SBD Dauntless (from scratch)

[Witold Jaworski]

To build the model from scratch you need a good reference. Initially I decided to use for this purpose detailed scale plans from the monograph published by KAGERO in 2007 (Authors: Krzysztof Janowicz, Andre Zbiegniewski, ISBN: 978-83-60445-25-9). It contains SBD Dauntless plans in scale 1:48, traced by Krzysztof Lukasik. They are quite detailed (up to the rivets on the aircraft skin). Apparently these drawings were made using Corel Draw or similar software. I scanned these drawings to do the basic verification. During this phase I did not find any flaws:



All the key locations of the fuselage are in the same place in the side view and the top view. The proportions of the length and the span of the top view is correct: 0.787. (This ratio comes from 996/1266. According the dimensions specified in this monograph, the length of the SBD-3 fuselage was 996 cm [32’ 8”], while the wing span of all the Dauntless versions was 1266 cm [41’ 6”]). This good impression disappeared, when I compared side views of two different Dauntless versions: SBD-3 and SBD-5:



According the monograph data, SBD-5 fuselage was 4 inches (about 10.1 cm) longer than SBD-3. (The SBD-5 and SBD-6 fuselage was 33’ long. Most probably it has slightly different engine cowling and the propeller. The airframe after the firewall was the same in all Dauntless versions). However, in this monograph they have the same length!

Maybe the textual data contains an error? In such a situation I try to find an “official”, archival drawing of the aircraft. They do not show many details, but contain the key dimensions. I have found on the Internet a BuAer Navy drawing of the SBD-5, from 1944:

From the front view you can read the precise wing span: 41’ and 6 5/16“. From the side view you can read the exact length: 33’ and 1/8“.

This BuAer drawing isn’t an ideal source: it does not contain such details as panel seams. You can also find here some manual errors, made by its draftsman. While the aspect ratio of the top view matches the span and length specified in the dimensions, the actual fuselage length on the side view is somewhat shorter. (The positions of the wing and horizontal tailplane match in the side and top view match each other. It seems that the part of the vertical tail contour was shifted). On the other hand, the BuAer top view is a little bit asymmetric, and the firewall line is moved forward a little.

The good news is that the wing and the tailplane arrangement on the KAGERO plans and the BuAer drawing match each other:


Then I compared the side views of these drawings (I marked the correct fuselage length measured on the BuAer top view in red):


The differences of the side views are overwhelming: this is not only the engine cowling but also the cockpit canopy, the fin, and the tailwheel. (In general: none of these drawings shows the correct tail).

Thus, I can conclude: never trust the scale plans! I need a better reference, to fix these drawings.

In the next post I will show how I verify these drawings using photos.

Note: for editing these images I use two free, Open Source programs: GIMP (it is similar to Adobe Photoshop) and Inkscape (it is similar to Corel Draw or Adobe Illustrator). You can find more about them in this e-book.

[Tailspin Turtle]

One problem that you sometimes encounter with the SAC drawings is that the views were not reduced by the same percentage. You therefore have to check each view against the same dimension on another. A second problem is that in the reproduction process, at some point the illustration may have been "stretched" or "shrunk" in one dimension relative to the other. That means you have to check both the length and the height of the side view against published dimension, for example.

[Witold Jaworski]

Definitely! Fortunately, I did not find such deformations in these drawings.

However, there is another "trap": Let's assume that we have only the SBD-5 drawings from KAGERO. if you try to apply the length of the SBD-5 (33' ¹/₈") and the span (41' 6 ⁵/₁₆") from the BuAer dimensions to KAGERO drawings, you can come to conclusion that they are deformed during the printing process, because the airplane seems too short on both: the side view and the top view! In fact, as in every experient, you can avoid such wrong assumptions by an additional verification against other drawings.

On the other hand, sometimes the published airplane dimensions differs between various sources. Strangely enough, it often happens to their lengths (the span is more "stable"). I will write about it in one of the next posts.

All of this resembles me the typical work of a historian: reconciling the information from various sources, to find the real state of the things ....

Edited November 30, 2015 by Witold Jaworski

[hendie]

I'm looking forward to this - this looks like the kind of thing I could get my teeth into!

[Tailspin Turtle]

Some more thoughts on drawings: http://tailhooktopics.blogspot.com/2012/05/accurate-three-view-drawings.html

I've got a pretty good Douglas drawing for an SBD desktop model but unfortunately I'm away from home through the first of the year.

[GAF]

Having been a draftsman in the era before CAD, I can say that you should not trust detail drawing views. Trust only the dimensions, and double check those.

That's why I cut manufacturers some slack when it comes to the accuracy of their models, as the information they work from can be quite misleading.

And don't expect CAD to be any better. GIGO.

Edited December 1, 2015 by GAF

[Witold Jaworski]

I'm looking forward to this - this looks like the kind of thing I could get my teeth into!

Great! I will do my best in this project!

You can also download my previous models. I created them in the free (Open Source) software. They are accompanied by a tutorial that describes basics of their use.

Some more thoughts on drawings: http://tailhooktopics.blogspot.com/2012/05/accurate-three-view-drawings.html

I've got a pretty good Douglas drawing for an SBD desktop model but unfortunately I'm away from home through the first of the year.

Thank you for this article! I read it with great interest - it seems that in parallel I came to the same conclusions.

During nearest weeks I will show in this thread how I use information from such references as SAC (I called them "general arrangement") drawings, as well as some other methods, to create decent scale plans (in fact, finally I will create my own drawings of the SBD). Then, in the effect of building the 3D model, I will discover most of their flaws .

Having been a draftsman in the era before CAD, I can say that you should not trust detail drawing views. Trust only the dimensions, and double check those.

That's why I cut manufacturers some slack when it comes to the accuracy of their models, as the information they work from can be quite misleading.

And don't expect CAD to be any better. GIGO.

Absolutely! In general we can trust the explicit dimensions, but it is even better when they are repeat on two or more drawings... For example: I remember the issue of the difference in the horizontal tailplane span in the P-36 and the P-40. It is dimensioned on the Curtiss arrangement (SAC) drawings. In general the P-40B,C is another version of the P-36, equipped with the Allison inline engine. Surprisingly enough the span of the tailplane in the P-36 is dimensioned as 13', while in the P-40 it is minimally smaller: 12' 9 ⁵/₈". In fact, they differ in the elevator details. The elevator in the P-36 had both: horn counterbalance at the tip and the tab counterbalance along whole hinge line. In the P-40 it had only the horn counterbalance at the tip. Thus in the P-36 the elevator hinge was mounted on small external arms, while in the P-40 these arms were hidden inside. However, I doubt that these differences have such an infulence on the tailplane external shape: usually the manufacturer tries to use as many of the parts from the previous version (ribs, spars, frames), as possible. I compared several available photos and still have impression that the real span of the P-36 tailplane was as specified in the P-40 drawings (12' 9 ⁵/₈"). I think that these rounded 13' in the P-36 general arrangement drawig aremade just by a careless draftsman..

I also agree that the tool (CAD or any other computer system) won't help when you are using simplified/wrong reference. I am afraid that even the model manufacturers have certain time restrictions which do not allow their staff to make the full research of the airplane they prepare.

Edited December 1, 2015 by Witold Jaworski

[Witold Jaworski]

Before I start a new model, I collect its photos — as many as I can find, everywhere: in the books, magazines, on the Internet. Some of these photos are high-quality, detailed photos of restored airplanes. One of them is this a high-resolution photo from the web page of Chino Planes of Fame Air Museum:

This is a special photo: it was made from a long distance using a “telescope” lens, which minimized the perspective barrel distortion. The airplane on this picture lowered its right wing, so its bottom parts are slightly shifted downward, but except this area it is a perfect reference!

I placed this photo in Inkscape, and set it horizontally (along the canopy frames). Then I mirrored it, for the comparison with the left side view:

In Dauntless there are two long parallel lines that were perpendicular to the fuselage centerline: the trailing edge of the wing center section, and elevator leading edge. On this photo they are also parallel (more or less). This is the proof that we can neglect the perspective (barrel) distortion.

Now let’s compare the BuAer drawing (see the previous post) with the reference photo:

In the first post I mentioned that this BuAer side view is too short. To make a fair comparison, I marked on this drawing the proper fuselage length (as on the BuAer top view). As you can see, the drawing matches the reference photo quite well!

It fits the photo even better when you correct the tail contour (so it matches the fuselage length in the BuAer top view):

It seems that the BuAer drawing from 1944 matches the contour of the real aircraft quite well. In fact, it is much better than the contours of the detailed KAGERO drawings from 2007 (see previous post), which most probably are based on the drawings made by previous authors:

I think that these KAGERO plans “accumulated” many decades of various errors. Do not be surprised: before the 1990 it was practically impossible to make such a “photo verification” like this one. Even today authors are used to redrawing earlier plans. They seldom compare their work with the real photos in the manner shown above.

Concluding: there is no good reference among the existing Dauntless drawings: the BuAer lacks details, while the KAGERO plans contain too many deviations. The plans from other authors have similar errors (I will not elaborate about it here).

It seems that I have to crate my own drawings!

In the next post I will show you the corrected side view.

[Witold Jaworski]

As I wrote a few days ago, I am working on a better drawing of the SBD-5. It is based on more than 1000 various photos. Below you can see the first version of the side view (click it to see the high-resolution version):

This is not an ultimate drawing: I suppose that it will be updated during my work, following the new findings about the airframe shape and/or details. The dotted lines mark the rivet seams, but size and spacing of these dots does not match the real rivets. I prepare these plans to build a model: that's why I removed the outer wing section and horizontal tailplane. For these parts the most important drawing is the top view. To build them, on the side view I need the precise contours of their key sections (i.e. their airfoils as well as the incidence angles and spar locations). I draw three profiles: first of the wing root, then the root of the outer wing section, and then the wing tip. Two different sources specifies different wing tip profiles: NACA-2409 (Performance Test Report, 1942) or NACA-2407 (BuAer drawing, 1944). However, the bottom contour of the NACA-2407 seems to be a little concave. Because I did not observe such an effect on the photos, I decided to use the thicker airfoil of NACA-2409. I still have to verify this detail when I build the wing. The airfoils of the tailplane were specified nowhere. I copied its root airfoil from a photo.
While drawing the side view you still have to think "in 3D". That's why you can see around this silhouette some auxiliary sketches: the front view of the engine cowling, and the contours of the center wing section. I draw the latter element just to mark the exact position of the first rib of the wing. It was hidden inside the fuselage. Note that this airfoil was a specific modification of the NACA-2415 shape: the part of the wing that houses the main landing gear was reshaped. In the effect, the leading edge of the center wing section has a small downward inclination.

Within a few days I will present you side views of two earlier Dauntless versions: the SBD-2 and SBD-3. I will discuss where are the differences in the length of the SBD-5 and SBD-3, 4, as well as the mystery of the "missing 7 inches" of the SBD-2.

In the next week I will present here the corrected/verified top view.

[Witold Jaworski]

In this post I will show you how do I create Dauntless side views. First I used the “semi-orthogonal” photo of the SBD-5 as the reference to draw the side view of this version. This is the most important picture, because it provides reliable “general reference”:


Then I used many other photos and sketched fragments of the other views to complete the side view details (note the multiple guide lines on these pictures):


Note the large B/W photo that I used to verify the shape of the propeller hub and engine cowling. I could not compare it with other areas — the cockpit canopy, for example — because its perspective (barrel) distortion was too intense.

However, when the barrel distortion is moderate, we can revert it! See for example this side photo of another Dauntless version: the SBD-3:


First I identified the undeformed fragment of the fuselage (in this case — around the firewall and the windscreen) then fitted this part of the photo into the drawing. Then, comparing the lengths of this photo and the side view, I concluded that it has a moderate barrel distortion. From the history of this design I know that the SBD-3 and SBD-5 had different engine cowlings. The other parts of their airframes had the same shape. This means that I could use the existing SBD-5 drawing from the firewall to the fin as the reference for the unwrapping process of this SBD-3. Then I unwrapped this photo using the GIMP. (Speaking more precisely – its “Lens Distortion” image filter. You can find all the details of this process in this book).


Note that this operation “flattens” only the airplane contour that lies on the symmetry plane of the fuselage. All protruding elements, like wing and horizontal tailplane remain deformed. But it’s OK, I need this just this contour. While drawing, I will compensate the small remaining deformation of the bulkhead lines. (For example, the leading edge of the NACA cowling from this photo should be a straight line, but it is a very flat ellipse).

Of course, it is always better to prepare more than one of such “flattened” pictures, to minimize inevitable errors:

NOTE: these two SBD-3 photos depict training aircraft, without the telescope gunsight. Such a gunsight was protruding through the windscreen in the combat airplanes.

On both SBD-3 photos you can see that the engine cowling is somewhat shorter than in the SBD-5. In fact, in the specifications you can find that these versions had different overall length:
- SBD-5: 33’ ¹/₈”;
- SBD-3: 32’ 8 ” (in some sources I also saw 32’ 8 ⁴/₅”)
However, on the scale plans authors attribute this difference to the longer propeller hub of the SBD-5 (It used different propeller: Hamilton standard hydromatic). Others did not bother about the different lengths of the SBD variants, and draw the profiles of all Dauntless versions alike.

Following the findings on the unwrapped photos I analyzed many other archival pictures. Below you can see the conclusion:


It seems that in the SBD-5 the engine, together with the NACA cowling, was moved slightly forward. All other bulkheads remain in the same places. This modification shifted forward the center of gravity. I suppose that this correction improved some handling characteristics that changed after the doubling of the rear guns. (The second gun in the rear was introduced in 1942 to the SBD-3 as the “field” modification. It shifted the cg backward).

In my next post I will finish the side view matter, delivering you the complete side views of the SBD-3, SBD-2 and SBD-1 variants. I will also write about other non-existent length difference, which you can find in the books about the SBD Dauntless.

[Giorgio N]

It's very fascinating to see the process you're going through, thanks for sharing !

[Witold Jaworski]

Giorgio N - thank you!

______________________________________________________

In addition to the side view of the SBD-5 presented in one of my previous posts, I have prepared side views of the earlier Dauntless versions: SBD-2 and SBD-3:

(Here are the links to the high-resolution profile images of: SBD-2, SBD-3).

When you look into Dauntless specifications, you will find that all its models have the same span, but they often differ from each other in the length. This is a typical case, because the wing geometry determines the aircraft behavior. Thus, once “debugged” in the prototype (the stall characteristics etc.) it remains unaltered between subsequent versions. The fuselage shape is not so important, so it is often modified. In the effect, the length of the airplane often vary between subsequent versions.

In the previous post I described how the photos confirmed the different length of the SBD-5 (33’ ¹/₈”) and the SBD-3 (32’ 8”), listed in their specifications. The reason was the different engine mount, modified in the SBD-5. The same sources specify the length of the SBD-2 as 32’ 2”. This is something strange, because I cannot find any evidence of this significant, 6 inch difference between SBD-3 and SBD-2 on the photos!

The SBD-3 was a “quick and dirty” adaptation of the SBD-2 to the recognized requirements of the modern war. Douglas added armor plates to the pilot and gunner seats, self-sealing fuel tanks (reducing their capacity), doubled the rear guns. All the key elements of the design: the airframe and the engine, remained the same. Where is there the modification that changed the overall of length of the SBD-3 by 6”!?

I started to look for the sources of this information (the subsequent publications copy their specification data from the earlier ones, up to an ultimate source document). Ultimately it seems that it comes from the BuAer Performance Data Reports. There are two of such documents, created in 1942: one for the SBD-2 and one for the SBD-3. On their last pages you can find the measured airplane dimensions. The difference is there: LENGTH, LEVEL: 32’-8” in the SBD-3 report, and LENGTH, LEVEL: 32’-2” in the SBD-2 report. (Unfortunately, they did not specified the length on wheels for the SBD-2, so there is no double-check). Note that all other dimensions are the same. I speculated that the reason of these differences lies in the propeller spinner: it was often removed. If the tested SBD-3 had this spinner, and the SBD-2 didn’t — what was the eventual difference? I tried to check this option, but it shortens the fuselage length by less than 4”.

What’s more interesting: the only survived SBD-2 is owned by the National Navy Aviation Museum in Pensacola. On their web page the owner specifies the length of this airplane as 32’ 8” — the same as the SBD-3! Thinking further about it, I noticed the manual corrections of typing errors in other SBD performance data reports. So I have following hypothesis:

• The SBD-2 and SBD-3 had the same length: 32’ 8”, as specified by the owner of the restored SBD-2 (NAM in Pensacola);
• The typist of the BuAer Performance Data Report made a mistake (most probably —deciphering the handwritten measure results he/she read “2” instead “8”). The authors of the first publications about SBD Dauntless used this source, and the others used their publications. So the initial error was multiplied;

Thus I assumed that the SBD-2 length specified in Performance Data report is wrong. Basically it was the same as the SBD-3. It also applies to the SBD-1:

(Here is the link to high resolution profile image of the SBD-1). The only external difference between the SBD-1 and SBD-2 is the larger air scoop on the top of the engine cowling.

Conclusion from this little investigation (in fact, it took me a few days): do not trust blindly the width and wing span of a historical airplane specified in the books! When you compare the different sources you will find that sometimes these figures are different. Always try to verify available data. The wing span is less error-prone because it usually does not vary between subsequent versions. As GAF mentioned in one of the previous posts in this thread, the most realiable are the explicit dimensions from the original general arrangement drawings. If you have no such source - remember that the photos are always the ultimate evidence.

In the next post I will present the updated/verified Dauntless top view.

[Witold Jaworski]

I finally got the “Instructions for the Erection and Maintenance of the Model SBD-6 Airplane” book – more than 600 pages about the Dauntless, published by Douglas in March 1944. Because of the lengthy title, I will refer to this book as the “SBD Maintenance Manual” or the “Douglas manual”. In spite that it describes the last produced version, it is also usable for the earlier models: as I mentioned in one of the previous posts, the SBD-1 airframe behind the firewall differs only in a few details (the double gun mount, gunsight type, lack of the YAGI antennas) from the SBD-6.

Inside you can find the SBD-6 general arrangement drawings, as well as the stations diagram:



Here are the links to the high-resolution versions: side view (cropped from the page), top and front view, stations diagram. As you can see these Douglas diagrams contain more dimensions than the BuAer drawings. Their chains on the side view allow for verification of the wing location, as well as the wing and tailplane incidence angles. They also allow you to determine the basic “trapeze” around the rudder and the fin. From the front view you can also read the dihedral angle of the outer wing panels (9⁰ 19’).


The dimensions from the top view allowed me to draw the basic trapezes around the wing and tailplane, as well as to determine locations of the aileron and elevator hinge axes. This information, combined with dimensions from the side view, allows for determining the precise location of the firewall, wings and empennage. I used them to verify my scale plans. Sometimes they just confirmed what I determined before (for example — locations of the wing or the last bulkhead). Sometimes they revealed the errors I made. I will write more about it in the next post. So here is the current, updated version of my drawings:



Because of the formatting issues I had to split this image into two parts:



(Click here to get these drawings as a single, high-resolution image). Note that I draw the outer wing panel without its dihedral (it is much easier to build its model using such a “flat” reference). Thus when you check proportions of this top view, its span/length ratio is somewhat greater than the expected value of 41’ 6” / 33’. What is interesting, the dimensions on the general arrangement drawing indicate that the “official” wing span does not include the size of the running lights:



To obtain the “physical” wing span value you have to add 1.5” to each wing. I used similar convention when I matched the fuselage contour against its dimension (33’ ¹/₈”). These dimension lines are more obscured on the side view, but for the matching purposes I skipped the length of the running light cover protruding from the tip of the tail (1”).

In general, after all these updates I feel more confidence in my drawings. I know which elements come from the explicit dimensions of the general arrangement diagram, which from the photos, and which are based on other drawings or just on an assumption. The only larger element that I was not able to verify is the fuselage width (i.e. its contour in the top view). It is copied from the Douglas drawing. I was able just to verify it at the 9^th bulkhead (station 140). I have a photo of this bulkhead from one of the Dauntless restorations, so I am sure that it fits properly into the fuselage contour on both views: the side view and the top view. However, I did not verify in any way the curved contour of the tail on the top view.

Frankly speaking, after this experience I am really glad that I am doing such a “slow start” to the modeling by preparing these drawings. It forced me to think twice (or even more times than twice) about every part of this airplane, resulting in better understanding of various nuances of its geometry. Sometimes I had to deliberate over a single line (like the gap between the elevator and stabilizer) for a whole day, watching and comparing hundreds of photos. In the effect I had to move a few lines around it on the plans. It was not a big deal. However, if I already started to build the model, adaptation of such findings would require a lot of work!

In next post I will tell you more on how I used the explicit dimensions from the Douglas drawings. They allowed me to find some flaws in my plans. Description of this case will give you an insight into the errors that you can make using the photos.

[helios16v]

Impressive digging. I'll be following this for sure.

[Whistlekiller]

Fascinating stuff Witold! I must have missed it somewhere but what scale will the model be?

[Witold Jaworski]

Thanks, guys!

(...) I must have missed it somewhere but what scale will the model be?

Well, when your model goes digital, it becomes fully scalable. For example: I do not state the scale in the drawings published here, because you can easily change it in any graphic program before you print them on the paper. For my 3D model I will assume that 1 unit = 1 inch in the real airplane. But it depends entirely up to you, how do you scale this 1 abstract, "computer unit". In case of such models people just describe their level of details. I am going to make a detailed computer model of the SBD Dauntless, recreating most of the cockpit details, as well as the engine compartment.

[hugogo]

jeez Witold, this puts 'checking your references' to a hole new level!

It's great nowadays to have such nice software available that can make you do stuff like this.

Good Luck!

[Witold Jaworski]

hugogo, thank you!

The ultimate shape of airplane from post #13 resulted from matching my initial drawings against the Douglas general arrangement diagram. I couldn’t do it before, because this diagram comes from the Dauntless maintenance manual, which I received in previous week.

In this post I will show you how I do such a matching using the diagram shown below:



When you use such a drawing, you can follow the general rules of the technical drawings. In particular:

• The ultimate contour of the depicted object is on the outer side of the drawing lines;
• When the shape on the drawing differs from the result of its explicit dimensions, the result of these dimensions prevails;

So, starting from the thrust line (i.e. the propeller axis) and from the firewall (the base of all dimensions), we can use the dimensions from this diagram to determine the wing chord position (points A and B on picture below):



We can read from the side view dimensions that A (the rib tip) is located 20.38” from the thrust line and 9” from the firewall. The end of this rib is located 2.5⁰ lower, and the chord length of this rib is 115.12” (this dimension you can read from the top view). This determines location of point B.

I used the scale of my drawing (3.02 px/in) to convert the dimensions listed above into drawing units. Then I used guide lines to find these points of the wing chord on my plans:


Fortunately points A, B on my plans occurred very close to wing leading and trailing edges.

You can use dimensions from the general arrangement drawings to sketch the basic trapeze around the wing (in the top view) as well as around the fin, rudder, and horizontal tailplane. These trapezes allow you to determine the basic shape and proportions of the airplane. I will show this method on the example of the fin and the rudder. Figure below shows their dimensions on the original drawing:



Using these dimensions you can draw the basic trapezoid around the rudder and fin. You can also locate the chord of the horizontal tailplane as we did for the wing.

When I mapped these elements onto my plans, they revealed a serious flaw in my drawings:



The whole tailplane seems to be shifted downward, and the rudder hinge is moved left! However, if the wing chord fits to the dimensions, then most probably this is the result of a random rotation. I have quickly verified this hypothesis using the reference photo:



When I set the pivot point of this transformation above the wing see figure below), it was enough to rotate this photo by 0.27⁰ to fit the rudder and fin into given contour:

It seems that I made mistake at the very beginning, trying to set this photo horizontally (in post #8). I estimated it using cockpit edge (as in figure above), because the better candidate for such a reference — the seam running on the side view along the reference line — is not visible on this picture. It seems that this fragment was too short for precise estimation of the horizontal direction. What’s more, I did not know at the beginning that in the top view this edge is not parallel to the fuselage centerline. Because the depicted airplane is slightly inclined toward the photo, I had to estimate location of this edge as the line lying between two cockpit edges visible on this picture. The BuAer drawing (copied from the Douglas general Arrangement Diagram) would help, if its draftsman did not made additional errors around the tail and empennage (see post #8).

Of course, the drawings that I published in post #13 did not contain any of the flaws that I have found here. I fixed all of them before. I just wanted to show you in this post what kind of errors you can do using a photo reference.

Conclusion: always try to find a general arrangement diagram of the airplane and use its explicit dimensions to verify your drawings. They often allow you to fix severe flaws in the geometry of the depicted aircraft!

[Witold Jaworski]

In this post I will shortly describe how did I create this top view. Drawing such vertical views (from the top or bottom) of the SBD Dauntless is more difficult than the side view, because there are no “vertical” photos which you can use to verify and enhance the available plans. The methods presented below can be useful when you want to draw or verify blueprints of an aircraft.

I started my top view using everything I could, for example some photos from the restoration done by the Pacific Aviation Museum:


The photo on the picture above has a strong barrel distortion. We cannot effectively “revert” it as we did for the side view. Why? Because the photo of the side view all contours of the aircraft lie on a single plane (the symmetry plane). This one contains are at least three important planes: the edges of the cockpit, the center of the fuselage (along its maximum width) and the wing contour. Each of them is located at a different distance from the camera, and each requires different distortion (fixing one of them you would spoil the others).

Nevertheless, taking all of this into account, this high-resolution photo is still useful to determine the rivets pattern of the center wing section, as well as the width of the cockpit frame. The edge of the Dauntless cockpit is formed by an important longeron: it determines the fuselage shape in this area. To precisely estimate the width of the cockpit canopies I draw auxiliary contours of their cross sections (you can see them on the picture above as the blue lines). Positions of the bulkheads are copied from the side view. On this top view I roughly approximated positons of the longerons below the cockpit edge. This is just a “workshop drawing”, not a regular scale plan: I will form the fuselage following its contour on the side view and a few key cross sections which I will draw later. Because of the barrel distortion of the reference photo I was not able to check the contour of the fuselage in the top view. This is the only element I had to redraw without any verification from the Douglas general arrangement drawing.

In next step I used dimensions from the Douglas diagram to draw the trapezes of the outer wing panels and horizontal tailplane:



Picture above shows all the lines which you can deduce from the general dimensions provided by the manufacturer. We can further enrich it using the information from the stations diagram:



The station diagram provides precise position of all wing ribs. Most of them are just a row of rivets, but along some of them you can find the panel seams.

All right, but this wing drawing is still missing its “vertical” elements: rivet and panel seams along the spars and stringers. How to determine their locations?

I had to review all the collected photos. Ultimately I chose one of the pictures from the web page of Chino Planes of Fame Air Museum:



I rotated this photo, aligning the wings of this airplane to the vertical guides. As you can see, it is made with a telescopic camera, so that it is very close to a perfectly orthographic projection. (The guides of the tailplane are not ideally parallel to corresponding guides on the wings, but this difference is minimal). The left wing is depicted at a relatively high angle, so you can see clearly the rivet seams along the spars and stringers. I decided that I can use this picture to map these lines onto my drawing.

I flipped this image from right to left, and stretched it, fitting its wing into the basic trapeze:


It allowed me to recreate the wingtip curve. In such a highly-deformed image the rib lines are bent. They match their “true” positions only on the wing edges. However, we can easily map from this image the spar and stringer lines. All of them continue from the center wing section. Combined with the ribs these lines form a kind of the “reference grid”, which cells allowed me to draw all the remaining details: the circular holes in the flaps, fixed slats openings, etc.

I used similar method to map the tip of the horizontal tailplane as well as its two spars. In the effect I obtained a detailed top view of the SBD Dauntless.

In the next post I will publish the bottom view.

[hendie]

incredible research - great work

[Witold Jaworski]

hendie - thank you for following

During previous weeks I was working on the bottom view and other details of the SBD Dauntless. For example — I added a modified side view that reveals the engine and the cowling hidden under the NACA ring:


Because of the formatting issues of this post I had to split the original square drawing into two parts:

(Click here to get these drawings as a single, high-resolution image). As in the case of the top view I draw the outer wing panel without its dihedral.

Detailing of the bottom view resulted in minor updates of the side view:
(See its high-resolution version).

I have already started working on the front view. One of the elements I need for the model are the key cross sections, thus I identified their shapes, and incorporated them into this drawing:

I did not draw the first sections of the NACA cowling here, because they will be visible on the front view. As you can see there are large gaps between sections 2 and 3 and between 8 and 9. Why I did not add these intermediate contours? Because nothing special “happens” between these bulkheads: the resulting shape will be automatically interpolated during modeling.

I sketched the engine and the inner cowling, because I am going to model these parts. Analyzing this area I discovered many differences between the earlier versions (SBD-2, -3, -4) and the later versions (SBD-5, -6) than were not mentioned in any previous publications about the SBD:

• Different cross section B (in the SBD-1…SBD-4 it had wider, elliptic shape);
• Different widths of the oil radiator scoop;
• Yet another carburetor air scoop: you can find in the books that in the SBD-5 it was removed from the NACA cowling and replaced by two intakes located between upper cylinders of the radial engine. However, they did not mention that they were just additional intakes for the filtered air (for the takeoff/landing from provisional ground airstrips). The main air scoop was still at the top of the fuselage, but since SBD-5 it was hidden behind the NACA cowling!

In the next post I will elaborate about these unpublished differences between the SBD versions, showing them on drawings. I will also prepare a simplified front view (for my model I do not need to redraw all the minor details there).

The drawings of this aircraft will be complete soon.

[Witold Jaworski]

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.

NOTE: 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 the 3D model: the wing.

[Witold Jaworski]

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.

[Witold Jaworski]

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).

[helios16v]

Awesome. I knew the -2 & -3 were close, now I know the couple little tweaks I should try to make on a build I have coming up. Cheers.