Comparing My SBD Model with Original Blueprints

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

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

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

Figure 110-1 My error in the wing location (0.12”)

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

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

Matching a 3D Model to the Photos (2)

Here is a printable (PDF) version of this tutorial. It contains much better images than the automatically processed copies, which you can see in this post.

In my previous post I “fitted” my model of the P-40B into modern photo of a restored aircraft. (Precise speaking, it was a photo of the P-40C, but there were no external differences between these two versions). In general, I used Blender camera object to “pose” the 3D model so in the camera frame it looks just like the aircraft depicted in the photo. One of the key information that I used for this “fitting” was the lens focal length used for making the reference photo. (Modern cameras save key technical parameters in the resulting image file). I could just read this length from the photo properties, write it to the corresponding Blender camera Focal Length property, and focus on determining the remaining unknowns: camera location and direction.

However, how to use the historical “analog” pictures for such a match? (For example –  this original Curtiss photo of the Tomahawk IA from November 1940:)

Figure 104-1 Original Curtiss photo of a Tomahawk IA (technically similar to P-40B)

Recreating the P-40B Side Contour (2)

As I mentioned in the previous post, I had to check if the “keel” under the wing that I draw according the P-40E blueprints and the “keel” in the P-40B were identical. I was forced to use the P-40E documentation, because the drawings of the earlier P-40 versions (B, C) are extremely rare and often dispersed among less important blueprints (like sketches or design proposals). Thus, to check the assumption that the P-40 “keel” was identical in the “short nose” and “long nose” Hawks, I had to use available photos.

The aircraft picture on most of the photos is deformed by the perspective distortion (which depends on the camera lens length) and barrel distortion (caused by imperfections of the optical system). You can quickly estimate the amount of these (combined) distortions on a side photo of an aircraft. Just look at the seam lines along the fuselage bulkheads. Usually they form “bulges”. If the seam lines on the aircraft nose are “bulged” in opposite direction than similar lines on the tail – then in this image you have a perspective distortion (as in this “Tomahawk” IIA picture, below):

Figure 100-1 Typical photo: an example of perspective deformation

Recreating Geometry of a Forgotten WWI Fighter

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

Figure 96-1 Fokker D.V

I was asked to take part in this project by C. West. He did all the research and provided all the materials: blueprints and photos. My part was recreating the geometry of this aircraft, especially its fuselage frame made of steel tubes. All what we had was a dozen of various archival photos, a poor general drawing, and the landing gear dimensions:

Figure 96-2 All remaining D.V blueprints

In this case I had to turn the available photos into the precise reference, as I did for the SBD, then use them to determine the required geometry details.

Creating Textures: Markings

The last texture for my model contains various elements that in the plastic kits are delivered as the decals: national insignia, radio-call numbers and various service labels. I prepared it as another vector drawing in Inkscape:

Figure 77-1 “Decals” texture for the SBD-3 model (the reference drawing is visible in the background)

I exported this picture to a raster file named color-decals.png. It has transparent background, because I will combined this image with the other components of the color texture, prepared in previous posts.

Creating Textures: Color Map (2) – Weathering

In this post I will work on the weathering effects of the color texture, while in the next one I will add scratches and some other remaining details.

The weathering effects that you can observe on the aircraft from WWII era are quite “dramatic”. The paints used in mid-20^th century were not as chemically “stable” as the contemporary coats, thus they could change their hues in few months of intense service. The archival color photos below show an extreme case of this effect (Figure 75‑1):

Figure 75-1 Examples of heavy weathered aircraft

These photos were taken by Frank Sherschel on 14^th November 1942, for the “Life” magazine. The SBD-3s depicted on the pictures belonged to VMSB-241 squadron, stationed at Midway in that time. Marines received these aircraft in July 1942, but all of them were already used before – most probably on the U.S. Navy carriers. I think that in November 1942 these SBDs had accumulated about 10-11 months of the war service. I will use them as an extreme case of the weathering. (It is always good idea to recreate such an ultimate case in the texture, because you can always make your model “newer” by decreasing intensities of the weathering layers. On the other hand, you cannot use more than the 100% of their intensities, thus you cannot make your model “older” than you initially painted).

Using Panel Lines to Verify the Fuselage Shape

In every creative process, after each “big step forward” you have to stop and carefully examine the results. Usually you have to make various corrections (sometimes minor, sometimes major), before taking the next step. This post describes such minor corrections that I had to make after mapping the key texture of the panel lines.

In my first post published in October, I drew the panel lines on the model, then compared them with the photos. Sometimes a minor difference between their layouts can lead to a discovery of an error in the fuselage shape. I in that post already found and fixed an issue in the shape of the tailplane fillet.

I also mentioned (see Figure 65‑9 in previous post) that I can see a difference in the bottom part of the wing fillet. Now I would like to resume my analysis at this point (Figure 67‑1):

Figure 67-1 Difference in the shape of the wing fillet bottom seam

Final Adjustments of the Model Shape

While working on the cowling details, I discovered that the SBD-5 from the Commemorative Air Force (“white 5”) uses a non-original Hamilton Standard propeller. It has larger hub and a pair of bolts in the middle of the hub barrel edges. (As I wrote in this post, the original Hamilton Standard hubs used in the SBDs were smaller, thus they had a single bolt in the middle of each barrel edge). What’s more, I also noticed that the centerline of my model does not precisely pass through the tip of the propeller dome visible in this photo. When I corrected this mistake, I also noticed that the edges of certain cowling panels in my model are minimally below their counterparts on the photo. I examined this difference and decided that I should fix it by rotating the camera of this projection around the fuselage centerline. It was really a “cosmetic” adjustment — the rotation angle was about 0.7⁰. However, suddenly everything in this model matched better the reference photo — except the horizontal tailplane (Figure 58‑1):

Figure 58-1 Improved matching of my model to the largest reference photo

Fixing the Barrel Distortion of the Reference Photos

In this post I describe a break in the modeling that I made this week, because I had to fix my reference photos before the further work. The reason for this fixing was simple: the NACA cowling of my model did fit only the long-lens photos. For the further work I needed more information. This information was available in the high-resolution photos made by the Pacific Aviation Museum Pearl Harbor. However, they are slightly distorted.

In the ‘mathematically ideal perspective’ calculated for the computer cameras all of the straight lines remains straight. Unfortunately, the real-world camera lens can slightly deform (bend) the straight contours. This is so-called ‘barrel’ (or ‘cushion’) distortion of a photo. Unless you are using a panoramic lens, this deformation is hardly noticeable for the naked eye. Unfortunately, these differences become evident when you place a photo behind a 3D model, projected by a computer camera.

In case of reference photos that I used to verify my SBD Dauntless, the differences caused by the barrel distortion are visible around the forward part of the engine cowling (Figure 42‑1):

Figure 42-1 The influence of the barrel distortion

Modeling the Empennage (1)

The horizontal tailplane has similar structure to the wing — but it is simpler. Thus I started it in the same way as the wing, by forming its root airfoil (Figure 32‑1):

Figure 32-1 Modeling the root airfoil of the horizontal tailplane

In the most of the aircraft the tailplane has a symmetric airfoil. So it was in the Dauntless. I did not find its signature (family) in any of the reference materials, thus I carefully copied its contour from the photos (its rear part — the elevator — seems to have modified shape, anyway). It has incidence angle of 2⁰, so I rotated the rib object and used a Mirror modifier to generate its bottom part.