Original SBD Dauntless Blueprints: Fuselage Geometry (1)

As I already mentioned in this post, my microfilm set does not contain the fuselage geometry diagram. (I suppose that it was included in the missing roll C). Thus, this part of my work will be much more difficult, because I even do not have complete set of the bulkhead drawings! Just found a structure assembly drawing (i.e. side and vertical views), skin panels assembly drawing, mid-fuselage bulkheads, and some bulkheads of the tail. In the picture below I marked these undocumented areas of the fuselage in transparent red:

Figure 112-1 Fuselage structure: documented and undocumented areas

Douglas blueprints refer to the fuselage bulkheads as “frames”. They are numbered from 1 (the firewall) to 17 (the mounting base for the tail wheel and horizontal stabilizer). Of course, the fuselage assembly drawings provide their positions, measured from the firewall. (You can find them in this assembly drawing of the skin panels). In this post I will refer to fuselage bulkheads using their ordinal numbers, shown in the picture above (for example: “frame #05”).

Original SBD Dauntless Blueprints: Arrangement Drawing Issues

In some aircraft it is difficult to provide the precise value of overall length. One of them is the SBD Dauntless, because of its easily demountable spinner used in the first three variants (SBD-1…-3). Also the length of the Hamilton Standard Hydromatic spinner hub, used in the later SBD variants, can vary – especially in the restored aircraft. Thus, for verification of model kits or similar purposes I would suggest checking the distance between two easily distinguishable points: from the firewall to the tip of the tail cone. This dimension remains the same in all SBD variants. Preparing the fuselage blueprints for my model, I could determine this distance using the tail cone assembly drawing:

Figure 111-1 Using the marked stations to measure distance from the firewall

The key information is provided by the stations marked in this drawing: their names describe distances from the firewall. (You can read them yourself from the high-resolution version of this drawing).

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”.

Original SBD Dauntless Blueprints: Wing Geometry

I am preparing data from the original Douglas blueprints to verify my model. For the beginning I chosen the wing. This is a well-documented assembly, because I found a master diagram in the NASM microfilm that describes SBD wing geometry (ordinals). Below you can see the first sheet of this diagram (dwg no 5090185):

Figure 109-1 Master diagram of the wing geometry (sheet 2/2)

Here you can download its high-resolution version (5MB). As you can see, it contains the ordinal tables of the wing bulkheads (ribs) and webs (spars). In the sketch on its right side Douglas engineers depicted various other dimensions of the wing center section. In the picture above I marked in red its key wing stations. Their names correspond to spanwise distance in inches from the aircraft centerline: “STA 10” is 10” from the centerline, while “STA 66” is 66” from the centerline.

Original SBD Dauntless Blueprints: Initial Findings

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

I chose about 1000 frames (mostly assembly drawings) from this microfilm set, and organized them into a tree-like structure as in Figure 108‑1:

Figure 108-1 Folder structure for the SBD blueprints

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

Original Douglas SBD Blueprints: First Impressions

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

Figure 107-1 Description of the SBD/A-24 microfilm set in the NASM order form

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

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

Figure 107-2 SBD-5 inboard profile

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

Curtiss YP-37 with the R-2600 Radial Engine

Reviewing the original P-36/YP-37/P-40 blueprints published by AirCorps Library, I also browsed the “uncategorized drawings” category. In general, many Curtiss drawings from this microfilm set are unreadable, especially these “uncategorized” images. Often all what you can see is just a blank microfilm frame with barely visible remains of the title block. However, in this “junk” category you can find interesting sketches of various design proposals. One of them is the YP-37 with the powerful R-2600 Twin Cyclone engine. Below you can see side view of the initial idea, from November 1938:

Figure 106-1 YP-37 with R-2600 engine: initial proposal from November 1938 (a fragment of drawing P-1660)

A New Finding about the “Long Tail” P-40s

Last month I was busy with my daily business, so in this post I would like to share just single detail, which I encountered in the P-36/YP-37/P-40 documentation.

This finding is related to the “long tail” P-40 variants. In August 1942 Curtiss decided to definitely resolve the directional problems of the “short-nose” P-40s. They extended their tail, adding an additional segment after station 16. It shifted the original “P-36 – like” fin and rudder back by about 20 inches. This modification was introduced to the Allison-powered P-40K-10, and to the Merlin-powered P-40F-20. (These two versions were produced in parallel).

Below you can see how these two tail variants are depicted in typical scale plans:

Figure 105-1 The “short” and “long” tail P-40 variant (P-40F), depicted in a classic scale plans

In the picture above I placed drawing of the P-40F-1 (“short tail”, in black) over the P-40F-20 (“long tail”, in red). As you can see, the tail is the only difference between these aircraft. Note the shape of the fuselage in the bottom view. In all scale plans of the long-tail variant that I saw, the width of the fuselage was wider than in the “short tail” version. These differences usually begin at station 12 and continue to the rudder.

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)

Matching a 3D Model to the Photos (1)

This summer I was asked by some readers for making a tutorial on my photo-matching method. This method allowed me to recreate the shapes of  various historical aircraft with greater precision than the classic scale plans. (For example – the Fokker D.V or SBD Dauntless). This is the first post on this subject (I decided to split this tutorial into two subsequent posts).

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.

The goal of the photo matching is to set up in your 3D environment a photo as the precise reference image (a more reliable equivalent to the scale plans). You can then use such a photo to verify, correct, and enhance the initial version of your 3D model. To begin, you need:

1. Initial 3D model. First you have to prepare an initial 3D model of the aircraft. You can do it in the classic way, using available scale plans and photos. This first approximation of the real aircraft does not have to be too detailed – prepare just the fuselage, wings and empennage. Eventually you can also add simplified landing gear (placing plain cylinders in place of its oleo struts) and the propeller blades;
2. High-resolution photo. Ideal reference photo should be detailed and free of barrel or pincushion distortions (i.e. it should depict the aircraft in a pure perspective projection). Of course, in practice such an ideal is not possible, but I will give you some hints how to identify a good candidate for the reference photo;

I built my models and matched them to the photos in Blender 3D program. In this post I am using Blender 2.80 (this is the actual version). I assume that the Reader knows the basics of Blender environment, in particular its UI and the navigation in 3D scene (“3D View” window). However, sometimes in this post I will describe some details of Blender commands that are obvious to its regular users. In this way I just want to minimize the risk that eventual Reader will “get stuck” in the middle of the described process.

For this tutorial I decided to use my old P-40 model, shown below. I built it several years ago in a “classic” way: using the scale plans.

In this old model I did not used any information from the P-40 blueprints, which I presented in my previous posts.

Figure 103-1 An initial model of the P-40