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

The amount of the distortion is proportional to the depth of the opposite “bulges”. You can also recognize this distortion by the direction of the elevator hinge line. (This axis of rotation is usually perpendicular to the fuselage centerline). You can use some software tools to “flatten” such a distorted image, but this process is quite prone to various errors. (Especially in the cases of historical photos, as the one above).

It is much easier to find a modern photo of a restored aircraft, made with zoom lens. These shots are taken from a large distance, and usually depict the airplane in flight or just before takeoff or landing. In such photos the bulkhead seam lines “bulge” in the same direction. Below you can see an example of such a zoomed P-40C photo, made during an air show in Duxford:

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Figure 100-2 A high-resolution photo with minimal perspective distortion

Usually such an image is semi-orthogonal, and you can easily match its silhouette to your side view by stretching it a little along the fuselage centerline. Just remember that the vertical seam lines remain “bulged” in this picture. Thus, if you want to use it for recreating any side detail (an access door, cockpit frame, etc.) you still have to adjust it for the corresponding offset.

Below you can see how I fit this photo to my side view drawing. First I flipped it to match the left side view, then I stretched it a little in the horizontal direction, to match the key points on the drawing contour. In this case these key points were the windshield profile and the fin leading edge:

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Figure 100-3 The same photo, matched with the side drawing

Then I checked if the contours that I drew in red match the aircraft silhouette from the photo. As you can see, they fit it very well, including the keel. I can see just a single detail: the forward and rear edges of the tail wheel opening do not match their counterparts from this photo. However, when you encounter such a difference, it is better to check this detail in another photo of another aircraft.

Sometimes the details of restored historical airplanes differ from the original. You should be especially suspicious in the case of the aircraft restored before 1990.

Fortunately, I have found another zoomed picture. This is a photo of the restored P-40B (also from Duxford):

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Figure 100-4 Matching my drawing with another photo

The wing leading edge of the depicted aircraft reveals that it is rotated by about 10⁰ toward the camera. Thus, all the details located on the fuselage port side, away from the symmetry plane, seem to be shifted to the rear. You can see this effect on the windshield frame or opened tail wheel doors. This effect could also cause the small difference, visible at the end of the “keel”. Anyway, this match also confirmed that all the P-40 versions used the same “keel” cowling. (I can conclude this, because the red drawing of the P-40E keel match the keel from both photos).

Comparing such details in the restored aircraft still leaves an error margin: as I mentioned in the previous post, there is no P-40B/C documentation, and the restoration teams could also restore this keel cowling using the same P-40E drawing that I used. Still, its shape seems to match the original, historical photos. Nevertheless, I need a 3D model for eventual further checks with perspective-deformed archival photos. At this moment I am preparing the reference drawings for such a model, thus I have to postpone these tests for the future, when I build it.

At this moment I will use these mapped photos to recreate some details around the cockpit that were unique to the “long nose Hawks”. In Figure 100‑5 I marked them in blue:

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Figure 100-5 Details of the cockpit frame (black lines belong to the P-36 skeleton drawing)

I did not find any drawings of the “early P-40” rear glass frames. However, using contours of the corresponding frames from the P-36 (visible on its skeleton drawing) and the photos, I could obtain a fairly accurate approximation of this part. As you can see in the drawing above, there were also other minor differences between the first batch of the 200 P-40 (the first version, also called P-40-cu) and the later “long nose Hawks” versions (“Tomahawks II” and the P-40B/C).

In the next post I will wrestle with the least documented of the P-40B assemblies: engine cowling.

 

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