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:
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.
In my previous post I “fitted” my model of the P-40B into modern photo of a restored aircraft. (Precise speaking, it was a photo of the P-40C, but there were no external differences between these two versions). In general, I used Blender camera object to “pose” the 3D model so in the camera frame it looks just like the aircraft depicted in the photo. One of the key information that I used for this “fitting” was the lens focal length used for making the reference photo. (Modern cameras save key technical parameters in the resulting image file). I could just read this length from the photo properties, write it to the corresponding Blender camera Focal Length property, and focus on determining the remaining unknowns: camera location and direction.
However, how to use the historical “analog” pictures for such a match? (For example – this original Curtiss photo of the Tomahawk IA from November 1940:)
This summer I was asked by some readers for making a tutorial on my photo-matching method. This method allowed me to recreate the shapes of various historical aircraft with greater precision than the classic scale plans. (For example – the Fokker D.V or SBD Dauntless). This is the first post on this subject (I decided to split this tutorial into two subsequent posts).
The goal of the photo matching is to set up in your 3D environment a photo as the precise reference image (a more reliable equivalent to the scale plans). You can then use such a photo to verify, correct, and enhance the initial version of your 3D model. To begin, you need:
Initial 3D model. First you have to prepare an initial 3D model of the aircraft. You can do it in the classic way, using available scale plans and photos. This first approximation of the real aircraft does not have to be too detailed – prepare just the fuselage, wings and empennage. Eventually you can also add simplified landing gear (placing plain cylinders in place of its oleo struts) and the propeller blades;
High-resolution photo. Ideal reference photo should be detailed and free of barrel or pincushion distortions (i.e. it should depict the aircraft in a pure perspective projection). Of course, in practice such an ideal is not possible, but I will give you some hints how to identify a good candidate for the reference photo;
I built my models and matched them to the photos in Blender 3D program. In this post I am using Blender 2.80 (this is the actual version). I assume that the Reader knows the basics of Blender environment, in particular its UI and the navigation in 3D scene (“3D View” window). However, sometimes in this post I will describe some details of Blender commands that are obvious to its regular users. In this way I just want to minimize the risk that eventual Reader will “get stuck” in the middle of the described process.
For this tutorial I decided to use my old P-40 model, shown below. I built it several years ago in a “classic” way: using the scale plans.
In this old model I did not used any information from the P-40 blueprints, which I presented in my previous posts.
In the previous post I finally identified Curtiss layout sketch L-10202 as description of the XP-40 geometry, as it was in February 1940. In that time Curtiss was finishing preparations for serial production of the P-40. (The first P-40 from this batch was accepted by USAAC in April 1940). This final variant of the XP-40 resembled the serial P-40-cu, except the tail wheel cover and rear glass frames, “inherited” from the P-36. However, the archival photos revealed minor differences between engine cowlings of these aircraft: the serial P-40 had longer spinner and deeper radiator cover.
It seems that all the original drawings and sketches of the early P-40s that I collected from the AirCorps Library resources describe the XP-40. Thus, first I will prepare the XP-40 side view using this original documentation. Then I will draw a P-40B side contour, using these XP-40 lines and available P-40-cu/B/C photos.
As I showed in one of previous posts, the XP-40 sketches are not only rare, but also in poor shape:
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:
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).
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):
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:
Before you organize the original blueprints of an aircraft, collect as many reference photos as possible, and familiarize yourself with the aircraft shape, main assemblies and – especially – their joints. You will need all this knowledge to quickly recognize the drawings you need. About 60% of the original blueprints depict various small, internal details (tubes, brackets, plates, etc.) which are necessary only when you would like to build a real, flying airplane.
To select a useful subset of these blueprints, I had to review all the drawings in the microfilm set, and copy some of them into one of the target folders:
You can do such a “review” using two File Explorer windows: one for the source drawing list (of course with the preview pane), and the other for the target folder.
Recreating geometry of a historical aircraft is usually a painstaking, iterative process. You can see this in my work on the SBD Dauntless. During the long hours of studying the photos and trying to figure out the precise shape of this plane I often wished to have its source blueprints! For many years the access to the original documentation was “the Holy Grail” of the advanced modelers. (Everybody wished to have this ultimate resource, but only few saw it. And even those, who saw these drawings, often did not know what they are seeing).
When the production of an aircraft is definitely closed, and it quits the service, its blueprints are packed into manufacturer’s archives. After a few decades most of these companies are sold, while the less successful ones are out of the business. The original technical documentation of an aircraft usually becomes a bunch of useless, unreadable paper rolls that disappear in trash bins.
This winter I was busy with my daily business and took a break from the SBD model. However, in February and March I spent few Sundays helping in another project: the Fokker D.V biplane, used in 1917 as an “advanced trainer” by German Air Corps:
I was asked to take part in this project by C. West. He did all the research and provided all the materials: blueprints and photos. My part was recreating the geometry of this aircraft, especially its fuselage frame made of steel tubes. All what we had was a dozen of various archival photos, a poor general drawing, and the landing gear dimensions:
In this case I had to turn the available photos into the precise reference, as I did for the SBD, then use them to determine the required geometry details.