Recreating the P-40B: 3D Reference of the Fuselage (1)

At this moment I am working on second volume of my book about 3D modeling.  It describes building a 3D model of a WW2 aircraft on the example of the P-40B. Preparing for this work, I discovered that the original documentation of this early P-40 variant (also known as “long nose Warhawks”) is missing. On the other hand – you can find plenty of the “short nose Warhawk” blueprints (related to the P-40D later variants), as well as some P-36 drawings. I started by picking over 1000 original Curtiss blueprints and sketches related to the P-40, XP-40, and the P-36 from the vast resources of the AirCorps Library. Then I analyzed their contents, comparing them to the available historical photos. I described this process in this and following posts, written in 2019. Ultimately I traced side view of the P-40B. I also concluded that a 3D visualization of the available ordinals will be a better reference. In the previous posts I built such a reference for the SBD Dauntless. In this and the next post will I describe similar work on the fuselage of the early P-40 variants (P-40-cu, P-40B, P-40C).

I prepared an empty Blender file. For the convenience, I placed there my side view (from this post, see Figure 102-15). As for the SBD model, I assumed that 1 Blender unit = 1 in. For the main part of this fuselage, spanning from the firewall to the rudder, I used two P-36 diagrams. First of them (dwg 75-21-140) provides locations of the fuselage stiffeners at each bulkhead. There is also its modified variant (dwg 75-21-836) for the XP-40:

Figure 120-1 XP-40 fuselage stiffener dimensions (dwg 75-21-836)

In fact, both layouts are identical. I can read the maximum width of the fuselage from horizontal dimensions of stringer #8, which runs along the fuselage reference line. In addition, this blueprint also provides data points for the side contour, because there were stringers #1 (upper contour) and #13 (lower contour). I suppose that the black “masks” in the XP-40 drawing correspond to the Prestone and oil radiators. In the first variant of this prototype, they were located behind the wing trailing edge in a “box” cover (like in the Hawker “Hurricane”).

There is also another diagram of the P-36 fuselage ordinates (dwg 75-21-020). However, this microfilm scan is partially unreadable:

Figure 120-2 P-36 Fuselage ordinates (dwg 75-21-020)

In particular, the sketches in the lower left part of this drawing are dimmed, so I could not determine the meaning of the parameters listed in the ordinate tables.  

I began by building the vertical and horizontal plane of the fuselage. Each vertex of these meshes corresponds to a point dimensioned in the layout drawing:

Figure 120-3 First reference planes

At this moment I do not want to speculate about the shape in between these points, so I connected them with simple straight edges. In this way this shape represents the “hard”, dimensioned data.

For greater readability of these reference objects, I added here a few additional faces on the important contours, for example – behind the wing trailing edge. I drew them following the blueprint contours, which can be slightly distorted. Thus, they are less “confirmed” than the dimensioned datapoints. That’s why I marked them in another color. When the fuselage surface in the final model reveals a contradiction in the reference planes, these additional vertices will be the first candidates for eventual adjustments.

Figure 120-4 Using colors for marking “less confirmed” areas in the reference planes

I decided to mark faces connecting the “hard” (dimensioned) data points in blue, while the faces created by copying the blueprint contours are in green.

On the other hand, let’s do not forget that these blueprints were drawn in the “analog era”. This means that all the explicit dimensions you can see in these sheets were ultimately measured on a “master drawing” of the aircraft geometry. Such a physical measurement always produces minor, random deviations. You can find them by looking at a high angle along any of these contours, especially along a straight segment:

Figure 120-5 Deviations among the “hard” data points

The bottom contour of the P-40 fuselage in the side view forms a long, straight line (see Figure 120‑5a). However, when you look along this shape in a large zoom, you will see the deviations of its vertices (Figure 120‑5b). It seems that typical tolerance of the measurements for this aircraft was +/- 0.02”. I think that this value is possible since the typical skin thickness was about 0.03”.  However, among these datapoints you can encounter a few deviations which are greater three or four times.

In this early stage of building the 3D reference you cannot determine, if such an “outstanding” ordinate reveals a real, minor feature of the aircraft contour, or is a result of significant measurement error. Thus, I did not make any adjustments, just marking edges around such a dubious vertex as “crease”, to easily find them later.

Sometimes you can identify such an ordinate as erroneous, when you find another blueprint which provides a different dimension for the same point. When I identified that the lower part of the tables from the partially unreadable diagram 75-21-020 (Figure 120‑2) contains stringer points coordinates, it became a great help for such verification.

I created a reference “plane” for each of the fuselage stringers. Curtiss numbered them from 1 to 13, so I named accordingly each of these objects:

Figure 120-6 Fuselage stringer planes

In addition, I found in diagram 75-21-020 a small table containing ordinates of the upper edge of the opening around the wing. I created from them another plane.

It is easier to find most of the “outstanding” data points when you build continuous faces from their ordinates, as in the illustration above. Then look along each edge of these contours. To give you impression, how many errors you can encounter in such a layout diagram, I am showing drawing 75-21-140 where I marked these identified wrong dimensions in red:

Figure 120-7 Erroneous dimensions in the P-36 stringers layout (marked in red)

During this verification I studied again the diagram 75-21-020 (Figure 120‑2), and finally identified that its upper table contains widths and heights of the bulkheads. They are measured in the equal steps of 3” from the fuselage reference line.  Using the readable areas of these tables, I was able to recreate tail bulkheads – from #5 to #16:

Figure 120-8 Bulkhead ordinates, according dwg 75-21-020

I used here the equally spaced widths from the fuselage diagram. Of course, I also used other blueprints. For example – drawings 99157 and 74-21-080 provided dimensions of the “turtledeck” and the glass spanning between frames #5 and #9.

Unfortunately, ordinates that describe frames #1..#4 are not readable. What’s worse, the stringer ordinates provided only 5 data points for each of these bulkheads. I had to seek additional information among various detailed blueprints. Ultimately additional dimensions of the cockpit frame allowed me to recreate shapes of bulkheads #2 and #3, and determine the location and twist of the fuselage longeron:

Figure 120-9 Bulkheads #1..#4

As you can see, I did not find any additional dimensions of the firewall (#1), but I copied this contour from its assembly drawing. It fits the stringer points, but I marked it in green, to be fair.  Dimensions of the frames #2 and #3 revealed that their contour between stringers 7 and 9 forms an arc. The upper contour of #2A is also a combination of two arcs. Knowing this, as well as the shapes of the stringers between frames #3 and #5, I concluded that frame #4 should be a linear interpolation between their contours.

The windshield can cause troubles when its intersection with the fuselage does not look like in the photos. This happens quite often. To avoid such surprises, I decided to check this edge in this 3D reference:

Figure 120-10 Windscreen-fuselage intersection edge in the P-40-cu

I assumed that the windscreen shape was identical in the P-36 and the P-40, so the P-36 drawings (75-26-001, -012, and -026) provided me its accurate dimensions. I formed the upper part of the fuselage basing on the bulkheads #1 and #2A and the cockpit frame. Then I compared the resulting intersection edge with the archival P-40 photos. I discovered that in the XP-40 this shape of the bottom cockpit frame was “angular”, identical to the P-36, while in the P-40-cu its rear parts became somewhat smoother and moved rearward by about 0.8”. This means that in the serial P-40s they modified the shape of the fuselage between stations #2A and #3A.

Two years later, when I projected this model onto some reference photos, I discovered that:

1. The P-40 sliding canopy was ~1” shorter than in the P-36. In the effect, its 3A station which marked the base of the rear windshield frame, was 40.75” from the firewall. (In the P-36 it was 39.75”).
2. The P-40 windscreen preserved most of the original P-36 geometry, but was longer by about 0.8” (That’s I wrote in the paragraph above that it was moved by 0.8”);
3. In the P-40 the rounded corners of the gun cowling cross-section at station #2A (and correspondingly, at #3A) were higher than in the P-36. This means, that while the overall dimensions of the cross section #2A (the width at the fuselage longeron and the height), are identical in the P-36, their contours in the P-40 are different. Most probably they modified them to better accommodate the pair of the M2 guns, mounted in the P-40.  

All these observations contributed to the different shape of gun cowling-windscreen intersection edge which I observed in the photos. I recreated all these findings in this 3D reference, creating the P-40 gun cowling and the cockpit as gray, surface objects.

In the P-40B/C Curtiss introduced another modification to the windscreen frame, enlarging the inspection doors above the fuselage guns:

Figure 120-11 Enlarged gun back plate doors in the P-40B/C

However, it did not alter the fuselage cross-sections, so I decided to skip this variant here. Because of this overlapped gun door, the windscreen frame in the P-40B/C is a quite complex shape. I think that it will be easier to form it starting with the previous, simpler variant of the P-40-cu, then apply the later modification.