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SBD Dauntless (from scratch)

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Truly outstanding work! Thank you for sharing your progress with us.

Thank you! In fact, I am not focused on the plastic kits. I am from another, new branch of scale modeling, that emerged during first decade of this century (see more here). The ultimate result will be a Dauntless model released on the CC license - like this P-40B model, which was already adapted as a paper model and for a flight simulator...

I would expect that the development of a highly accurate computer model could potentially have quite a number of applications. You've identified two uses: paper models and flight simulators. Another use may be computer generated artwork. Understandably, a kit manufacturer that can access a highly accurate and detailed computer model may, ultimately, develop a highly accurate kit that meets the demanding requirements of contemporary modellers.

If, for example, this model were converted into a kit, would there be any "true" expert that could criticise it having regard for the public documentation of its development? I suspect that there would be "critics" nonetheless, who by reference to inaccurate documentation you have already identified or the naturally occurring distortion within images that you have already demonstrated, may in ignorance criticise a new release anyway.

If anything, your work has opened my eyes to the limits of the available material we so often rely upon as being authoritative in our discussions concerning accuracy. It makes me wonder if the best reference material is in fact an intact example of the relevant aircraft which can be measured and examined in detail for all the subtleties that really only become evident to the mind when one examines them very closely.

Keep up the outstanding work. Perhaps in the future there will be a community of committed artists such as yourself who develop truly accurate computer models that are adapted by kit manufacturers for consumption by your audience in these forums.

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Thank you!

(...) It makes me wonder if the best reference material is in fact an intact example of the relevant aircraft which can be measured and examined in detail for all the subtleties that really only become evident to the mind when one examines them very closely. (...)

Well, I look for the original factory blueprints of various Dauntless elements, but so far I did not find any. Unable to "touch" a real airplane, I use over 1500 photos of its various parts. Half of them were made by my friend, during his trip to several air museums in the U.S.

(...) I would expect that the development of a highly accurate computer model could potentially have quite a number of applications. You've identified two uses: paper models and flight simulators. Another use may be computer generated artwork. (...)

Didn't I mention it? In fact, my ultimate result are visualizations! Usually I try to recreate something that really happened (for me this is a kind of the ultimate test: if the model looks like a real thing in the real environment, then I obtained the effect I want). Going a little bit off topic, I will show it on an example. Here is my previous model, the P-40B:


(Click here to see larger version of this image)

You can also apply an alternate camouflage: in this case it is an AVG fighter, standing on the Kweilin airfield:


(Click here to see larger version of this image)

What's more, you can make dynamic, in-flight scenes like this one:


(Click here to see larger version of this image)

I titled this one "Unexpected end of Saturday's party". These two P-40B from 47th PS are taking off from provisional Haleiva airstrip (Hawaii, Oahu island) on 7th December 1941. The 47th PS avoided the direct attack, on that Sunday. A few days earlier, they were relocated for the gunnery training to provisional Haleiva airstrip, at the north of Oahu island. Two pilots from this unit – George Welch and Kenneth Taylor – managed to return to the airfield and get airborne. (All flying staff had left this field base for the weekend, and Japanese fighters could “hunt” this day even the cars on the roads!). Both survived this day, destroying some Val diving bombers. They fly two times, that day. On the second time, they were caught by the Vals just over the Wheeler Field runway. They downed the Taylor’s machine, and its pilot got hurt. Welch shot down the Japanese airplane that attacked Taylor, and flew again over the port.

(...) Understandably, a kit manufacturer that can access a highly accurate and detailed computer model may, ultimately, develop a highly accurate kit that meets the demanding requirements of contemporary modellers.(...)

(...) Perhaps in the future there will be a community of committed artists such as yourself who develop truly accurate computer models that are adapted by kit manufacturers for consumption by your audience in these forums. (...)

Well, why do we have to wait for a kit manufacturer? Instead of being consumers, let's become "prosumers"! If you wish to make a plastic model, the 3D printing services are becoming affordable. You can build these models using the free, Open Source software: Bleder, GIMP, Inkscape. What's more, we can share our computer models - so do I, here. They are accompanied by a free "getting started" booklet, which explains how to install these software and its basic usage. Of course, a much thicker guide that describes everything is also available. So I encourage everybody who would like to try! It is a great fun! :)

Edited by Witold Jaworski
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OK, after the digression in my previous post, let's go back to the main topic.

To recapitulate my work on the Dauntless plans, I decided to draw all the external differences between its subsequent Navy versions. Because of the numerous changes that occurred in the SBD-5, I decided to split this description into two posts. This is the part one describing changes from the SBD-1 to the SBD-4. The part two (about the SBD-5 and the SBD-6) will be ready in the next week.

: All airplanes on the drawings below are equipped with the small tailwheel with solid rubber tire for the carrier operations. However, for ground airfields Douglas provided alternate, pneumatic, two times larger wheel. These tail wheels could be easily replaced in workshops.

Starting from the beginning: here is the SBD-1, the first of the Douglas Dauntless series:


(See the high-resolution SBD-1 left & top view).

US Navy originally ordered 144 SBD-1s in March 1939. The first of these aircraft took off from Douglas airfield in May 1939. However, the Navy was not satisfied with their relatively short combat radius. Probably the outbreak of the war in Europe (September 1939) forced the Navy to accept first 57 SBD-1s “as they were”, assigning them to the Marines squadrons. For the 87 remaining airplanes from the original contract, the Navy requested longer range. To improve Dauntless combat radius, Douglas installed additional fuel tanks in the external wing panels. They also equipped these airplanes with the Sperry autopilots. This new variant was named SBD-2. It was delivered in 1940 to carrier squadrons of the US Navy. Externally, the SBD-2 had lower carburetor air scoop than the SBD-1:


(See the high-resolution SBD-2 left & top view).

The next Dauntless version — the SBD-3 — was originally ordered in 1940 by French Aeronavale. SBD-3 was updated for the identified requirements of contemporary battlefield. It had armor plates protecting pilot and gunner seats, armor glass plate inside the windshield (I did not draw this and other cockpit internal details). Douglas installed also the self-sealing fuel tanks. After June 1940 all 174 ordered aircraft were taken over by the US Navy, which then ordered additional 411 airplanes. The Navy workshops doubled in these machines their rear guns. This modification was adopted by Douglas in the later series of this aircraft. Externally — the boxes containing flotation gear (“balloons”) were removed from the engine compartment:


(See the high-resolution SBD-3 left & top view).

The side slots of the SBD-3 cowling were slightly larger than those in the SBD-1 and SBD-2:


The next version — SBD-4 — received new, 24V electric installation, which allowed for installment of the radar and broader range of other electronic equipment. However, in the 1942 the Navy was short of these devices, and the factory-fresh aircraft did not have any of them. (The Navy workshops installed radars on some SBD-4s later). Externally you can recognize this version by the new Hamilton Standard Hydromatic propeller:


(See the high-resolution SBD-4 left & top view).

The previous SBD versions (-1, -2, -3) used the Hamilton Standard Automatic propeller. As you can see in drawing below, the blades of these propellers had different shapes:


(See the high-resolution SBD-4 front view, SBD-3 front view).

Below you can see another drawing of the SBD-4, consisting the bottom view as well as the side view without the NACA cowling:


(See the high-resolution SBD-4 bottom view).

Comparing it to similar drawing of the SBD-5 published in the previous post, note the different profile of the internal cowling (the cowling behind the engine cylinders). For this version I had no photo of its upper part! The shape of this element is deduced from the shape of similar part in the SBD-5 and from the size and location of the Stromberg-Bendix injection carburetor, located just behind this cowling.

Next week I will describe the external differences between SBD-4 and SBD-5. It will be the last post about the “general” reference drawings. Then I will report my progress on the first element of this model: the wing.

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In this last post about scale plans I will write about the modifications introduced in the SBD-5 Dauntless version.

For the reference, I placed below the drawing of the previous version: the SBD-4:


(See the high-resolution SBD-4 left & top view).

In February 1943 Douglas started to produce another Dauntless version: the SBD-5. It used more powerful Wright R-1820-60 engine (performing 1200 HP on takeoff: 20% more than the R-1820-52 used in the SBD-4). The engine was moved a few inches forward, and the whole area in the front of the firewall was redesigned


The old telescopic sight was replaced by modern reflector sight. The SBD-5 had heated windscreen (because it sometimes misted over in dives). (See the high-resolution SBD-5 left & top view).

The engine in the SBD-5 was moved forward by 4 inches, together with its NACA cowling. The overall shape of the NACA ring was the same as in the previous versions, except the removed carburetor air scoop. (The cross sections A are the same in both versions):


The shape of the firewall (section C in the figure above) remains unaltered. However, there is a difference in the width of the gap behind the NACA ring. In the SBD-1 … 4 this gap was relatively narrow, and the cross section of the fuselage below (section b in the figure above) forms a regular ellipse. Thus in the previous versions the upper part of the NACA ring had six flaps that controlled the flow of the cooling air through the engine. In the SBD-5 the fuselage was a little bit “thinner” here, and the bottom part of its cross section (section B in the figure above) had slightly different shape. The larger gap between the NACA cowling and the fuselage increased the constant amount of the incoming air that cooled the engine. It allowed Dauntless designers to reduce the number of cowling flaps from 6 to just 2.

The figure below reveals more differences between the SBD-4 and SBD5 engine cowling:


(See the high-resolution SBD-5 bottom view).

Some of these changes are well known, like the removal of carburetor air scoop from the top of the NACA cowling or the different shape of the side ventilation slots. However, while studying the photos, I have found two minor differences that were not yet mentioned in any source:

  • The oil radiator air scoop was in the SBD-5 was wider than in previous versions (as well as its panel);

  • The bottom seam of the NACA cowling was in the SBD-5 shifted left, while in the previous versions it was running along the symmetry plane;

Finally, I would also like to share with you my findings about the carburetor air intake in the SBD-5. As I mentioned earlier, it disappeared from the cowling, as you can see it on the front views:


(See the larger SBD-5 front view).

But where did they place this air scoop in the SBD-5? Studying the photos and descriptions in the books you can find two air intakes located between engine cylinders (as in figure a, below). However, in the original SBD Dauntless maintenance manual I discovered that the central air intake remained — just hidden under the NACA cowling:


The side air scoops were filtered, while the central air scoop was not. I used the Pilot’s Manual to find that there was a switch to flip the carburetor air intake between the filtered and non-filtered air. The filters were auxiliary devices, intended for takeoff and landing on dusty ground airstrips. (You can see similar solutions in contemporary designs from 1943: P-40L and P-51C). In the Pilot’s Manual you can read that you should switch into the non-filtered (i.e. central) air scoop to get the full power from the engine.

I must say that I was used to more streamlined carburetor air ducts. Such a location of the main air scoop is quite strange. It seems that the designers of the SBD-5 concluded that there is enough air behind the single-row radial engine to feed its supercharger. (In an airplane flying 100mph or more the amount of the air passing around the engine is several times larger than during takeoff. Thus such a solution could work if we assume that for the takeoff pilots used the less obscured side air scoops).

I did not prepare drawings of the last Dauntless version — the SBD-6. It had even more powerful engine (R-1820-66, rated 1350 HP on takeoff). Douglass built 450 of these airplanes between April and July 1944. Their radars were fitted in the factory. However, there is no external difference between the SBD-5 and the SBD-6!

In the next post I will report my progress in building the first part of this airplane — the wing.

Edited by Witold Jaworski
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I'm impressed, you're not building one model, you are actually building five!

To be frank, learning all these new methods to build a complete aircraft seems somewhat daunting. However, I realised I was looking at it the wrong way.

These methods could be used on a smaller scale by a novice to begin with. For example, designing a single component such as a missile, or a weapons pylon, a side console for the cockpit or a more accurate wheel bay. These individual components could then be printed using 3D printing technology. It's definitely something I'm keen to learn more about so I will continue following your progress.

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To be frank, learning all these new methods to build a complete aircraft seems somewhat daunting. However, I realised I was looking at it the wrong way.

These methods could be used on a smaller scale by a novice to begin with. For example, designing a single component such as a missile, or a weapons pylon, a side console for the cockpit or a more accurate wheel bay. These individual components could then be printed using 3D printing technology. It's definitely something I'm keen to learn more about so I will continue following your progress.

Definitely! You are right! Although I have not experimented with the 3D printing (yet?), I learned some details from those who do it. As every technology, this process has its own quirks. For example - heat dispersion. Like in the casting, the spatial orientation of the printed element is important. One of my colleagues told me that you have to avoid situation, when there are layers containing just one small section. In such a case the delays between two successive passes of the printing heads are too short to let the previous layer become solid before placing another layer. Another factor is small, but observable shrinking of the plastic material (after few months). To reduce this effect, you have to prepare internal reinforcements. So - the best way to start are various small elements. They allow you to learn this craft. Then you can create larger assemblies.

As I wrote, I practice another branch of this hobby - visualizations. However, the same skill in 3D modeling is required in both cases. Those, who would like to learn it, can find many tutorials and books around. For the hobbyists like me the logical choice are the free (Open Source) tools. I wrote a guide about it - just for those who would like to use these programs for aircraft modeling. Details of the process are explained in the step-by-step manner in its vol. II.

Edited by Witold Jaworski
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I started by setting up the initial scene in Blender:


Although Blender allows for arranging the reference drawings on the three perpendicular planes like in the 3D Max, I prefer the alternate way: the Background images feature. Using them, I can assign appropriate image to the corresponding view, and simultaneously use all the six views (bottom, top, left, right, front, rear). They appear just when I set appropriate projection.

This is also the moment to determine the “scale” of this model. Because in the SBD drawings that I have all the dimensions are in inches, I decided to assume that 1 unit in this Blender scene = 1 inch on the real airplane. However, I have no experience with the Blender Units setting, so I left them set to None. If you want to check details of this setup, here is the original *.blend file.

I started modeling the wing by forming the contour of its root rib. (For this purpose I draw the shape NACA2415 airfoil on the reference drawing). I smooth most of the model meshes with Subdivision Surface modifier (it uses the classic Catmull-Clark scheme). The shape of a single edge loop smoothed by this scheme is a piecewise Bezier curve (or, if you wish, a NURBS curve – this is just an alternate math representation). The edge vertices are its control points, so I can easily shape this contour. You can see the result in the figure below. (In this image you can see that the vertices lie on the rib contour, because the mesh drawing mode there was switched to draw the resulting surface):


The theoretical shape of the NACA-2415 airfoil has a thin, sharp trailing edge. However, in the real airplane it was rounded because of the technological reasons. I tried to determine its radius from the photos. As you can see in the enlarged fragment of this picture, it forms a small wedge with rounded corner. It is shaped using five vertices. (Their number corresponds the number of the leading edge vertices — I will explain the reason further in this text). The Dauntless inherited many solutions from its Northrop Delta lineage. For example — its wing spars are not perpendicular to the wing airfoil chord. Instead, they are perpendicular to the fuselage centerline. (In the SBD, like in the earlier Northrop designs, the center wing panel and the fuselage form a single unit. I suppose that it was easier to put together the wing spars and fuselage bulkheads when they shared the same technological bases).

To provide as many “technological bases” for my model as possible, the X axis of the wing object is parallel to the wing chord. I can set it “in the Northrop way” by setting the object incidence angle to 2.5⁰. In this position I can work with the wing mesh, moving vertices along the global coordinate axes (i.e. the axes of the fuselage), and then switch to the local wing object axes when needed.

In the next step I formed the basic wing trapeze. I did it by extruding the wing root edge, and shrinking the airfoil located at the wing tip:


Now you can see why I draw this wing section on the plans without dihedral. This drawing would be useless if it depicted the wing “properly”! From the reference images and descriptions it seems that the wing tip had the NACA-2409 airfoil. In the first approximation I scaled down the rib of the tip, fitting it to the reference drawing. (To fit this mesh to the front view I temporarily rotated the wing by its dihedral angle — 10⁰ 8’ — as in the figure below). However, although scaling down the original NACA-2415 coordinates produces the NACA-2409, it does not work precisely for the airfoil shape recreated with the Bezier curves. To fix these small differences I prepared an auxiliary “guide” rib of the NACA-2409 airfoil and placed it in the tip. (see the figure above). Then I modified the wing tip airfoil, fitting the wing surface to the contour of this guide rib (you can see on the picture that it minimally protrudes from the wing – as a very thin line).

Then I rotated the root airfoil, adjusting it to the wing dihedral:


In the SBD Dauntless all the wing ribs were perpendicular to the wing chord plane, except the root rib of the outer panel. To easily insert properly oriented ribs in the middle of this wing, I inserted another rib after the skewed wing root rib. It is perpendicular to the chord plane. I marked this rib edge as “sharp” (by increasing its Crease weight to 100% — you can recognize it on the picture by different edge color). In this way I ensured that the skewed root rib has no influence on the new edges I will add in the middle of this mesh.

In the Catmull-Clark subdivision surfaces, you can use the Crease weights to obtain a local sharp edge or to separate a mesh fragment from the influence of the outer mesh vertices. I learned this method from a Pixar paper, presented on SIGGRAPH 2000 by Tony DeRose. (Before I started my first model, I studied the subdivision surfaces math, to know better properties of the basic “material” used in the digital modeling).

I had an occasion to learn that it works as expected in the next step: forming of the rounded wing tip. First I inserted into the tip area a few new ribs (using the Loop Cut command). Then I started bending their trailing and leading edges, to finally join them into an arch:


As you can see in this picture, I also removed some of the internal mesh faces. I did it because I had to alter the topology of this area. (It is easier for me to determine the new faces when the old ones are removed).

Note that it was a good idea to have the same number of vertices on the trailing and leading edge. Now I can easily join them at the wing tip.

The figure below shows the resulting surface:


Note that the wing tip edge lies on the wing chord plane. As we can see from the reference drawing, in the real airplane the wing tips were slightly bent upward. We can easily obtain such an effect by moving upward (and slightly rotating) last vertices of the tip:


In the figure below you can see the control (i.e. not subdivided) mesh of this wing:


Note that I tried to align as many “longitudinal” mesh edges as possible to the stringers and spars visible on the reference drawing. This will be extremely useful when I draw skin details on the wing surface unwrapped in the UV space (for texturing).

In this source *.blend file you can check any detail of the mesh presented in this post. The next post will describe further steps of the wing modeling: separation of the aileron and forming of its bay in the wing.

This thread provides just an overall picture of the process. If you want to learn more about Blender, digital aircraft modeling and subdivision surfaces, see this guide: “Virtual Airplane” (vol. II).
Edited by Witold Jaworski
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In the previous post I have formed the general shape of the Dauntless wing. Now I will work on its trailing edge, separating the aileron and flaps. They were attached to the internal wing reinforcements. These reinforcements were distributed in parallel to the trailing edge:


In the first step I will split the wing mesh along this line. However, before I do this, let me mention a certain geometrical effect which can be surprising for many modelers. (Frankly speaking: it was also surprising for me — I knew that such an effect exists, but I thought that its results can be neglected for this wing area).

When you place on the wing a plane shaped like the "cutting line" shown on the picture above (see below, left), you will discover that the resulting intersection edge on the wing surface forms a curved contour (see below, right):


The curve on the wing tip is not a surprise, but why the intersection of the flat plane and the wing trapeze (i.e. the line between point 1 and 2) is also curved? The answer is: because this wing is like a section of an elliptic cone. The only straight line on the cone surface connects its base and apex. Any other direction (like our cutting plane) produces a curve. When the curvature of the wing airfoil on this area is low, the deviation from the straight line can be neglected. However, in this wing it produces a 0.23” deviation at the aileron root rib. You had to adapt contours of the spars and stringers used there.

Obtaining such a gently curved shape on a relatively long element is difficult from the technological point of view (i.e. costly). It can be applied if the high performance is on the stake (as in the Spitfire case). However, even the Spitfire designers had to make a compromise with the workshop and made the bottom of their wing flat. (In this way they provided a technological base).

What could do a pragmatic Northrop (then Douglas) designer in such a case? I have no direct photographic proof, but it seems that they approximated this shape with two straight segments. They are split at the aileron root section:


In the next post I will show you that in this wing each of these two segments was made in a different way. The flaps were attached to a reinforced vertical wall (a kind of a partial spar), while in the front of the aileron there was a lighter structure matching the shape of the aileron leading edge.

After these deliberations we can cut off the trailing edge from the wing:


(I did it in two steps. In the first step I created a new edge along the intended split line, using the Knife tool. In the next step I separated the rear part of this mesh into a new object).

We will deal with the red elements in the next post. In this post let’s recreate wing details along the flaps and aileron bay:


The ultimate edges of aileron bay are located a little bit further than the “reinforcement line”. I extruded them from the original mesh.

When a part of the original control mesh is removed, the shape of the resulting object can have small deviations from the original shape of the complete wing. Thus before I separated the trailing edge I copied the complete wing into an auxiliary, “reference” object. Now I am using it to ensure that all these newly extruded vertices lie on the appropriate height:


On the picture above you can see solid red areas around the modified vertex. This is the result of the approximation of the curve section (the flap hinges have to be straight lines).

To determine exact shape of the aileron bay edges I placed an auxiliary “stick” along the aileron axis, as well as some circles around it. The radii of these circles match the shape of the aileron leading edge (+ the width of the eventual gap — see picture below, bottom left). Then I set the view perpendicularly to this aileron axis object, and used auxiliary circles to determine the shape of the aileron bay edges:


Finally I closed the aileron bay with a curved wall that matches the shape of aileron leading edge:


In this source *.blend file you can check all details of the mesh presented in this post. The next post will report further progress on the wing trailing edge details (I will form and fit the aileron).

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I'm impressed, you're not building one model, you are actually building five!

To be frank, learning all these new methods to build a complete aircraft seems somewhat Dauntless. However, I realised I was looking at it the wrong way.

FIFY w00t.gif !

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In the previous post I have modeled the aileron bay in the SBD Dauntless wing. However, it was one of the cases when I followed my intuition and the mathematical precision of the computer models instead checking how this detail looks in the real airplane. So let’s do it now. I have reviewed many photos. The figure below shows the one which is the most useful (made by my friend in 2014 in one of the air museums):


We can see here that the flaps are attached (via a very long hinge) to a reinforced structure which resembles a spar. It ends at the first aileron hinge. On the other hand, the aileron is mounted on three “point” hinges which protrude from the ribs. Thus the curved sheet metal that closes the aileron bay has much lighter structure, because it is merely a cover. It is riveted to the ribs and other wing skin panels. The “sharp corner” at the upper edge of the aileron bay is obtained by a fragment of the upper wing skin that overlaps (by about half of inch) the bent, rounded edge of the internal wall.

I recreated in my mesh the auxiliary spar along the flaps and the fragment of the wing skin that overlaps the upper edge of the aileron bay:


I will model the bent upper edge of the internal wall later, during the detailing phase. The lightening holes in the spar will not be modeled. For such less important openings I will use transparency textures.

At the beginning of the previous post I cut off the wing trailing edge. Now I split it into two objects: the aileron and the flaps. Then I started to work on adapting the aileron mesh. First I simplified its topology: I slid its upper longitudinal edge forward, where the curved leading edge begins (Figure a), below). I do not need its bottom counterpart, so it will disappear. In the effect the aileron cross section resembles a triangle, as in the real airplane. (Such simplifications of the theoretical trailing edge geometry were common in this aircraft generation).


To form the curved shape of the aileron leading edge I extruded vertically from its bottom edge two face rows (Figure B)/>, above). Then I closed the remaining gap with another row of faces.

After small adjustments of their vertices at the wing tip I obtained the rounded shape of the aileron leading edge:


Then I did some further adjustments, checking if the gap between the aileron and the wing is wide enough (0.2”) for the whole aileron rotation range (from -10⁰ to +17⁰). You can see the result in the figure below:


However, comparing this result with the photos, I discovered that I fitted it too tightly! What’s more, I also noticed differences in the shapes of the aileron tip and its bay between various restored aircraft:


The outer wing panels were the same in all the SBD versions (at least their external details — see this post) — so I cannot explain these differences as the differences between various aircraft versions. Well, it seems that one of these restored aircraft was modified afterward. But which one?

Restored aircrafts are great resource of information for all modelers. However, some of them contain various modifications. Most of such differences you can find in the airplanes restored before 1990. Since that time the average level of restorations has significantly improved.

To determine which case is wrong, you have to look at the archival photos:


In the picture of a factory-fresh SBD-1 you can see that the tip of the aileron was curved. Nevertheless, I had to widen the gap between the aileron and the wing tip, reproducing the case I can see on the archival photo:


In this source *.blend file you can check all details of the model presented in this post. In the next post I will recreate the flaps.

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  • 3 weeks later...

Doctorgaz, Emvar - thank you! OK, back from my trip, I continue the work on the wing:

Perforated split wing flaps were the hallmark of the SBD Dauntless. Their inner side was reinforced by the “grid” made of stringers and ribs. Because these flaps were often wide open — during landing or in dives — I have to recreate their internal structure. In this and the next post I will describe how I did it.

All the SBD flaps had fixed chord (they were made from perforated sheet metal of rhomboidal shape). After studying many photos I assume that all their ribs have the same size and shape — also the parts attached to the trapezoidal, outer wing section. It seems that Douglas factories built all five flaps of the SBD in the same way, using unified components. The flaps for the external wing panels had to be twisted a little during riveting — most probably on appropriate mounting pads. The trailing edge of the upper flap is the trailing edge of the whole wing. It was a thin wedge, profiled from a sheet metal and riveted to the flap skin:


(Similar wedge is riveted to the upper skin of the center wing — see the picture above). The chordwise contour of these flaps looks flat on the photos. In fact there is only a small difference (less than 0.2 inches) between the theoretical contours of the wing airfoil and a straight line on the area around the trailing edge. I think that for the designers such a technological simplification was not a big deal — they had already made a more serious modification by perforating the flaps.

I started building the SBD flaps by creating their upper and lower planes. (I created them by simplification of the mesh fragment that I previously cut off from the wing). I used the Solidify modifier to give them thickness of a sheet metal. (I used this modifier for all parts which I will create in this post). Then I added the wedge (another object) along their trailing edge:


I started this wedge as a single contour, which I extruded along the whole span of the flap. Because of the trapezoidal shape of this wing, I had to twist a little the outer end of this wedge, fitting it better to the upper flap. Then I shortened the trailing edge of the bottom flap, fitting it into the wedge when it is closed.

When it was done, I added the main “spar” of the flap (in fact it was a U-shaped stringer). I did it in the same way as I created the trailing edge: shaping the profile, then extruding it lengthwise:


Once extruded, I had to rotate this object and twist its end, lying its outer edges on the inner surface of the flap skin. To facilitate this process I assigned this object a contrast, red color.

While fitting this spar, I discovered that the twisted, four-vertex face of the flap skin has small elevation along its diagonal (as in picture above). It is not something “real” — just an effect of the internal decomposition of all quads into triangles made by Blender.

To eliminate this artificial effect I had to divide this sigle, large face into several smaller pieces:


It minimized the influence of Blender internal “triangulation” and allowed me to properly fit the stringer to the flap. As you can see in the picture above, the end profile of this spar is twisted, following the twist of the flap skin.

After the first stringer I created in a similar way two other reinforcements on the flap edges:


As you can see, I used two clones of the rib contour. (I needed them to determine slopes of the front and rear reinforcements in the side view — as in picture above).

When the flap lengthwise reinforcements are in place, I can add the ribs:


All the internal ribs are clones of a single mesh. The external ribs have the same contour, but each of them has its own mesh (because they do not have the cutout for the central spar, as the internal ribs). These flap ribs have quite complex shape, but I managed to keep their mesh quite simple. It was possible, because a part of this complexity (the sheet metal thickness, rounded edges) is created by the Solidify and Bevel modifiers.

When the ribs were in place, I added the last stringer. It was a “L”-shaped beam:


Modeling internal structures of the flap forced me to carefully measure anew all of its details, especially the width and location of its spars. In the effect you can see that my wing drawings are not as precise as you could expect:


In this source *.blend file you can check all details of the model presented in this post. In the next post I will continue my work — this time on the upper flap.

Edited by Witold Jaworski
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Nice to see a fellow Blender user (and I actually got your book).. I have managed to miss this thread before, but I will follow and learn hopefully learn something. :-)

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Erik_g, thank you for buying the "Virtual Airplane" guide! Feel free to ask me (via PM or in any other means) when you have any question about this book! This thread is less detailed than that publication, but it may be useful as another example of using the same techniques...

Is this Draken from your post footer a 3D model? Looks really interesting! (It is one of the jest planes that I like much more than the others :))

Edited by Witold Jaworski
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In this post I will create internal structure of the upper split flap. Structures of both flaps are similar, thus I started this job by copying stringers from the bottom flap, finished in the previous post:


Every copied stringer is a duplicate of its counterpart from the bottom flap (I just used the negative scale: -1). I had to rotate these objects, placing them on the internal side of the upper flap skin. I copied the internal ribs in the same way (see picture below). (All of them are clones, which use the same mesh):


As you can see in the side view (see in the picture above, upper left), there is just a small vertical distance between the last ribs of the upper and lower flap (i.e. at the aileron). This is the thinnest place of this structure.

At the trailing edge of the upper flap there is the profiled wedge (I described it in the previous post). The upper flap is little bit wider (it has longer chord length than the bottom flap). Because of this the ribs of the unified size used in these flaps are too short to reach the closing wedge (see picture above).

We can observe this effect on the photos. To make these ribs longer, designers added at their ends small “U”-shaped profiles (see picture below):


I recreated these elements in my model (see in the picture above, right).

The upper flap has a cutout in its inner edge. Thus there is “one and half” of the external rib here:


I recreated this structure in my model and modified the mesh of the upper skin:


These flaps were attached to the wing by two long hinges. I recreated them as two very long cylinders and placed between the flaps and the wing:


Now, when I rotate the hinge along its local Z axis, the whole flap rotates, like in the real aircraft:


This is a preparation for the future animation of this movement (during the detailing phase).

In this and the previous post I built the split flaps and their basic skeleton. I recreated these ribs and stringers because they are visible when the flaps are extended. The additional benefit of this work was the verification of my reference drawings. (Now I know that I have to shift a little the perforation and rivet seams on both flaps. I will do it when I prepare their textures). However, on this stage it is too early to finish all remaining details of these flaps. It still may happen that I will discover something which will force me to modify the geometry of this wing and its flaps. Thus in the picture below I marked what I prefer to postpone until the detailing phase:


As you can see in this picture, I will create the openings in the flap skin later. At this moment I am going to recreate them using the same technique as for the lightening holes: textures (the bump map and transparency map). However, if this idea fails, I will model these openings in the flap skin mesh. (This method requires much more time than the textures).

In addition to these openings I will also recreate all the minor details of the flap structure. For example — I will split the “L”-shaped auxiliary stringer between the ribs. I have also to split the flap forward reinforcements into separate segments.

The complex system of the flap actuators will be also a challenge for the detailing phase (however, I already analyzed how it works).

In this source *.blend file you can check all details of the model presented in this post. In the next post I will create the fixed slats and finish this outer wing panel for this “general modeling” stage of work. Of course, I will work on it again later, during texturing and detailing.

In the next post I will add fixed slats, completing this outer wing section.

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In one of the previous posts I showed the details of the aileron bay. Now I separated the corresponding wing mesh fragment into a new object. I bent its upper edge like it was depicted on the photo:


On some photos I could see that this wall was built of two pieces of sheet metal. Their seam was located below the aileron pushrod.

The reason for such split became obvious after the comment I received from one of the readers (thank you, Brian!). It happened that a few weeks ago he visited the Yanks Air Museum in Chino, and had an occasion to examine wings of their SBD-4. He reported that while the bottom edge of the aileron bay is a straight line, the upper edge has a break at the pushrod. The difference from the straight line at this point is about 0.1-0.2 inches. Checking this tip, I examined photos of this particular SBD-4, then I verified photos of the other SBD version:


This nuance of the aileron edge is hardly visible in a perspective view. It explains why I missed it studying the photos!

Finally I recreated this detail in my model:


(Doing it, I had to modify shapes of three objects: the wing, the rear wall of the aileron bay, and the aileron).

I could not resist the temptation to recreate the rounded corner of the wing skin at the aileron root:


Frankly speaking, I should model such a thing during the detailing phase. I allowed myself to use some n-gons (faces that have more than 4 vertices) here, because this surface is flat so these n-gons will not deform the smoothed result.

However, looking on the photo above I noticed that the aileron bay edge seems to lie on the same line as one of the rivet seams on the flap. (The seam that runs along the rear edge of the hinge reinforcements). So it was on the reference drawing. However, do you remember that I had to modify these flap reinforcements, shifting them forward (in this post)? So now I know that this rivet seam is in another place on this flap, different from the place where you can see it on my drawing. Now I have to update accordingly the location of the aileron edge!

To preserve its vertical shape, I did it by two rotations: first I rotated it along Z axis:


Then I had to make a small rotation along Y axis (along the same pivot point), elevating these faces back onto the wing surface.

The updated layout of the flap ribs and struts means that I will have to move forward not only the rivet seams, but also the rows of the circular openings placed on the flaps (I mentioned it in one of the previous posts). What’s interesting, the auxiliary “L”-shaped stringers on the upper and lower flap have different chordwise locations. In the result, the last row of the holes in the upper flap does not match its counterpart on the bottom flap (see picture above).

The last detail I will recreate during this stage of work is the fixed slat. It requires six openings in the wing skin: three on the upper surface and three on the bottom surface. I did not modify the wing mesh for this purpose, because additional edges around these openings would seriously complicate its topology. I decided to create them in another way: it may happen that ultimately I will make these holes using transparency textures, but for now I will do it using the Boolean modifier. First I prepared an auxiliary object — the “cutting tool”


I set the wing as its parent, and placed on a hidden layer. Then I used a Boolean modifier to dynamically cut out these openings in the wing:


Note that I placed the Boolean modifier after the Subdivision Surface modifier, to cut these holes in the resulting, smooth wing surface. As an additional bonus, this modifier also creates their internal walls (they come from the auxiliary object).

Although the “rib” walls obtained in this way are OK, I decided to create the front and rear walls of this slat as a separate object. Why? Because it is easier to modify its shape when it is not split into three “boxes”, as the “cutting tool” object is:


I will join all these internal faces of the slats during the detailing phase. Currently I am leaving them in the current state, just in case I will have to modify the wingtip geometry.

This was the last element of the outer wing panel I wanted to create during the “general modeling” phase. I will recreate all of remaining parts (landing light, approaching light, Pitot tube, aileron axis arms, etc.) later, during the detailing phase.

In this source *.blend file you can check all details of the wing presented in this post.

Note: When you open this file, the Boolean modifier may not work properly. The slats will appear when you enter the Edit mode of the wing object, then switch back to the Object mode (i.e. select the wing panel and press twice the [Tab] key). It seems to be a minor bug in Blender: it happens when the object having the Boolean modifier is simultaneously the parent of the “cutting tool”. (More on various modeling issues you can find in Vol. II and Vol. IV of the "Virtual Airplane" guide).

In the next post I will start working on the centerwing. It will be occasion to find another parent for the “cutting tool” object, resolving the issue of disappearing slats.

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On the first glance the SBD center wing section seems to be a simple rectangular (i.e. constant chord) wing, with modified leading edge:


However, the landing gear openings visible on the photo can be difficult to recreate in a mesh smoothed by the subdivision surface modifier.

Additional photos from one of the SBD restorations made by Vulture Aviation in 2012-2013 reveal that the fuselage was mounted on the top of the wing (see the a) picture below):


A part of the upper wing surface was simultaneously the cockpit floor. Note the rectangular cutout in the middle of the leading edge. The SBD had a small window on the bottom of the fuselage, in the space between the two root ribs.

On the photo of the bottom of this wing (as in the B)/> picture, above) you can see that these root ribs had a modified airfoil shape: it bottom contour has a straight edge from the leading edge to the main spar.

I started to form the center wing section by preparing the single curve of its external rib. (I copied it from the root rib of the wing reference object, which I used during modeling of the outer wing panel):


I think that creation of these large landing gear bays (as the first picture in this post) will require a lot of modification in the wing mesh. Thus I decided to separate the mesh fragment that contains these openings (from the leading edge to the main spar) into a separate object. (It is always easier to modify topology of such a medium-size mesh part, than the whole wing). To ensure a smooth, invisible seam between this forward and the rear part of the wing, I had to accordingly prepare the control polygon of the initial airfoil. I added an additional point on each side of the vertices located above and below the spar line:


What’s important, such three points have to be collinear. The resulting subdivision surface “touches” the middle point of such a fragment of the control polygon, and it is tangent in this point to these two adjacent control polygon segments. (This is just one of the mathematical properties of the Catmull-Clark subdivision surfaces, which are implemented in Blender).

However, these four new control points altered the shape of the airfoil curve. Now I have to fit this shape to the original NACA-2415 airfoil of the outer wing panel:


Fortunately, the Catmull-Clark curves/surfaces have another property similar to the NURBS: so-called local change. Their formula ensures that influence of a single control point does not exceeds two subsequent segments of the control polygon (two segments in both directions — see picture above, right). It is easier to focus on the modified mesh fragment, when you know this rule.

Once the initial rib shape fits the outer panel, I can extrude it forming the center wing section:


To shape the leading edge I had to stretch a little bit the forward part of this mesh. As you can see (in the picture above), I placed this new edge loop in the place of the wing root rib.

However, comparing the resulting object with the photos I discovered that the leading edge of the center wing section should have constant radius (at least approximately):


In this way I have found another error in my reference drawing: the wrong shape of the root airfoil:


The tangent direction at the wing spar differs from the direction estimated on the drawing, thus the bottom, straight segment of the root airfoil has a slightly different slope. The leading edge is much thicker than I draw on these plans.

Adapting the well-known von Moltke’s sentence: “no plan survives contact with the enemy” to this situation, we can say that “no scale plans survive contact with their 3D model”. :)/>

I created a first approximation of the main wheel (it lacks the details) to check if it fits into the space between the leading edge and the main spar:


When I was sure that the shape of the wing is OK, I separated the forward part of this mesh (by splitting it along the main spar). As you can see (in picture above, right) these two parts join in a seamless way. It was quite simple to prepare such an effect in the initial curve by adding additional control points (as I described in the beginning of this post). It would be much more difficult to introduce similar modifications into the extruded mesh.

If you want to learn more about properties of the Catmull-Clark subdivision surfaces, as well as the details of the modeling workflow, see Vol. II of the “Virtual Airplane” guide.

In this source *.blend file you can check all details of the wing presented in this post.

In the next post I will create the opening for the landing gear.

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In this post I will cut out the opening of the landing gear bay in the wing. In the SBD Dauntless its shape consists a rectangle and a circle:


However, when you look closer, you will notice that the contour of the main wheel bay is not perfectly circular. There is a small deformation of its shape on the leading edge (see picture above). I think that it looks in this way because of the technological reasons. Another feature of this opening is the fragment “cut out” in the bottom part of the fuselage, below the wing. (We will make it when we will form the fuselage).

I started by applying all the information that was confirmed by the general arrangement drawing and various technical descriptions: the main wheel used 30”x7” tire. Its center was placed 18.5” from the firewall (measured along the global Y axis)


The X coordinate of the wheel center can be determined by the location of the root rib (10”) + small gap + tire radius (30”/2) ≈ 26”.

Then I tried to put around the main wheel a test contour of the opening in the wing:


Initially I thought that I will recreate this opening by embedding a subdivided octagonal hole in the wing mesh, as I did in my P-40 model (see Vol. II of the “Virtual Airplane” guide).

A subdivision curve based on an octagon produces nearly perfect circle. It does not matter if vertices of this octagon lie on different depths — as long as they form an octagon in the vertical view, the curve based on such a control polygon looks like a circle in the vertical view. (The mathematicians call this property “projective invariance”, it also applies to the NURBS curves). When you know it, it is much easier to model various mechanical shapes.

However, when I created an appropriate octagon around the wheel, I discovered that one of its vertices lies outside the wing mesh (see figure a), above). You cannot compose such a contour into the wing. Therefore I decided to create this opening using another Boolean modifier, as I did in the case of the fixed slats (described in one of the previous posts). I prepared the basic contour of the “cutting tool” — a smooth circle based on a 16-vertex polygon (as in figure B)/>, above).

The fragment of the main wheel opening that “touches” the wing leading edge seems to be flatten a little (see the first picture in this post). To obtain such an effect I rotated the “cutting tool” object (the ring) by a few degrees so its Y axis was perpendicular to the leading edge. Then I shifted a little the single edge of this ring along the Y axis, fitting it into the wing:


By small movement of these two vertices I was able to precisely recreate the shape of this opening visible on the photos:


If I do not want to get the inner part of the “cutting ring” inside the resulting opening, I have to assign to this wing mesh a sheet metal thickness (using the Solidify modifier – as in picture below):


Because the forward and rear part of the wing are separated, I can use this Solidify modifier only in the front part. In this way I do not increase the polygon count of this model with unnecessary faces.

As you can see in the picture above, I also created a second “cutting object” — a box. I will use it to recreate the rectangular opening around the landing gear leg. Both of these tool objects are located on a single layer (9) which will be hidden during rendering. Their parent is the rear part of the center wing section (to avoid dependency conflict with the front part of the wing).

Finally I assigned both of these “cutting” objects to the Boolean (Difference) modifiers of the wing skin (The same method as used for the fixed slats). You can see the result in picture below:


It would be quite difficult to recreate such an opening by altering the control mesh of the wing skin. It also would make its shape more complex, and difficult to unwrap in the UV space (for the textures).

The openings created by Boolean modifiers have another advantage: it is very easy to modify their contours. I had to do this just after I created these holes. I discovered that I made minor error in the reference drawing: the landing gear leg opening should lie a little bit back. (Its centerline should pass through the landing gear wheel center


All what I had to do was to shift back the auxiliary box object, which creates this opening. So easy!

On the other hand, I observed small shadows caused by triangular faces created by the Boolean modifiers along edges of this opening. It was impossible to remove them in the typical way — using the Auto Smooth option or the Edge Split modifier. The only solution was to increase (from 2 to 4) the level of the Subdivision Surface modifier assigned to the wing surface object. It increased 16 times the number of resulting smooth faces created from this mesh. Fortunately, I split the wing into two parts, so I could set keep such a dense mesh only around the area where it is needed.

In this source *.blend file you can check all details of the wing presented in this post.

In the next post I will create main spars and ribs, visible inside this opening.

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Inside the Dauntless landing gear bay (which I cut out in the previous post) you can see fragment of the wing internal structure. Because I plan to create this model with retractable landing gear, I have to recreate these details. During this “general modeling” phase I will create here just the few key ribs and spars. I will show it in this post. The remaining details have to wait for the detailing phase.

Examining the photos I identified two auxiliary spars and three ribs as the key elements of this structure:


The spar is relatively simple to recreate. Initially I created a rectangle. Then I split it into six faces. Then I removed one of these faces, creating the space for the wheel bay:


Then I added flanges, rounded corners, and the sheet metal thickness, to give this spar a more realistic look, as you can see in picture "a", below:


However, when you examine this object, you will discover that the mesh of this spar is very simple (as you can see in picture "b", above). I obtained all these effects using a Solidify and two Bevel modifiers. It even did not require any special smoothing (I did not use the Subdivision Surface modifier here).

In the same way I created the second spar:


While working on these spars I also decided to recreate the wing skin that covered the gap between the main spar and Spar 1:


It would be possible to shape such a hole by altering the wing mesh. However, if I already used the Boolean modifier in this object to cut the opening for the landing gear, it was much simpler to extend it for this purpose. Thus I extruded the whole leading edge section, up to the centerline. Then I cut out a part of this newly created surface using the modified “cutting tool” object that I used to form the landing gear bay (as you can see in the picture above).

The mesh of the wing skin already contains a “rib” edge loop in place of the root rib (see picture "a", below). Thus it was easy to duplicate this edge into a new object, and extrude it by an inch into a flange. I offset this flange by a metal sheet thickness, placing it below the wing skin. (I did it by applying a temporary Solidify modifier — in Blender it produces better results than the Offset command). Finally I created faces between the vertices of the upper and lower rib edges (as in picture "b", below):


As in the spar, the rib object uses a Solidify modifier to recreate the sheet metal thickness and a Bevel modifier to round flange edge. It also uses Subdivision Surface modifier to fit it tightly into the wing.

A new rib that fits a trapezoidal wing segment requires somewhat more work. To create it, I prepared auxiliary “cutting tool”: two parallel planes (as in picture "a", below):


I used this helpful Intersection Blender add on (I created it for similar purposes) to find the intersection of these two planes and the wing mesh. I separated the result of this operation — two edge loops (see picture "b", above) into new object. Then I continued as in the case of the previous rib: created the flange (see picture "a", below) and offset it from the wing skin. In this case I had to modify the bottom part of the rib, creating space for the wheel bay. Finally I created the vertical walls (see picture "b", below):


In similar way I created another rib. You can see the result in the picture below:


At this moment I am not recreating in this structure the lightening holes — I will obtain this effect using bump and transparency textures.

I mentioned the “textured holes” many times in the previous posts. However, I will apply it much later, during the texturing phase. Thus, if you want to find more about this particular method, you can find its detailed description in Vol III of the “Virtual Aircraft” guide. (It is an introduction to materials and textures).

In this source *.blend file you can check all details of the wing presented in this post.

In the next post I will create the remaining elements of the wing.

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