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P-40B (based on the original Curtiss blueprints)

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  • Member since
    June 2014
P-40B (based on the original Curtiss blueprints)
Posted by Witold Jaworski on Sunday, August 4, 2019 11:22 AM

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). For example - below you can see the original so-called "side profile" of the P-47:
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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.

Such a “total destruction” could occur, because in the “pre-computer” era there was just a single master drawing of each part, made (if there was enough time) in black ink on the tracing paper. (If there was not enough time, it could be even a pencil sketch). For the “everyday” use, the factory made cheap (and volatile!) workshop copies of these drawings, using the old blueprint or the newer diazoprint methods.

Both of these copying methods produced 1:1 duplicates of the original drawings. The older blueprint method was passing out in the 1930s, because it produced white lines on the Prussian Blue background (as in figure above). The diazoprint copies were preferred, because of their bluish lines on white (more-or-less) background (as in figure below):
 
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Unfortunately, the contrast of the blueprints and diazoprints gradually faded after a few months of use (they are sensitive to light). Because of this factor, coupled with the usual wear and tear, the aircraft manufacturer had to continuously provide new replacements of the workshop drawings for the production and service purposes. In the WWII period they started to reproduce these drawings on microfilms. (This compact form was more economic and durable for delivering the technical documentation to all the service facilities, especially to those located overseas). Some of these microfilm rolls were later deposited in the national archives. This process was continued after the war, thus nowadays in the archives you can find microfilm rolls that contain documentation of various historical aircraft. However, this archival “environment” is better known to the professional historians than to the hobbyists modelers. (Historians know better, how to query a museum/library for the materials on a certain subject). Even if you manage to get the requested copies, there are also other questions: how to convert these microfilms into usable drawings? How to find among several thousand of drawings the one that you need?

Answering the first question: generally, there are special devices that scan the microfilm frame and store its digital image on the computer disk. You can buy such a low-end scanner, intended for scanning family slides (resolution: 3000dpi), in a computer store. It costs about one hundred dollars. The advanced microfilm scanners (of higher resolution: 4500dpi or more) are three to four times more expensive. (In the further text you will see why the resolution is so important). There are also companies which offer professional microfilm scanning services to various archives/libraries, as well as to the individuals.

To answer to the second question – how to use the technical documentation without going crazy (or lost) I need an example: scanned microfilm rolls. Fortunately, today you can find several Internet portals that offer the scanned microfilms of a few aircraft that are most popular among the aviation enthusiasts. Most of them are the WWII U.S. fighters, but you can find a few German (the FW-190, Bf-109, Me 262) and British planes (Avro Lancaster, Supermarine Spitfire, Sopwith Camel).

Ten years ago, there was a single site that offered these blueprints, and its prices were prohibitively high: as I remember, about one thousand dollars for a single aircraft. Fortunately, now they have competition, and the cost of a single set dropped to about one hundred dollars (depending on the site).

 

I will show you how I work with the original documentation of an aircraft on the example of the P-40. Scans of the blueprints that describe its later versions (P-40D..N) are widely available. This spring I decided to update my P-40B to the new Blender version (2.8), so I thought about buying a set of the P-40 blueprints, to improve the quality of this digital model. However, I did not expect that finally I will buy three different P-40 sets from three different portals! In this post I will describe, how it happened. In this way you will learn on my mistakes. In the next post I will describe how I organized these scanned images and used them for preparing the references for my model.

Initially I knew two blueprint portals: www.plans.aero and www.aircraft-manuals.com. Both of them offered about 7 thousand drawings of the P-40D..N. It seems that these blueprints were scanned from the same set of 8 microfilm rolls, labeled from “A” to “H”.

I started by ordering the cheaper set from aircraft-manuals.com: 68.85 USD + 7.85 USD of the shipping costs (they are located in Italy). After ten days I received a CD with these drawings. Technically they were monochromatic *.tif files, scanned at 2500dpi. (Size of each image: 3200x2368px). These files were organized into eight folders, which names corresponded the microfilm rolls (from A to H). The first frame of the microfilm contained the roll title page. You can find it in each of these folders under TIT~*.TIF name (for example – below you can see the TIT~2348.TIF file from folder E: )

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You can learn from this picture, that these drawings describe the state of the P-40 E, F, K, L, M, N documentation as on August 26th1943.

The easiest way to browse contents of these folders is enabling in your File Explorer the “Extra large icon” view option (as in figure below). All scan files (except the title) are named <N>_<o>.tif, where <N> is the original reference number (“punched” on each microfilm frame). The assembly blueprints, made on long paper rolls, can span over several subsequent microfilm frames. In such a case their frames share the same reference number <N>. However, their scans have different <o> suffixes. (It seems that the <o> is an ordinal number of the scanned file).

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For example: on the roll “E” there were two subsequent frames marked as “E177”. They contained the wing skeleton assembly drawing, made on a non-standard format. However, there are four files named 177_*.TIF in the folder E (see figure above). It seems that the person who made these scans, first created the 177_232.TIF and 177_233.TIF files. I suppose that later somebody reviewed these scans and found that the left edge of the wing in file 177_232.TIF was clipped. Thus, they made two additional scans of these frames. These corrected pictures are named 177_751.TIF and 177_752.TIF (However, I do not understand why they did not remove the first, spoiled pair of files. Maybe because in 177_752.TIF some lines fade into the background?).


My basic idea of organizing these drawings into something usable for the further work was a tree-like folder structure. On the first level I created folders for the basic assemblies: Fuselage, Wing, Empennage. Then in each of these folders I created directories for their subassemblies. For example, in the Fuselage folder I created the Spinner, Engine Cowling, Cockpit and Tail Wheel directories. Then I was going to review and move the scans from the source (Roll AH) folders into this newly created structure. I thought that I rename each moved file, giving it a descriptive name (like “General Assembly”, “Skeleton”, “Bulkhead at STA#1”, and similar). In each of these target folders and subfolders there should be no more than 100 drawings. (Eventually I can also create additional subdirectories, when needed). Of course, I will skip all the unnecessary drawings of small inner details, focusing mainly on the assemblies.

However, there was a drawing identification issue. Recognizing the main assemblies, like the one below, was easy:

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But when it comes to the subassemblies, you often cannot identify the depicted part at the first glance. For example, look at the drawing from frame E261:

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I could not identify this part just by its shape. So I looked at its description (they are always placed in the lower left corner). There was an unpleasant surprise: the original drawing was so large that its title block is nearly unreadable. (The scanner resolution was too low!). 

Fortunately, aircraft-manuals.com also provided scans of the index pages that originally accompanied these rolls:

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It was intended for searching by the drawing number or part name. I did not know the drawing number (last digits in the title block are unreadable). The name index is divided into seven main sections: “Wing”, “Tail”, “Body” (i.e. fuselage), “Power Plant”, “Flight Control”, “Equipment”, “Armament”. I was afraid that searching this index is not especially practical, since the part names are not unique: the first four pages of the “Wing” section list items simply named “Angle”. Fortunately, in the “Body” section you can find also the bulkhead station numbers. Using them I finally confirmed that the first word “B’LK’D” is a Curtiss shortcut for “BULKHEAD”, and that this E261 frame contains the STA#3 bulkhead assembly. (From the index entry I could read its drawing number: 87-21-603). Note that even in this index some frame references in the “FILM INDEX NO.” column are unreadable, because of the low image resolution!

When I started to study details of the identified drawings, the low resolution of the assembly images became a serious issue. The references to the detailed drawings were unreadable, as well as many key dimensions. For example – look at this middle part of the fillet between the wing and fuselage (E201), depicted in the blueprint below:

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In this case the title block was completely unreadable, so I had to confirm its identity searching the fuselage name index for the “Fillet” term. In this way I identified the number of this drawing: 87-06-503. Strangely enough, in this index you can find the next segment of this fillet (drawing number 87-06-502, E200) several pages later, under name “Skin”. The geometric data that describe shapes of these fillets are placed in the upper left corner of their drawings. As you can see in figure above, they are completely unreadable.

The poor image quality of these scans is not only caused by the inadequate resolution. They also lack shades of gray, which could improve a little bit the overall readability. (They are high-contrast black-and-white images, without any halftones).

At this moment I realized that scans from aircraft-manuals.com are of little use for me, and started looking for better ones. Thus, I read again the description of the P-40 package offered by the other portal: plans.aero. They claim that “Roll E has now been completely re-mastered at super high resolution because the original records were overexposed.. We have scanned Roll E @ 4,500ppi and darkened the scans to bring up all detail in the film”. Now I understood what they are saying! (Roll E contains most of the assembly drawings, so this is the “critical resource” of this microfilm set).

Thus, I bought my second P-40 documentation set from plans.aero. It costed 119 USD, and there were no shipment fees, because you can immediately download these files. The total size of this set is about 6GB (ten times more than the set from aircraft-manuals.com). For the user convenience it is split into four parts for separate downloads. For the higher price than aero-manuals.com, plans.aero provide not only the scanned rolls, but also all the available P-40 manuals, in particular the P-40N “Erection and Maintenance” and illustrated “Parts Catalogue”. (These books are a useful addition, and they would cost much more when purchased separately).

The scanned files are grouped into the “roll folders”, as in the set from aircraft-manuals.com. It seems than plans.aero provided the gray-scale pictures for rolls A and F (that’s why in some of them you can see white lines on a black background). There are also two folders for roll E. One of them, titled “old scanning” contains the same pictures that I got from aircraft-manuals. The another, titled “new scanning”, contains PDF files. Each of these PDF is a scanned film frame:

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As you can see on the picture, all the PDF files have a “P-40” prefix, followed by the full microfilm reference number (with the roll letter prefix: “E”). For the drawings that span over multiple frames, the sequential suffix is used (“-a”,”-b”, “-c”, and so on). Using this rule, I assumed that there should be two files: “P-40 E 177-a.pdf” and “P-40 E 177-b.pdf” for the wing skeleton drawing (E177). To my surprise there is only one, named “P-40 E 177.pdf”. It contains the right side of this drawing. Where is the left side? I quickly found that you can find it in file “P-40 176-e.pdf”. (An evident mistake of the scanner operator, but once identified, you can easily correct it by renaming these two files).

Of course, the first thing that I have checked in these new files was the table with the geometry data in the upper left corners of drawings E200 and E201:

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It was definitely better: now I was able to read this simple diagram, the fillet ordinates table, as well as all other dimensions and drawing references.

I could close this blog post here, concluding that the www.plans.aero offered better blueprints of the P-40 (and additional manuals). However, when I described this case to my friend, he suggested me to visit yet another site: www.aircorpslibrary.com:

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AirCorps Library portal was set up in 2015. For 5 USD per month (or 50 USD per year) it provides online access to the original documentation of various aircraft, including manuals and drawings. At this moment you can find there documentation of 29 aircraft (all of them are U.S. designs from WWII-era, with a single exception: the Spitfire). Among them is the “P-40 Warhawk” section:

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Instead of the indirect “roll index numbers” this site uses the original drawing numbers from the drawing title blocks. (I suppose that they used an OCR process to “extract” them from the scans). Using the drawing numbering rules specific to the manufacturer of each aircraft, AirCirps Library automatically splits the scanned drawings into subassemblies. For example: it seems that in Curtiss the drawing number was built of 7 digits divided into three sections: TT-AA-NNN. The “TT” was two-digit type designation. In particular, the “75” prefix is for the “Hawk 75” (i.e. P-36), while “81” is for the YP-37, and  “87” is for the “Hawk 87” (P-40D and later versions). (Frankly speaking, I would expect that “81” stands for the “long nose Hawks”, i.e. early P-40 versions, designated by Curtiss as “Hawk 81”. However, this is not the case). The “AA” (after 1942 also three digit: “AAA”) section indicates the subassembly, for example: “03” – Wing components, “05” – Aileron, “06” – Wing Assembly, “08” – Wing Flap, “21” – Fuselage, “23” – Fairings, and so on. (The diagram of the P-40 components is based on these two or three middle digits of the drawing number). Finally, the NNN segment seems to be a kind of an ordinal number. The assembly drawings usually have one or two leading zeros in this last segment (for example: “001”, or “010”).  Thus, in this graph you can find the fillet between the fuselage and wings in the group “06 – Wing Assembly”, because their drawing numbers are 87-06-502 and 87-06-503. (I just do not know why in the microfilm index these parts are assigned to the “Body” section – i.e. fuselage – instead of the “Wing” section). In this “analog era” you could encounter a trace of human error everywhere: thus, in the “06” section you can also find an forgotten P-36 “Markings & Insignia Airplane” drawing (75-06-013).

I am not sure, but it seems that different aircraft versions (for example – the P-36 and the P-36C) can be recognized by the fifth digit of their drawing numbers. For example: the basic wing design is described by drawings numbered from “75-03-001” .. to “75-03-3XX”. The P-36C had different leading-edge section, which is documented by “75-03-4XX” drawings. Similar, you can find the two-gun wing assembly of the export Hawks 75 in the “75-03-6XX” drawing series).

Note that I have just mentioned another aircraft: the P-36. This is not a mistake: this “P-40” section of the AirCorps Library contains 11 thousand blueprints of at least three (!) different (although related) aircraft types:

  1. P-36 (internal Curtiss name: “Hawk 75”, drawing prefix: “75-*”);
  2. YP-37 (drawing prefix: “81-*”);
  3. P-40 cu/B/C (the “long nose Hawks” - internal Curtiss name: “Hawk 81”)
  4. P-40D…N (the “short nose Hawks” - internal Curtiss name: ”Hawk 87”, drawing prefix: “87-*”)

Depending how you count p. 3 and p. 4, there are three or four aircraft. It seems that Curtiss prepared the P-40 project “in a hurry”, because there are just a few XP-40/P-40cu drawings, most of them of poor quality. (Their light lines fade into the background – as if they were originally made in pencil instead of the tracing paper and ink). Strangely enough, all these early P-40 drawings have “75-*” suffix and “8” as the fifth digit: “75-XX-8XX” (as if this aircraft was another version of the P-36). Could it happen that it inherited the “Hawk 81” name (used internally by Curtiss) after the unsuccessful YP-37 project?

When you click one of the assembly graph items or type a mask expression in the search box (for example an expression like: “75-*-8*”), you will obtain the blueprint result list similar to the picture below:

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In other aircraft sections (for example – the P-51) you can also search by drawing name. However, most of the P-40 drawings have no description, yet. When you click the drawing miniature on the left or one of its buttons on the right, you will open this blueprint in the drawing viewer. Figure below shows how it looks like:

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The online viewer is quick and intuitive. It allows you to switch between subsequent microfilm frames. It also provides the pull-down menu where you can see their miniatures. You can easily zoom and pan across the image using the mouse (in the same way as you do it in the Google Maps). The portal dynamically adjusts resolution of the displayed image to the current zoom settings.

For me, the most important feature is the possibility to print this drawing locally, in a decent resolution, without any additional charges. Below you can see how such a printout looks like:

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The paper size of your printout does not matter – it can be A4 or A3, it does not have any influence on the picture details. This portal simply sends to your printer a raster, grayscale picture which is about 6000px wide and 3870px high. For my purposes I need these pictures in an electronic form, thus I used a popular print capture utility to convert them “on the fly” into PDFs.

When I visited AirCorps Library site for the first time (at the beginning of June 2019), there was a single purchase option: a full-scale printout, which was not especially interesting for me. When I am writing this post (two months later), they introduced some modifications. Now you can buy and download copies of the manuals and drawings that they expose online. Thus, when you click the “Purchase” icon in the drawing viewer, you will see two options: “Purchase Hi-Res” and “Download”. Selecting the “Purchase Hi-Res” option you can order a paper printout (on a 24”, 36”, or 42” sheet). The new “Download” option allows you to buy the image you are viewing (size of the downloaded image: 9888 x 6984px). However, this means that in the case of multi-part assembly drawings you have to buy each microfilm frame separately. As long as the high-res printouts are available, I am not interested in the downloading option. (I have just bought a single picture, and concluded that the local printout resolution is enough for my purposes). 

Below I compared the test images (this is the fillet ordinates scheme and table from the upper-left corner of drawing 87-06-503). I obtained these pictures from the aircraft-manuals, plans.aero (“new scanning” PDF), and AirCorps Library (using the local printout function):

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As you can see, the best images are from AirCorps Library. I suppose that they scanned the Curtiss microfilm at 7000 dpi, and the original size of each scanned frame is as in the purchased image: about 10000 x 7000px. For the local printouts this image is scaled down to 6000x3860px, but this operation only slightly degrades its quality. (The image on the right is the local printout fragment).

Conclusion: if your are looking for the blueprints of one of the U.S. aircraft (or the Spitfire) – start by buying a monthly subscription of the AirCorps Library online resources. Even if they remove the local printing function (this portal still evolves!) you can use it to learn what the preserved microfilms contain. A complete package of the blueprints and manuals you can buy from plans.aero – and eventually use the AirCorps Library to check some details in the higher resolution.

Note that the collections of AirCorps Library and plans.aero are not identical. The plans.aero portal offers only the most popular U.S. fighters: F4U-1, P-40, F4F, F6F, P-51, P-47, and a single bomber: TBF-1. However, you can also find there the blueprints of some foreign aircraft that are not present in the AirCorps Library: Bf-109, FW-190, Me 262, Avro Lancaster, Sopwith Camel and Pup.
 
Finally, if a certain aircraft is available only in aircraft-manuals.com – you can use this portal, but first try to ask for a sample of a large assembly drawing from the package you need. (I do not know if they will answer, but at least it is worth to try such a check before purchasing). Maybe they scanned the microfilms of other aircraft using better resolution? You can also use as an “ad-hoc” indicator the number of DVDs they offer in the set divided by the number of scanned drawings it contains. The lower value of this ratio, the better.

In the next post I will describe how I use these original P-40 blueprints, preparing the reference drawings for my model. (In this way I will try to answer the second question from this post: “how to use the technical documentation without going crazy”).

  • Member since
    January 2015
Posted by PFJN on Monday, August 5, 2019 7:05 PM

Hi,

Can't wait to see more.

Pat

1st Group Build

  • Member since
    June 2014
Posted by Witold Jaworski on Wednesday, August 21, 2019 4:28 PM

Thank you, Pat!

____________________________

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.

To quickly find any of the selected drawings during the further work, I organized them into a tree-like folder structure. As you can see in the picture above, each target folder represents an aircraft assembly. Initially there are just three: Empennage, Fuselage and Wing. Then each of these main assemblies (folders) splits into subassemblies. For example – Empennage contains separate subdirectories named Fin, Rudder, Stabilizer and Elevator. I copied the interesting files from the source list into the appropriate subassembly directory. Then I renamed each of these copied files, giving it a descriptive name, and an ordinal number (for example – “Assembly-01”). The ordinal number is needed, because you can often encounter several variants of the same drawing, coming from various versions of this aircraft. If I encountered another assembly drawing of the stabilizer, I would copy it into this directory and name “Assembly-02”. Note that these file names are relatively simple, because the tree-like folder structure provides the necessary context (I know, that this “Assembly-01” file contains the drawing of the stabilizer assembly, because it is located in the Empennage\Stabilizer\ path). In the root folder (Empennage) I can place drawings that are related to whole assembly, like the empennage erection scheme, which describes how to mount the fin to the stabilizer and the stabilizer to the fuselage.

Of course, the structure of these folders and subfolders reflects the structure of the particular aircraft. For example, in the case of the Soviet La-5 fighter the fin is an integral part of the fuselage, thus there would be no separate “Fin” directory

In my case the I used the AirCorps Library site as the source of the blueprints. I did it, because it already grouped the original drawings into categories, based on their unique numbers:

In practice, drawing numbers of each manufacturer contain a segment which describes the category (assembly) of the depicted part. Of course, every manufacturer used its own rules for this purpose. Sometimes these rules are difficult to identify. For example, the P-39(/P-63) drawings in AirCorps library are categorized by first three digits, but these categories are not named. You have to use the name index of the original microfilm (provided on the same page) to find the key assembly drawings of these aircraft.

 

Fortunately for me, in the case of the P-40 AirCorps Library properly identified the middle section of the Curtiss drawing number as the assembly id, and even provided appropriate assembly names. In most cases these categories are more detailed than I need. For example: I used just the single assembly drawing from the “Engine Mount Install” category, and skipped the entire “Engine Starter” category. However, note the large number of the “Uncategorized” blueprints: 3403! I reviewed this group as the last one. First I tried to complete the assemblies using the drawings from the identified categories. Then I learned, which blueprints were missing, and looked for them in this “unidentified” category.

I reviewed the AirCorps Library blueprints using their web page result list, and “printed to PDF” the drawings that I selected. (For details, see the last part of my previous post). I saved these PDFs into corresponding target folders.

In the figure below I am showing an example of such a “target” directory, which describes a single subassembly: the stabilizer. As you can see, I chosen just few drawings that describe this part: except the general assembly (which you can see in the first figure of this post), I also chosen the blueprint of its ribs:



(The last rib, located at the stabilizer tip, is documented in another drawing, which I named “Ribs-02”). Note that each of these AirCorps Library pictures contains a large drawing number. I can use it for re-checking the source list. (Sometimes this information allowed me to skip duplicates).

Another stabilizer drawing – “Webs-01” – describes the stabilizer webs (spars):



As you can see, I also selected two important details: elevator hinges. (In the P-40 they are visible in the small openings in the elevator leading edge).

Some drawings were re-used between subsequent aircraft types. In this case the detail drawing of the stabilizer tip edge is located in the parallel folder tree of the P-36:



This edge was a simple “V” – shaped beam, bent along an arc. It was identical in the P-36 and P-40.

As I mentioned in previous post, the microfilm scanned by AirCorps Library contains mixed drawings of the P-40, YP-37 and P-36. I skipped the YP-37, but for the P-36 I prepared a parallel folder structure. Fortunately, you can recognize each aircraft type by the prefix of its drawing number: “75” is for the P-36, “81” is for the YP-37, and “87” is for the P-40.

Of course, the more complex assemblies, like Fuselage, contain more drawings:



I created some of the subassembly folders (for example: Antenna) just because the number of the antenna drawings exceeded seven (there were several variations of the antenna mast). Note that there are many assembly drawings (“Assembly-02”, “Assembly-03”, and so on). Curtiss engineers had to prepare a new one for each of the P-40 versions (D, E, F, K, M, L, N). I even created a separate subfolder for the variants with extended fuselage (later variants of the P-40F, K, M, L and all of the P-40N).

When a blueprint spans over several microfilm frames, I saved them with additional “a”, “b”, “c” suffix:



In general, in this Fuselage folder I placed the assembly drawing, skeleton drawing, and the drawings of each bulkhead. (Usually such a bulkhead is also built of many components, thus in the original documentation they are also referred as the assembly drawings). Sometimes there are more than one drawing for a bulkhead (especially the most complex one: the firewall). I skipped the lengthwise stringers, because they are simple L-shaped beams, slightly bent between subsequent bulkheads. For similar reason I did not copy from the source list the fuselage longeron drawings. I also selected some important details – for example the wing attachment fittings. (although they were hidden behind the fairings, they were visible from within the cockpit). There are separate subfolders for the spinner, engine cowling, cockpit, and tailwheel. The engine cowling had several variants, thus I created additional subfolder: one for the “long nose Hawks”, i.e. P-40cu, P-40B and C, one for the “short nose Hawks” (P-40D and later versions), which in turn splits into the cowlings of the Allison-powered (D, E, K, M, N) and Merlin-powered (F, M) aircraft.

Similarly, in the Wing folder I placed the assembly drawings, skeleton scheme, and drawings of the ribs and webs. There are also separate subfolders for the flaps, ailerons, wingtip, fuselage fairings, keel (the “continuation of the fuselage” under the wing, specific for the P-40), guns, landing gear and its fairing.

Ultimately I selected about 900 P-40 drawings, organized into 36 folders and subfolders. There are also about 450 P-36 drawings, organized into 23 folders and subfolders. However, I realized that the documentation from this microfilm set is not complete: I could not find some important drawings that are listed on the assembly blueprints.

What’s more, I found just a few of the “long nose Hawk” (P-40-cu/B/C) drawings. For example, the largest P-40/B/C part that I identified is the Allison-1710 C15 engine mount:



The rest of the identified “long nose Hawk” blueprints describes the XP-40 prototype. Judging by the drawing numbers, the XP-40 was treated just as another P-36 variant: these numbers have “75” prefix and “8” in the fifth digit. For example – you can find in the AirCorps Library the drawing of the XP-40 radiators support under number: 75-50-858:



Note the hardly visible, thin lines of this drawing. In some areas they disappear into the background. This is the rule, not exception: most of the XP-40 drawings is even less readable!

There are also many XP-40 drawings where the only recognizable element is the title block:



Abut 25% of the scarce XP-40 blueprints is unreadable! It seems that because of the poor contrast of the original drawings and improper microfilm camera settings these pieces of information are lost forever.

This is a serious problem. I started this project to improve my P-40B model. I can use the blueprints of the P-36 four-gun wings and the P-36 fuselage (the part behind the firewall) as the first approximation of the P-40B airframe. They can be augmented by some details from the P-40D/E drawings: the “keel” under the center wing, the flat, larger tailwheel cover, larger main wheels and their simpler fairings. However, I desperately needed the geometry data of the unique P-40/B/C engine cowling!

It seems that I am not the first one who encountered this problem. In the “Classic Wings” magazine, issue 80 I found an article about a P-40B restoration made recently by AvSpecs team. Because of the lack of any documentation, they restored the engine compartment using just the photos of another restored P-40! (See the marked fragment):



In such a case there is a chance that they are also copied some non-original details, introduced during restoration of this P-40B from the Flying Heritage Collection.

Having all this in mind, I started to review the 3400 uncategorized drawings from the AirCorps Library. First I found there an additional P-36 category, omitted probably by a mistake: drawings 75-28-XXX that describe the NACA ring of the Double Wasp engine. Of course, they allowed me to fill this gap in the P-36 documentation, but it was not my main concern. The other drawings in this “uncategorized” list seemed to belong to the Y1P-36 (drawing no. prefix: 85-* and 99-*), XP-37 (prefixes: 84-* and 90-*), and P-40F (prefix: 96-*). There were also special classes of “layout” drawings (prefix: “L”), working sketches (prefix: “SK”), and design proposals (prefix: “P”).

I have found there some XP-40 sketches, like this one (SK-2698):



In the title block of this sketch I could read “Cooler Duct – Rear”. Looking close at these thin, vanishing lines, I was able to recognize the rear view and side view. However, between October 1938 and April 1940 the XP-40 engine cooling system was redesigned four times, and only the last of these modifications, made after December 1939, match precisely the ultimate shape of the P-40/B/C cooler. I was not sure if this drawing depicts this last update.

I also stumbled upon a layout sketch (number: L-10202). In its title block you can read “Basic Cowl Lines”. However, the aircraft type is unreadable, as well as the dates (the first date could be “1/6/39” and the last date “6/7/…”, but I am not sure):



Looking on its side profile with relatively small air scoop “chin” and missing cooler duct outlet, I thought that it depicts the “middle” arrangement of the XP-40, from January 1939. However, later I noticed that the shape of the air scoop in the front view seems to be much larger, and it looks familiar – like those in the P-40B/C! (None of the earlier XP-40 variants had the scoop divided into three separate air ducts, like the one you can see in this drawing). This sketch also contains a partially readable ordinates table. It provides numerical coordinates of the engine cowling, expressed in a cylindrical coordinate system:



The distances along the thrust line are measured from the firewall. All dimensions are in inches. Looking at this table I realized that its left side describes the “smooth” body of the initial engine cowling (as in the original XP-40 configuration, with the box-like radiator cover placed behind the wing). The appendix on the right side of the ordinates table describes an update (the final version?), with the cooler duct placed under the engine. This drawing simply skips the outlet of this cooler air duct (because it is depicted in another sketch?). It also lacks the carburetor air scoop and gun covers, placed on top of the cowling. However, maybe this is just a “conceptual” drawing, which did not need to take these details into account?

I decided to fit the shape described in this sketch to the P-36 fuselage, and check if such a combination match the P-40 photos. Maybe this is the real geometry of the P-40/B/C engine cowling? I will discuss the results in the next post.

 

  • Member since
    June 2014
Posted by Witold Jaworski on Saturday, September 21, 2019 12:06 PM

This post is first of the four dedicated to composing a side view of the P-40B from the original blueprints. As I already wrote in this thread, 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:



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 above) 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):


This way I discovered that there are minor differences between these exact locations, and the position of some stations on the original drawing (as you can see in figure above).

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


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 above 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:


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:



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:



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 (see figure "a", below):


To verify this shape, I used the P-40N rudder blueprint (see figure "b", above). 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:



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



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 the XP-40 general assembly drawing, in one of the earlier pictures in this post).

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



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:



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):



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”:



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 below shows such a table that describes the middle segment of this cowling:



For drawing this 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 below (the precise contour runs along the outer edge of this outline):



At this moment my side view drawing looks like this:



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.

  • Member since
    June 2014
Posted by Witold Jaworski on Friday, September 27, 2019 12:05 PM

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):



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:



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:



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):



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 below I marked them in blue:



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.

  • Member since
    January 2015
Posted by PFJN on Friday, September 27, 2019 5:35 PM

Hi,

It looks like you are really making progress here.  I always liked the early P-40s the best.  Can't wait to see more.

Pat

1st Group Build

  • Member since
    June 2014
Posted by Witold Jaworski on Monday, October 7, 2019 6:02 AM

Pat, thank you for following.

Here is the next part:

________________________________

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 team from 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).

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:



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:



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):



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):



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:



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):



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:



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:



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:



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):



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 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.

 

  • Member since
    June 2014
Posted by Witold Jaworski on Thursday, October 17, 2019 10:39 AM

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 close 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:


Their lines are fading into background, so you can see only fragments of the original blueprints. Frankly speaking, it was often hard to identify what part they are depicting. Thus, first I had to outline their key contours and reference axes in a separate Inkscape drawing, making them more visible:



Then I fitted such enhanced drawings into my “main assembly”. They allowed me to recreate the contours of the cooler inner ducts. You can see the result in the picture below:



The blue component in the color of these lines determines how much they are based on the information from the photos. (This means that the red lines are fully confirmed by the blueprints, purple lines – partially confirmed by the blueprints, while the pure blue lines are based solely on the photos).

The L-10202 cowling assembly layout does not provide contours of the carburetor air scoop and the gun fairings, located on the top of the P-40 engine cowling. Fortunately, I have found sketches of their key parts among AirCorps Library files:



Both drawings contain not only the key dimensions, but also references to the engine cowling stations, which allowed me to determine their X coordinates on my drawing. In addition, the carburetor air scoop contains reference to the cowling upper contour, which determines its vertical location (its Y coordinate on my drawing). To find similar coordinate for the gun fairing, I had to find the position of the gun barrel axis. For this purpose, I used the P-36/Hawk 75 drawings (as you remember, the P-36 and the early P-40s shared the same fuselage behind the firewall. This means identical gun barrel axes). Below you can see the gun mount blueprint from an export Hawk 75.

(I also have similar drawing of the P-36, showing that the barrel axes of the the 0.5” and 0.3” guns are symmetric. However, its lines are fading into background, thus I am showing here the more readable of these two scans):



The gun barrel axis location is specified in the front view. From this blueprint I learned that it was placed 22” over the fuselage reference line. Using this information, I could precisely place the gun fairing reinforcement from SK-2605 in my drawing.

  • Looking at the picture above I also noted that in this Hawk 75 (as well as in the P-36) the space separated by wing spar #1 and #2 and the fuselage ribs was used as the container for the used cartridge cases (for re-using). Such a frugality was common in the pre-war USAAC (and in some air forces of other countries). In this front view you can see at the bottom of the fuselage the case ejector doors (in the open position). You can also encounter similar doors in the P-40-cu. They could be bolted, if you wanted to bring back all the cases to the airfield. In the P-40B/C they removed these doors. I suppose that inside the wing they also added a “pass-through” segment of the case ejector duct, but I am not sure.

In the drawing below I drew the details of the P-40 upper cowling using the SK-2603 and SK-2605 sketches. They allowed me to determine the location and size of their fairings:



As you can see in the picture above, I also speculated about the shape of the case ejection duct in the P-40B/C wings.

The next missing element was the location of the engine exhaust stacks and their opening. On my photos this detail is always shifted vertically and horizontally, following the camera position, which was never ideally “inline” with the fuselage axis. (It could not be). Thus, to “pin” more precisely this opening, I used the P-40-cu engine mount drawing (drawing no. 87-22-001):



I do not have any decent drawing of the Allison V-1710-C engine, used in the early P-40s. Instead I used here the contours of its later variant (V-1710-F), from the P-40D. (They used different gear boxes, but it seems that their key dimensions and exhaust stacks locations were identical). I carefully “attached” this engine profile to the bolts visible in the P-40-cu engine mount. Finally, I could use this picture for adjusting the location of the exhaust stacks opening contour, which I prepared according the photos. (As you can see, in my drawing I outlined this opening in purple, which means that this is a “partially confirmed” element).

I discovered that the front side view in the main L-10202 layout also contains some faded lines which, after matching with the photo, occurred to be the split lines of the cowling panels:



L-10202 indicates that these lines run across the cowling in the radial directions, as I marked in the picture above. What is interesting, they fit the photos of the productive P-40-cu/B/C. In the XP-40 the seam line above exhaust stacks runs a little bit higher than in this layout. (Was it an effect of a tooling problem?)

There were some interesting details around the spinner. I highlighted them in the picture below, which compares the early and late variants of the “long nose Hawks”:



Note that the spinner rear edge overlaps the engine cowling. Such a solution was used in all P-40 versions. (I think that it could minimally reduce the aircraft drag). In the pictures above you can also see that the shapes of the cutouts in the spinner around the propeller blades differ between these two aircraft. In the P-40-cu the propeller blade base cross section is circular, so its cutout in the spinner has a circular shape. Similar opening in this AVG “Tomahawk” IIA from the photo above seems to have an oval shape. In Dana Bell’s “Aircraft Pictorial #5” book I have found a mention that the P-40-cu used the hollowed steel propeller blades. Shortages of these blades during further production of the P-40B/Tomahawks forced Curtiss to use a substitute: solid aluminum blades, which were somewhat heavier. Maybe this AVG aircraft used such an aluminum propeller?

In the P-40-cu photo you can see the long blast tubes. They occurred to be too long: engine vibrations and other random circumstances could deform these tubes during flight. In some extreme cases, the fired .50 rounds could shatter ends of these tubes. In response, Curtiss decided to them close to their fairings, as you can see in the AVG aircraft, and reinforce the cowling panels in the front to withstand the blasts. This change was also retrofitted to the previously produced P-40s.

Below you can see my sketch of the XP-40 spinner:



The L-10202 layout marks the propeller blade axes plane at 99 11/16” from the firewall. (BTW: the general assembly drawings of the early XP-40s, from fall 1938, placed this plane at 99”. However, that initial variant used a different, shorter spinner). I have also found a drawing of the engine cowling bulkhead (drawing no. 75-29-834). This scan was in a poor shape, but it allowed me to determine the precise location and size of this disc. However, none of these drawings does provide the location of the spinner rear edge. I had to determine its X coordinate using photos (that’s why I drew this contour in blue).

Note that in the previous XP-40 picture I drew the gun blast tubes, which is simply wrong. (The XP-40 was not armed in the photo from February 1940). In this picture I corrected this mistake.

I also found faded blueprints of the spinner assembly and the front bulkhead (drawing no 75-42-803 and 75-42-823). The assembly, dated from 1939, seems to describe a shorter, non-overlapping spinner. (It shows a 5/8” wide gap between the spinner rear edge and the fuselage cowling. I suppose that the later spinner variant was extended by about 1” behind the base bulkhead, covering this gap and a thin strip of the cowling). From this assembly drawing I also copied the circular contour of the propeller blade opening.

Picture below shows the resulting XP-40 profile (you can download its high-res version from here):



As I discovered two weeks ago (see this post), the serial P-40s had longer spinner and deeper cowling around the radiators. In the Allison-powered aircraft there were two radiators of the engine cooling liquid (in Curtiss docs it appears under its commercial name: “Prestone”) and single oil radiator. In the photos of the cowling details (Figure 102‑9) you can see that these coolers had simple cylindrical shape. I have found two blueprints of their mounting brackets: one for the XP-40, another for the “short nose” P-40E. While the diameter of the oil radiator remains identical in both drawings (11.12“), the Prestone radiators in the XP-40 are smaller (13.12” in diameter) than their counterparts from the P-40E (14.63”). I compared the proportions of the oil/Prestone coolers diameters in various photos of P-40-cu/B/C and came to conclusion that most probably their Prestone radiators were as large as in the P-40E. It seems that the cowling from L-10202 layout was wide enough to fit these larger coolers. The vertical distance between the axes of the oil and Prestone radiators increased from 9.5” in the XP-40 to 10” in the P-40. Assuming that the upper part of the cooler air duct remained intact, I had to lower the axis of the Prestone radiator by 0.755”. In the result, the axis of the P-40 oil radiator occurred 1.25” lower (0.755” + 0.5”) than in the XP-40. Such a configuration fits the archival photos. (In the picture below I marked the XP-40 radiator axes in black and the P-40 radiator axes in white):



Matching the aircraft contour from an orthogonal side view to a perspective-deformed photo is never easy or error free, especially in the case of the photos made with the typical lens length (about 35mm). In the case of the “Tomahawk” IIA photo above, the details on the upper part of the cowling seems to be shifted up, and the spinner is a little bit longer than it was. However, it seems that the area around the cowl flaps and the landing gear struts is not deformed (it was in the center of the original photo). Basing on these assumptions and the sketch of the XP-40 duct I drew the P-40 contours (in blue, because they are based solely on the photos). Note that the part of the cooler air scoop is still in red – because it was lowered by about 0.5”, but did not move from its original station (74” from the firewall). The complete arc of the cowling flaps was lowered by 1.2”.

Of course, I checked these contours against several other historical photos, to reduce the error margin, unavoidable in such a method.

I also compared this contour with the long-lens photo of the restored P-40B:



In general, they match each other. However, as I learned from the Classic Wings article, the restoration team of this aircraft did not have any original drawings of the P-40B engine cowling. They restored it using multiple photos of another restored P-40. In general, it seems that such an approach was quite accurate, except the rear cowling segment, located behind the radiator (see the picture above): it is definitely too flat.

Figure below shows the resulting contours of the P-40B engine cowling:



In the next picture you can see the complete P-40B side contour:



Use this link to get the high-resolution version of the picture above. You can use it as a reference for your model. Note also that the blue lines in this drawing are still based solely on the photos (i.e. they are not confirmed by the blueprints). This is due the lack of the documentation of this P-40 version. In the case of the “short nose” P-40D or later versions, as well as in the Pratt-Whitney powered variants of the P-36, you would see all (or nearly all) lines in the “confirmed” red color.

Of course, this is not a complete side view: I prepared this image for my own purposes, as the reference for building a precise 3D model of the P-40B in Blender. This work allowed me to identify which of the collected Curtiss drawings are useful, but I also learned about their limitations.

Thinking about further steps I realized that this “classic” approach:

  1. Create from the original assembly blueprint a 2D reference drawing (I did it here);
  2. Create from the 2D reference drawings a 3D model (I am going to do it);

is not optimal.

For example: the contours of a real object are a mathematical abstraction: infinitely thin lines. On the other hand, in a 2D drawing I always have to draw them using lines of a non-zero thickness. I can “work around” this issue by assuming that the precise contour lies on the outer side of its outline. However, inside these thick lines I could inadvertently create some small, but important, misalignments and discontinuities.

I started to think that this work on a 2D side view was just an “reconnaissance survey” of the Curtiss documentation. So far I used only fraction of the available information: most of the layout drawings provide complete 3D coordinates. For this side view I used only two rows from their ordinates tables, (the rows that describe the upper and lower contours). For the top / bottom 2D views I would also use just a single row from these tables! (The row that describes the symmetric side contour). Of course, I could also trace on the reference drawings additional lines “in between” these top and side contours. However, I am afraid that they would not be as precise as in the source ordinates table, because I would have to trace the same line twice, in each view. Everybody makes errors, but these mistakes would not be evident when they are split between two 2D views. Much better idea is to use the coordinates provided by the ordinate tables for creating a kind of 3D “reference grids” in Blender. Then I will use them for building the ultimate model. Such a “reference grid” of the fuselage would contain contours of its cross-sections at selected stations. Similarly, the wing “reference grid” would combine contours of the key ribs, webs (spars), and the wing tip outline.

In Blender 2.7 (and earlier versions) the limited system of 20 “flat” layers was an obstacle in effective organizing the resulting model surfaces, their 3D references and the multiple source blueprints. Fortunately, the new hierarchical collection system in Blender 2.8 finally allows for such a kind of work.

  • Member since
    December 2002
  • From: NYC, USA
Posted by waikong on Thursday, October 24, 2019 4:53 PM
the effort you put into this is mind blowing, Your SBD Dauntless was great by the way. Now, I'm wondering whether any of your work can be used to print 3d parts

My website: http://waihobbies.wkhc.net

   

  • Member since
    June 2014
Posted by Witold Jaworski on Monday, November 11, 2019 7:40 AM

waikong - thank you for visiting these threads!

_______________

This post in on a partially off-topic subject, so I just place a brief description:
In October I wrote two tutorials on fitting a 3D model into a photo. (You can then use such a photo like a precise reference. Of course, such a "real-world" reference is always better than any scale drawings).
As I mentioned in previous posts, I am going to use this method to make final checks on the P-40B engine cowling shape, that I deduced from the XP-40 (late) sketches from 1939.

In this tutorial I used my old P-40B model, which I finished in 2011. It was based on the scale plans and a few low-res blueprints, that were available in the previous decade:


Below you can see how many errors I discovered, when I matched my model with a photo of a real aircraft:


This photo-matching method is extremely useful in the cases when the photos are the only documents that remained after a historical aircraft. (For example - in the case of a less-known WWI fighter)


Here are links to the PDF files of this tutorial: Part 1, Part 2.

  • Member since
    June 2014
Posted by Witold Jaworski on Friday, December 20, 2019 12:10 PM

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.

However, the original Curtiss assembly blueprints reveal a more prosaic truth:



The fuselage up to station 16 in “short” and “long” tail variants was identical! The only difference was made by the additional segment. Note the sharp edge (a “corner”) formed in the top view along Sta. 16:

The P-40 blueprints are available online in AirCorps Library, so you can check details of this drawing (87-21-451) yourself. You can also find there several similar blueprints for the P-40L, M and N variants.

This finding should not be any surprise: in the mass production, you always try to minimize the influence of a modification on the existing tooling. You can observe similar sharp edges along the additional tail segment in the Focke-Wulf 190D. However, while these edges are properly depicted in the scale plans of that German fighter, this detail of the later P-40s remained obscured under the tailplane. For example – it is practically invisible in such a side-view picture as in the picture below:



However, you can (hardly) notice it in the pictures of the flying aircraft:







So - does your “long tail” P-40 model (for example: the P-40L, P-40M, or P-40N) have this feature?

I did not mention that the rear part of the fuselage in the Curtiss blueprints seems to be generally wider than in Kagero scale plans. However, this is another issue, for another post.
  • Member since
    June 2014
Posted by Witold Jaworski on Sunday, February 23, 2020 3:58 PM

I am still busy with my daily business, but I managed to write an article about unknown Curtiss project: the YP-37 with the R-2600 radial engine:
________________________

Reviewing the original P-36/YP-37/P-40 blueprints published by AirCorps Library, I also browsed the “uncategorized drawings” category. In general, many Curtiss drawings from this microfilm set are unreadable, especially these “uncategorized” images. Often all what you can see is just a blank microfilm frame with barely visible remains of the title block. However, in this “junk” category you can find interesting sketches of various design proposals. One of them is the YP-37 with the powerful R-2600 Twin Cyclone engine. Below you can see side view of the initial idea, from November 1938:



(Here you can download the complete P-1660 drawing). The Curtiss-Wright R-2600 engine (1600hp) offered 40% more power than the inline Allison V-1710 (1150hp), used in the YP-37 prototypes. However, this additional power was gained at the price of additional weight (R-2600 dry weight was 2045lb against the 1400lb of the V-1710). In this aircraft this radial engine was covered with a special cowling which resembled cowlings of the inline engines. It was a patent of the chief engineer, Donovan Berlin. Below you can see another Berlin’s prototype – XP-42, in which he used similar cowling for the two-row radial R-1830 engine:



Basically, the XP-42 “inherited” everything behind the firewall after serial P-36, in the same way as the aircraft from drawing P-1660 inherited the airframe after the YP-37. I also found inboard profile of this YP-37/R-2600 proposal (drawing P-1661):



(Here you can download high-resolution version of drawing P-1661. I composed this image from three subsequent microfilm frames). Like the YP-37, this fighter would be equipped with a turbo-supercharger, which delivered compressed air to the engine carburetor (via an intercooler). When I analyzed this drawing, I wondered about the purpose of a strange empty space between the engine and the oil/auxiliary fuel tank. Fuselages of fighter planes were usually packed with fuel, ammunition, and other equipment, thus it was quite unusual to leave such a plenty of useful space close to the aircraft mass center! You could put there another fuel tank, or a pair of 0.5 cal. guns with large ammunition boxes…

This design also inherited after the YP-37 the extremely poor forward visibility during taxiing. It could be much worse than in the later F-4U or even than in the original YP-37, because in place of the sleek inline engine you had a much wider radial powerplant.

Another design flaw soon became apparent thanks to the first XP-42 tests, conducted in the beginning of 1939: Berlin’s cowling did not cool properly the radial engine cylinders. The small air scoop, which you can clearly see in the XP-42 photo below, did not provide enough air for 2x7 cylinders, used in the R-1830:



Thus Curtiss engineers prepared another proposal. I found it on drawing P-2131, dated on 10th April 1939, titled “two stage icing nose”:



(Here you can download the complete P-2131 drawing).

This time they proposed a cooling air scoop in the middle of the spinner – an idea also adopted in the Focke-Wulf FW-190 prototype, designed in the same time in Germany:



However, there was a significant difference: Focke-Wulf engineers placed a fan behind the propeller. This fan rotated three times faster than the propeller blades. It “pumped” additional amount of the cooling air onto the cylinders of the BMW-139 engine:



Even with this modification the first series of the FW-190 faced troubles with overheating second cylinder row. In late 1940 the whole FW-190 project was almost cancelled due to these issues. What is interesting, they were finally resolved not by the Focke-Wulf design bureau, but by Luftwaffe field technicians.

At the end of 1940 Luftwaffe created an experimental unit in France for field evaluations of the pre-production FW-190 A-0s. These aircraft were equipped with a more powerful BMW-801 engine and a classic engine cowling with small spinner. Early FW-190s were plagued by the overheating engine cylinder issues. Technicians from this unit experimented with various cylinder deflector shapes, ultimately finding a solution for effective cooling.

Thus – most probably the design from drawing P-2131 was doomed from the start, because of the engine cooling problems.

I also found inboard profile of this YP-37/R-2600 variant (drawing no P-2133):



(Here you can download high-resolution version of this profile). For this aircraft designers proposed a R-2600 engine with two-stage mechanical supercharger. Compressed air from the first stage was cooled in a large intercooler, then directed toward the carburetor, placed before the inner (second-stage) supercharger. The inboard profile reveals that first stage of the supercharger could be turned off, and additional air scoop could direct the outer air directly toward the carburetor and the inner supercharger. This air scoop is marked on the drawing as “cold air”.

In overall, this solution was simpler than the first, turbo-supercharged variant, but still quite complex. The engine proposed in this variant was most probably the R-2600-B (1800hp at take-off).

There is also the last, third YP-37/R-2600 variant, depicted in drawing no P-2193 (dated on 28th April 1939):



(Here you can download the complete P-2193 drawing). This aircraft is named “short-nose”, because it uses a classic NACA cowling around the short-geared R-2600 engine. It seems that this is the standard production engine featuring single stage, two-speed inner supercharger, rated at 1700hp (similar engines were used in the A-20s). Drawing P-2191 reveals internal details of this powerplant installation:



(Here you can download high-resolution version of this profile). This is the simplest solution: you can see in this drawing just a straight carburetor air scoop, without any additional supercharger or intercooler. In the result, an empty, unused space behind the engine is back!

(continued in the next post....)

  • Member since
    June 2014
Posted by Witold Jaworski on Sunday, February 23, 2020 4:00 PM

(continuation of the previous post....)

What about the possible performance of the YP-37/R-2600 fighter? Could it be significantly better than the YP-37, or its direct predecessor, the P-36?

Well, we can estimate the answer basing on other aircraft designs in which similar inline engine was replaced by a similar two-row radial. For example – such a thing happened to the Soviet LaGG-3 fighter which used 12-cylinder M-105P inline engine (improved version of the Hispano-Suiza) rated at 1100hp on takeoff. In 1942 it was replaced by 14-cylinder, two-row radial M-82 engine (which closely resembled R-2600) rated at 1700hp on takeoff. The result was named La-5. Below you can see inboard profiles of both aircraft:



The LaGG/La airframe was 20% smaller than the P-36/YP-37/P-40 (its wing area was 187sq. ft, comparing to 236 sq. ft of the Curtiss fighters). The max. takeoff weight of the LaGG-3 (1941/42 variant) was 7 270lbs. Its max. speed was about 342mph at 16 000ft, and time to climb to 16 400ft was 7.4min. Takeoff weight of the La-5 (first series) was 7 450lbs. It reached 360mph at 21 000ft, and time to climb to 16 400ft was 5.7min. Note the minimal increase of the takeoff weight: it seems that the weight of the radial M-82 engine (dry: 2050lbs) was comparable to the weight of the M-105P coupled with the necessary cooling liquid installation (mainly the large radiator).

The new engine mounted in this Soviet airframe improved its climb rate by 23%, while the maximum speed increased by 5%-8% (depending on the altitude). Note that in spite of their smaller size, the weight of these LaGG/La was comparable to maximum weight of the YP-37. Generally this is because these LaGG/La were equipped with self-sealing fuel tanks, armor plates, and carried much more armament than the YP-37. In addition, they were made of plywood – which made these fighters somewhat heavier, but much cheaper (i.e. easier to produce) than the classic all-metal riveted airframe. (One of the factories assigned to Lavochkin’s bureau, no. 31 located in Tbilisi, was a former furniture manufacturer).

Taking into account the performance of the Allison-equipped YP-37 (rated at 1090hp on takeoff), as well as the XP-40 and P-40-cu, one could expect similar improvements from the “short-nose” YP-37/R-2600. I estimate that its takeoff weight could be about 7 200lbs (without the self-sealing tanks, and armor). For this weight, it could reach 15 000 ft in 4.5min, and obtain the maximum speed of about 370-380mph. The climb rate would decrease after adding the additional weight of the armor and self-sealing tanks (by about 10%), while the maximum speed could be preserved tanks to introduction of a more smooth, flush-riveted skin. (It was introduced in the P-40-cu, increasing its speed by about 10-15mph). Of course, the maximum range of the R-2600-powered aircraft without external fuel tanks would be shorter.

The YP-37/R-2600 project seems to be a typical example of the simple engine upgrade. The engineers put a new, more powerful engine into the existing airframe, without any additional improvements. The new radial engine increased the area of the fuselage cross-section, in the effect producing more drag which “consumed” certain part of the increased engine power. Usually the length of the “inherited” landing gear legs in such designs restricted propeller diameter to a sub-optimal size. (The more powerful engine requires larger propeller. For example – the diameter of the F4U propeller was 13’ 2”. Curtiss for this YP-37/R-2600 planned a 11’ 6” propeller, but it already seems too large, leaving just 5’ clearance between the blade tips and the ground).

Let’s look at different case: how much you can improve aircraft performance when you increase engine power without changing the airframe geometry and its weight? In 1939 Curtiss introduced such a modification to the P-36, upgrading the R-1830-13 engine used in the P-36A (rated at 1050hp) to a more powerful variant: R-1830-17, rated at 1200hp on takeoff. The design changes were minimal: this new engine variant used a 100-octane fuel, while the previous variant used 92-octane fuel. This change increased the maximum engine power at takeoff by 15%. The new model was named P-36C:



The maximum speed of the P-36C increased by 11mph, to 311mph at 10 000 feet, (3.5% improvement comparing to the P-36A). Without the additional wing guns and ammunition (they added 140lbs to the total weight and generated additional drag) it could reach 313mph (4% improvement). In the ideal case the maximum speed should increase by cubic root of the increase of the engine power, which means 4.5%. (At 300mph you can still neglect the effects of the air compressibility). However, this ideal case would require somewhat larger propeller, while the P-36C used the same propeller as the P-36A (10’ in diameter). The climb time to 15 000ft slightly degraded in the P-36C to 4.9 min instead of 4.8min in P-36A, most probably because of the increased weight (max. gross weight: 6 150lbs, while in the P-36A it was 6 010lbs).

Finally let’s analyze opposite case: instead of the engine upgrade, what results you can obtain from the same/similar powerplant, using various minor improvements? (For example – aerodynamic enhancements)?. There is an interesting historical evidence on this subject. In September 1940 Pratt & Whitney Aircraft Division bought a complete P-40-cu airframe (precisely: H81A-1, less the engine). Practically, it was a somewhat heavier, flush-riveted variant of the P-36 airframe. In this aircraft Pratt-Whitney engineers carefully mounted the R-1830-SSC7-G engine. This high-altitude variant featured two-stage mechanical supercharger. (I suppose that similar engine was used in the F4F-4 under the military symbol R-1830-86. However, this SSC7-G variant was tuned for even higher altitudes than the “Wildcat” engine). It was rated at 1200hp on takeoff – just like the R-1830-17, used in the P-36C.

This was not a high-priority project, and the aircraft was finally ready in mid-1942. You can see it in the photo below:



As you can see, Pratt-Whitney engineers completely redesigned the powerplant section, up to pilot’s cockpit. The side outlets of the engine cowling resemble a little bit those in the FW-190. Because in the R-1830 carburetor was mounted on the top of the rear crankcase, I suppose that the upper air scoop visible on this aircraft cowling is the carburetor air intake. However, it seems to be too large for this solely purpose. Maybe it was also used for additional cooling of the rear cylinder row?



Note also that Pratt-Whitney introduced an antenna mast – because the slanted radio wires used in the serial P-36 and P-40-cu/B/C generated a lot of unnecessary drag! (Starting from the P-40D Curtiss also introduced antenna masts to their design). For this aircraft Pratt-Whitney developed an exhaust system which provided jet propulsion effect. Similar solution was used in German Focke-Wulf FW-190 and – later – in Soviet La-5FN. In this system each cylinder has its individual exhaust stack, and these stacks run in parallel on both sides of the engine cowling. (There was no exhaust collector, used in the classic designs). It was an important improvement: in each of these fighters it increased the maximum speed by about 20mph. The gross weight of this aircraft was 7 100lbs (so it was 1 000lbs heavier than the P-36C, while the eventual self-sealing tanks, armor, and wing guns would further increase this weight to 8 300lbs).

Nevertheless, this P-40/R-1830 could reach 388mph at 25 000ft (at this altitude its engine still maintained 1015hp in the “military” power mode). This is an excellent result, when you compare it to the 311mph of the P-36C. Pratt-Whitney calculated that for the fully-equipped aircraft weighting 8 300lbs it would decrease by 10mph, to about 378mph. Of course, you can argue that this aircraft featured a special, high-altitude engine variant, so it could reach this speed in the area where P-36C already lacked power. Thus a better choice for the just comparison between the P-40/R-1830 and the P-36C is the maximum speed at the sea-level, where the output of both engines was identical (1200hp). The P-36C reached 272mph at sea level, while the P-40/R-1830 - 315mph. (43mph more! An increase by 16%). However, I suppose that the climb time to 15 000ft of the Pratt-Whitney aircraft would be worse than in the case of the P-36C, because of the greater weight. I estimate that for the “military” weight of 8 300lbs it could be comparable to the P-40F: 6.9 min. (In 1942 this result was “below the average” for a new design).

When this aircraft appeared, production of the Allison-equipped P-40K and the Merlin-equipped P-40F was in the full run. The performance improvements offered by the P-40/R-1830 were not a “breakthrough”, and in the 1942 the undisturbed output of the Curtiss fighters was critical to the war effort. (In 1942 serial P-40Fs reached 370mph at 20 000ft. Switching production to such a new variant as the Pratt-Whitney prototype would slow down the existing output from the Curtiss factory. In that time it was better to immediately dispose a dozen of even sub-par aircraft like the P-40F or P-40K, instead of waiting for improved fighters in the future. On the other hand, in that year other suppliers already begun full-scale production of more promising projects like the P-38, P-47, P-51B, F6F and F4U). Thus, further work on this Pratt-Whitney P-40 was discontinued in mid-1943.

Do you wonder, what kind of aircraft could be created if Curtiss would apply in 1939 the exhaust jet propulsion effect to their YP-37/R-2600 project (as did Kurt Tank in the Focke-Wulf FW-190)? Well, I do not expect that it would be useful in the incoming war, anyway. The main disadvantage of this design would be limited capacity of the YP-37/P-40 fuel tanks. Increased fuel consumption of the R-2600 engine would restrict its maximum range, which was just average with the less-powerful Allison inline engine. Even in the case of the R-2600 mounted in the P-40 airframe, there would be no remedy for this issue. (In the P-40 pilot’s cockpit was located just behind the firewall, thus it already used the empty space I marked in my drawing for the fuel tank placed behind the cockpit. The space in the P-40 wings was already used in by the landing gear and guns). In the best case such a P-40/R-2600 would be shipped as a lend-lease product to the Soviet Union, as it happened to the P-39. (The Eastern Front was the only WW2 area where the fighter range capability was not a key issue – most of the Soviet fighters had enough fuel for less than one hour of combat mission).

However, the Pratt-Whitney P-40/R-1830 design is an evidence how much the final result depends not only on the funds allocated to the project (I suppose that Curtiss in 1941-42 had enough money at hand), but also on the quality of the engineering team. It is interesting that in the same year – 1940 – the North American team took similar challenge and designed “a better P-40” (the “Mustang”) which initially featured the same engine and wing area as the Curtiss design. However, they did it quickly, for a powerful project sponsor (British RAF). We can only speculate, what could happen if the Pratt-Whitney project received similar boost and delivered this aircraft after one year, instead of two (i.e. in 1941, instead of 1942). Maybe it could be put into production in place of the P-40D? In that year the P-40/R-1830 performance could be considered as much better alternative than it was in 1942…

Anyway, it seems that since 1940 Curtiss engineers could not “catch-up” with the general progress in the aircraft development. This finally led to the general decline of the entire Curtiss Aeroplane Division. In 1948 it was sold to North American Aviation.

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