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:

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

I had to combine here two microfilm frames (Frame 1 and Frame 2 in the figure above) of the P-36 skeleton blueprint (drawing number: 75-21-606). For this and further operations described below I used a vector-based 2D drawing program (Inkscape). Conceptually Inkscape is similar to the popular Corel Draw suite. It allows for easy manipulation of the linked raster pictures: you can quickly move, rotate and resize even very large images (microfilm scans). For this side view, I placed the fuselage reference line at Y=0, and the firewall (station 1) at X=0. I scaled down the blueprint images, so that 1 drawing unit (“px”) = 1 inch. In this way the coordinates in this drawing (expressed in “px”) match 1:1 the dimensions that I can read from the blueprints.

It does not matter that the resulting side view image will be just about 1000 units (“px”) wide. This is a vector drawing, and you can scale it up (almost) at will. The only limit is the resolution of the source raster images. That’s why each microfilm frame scan used in this composition is 6000px wide

Initially I placed the auxiliary guide lines (the blue lines in Figure 99‑1) at the fuselage reference line, and at the three fuselage stations: STA 1 (firewall), STA 16 (the last bulkhead) and somewhere in the middle – at STA 8. Then I could scale each of the microfilm scans to fit the two nearest vertical guide lines. I scaled Frame 1 to fit the STA 1 and STA 8, and Frame 2 to fit the STA 8 and STA 16. Simultaneously I adjusted their locations and orientations to match the horizontal fuselage reference line.

Later I placed additional guide lines at each fuselage station. (Their positions on the X axis follow the explicit dimensions, given on the blueprint):

Figure 99-2 Deviations of the fuselage station locations in the blueprint

This way I discovered that there are minor differences between these exact locations, and the position of some stations on the original drawing (Figure 99‑2).

What’s more, similar differences occurred in the area where these two microfilm frames overlap each other. To see it better, in Figure 99‑3 I marked the right scan image (Frame 2) in red:

Figure 99-3 Overlapping of two subsequent microfilm frames

As you can see above, these two pictures are a little “out of sync”. The original blueprint depicted in these two microfilm frames was a single paper sheet. The result which you can see in Figure 99‑3 means that the microfilm photos are slightly distorted (especially in the areas around their outer edges).

Another source of the further distortions is duplication process of the original drawing, which you can see on these photo. Usually the depicted blueprints were not the original drawings made in ink on tracing paper, but their copies. These copies could be distorted during the “wet” duplication process. (See my post from August 2019 for more details). It seems that the most of the P-40 documentation drawings were “positive” diazoprints, while from time to time you can also encounter some classic blueprint “negatives”).

Using the original documentation of an aircraft, you have to rely mainly on the explicit numeric values given in its dimensions and ordinates. (In fact, this is the general rule for the technical drawings). The object contours, sketched on these drawings, are there just for the illustrative purposes. I will use them in the last resort, when I do not find their explicit dimensions.

Using the dimensions provided in the assembly drawings, I could also combine this fuselage blueprint with the fin and the rudder:

Figure 99-4 Matching the P-36 empennage assembly drawings (The P-40B used the same fin and rudder)

Basically, the P-40B rudder and fin were identical to the P-36. The only modification is the pushrod of the trimmer tab, introduced in the P-40D. Except this detail, the rudder remained the same in the all the P-40 versions.

In the P-40K-1 the fin was enlarged and coupled with additional large fillet to counter the directional problems of the “short-nose” P-40s. However, in August 1942 Curtiss decided to solve this problem in a radical way: by extending the fuselage length. They moved 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 the same time).

For this side contour I  used the P-36 fin assembly drawing (75-12-001) and the corresponding rudder assembly (75-14-001). I determined the precise scale and vertical location of these two images using dimensions of the rudder hinge locations. (These locations are used as horizontal datum lines on both of these drawings).

To finally “pin” these images to the fuselage I also had to determine their horizontal locations (position on the X axis). On the fin and rudder assembly blueprints the datum line for the horizontal dimensions is the rudder hinge line (axis of rotation). To determine its precise distance from the firewall (the X coordinate on my fuselage drawing) I had to use the XP-40 general assembly sketch:

Figure 99-5 Determining the rudder axis location (on an early XP-40 drawing, no. P-1142)

In figure above I highlighted the dimensions that I used. You can see that in this assembly the datum line is placed at the tip of the center wing section, located 9 inches from the firewall (STA 1). Thus, the distance of the rudder axis from STA 1, expressed in inches, is: 20’*12 + 1.5” – 9” = 241.5” – 9” = 232.5”.

However, I calculated this value basing on the information from the general assembly of the XP-40 prototype. I have no similar drawing for the early-production P-40s. To ensure that this distance remained the same in the later P-40 versions, I could only verify it on the general assembly drawing of one of the later “short-nose” Hawks – the P-40E:

Figure 99-6 Verification of the rudder axis location (P-40E – drawing no. 87-517)

As in the previous drawing, I highlighted here the dimensions that I have used. The resulting distance is the same: 232.5”. (Note the oddly written “9”, highlighted in the blueprint above – if you saw it for the first time, you could wonder whether this is “4” or “9”).

The lines of the empennage images are much thinner than the lines of the fuselage skeleton drawing. To make these parts more visible, I decided to outline their contours in Inkscape. In the P-36 drawings I found dimensions (radii and centers) of the arcs that were combined to compose this curve. These dimensions helped me to precisely recreate its shape, despite of the deformation in the lower part of the rudder blueprint (Figure 99‑7a):

Figure 99-7 Outlining the fin and rudder contours

To verify this shape, I used the P-40N rudder blueprint (Figure 99‑7b). As you can see, the red outline that I sketched in Inkscape fits both images. (It fits the P-40N blueprint even better than the P-36 drawing, because this P-40N microfilm scan is not deformed).

It seems that the P-40 curves were described using the old, conservative methods. In some cases their shapes was constructed from several adjacent arcs (as the rudder outline). For the other cases Curtiss used classic ordinates tables, which described curves using just a few points. The contour shape between these points is up to you.

When a Curtiss competitor – North American Aviation company – started their “Mustang” project, they promised to build “a better P-40”. Initially it featured the same engine and armament, and its airframe had similar size and the same wing area as the P-40D. One of the NA improvements was a more flexible mathematical model used for describing the aircraft geometry: conic sections (also known as “second-order curves”). North American also provided the algebraic formulas of these curves. Thus, all the P-51 contours were described by explicit mathematical functions. Such an approach allowed for calculating as many curve points, as needed. This feature was very important for building appropriate tooling sets and better fitting of the resulting aircraft parts.

I found two versions of the “structural layout” drawing that provide the key dimensions of the early P-40/P36 fuselage. Below you can see the first version (drawing no. 75-21-140), from the P-36 documentation:

Figure 99-8 Layout of the P-36 fuselage (drawing no. 75-21-140, microfilm frame 1/2)

There is also another variant (drawing no. 75-21-836), prepared for the XP-40 prototype:

Figure 99-9 Layout of the early XP-40 fuselage (drawing no. 75-21-836, microfilm frame 1/2)

The lines in this blueprint are thicker than in the P-36 drawing, so it can be useful for checking the less readable dimensions. Note the black areas in the fuselage behind the wing. It seems that the Curtiss engineers re-used here the P-36 drawing, just “masking” the areas that they were going to change. I think that this drawing was intended for the initial XP-40 prototype, which featured the box-like Prestone cooler behind the wing (you can see it in Figure 99‑5).

Using dimensions from these layout drawings, I was able to draw the precise upper contour of the P-40B fuselage:

Figure 99-10 Upper contour of the P-36/P-40B fuselage (according the explicit dimensions)

Drawing these lines I followed the old rule for modeler’s scale plans: the precise contour lies on the outer edge of the outline. As you can see, the P-36 skeleton drawing agrees very well with this “dimensioned” line: eventual differences do not exceed the blueprint line thickness.

I did the same for the bottom contour:

Figure 99-11 Lower contour of the P-40B fuselage (according the explicit dimensions)

The explicit dimensions of this line revealed that it was a simple, straight segment, spanning from the wing leading edge to the rudder. Note that in the blueprint above the lower contour of the fuselage is slightly bent up at the wing trailing edge. This is the effect of a slight distortion in the scan of the left microfilm frame. It could be misleading, if I did not verify it using the explicit dimensions!

Assuming that the tail wheel bay did not change between subsequent P-40 versions, I copied its contours from the P-40E fuselage drawing (no. 87-21-401):

Figure 99-12 Copying the tail wheel cutout from the P-40E fuselage assembly drawing

However, from Dana Bell’s “P-40 Warhawk” (Aircaft Pictorial #5) I learned that the original tail wheel cutout was smaller. (In autumn 1941 Curtiss sent to all USAAC units field modification kits which extended the tail wheel and its cutout. This modification had to reduce the possibility of ground loops, which haunted the P-40s. The modified doors had a cutout for the bottom part of the tail wheel, which was not entirely retracted. It seems that this modification was not applied to the export Tomahawks, including the AVG fighters). You can see the original cutout contour in the figure above. I sketched it in different color – blue – because this contour is based solely on the photos. (This means that this line is an assumption, not confirmed by the original blueprints/dimensions)

During further work on this view I will mark in blue details that are not confirmed by any blueprint. Sometimes I can also draw elements that were partially confirmed. In such a case I will mix the blue and red. Thus, the share of blue in the line color reveals the % of the photo-based assumptions!

In the P-36 and the P-40 there was an additional cowling under the wing center. In the blueprints, Curtiss referred to this element as “keel”:

Figure 99-13 The “keel” under the wing center of a “Tomahawk” IIA (basically – the P-40B).

It masked the flange that connected the left and right wing and provided a cover for the fuel and oil lines. (The “long nose” Hawks had their oil tank in the tail, behind the fuselage fuel tank).

I found blueprint of this cowling in the P-36 and P-40E..N documentation. In the P-36 it was shorter and wider, thus I decided to assume that the P-40 “keel” shape was identical in all of its versions. (It seems so, but later I will carefully verify this assumption using some high-resolution photos).

According P-40E documentation, its keel was divided into three segments (drawing no: 87-23-502, 87-23-503, and 87-23-504). The geometry of these segments is described by ordinate tables. Figure 99‑14 shows such a table that describes the middle segment of this cowling:

Figure 99-14 Ordinates of the P-40 “keel” (middle segment: 87-23-503)

For drawing its side view contour, I needed just values W and V from this table. For example: from the row that describes station at X (distance from the firewall) = 83, I can read that W = 8.24. This means that the upper edge of this cowling is at Y = 8.24+20 = 28.24 (precisely speaking: -28.24, since it is below the fuselage reference line). Similarly, when V = 9.93 then Y coordinate of the bottom contour is -29.93. I put all these key points on my drawing and connected them with a line.  You can see the result in Figure 99‑15 (the precise contour runs along the outer edge of this outline):

Figure 99-15 P-40 “keel” contour

At this moment my side view drawing looks like this:

Figure 99-16 Current state of my P-40B side drawing

In the next post I will do a basic matching with photos to recreate some details in the mid-fuselage that were specific to the P-40cu/B/C. Then I will recreate contour of the most difficult (because of the missing documentation) part of this aircraft: the engine cowling with its Prestone/oil coolers, and propeller spinner.