For over ten years Hugh Thomson has published marvelous posts in his blog about the historical aircraft. Just look there to see the P-51 Mustang, F6F Hellcat, F4F Wildcat and B-25 Mitchell CAD models and – what is sometimes even more important – compiled datasheets of their ordinates. The true, accurate geometrical data are usually dispersed and difficult to reconcile in the thousand sheets of the faded out, barely readable original blueprints. Hugh studied them all and is providing this information in the easy-to-use form. If you need such data on any of these aircraft – visit his page and choose any of these packages, or just make there a donation, to support his future projects!
This June I started working on a new (fourth) edition of my book about aircraft computer models. Actually, I am finishing its first volume (“Preparations”). It describes how to prepare accurate reference drawings of a historical airplane, on the example of the P-40. Below you can see two of its pages (as they appear my screen):
Comparing to the third edition, I altered here the proposed workflow, using Inkscape as my basic tool. I also wrote more about eventual errors, which you can find in typical scale plans. In the appendices I included a section about the original P-40 blueprints, which is based on the posts from this blog. Here is the link to the excerpt from this publication. It contains the table of contents. I expect to release this book in January 2021. (I will write a post, when it will be available).
As I already mentioned in this post, my microfilm set does not contain the fuselage geometry diagram. (I suppose that it was included in the missing roll C). Thus, this part of my work will be much more difficult, because I even do not have complete set of the bulkhead drawings! Just found a structure assembly drawing (i.e. side and vertical views), skin panels assembly drawing, mid-fuselage bulkheads, and some bulkheads of the tail. In the picture below I marked these undocumented areas of the fuselage in transparent red:
Douglas blueprints refer to the fuselage bulkheads as “frames”. They are numbered from 1 (the firewall) to 17 (the mounting base for the tail wheel and horizontal stabilizer). Of course, the fuselage assembly drawings provide their positions, measured from the firewall. (You can find them in this assembly drawing of the skin panels). In this post I will refer to fuselage bulkheads using their ordinal numbers, shown in the picture above (for example: “frame #05”).
In this posts I will analyze differences between my 3D model (built from 2015 to 2019) and the SBD geometry data obtained from the original documentation. Actually, I can perform such a ultimate comparison for the wing, because I found its original geometry diagram in the NASM microfilm. In previous post I used it for preparing a “reference frame” for such a verification. Results of this comparison will allow me to determine the real error range of my previous methods described in this blog, in particular – the photo-matching method.
Unfortunately, the incomplete microfilm set from NASM does not contain any other geometry diagram, so I will not be able to prepare such a precise reference frame for the SBD fuselage or empennage.
At the beginning, I identified an error in the wing location. It was determined by the position of leading edge tip of STA 66, marked as point A in the picture below:
In this post from 2015 I determined this location using the general arrangement diagram that I found in the SBD maintenance manual. As you can see above, there were issues in deciphering some of its dimensions. One of them was the distance from the thrust line to point A. I identified it as 20.38”, which means that in my model this distance from the fuselage ref line is 26.38” (6” + 20.38”).
A high-resolution scan of another arrangement diagram from Douglas microfilm (dwg no. 5120284) shows that this distance was 26.52”. (You can see this dimension in the picture above). Thus – this is the first identified error in my model, caused by a mistake in reading available drawings: 0.12”.
I am preparing data from the original Douglas blueprints to verify my model. For the beginning I chosen the wing. This is a well-documented assembly, because I found a master diagram in the NASM microfilm that describes SBD wing geometry (ordinals). Below you can see the first sheet of this diagram (dwg no 5090185):
Here you can download its high-resolution version (5MB). As you can see, it contains the ordinal tables of the wing bulkheads (ribs) and webs (spars). In the sketch on its right side Douglas engineers depicted various other dimensions of the wing center section. In the picture above I marked in red its key wing stations. Their names correspond to spanwise distance in inches from the aircraft centerline: “STA 10” is 10” from the centerline, while “STA 66” is 66” from the centerline.
In general, the set of 7 SBD/A-24 reels from NASM contains 3308 unique microfilm frames, belonging to 3022 drawings. On reels “XA” and “XB” you can usually find updated copies of the previous reels (“A”, “B”,.. “F”). However, 350 frames from “XA” and “XB” are unique – most probably this is a part of the missing roll “C”. Duplicates from these “X*” reels are also useful, when a drawing from one of the previous reels is unreadable.
I chose about 1000 frames (mostly assembly drawings) from this microfilm set, and organized them into a tree-like structure as in Figure 108‑1:
To preserve disk space, I placed in these folders shortcuts to files located in the original directories (These original directories correspond to microfilm reels: “A”, “B”, …, “XB”). I practiced that when I click such a link, it opens the image in Photo Viewer, as if it was the original file.
In June 2019 I followed C. West suggestion and ordered a set of Douglas SBD original technical documentation from U.S. National Air and Space Museum. Technically these blueprints are stored on several microfilm rolls. In that time all what I knew about this package (NASM id: “Mcfilm-000000408”) was the information printed on the order form:
As you can see, this set has no index, which I could order earlier to examine its contents. When I finally received these microfilms in November 2019, I also discovered the meaning of enigmatic “(roll C” in the item description: it was truncated phrase “(roll C missing)”!
Well, this set was incomplete, but anyway I ordered its high-resolution scans from a local company that provides professional microfilm scanning services to museums. In January I received these data (4700 high-res, grayscale images in LZW-packed TIFF format – in total, about 300 GB). Finally I was able to scroll these blueprints. Frankly speaking, I was afraid that the most important drawings were lost with the missing roll C. Fortunately, during the initial review I noticed many detailed assembly blueprints among the scanned images. I even found a complete inboard profile of the SBD-5:
Here you can download the high-resolution version of this inboard profile (about 70MB).
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 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: