep troughs, “cut out” in the cylinder fins. I was surprised by the photos showing that in the R-1820-52 these pushrods would not collide with the cylinder head, even if this head did not have the minimal, shallow troughs. I studied this cylinder head closer: the spark plug hollows also seemed to be shallower, and the upper contour of the fins (as viewed from the front) was lower in its middle section. I started to compare proportions of these cylinders. Finally I decided that the fins in the heads of the R-1820G and R-1820G100 series were shorter than in the R-1820G200 (i.e. in my R-1820-60). I estimated that the G100s cylinder heads had 10% smaller diameter than the G200s heads. (It means that cooling area of the G200 cylinders was about 30% larger than the cylinders used in the G100s. It matches the differences in their power).

Well, now I had to apply these findings to my model:



Fortunately, the “pattern” of the cylinder fins seems to be nearly identical in the G200s and G100s cylinder heads. (I have found just a single minor difference in their forward part). Thus all what I had to do was to prepare new, smaller “fin boundary surface” (Figure "a", above), then apply it using Boolean (Intersection) modifier to the same mesh of the fin planes. I could reshape the intake pipe by altering the shape of its control curve (used in the Deform Curve modifier of the intake pipe). You can see the results in Figure "b", above).

Figure below shows the actual state of the R-1820-52 model:



Cylinders 2-9 are instances of the object group named F.G05.Cylinder. The source of this group are the components of the Cylinder 1. When I modify the source Cylinder 1, Blender immediately updates the remaining eight cylinders. Components of Cylinder 1 lie on two layers: 3 and 13, while the group instances belong to the single layer: 3. I have found such an arrangement most useful for the constant work on the cylinder details – I often did it on layer 13. Note that I also modified the bases of the intake pipes (I had a single, poor quality photo of this area). In general, it seems that the rear crankcase section of the G200s that I roughly recreated in the R-1820-60 is similar in the R-1820-52. The same applies to the magnetos and oil pump.

This engine still lacks the cylinder deflectors, oil slump, and spark plug cables. In the next post I will finish all these details and fit it into the NACA cowling.

You can download the model presented in this post (as in figure above) from this source *.blend file.

Hi,

That looks great.  Thanks for the detailed explanation of how you did everything.  I haven't experimented with Blender yet.  For most things I work on I have just been doing in ViaCAD, but its not really suited for such complex models.

Can't wait to see more.

Pat

 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.

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

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).}

Cool, thanks

Pat

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?

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.

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. 

PF

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)

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:

1. 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;
2. 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]-) 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.

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.

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

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.

Hi,

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

Pat

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.

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.

Cool,

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

Pat

Witold Jaworski

Finally (after 5 months) I received the Douglas SBD microfilms from NSAM (7 rolls).
...
I will keep you informed on the progress.

  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

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).

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:

1. when the spinner was mounted: 32’ 6.3” (+/- 0.3”);
2. 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).

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).

Wow,

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

Pat

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!

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.

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.

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.

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.

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!

This is a bit like calculating Pi to a million places.  An interesting exercise, but completely impractical.

Mark Lookabaugh

(...) An interesting exercise, but completely impractical.

Of course, you mean scale modeling, in general?

I definitely agree!

But we do not follow our hobbies for practical reasons. Think, how much we can learn along this way about the history of these aircraft, and the people who build, serviced and fly these machines?