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:

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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”.

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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):

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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.

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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:

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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.

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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:

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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:

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Figure 107-2 SBD-5 inboard profile

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

Continue reading Original Douglas SBD Blueprints: First Impressions

Recreating Grooves in the Sheet Metal Panels

This post is dedicated to a minor feature, which I have found surprisingly demanding: modeling the grooves pressed in the curved surfaces of the aircraft panels. In the SBD you can see some of such reinforcements on the inner cowling, behind the cylinder row (Figure 95‑1):

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Figure 95-1 Reinforcements of the inner cowling: the grooves, pressed in the sheet metal panels

They are 0.7-1.0” wide (Figure 95‑1a) and span over the inner cowling along its radial directions (Figure 95‑1b, c). In the SBD-5 and -6 these reinforcing grooves occur only on the lower part of the cowling (Figure 95‑1b), while in the earlier versions (SBD-1, -2, -3, and -4) they are also present on the  upper part (Figure 95‑1c).

Even when the flaps on the NACA cowling are closed, you can still see rounded endings of these grooves around the cowling rear edge (Figure 95‑2):

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Figure 95-2 Reinforcing grooves visible behind the NACA cowling

In the earlier versions (SBD-1..SBD-4) they appear on the narrow strip behind the NACA cowling (Figure 95‑2a). You can see more of the upper grooves when the NACA cowling flaps are set wide open. In the SBD-5 and -6 the engine and the NACA cowling were shifted forward by 3.5”, and the gap between the NACA ring and the inner cowling is wider. Thus, in these versions you can see even longer fragments of the grooves behind the NACA cowling (Figure 95‑2b).

Such grooves appear on many sheet metal elements, so I decided to write this post as a small tutorial that teaches how to recreate these elements. Thus, do not be surprised when I list the detailed Blender commands in the text below.

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Differences between Dauntless Versions: Carburetor Air Duct

After “mounting” the R-1820 engines into my SBD models, I decided to recreate some details of the inner cowling (the cowling panels placed behind the cylinder row). In this post I will form the missing parts of the carburetor air ducts, hidden under the NACA ring. There are significant differences in this area between various SBD versions, which never appeared in any scale plans, or in any popular monograph of this aircraft. I think that the pictures presented below highlight these differences. They can be useful for all those scale modelers who are going to build the SBD “Dauntless” models with the engine cowlings opened. (Sometimes you can encounter such advanced pieces of work on the various scale model contests).

Let’s start with the SBD-5s (and -6s), which are better documented (because they were produced in much larger quantities). They had a dual intake system, of the filtered/non-filtered air, which I discussed it in the previous post. I already recreated the two intakes of the filtered air, placed between the engine cylinders. Now I have to create the central, direct air duct and its opening at the top of the internal cowling.

Figure 94‑1 shows the initial state of my SBD-5 model:

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Figure 94-1 SBD-5: the carburetor and its cowling (initial state)

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Mounting the Engine

In my previous post I have finished the second variant of the R-1820-52 “Cyclone” engine, which was used in the SBD-3 and -4. (It looks like the earlier R-1820-32 model, mounted in the SBD-1 and -2). In the resulting Blender file linked at the end of that post you will find two “Cyclone” versions: the R-1820-52 (for the earlier SBD versions, up to SBD-4) and the R-1820-60 (for the SBD-5 and -6). Each of these engines has its own “scene”.

To “mount” these engines into my SBD models, I imported both scenes to the main Blender file. I defined each engine variant as a group, to facilitate placing them in the aircraft models as the group instances. I also added the firewall bulkhead and updated the shape of the cowling behind the cylinder row. (I will refer to this piece as the “inner cowling”). So far I did not especially care for the shape of its central part, hidden below the NACA ring. Now I updated it for the real size and shape of the engine mounting ring (Figure 93‑1a):

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Figure 93-1 Mounting frame details

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Creating the Tail Wheel

The Dauntless had fixed tail wheel of a typical design among the carrier-based aircraft. The tail wheel assembly consisted a fork connected to two solid-made beams, which movement was countered by a shock strut. The beams and the shock strut were attached to the last bulkhead of the fuselage (Figure 82‑1):

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Figure 82-1 Tail wheel assembly

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Rigging the Main Landing Gear

In previous post I discussed how the SBD landing gear retracts into its wing recess:

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Figure 81-1 Landing gear retraction in the SBD Dauntless

In principle, it is simple: the landing gear leg rotates by 90⁰. However, the parts responsible for shock strut shortening during this movement increase mechanical complexity of this assembly. The figure above does not even show the deformations of the brake cable, which follows the shock strut piston movements.

For some scenes I will need the landing gear extended, while for the others – retracted. In practice, moving/rotating each part individually to “pose” my model would be a quite time-consuming task. That’s why I created a kind of “virtual mechanism”, which allows me to retract/extend the landing gear with a single mouse movement.  In the previous post I already presented its results in this short video sequence. In this post I will shortly describe how I did it.

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Details of the Main Landing Gear

The SBD shock absorbers had to disperse a lot of the kinetic energy of landing aircraft, minimizing the chance that the airplane accidentally “bounce” back into the air. (This is a key requirement for the carrier-based planes). For such a characteristics you need a relatively long working span between the free (i.e. unloaded) and the completely compressed (i.e. under max. load) strut piston positions. Indeed, you can observe that the Dauntless landing gear legs are much longer in the flight than in their static position on the ground (Figure 80‑1):

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Figure 80-1 The fully extended shock strut

The working span of the SBD shock strut piston was about 10” long, while the difference between the static and the free (extended) piston positions was about 7.5”.

Continue reading Details of the Main Landing Gear