Original SBD Dauntless Blueprints: Fuselage Geometry (2)

In previous post I started creating 3D reference objects for the SBD fuselage. In this post I will complete this work. I will focus here on the difficult part: the wing fillet. It spanned along more than half of the SBD fuselage length. In this post I am going to prepare reference geometry that describe its shape from bulkhead #4 to bulkhead #13. Unfortunately, in drawings from the NASM microfilms I found just a few contours related to this feature:

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Figure 113-1 Identified wing fillet contours

Continue reading Original SBD Dauntless Blueprints: Fuselage Geometry (2)

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:

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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”).

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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:

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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).

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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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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:

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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.

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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):

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

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Recreating the P-40B Side Contour (1)

As I wrote in the previous post, it is impossible to find a complete documentation of the early P-40 variants (so-called “long nose Hawks”: P-40cu, P-40B and P-40C). I collected all what is currently available from the Internet portals: blueprints of their direct predecessor (P-36) and drawings of the later variants (the “short nose” P-40D … P-40N). Using these scanned microfilm frames, archival photos and technical descriptions you can recreate the wings, empennage, tail and mid-fuselage of these aircraft.

I started with the most obvious part of the side view: the fuselage.  Behind the firewall it was basically identical to the P-36, except the tail wheel cover:

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Figure 99-1 The P-36 fuselage (behind the firewall) with modified tail wheel cover as for the P-40B

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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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Seamless Joining of Two Objects

This post is a small digression about a modeling technique that you may find useful.

There is a detail on the bottom surfaces of the SBD center wing: an opening, made partially in the cover of the fuselage belly (Figure 72‑1):

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Figure 72-1 The detail that I have to recreate

The difficult part of this detail is its flange, stamped in the fuselage cover. I just have two photos of this element, both of average resolution. On both of them you can see a typical circular recession, made around the opening in the belly cover. In fact, such a feature is quite common in the sheet metal design (you can see plenty of such stamped flanges in various places inside your car). This is a minor detail, too small for any serious modeling, but too large for recreating it with the textures.

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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):

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

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