Verification of an Unidentified Layout Drawing

Generally speaking, the early P-40s (-cu, B, C) were “P-36 airframes with inline engines”.  Thus, the only unique first-order assembly in these P-40 variants was their engine compartment. So far it seemed that the documentation of this area was lost, and the restoration teams had to rely on archival photos and other restored P-40B/C. (A P-40B restoration teamfrom New Zealand mentioned this in their interview).

In my post from August 2019 (Fig. 98-13 and Fig. 98-14) I described a previously unnoticed layout sketch, that I found among the “uncategorized” P-36/P-40 drawings in the AirCorps “P-40” microfilm set:

Figure 101-1 Engine cowling layout sketch (L-10202)

It can describe the geometry of the “long nose Hawk” engine cowling. In the same AirCorps Library uncategorized “pile” I also found some regular XP-40 drawings (engine mount, radiator support) and other sketches. However, the lines in all these images are faded, making them nearly unreadable. The L-10202 sketch is the most promising blueprint that I have found. In this post I will try to match this layout to the P-40B fuselage that I prepared in my previous post. I will also use photos to evaluate the results (i.e. for checking if the sketched engine cowling layout matches the real aircraft).

Because the scanned images can be deformed by the perspective/barrel distortion, I drew the side contour of this cowling using the ordinates from L-10202. Below you can see the results:

Figure 101-2 Contours of the spinner and engine cowling, according L-10202 ordinates table

This contour perfectly fits the firewall. However, it does not contain any hint about location of the gap between the spinner and engine cowling. What’s worse, the overall length of this contour is 116.5” – it is 1.688” short of the P-40 cowling length. (According the general assembly dimensions it should be 118.188” from the firewall to the spinner tip). Let’s check this result with the photos:

Figure 101-3 Checking L-10202 contour against the photo of a restored P-40C

The aircraft in the photo above is a restored P-40C, from Duxford. As you can see, its spinner has a thinner tip. It is also longer by about 1.5”. (Thus, the difference in overall length is caused by the wrong spinner contour). The cooler inlet seems to be in the proper location. (It is difficult to precisely determine its location on this photo, because this aircraft from has significant deflection from the camera, and all its bulkhead lines are accordingly “bulged”). However, the photo reveals yet another mismatch: the radiator cover is somewhat deeper (by about 1”) than in L-10202 contour.

Of course, when I found a difference on the first photo, I had to check if it also appears on the other pictures. Below you can see the results of another match (this is another reconstructed P-40, in this case – the P-40B, also from Duxford):

Figure 101-4 Checking the contour against the photo of a restored P-40B

These two photos were modern pictures of restored aircraft. Just to make sure I also checked this contour on a historical Curtiss photo (this is “Tomahawk” IIA – an equivalent of the P-40B):

Figure 101-5 Checking the contour against a historical photo (“Tomahawk” IIA)

After these three matches, we can confirm both differences: in the spinner length and in the radiator cowling depth. The difference of the bottom contour in the front of the cooler inlet requires further investigation, because it is clearly visible only on this last (historical) photo.

The difference in the spinner length I can explain by comparing one of the first general assembly drawings of the XP-40 with the general arrangement drawing of the P-40:

Figure 101-6 The difference in the overall length between the initial XP-40 and the P-40

The first XP-40 variant had to have overall length of 31’ 6.875”, while the production P-40 was 31’ 8.5625” long. The difference – 1.6875”- precisely matches the difference of the cowling length between the L-10202 drawing and the P-40.

Note that the USAAC general arrangement drawing hardly resembles the real aircraft. However, this is common among such “general  layouts”. The goal of this picture was just to show that this is a single-seat, pursuit aircraft with low, cantilever wing and an inline engine, and provide a few overall dimensions.

Indeed, the XP-40 spinner seems to have a more oval tip in its photos (unfortunately, I had only a low-resolution picture):

Figure 101-7 The final configuration of the XP-40 prototype

What’s more, when I stretched the engine cowling area from this photo between the gap behind the spinner and the firewall, I discovered that it fits the L-10202 layout contour:

Figure 101-8 Comparing the silhouette of the XP-40 prototype with the L-10202 contour

It looks like that these were minor differences between the prototype and the production aircraft! However, I had some doubts about the precision of such a simple “stretching” of a perspective-distorted photo over an orthogonal drawing. The more complex reversion of the perspective distortion is also prone to various errors, especially in the case of such a historical photo (as I mentioned in the previous post). Then I came up with an idea of taking the advantage of Curtiss photographers’ habits: for their shots, they set up this XP-40 and the “Tomahawk” fighter in nearly the same “pose”. In both photos the left landing gear leg obscures the right leg, and one of the propeller blades is pointing straight down. If you place one of these photos over another to match both silhouettes, it will reduce eventual adjustments of the compared images to the minimum. What’s more, both photos have similar perspective/barrel distortion (they could be even made using the same camera), so I do not need to compensate this deformation. This means, that such a method can be quite precise in revealing the differences in the spinner and engine cowling shapes.

Below you can see the result of this match:

Figure 101-9 Comparing the silhouettes of the XP-40 and P-40B (“Tomahawk” IIA)

To make a fair test, I had to use for the “anchor points” different elements than in the previous match. Surprisingly, the propeller bottom blade occurred to be an ideal matching point on the aircraft nose. Using it as the first “anchor”, I rotated and slightly stretched the P-40B image, until its rudder shape matched the XP-40 rudder (from the Curtiss documentation I know that they had identical shape). In the figure above I outlined these “anchor” contours using white, dashed line. For easier identification, I also colored the XP-40 picture in red. In the figure above you can recognize the differences between these two pictures as the lighter areas (for example – at the wingtip) or darker areas, which look like shadows (for example – behind the landing gear). The white area at the wing tip reveals that there was a slight difference in the orientation of these aircrafts toward the camera. (Of course, nobody required a high precision in this “outdoor photo studio”). This difference causes a slight deviation of the vertical contours, proportional to their distance from the symmetry plane. They are visible in the cockpit canopy frames and as the “shadows” of the landing gear. As you can see, the silhouettes (contours on the symmetry plane) of these two aircraft match each other except for the bottom edge of the radiator cowling. (This ultimately confirms my hypothesis from the previous picture!). However, the lighter XP-40 spinner “sinks” in the larger and darker P-40B cone, so you cannot see the difference in the spinner lengths in this picture.

To better illustrate these differences, the picture below shows the XP-40 engine cowling and the P-40B contours outlined by dashed white line:

Figure 101-10 P-40B cowling and spinner contours drawn over the XP-40 photo

Conclusion: layout L-10202 describes the XP-40 geometry of as it was in February 1940. In the production P-40s the spinner was longer by 1.688”, and the radiator cowling was slightly deeper (by about 1”). The L-10202 ordinates table matches the upper part of the cowling (180⁰ of each cowling cross-section above the thrust line). The differences along the engine cowling sides (down to -60⁰ below the thrust line) are probably minimal. Well, it is always better to have even such a partial ordinates table than nothing.

Figure below shows the resulting cowling contours (fragments confirmed by the blueprints/ordinates are in red, recreated from the photos – in blue):

Figure 101-11 The XP-40 and P-40 engine cowling contours

In the L-10202 drawing you can find the centerlines and sizes of the cylindrical Prestone and oil radiators, used in the XP-40. Consequently, I think that in the production P-40s these radiators were mounted a little bit lower.

After this experience with the unidentified sketch, I will assume that all sketches from the AirCorps Library microfilm set that are missing the type description in their title blocks (like L-10202) describe the XP-40.

In the next post I will complete this P-40 side contour.