Original SBD Dauntless Blueprints: Initial Findings

In general, the set of 7 SBD/A-24 reels from NASM contains 3308 unique microfilm frames, belonging to 3022 drawings. On reels “XA” and “XB” you can usually find updated copies of the previous reels (“A”, “B”,.. “F”). However, 350 frames from “XA” and “XB” are unique – most probably this is a part of the missing roll “C”. Duplicates from these “X*” reels are also useful, when a drawing from one of the previous reels is unreadable.

I chose about 1000 frames (mostly assembly drawings) from this microfilm set, and organized them into a tree-like structure as in Figure 108‑1:

Figure 108-1 Folder structure for the SBD blueprints

To preserve disk space, I placed in these folders shortcuts to files located in the original directories (These original directories correspond to microfilm reels: “A”, “B”, …, “XB”). I practiced that when I click such a link, it opens the image in Photo Viewer, as if it was the original file.

I think that Douglas did not use any sophistical drawing numeration (at least in this project). The SBD/A-24 drawing numbers seem to be assigned as they were ordered: for example, drawing numbers of subsequent wing bulkheads belong to number series that begins with: 206*, 209*, 212*, 406*, 409*, without any visible order. Maybe this is due the fact that part of these drawings came directly from the Northrop Co, without any renumbering? (You can still find “Northrop Aviation Division” name in the title blocks of some standard parts from this microfilm set).

The documentation from NASM microfilm is missing many important details – I suppose that they were on the lost reel “C”. For example, Figure 108‑2 shows the identified and missing wing bulkheads:

Figure 108-2 Identified and missing bulkheads of the wing structure

Fortunately, reel “E” contains also wing geometry master diagram (ordinals), so I can use it for recreating shapes of these missing elements.

Fuselage structure also misses many bulkheads (Figure 108‑3):

Figure 108-3 Identified and missing bulkheads of the fuselage structure

However, there is no master diagram for the fuselage. I will have to recreate its shape basing on the few reference dimensions placed in the identified assembly drawings. I have also found some contours of these missing bulkheads, drawn as additional information in various installation drawings. However, these blueprints are not as precise as you think – due to barrel distortions of the photo lens and draughtsman mistakes, I estimate their tolerance to 2-3% of the overall size. Unfortunately, there are no data about the wing fillet shape, especially its outer edge.

Several years ago I analyzed the SBD photos and concluded that SBD-5 (and -6) engines were mounted a few inches forward than in the previous Dauntless versions (SBD-1..-4). In this post I estimated this difference in length as 4 inches. Now I found the proof of this observation in the SBD-4 and SBD-5 engine mount dimensions (drawings 5055954 and 5159336):

Figure 108-4 Differences in the fuselage length between SBD-4 and SBD-5

The explicit dimension of SBD-4 engine mount (dwg no 5055954) specifies its length (distance from the firewall to the back faces of the engine mounting lugs) as 34.1875 (I switched original fractional dimensions to decimals). Similar dimension of the SBD-5 engine mount (dwg no 5159336) declares its length as 38.1875. Thus we have the 4” difference!

However, to determine the ultimate difference in the fuselage length (marked in the figure above with the question mark) I also have to determine the overall lengths of the elements in the front of the engine mount. Both versions (SBD-4 and SBD-5) used the same propeller (length: 21.75”). From the drawing of the SBD-5 NACA ring I can read the overall length of the entire engine cowling: 60.8125” (Figure 108‑4). Unfortunately, there is no such information in the SBD-4 drawings. I have to determine this dimension in an indirect way. Let’s try it. The blueprints show that the length of the NACA cowling was identical in all versions (31.5”), as well as the distance from the R-1820 cylinders plane to the front of the NACA ring (14.3125”). (The NACA ring was attached to the mounting points on the R-1820 cylinder heads. These points were placed on the engine cylinders plane). Thus the potential source of the eventual further fuselage length differences is the distance from the back faces of the engine mounting lugs to the cylinders plane. From the SBD-5 blueprint I can calculate that it was 8.3125”. This result is close to the dimension specified in the R-1820 installation drawing (8.24”):

Figure 108-5 Installation drawing (side view) of the R-1820-60 (or similar G200 variant)

However, this installation drawing describes the R-1820 G200 (also known as C9GC), which was used in the SBD-5 under military designation R-1820-60. (For more information on the Wright “Cyclone” engine variants see this post). The R-1820-52, used in the SBD-4 and earlier Dauntless versions, belonged to the earlier R-1820 G100 series (also known as C9GB). Crankcase of the G100 family significantly differs from the crankcase used in G200. This could also mean differences in the coaxial location of the engine mounting lugs.

Unfortunately, I did not find any blueprint which could reveal this dimension for the R-1820-52 engine. I found installation drawing of an earlier R-1820 major version (R-1820 F series, from 1934), which central crankcase segments had similar shape to the R-1820-52. In the R-1820 F this distance was 7.875”. All what I could find about the R-1820 G100 the was the side drawing without any dimensions:

Figure 108-6 Estimating the distance from the mounting points to the cylinders plane in the R-1820-52

The B dimension marked in this drawing is the piston bore. This is well-known parameter of this engine, specified in the manufacturer documentation as 6.125”. I can estimate the sought distance A by comparing it to known B. The A/B ratio that I measured in a high-resolution copy of this drawing is 1.286 (+/- 0.8%). This means that A = 7.875” (+/- 0.063”) – i.e. the same distance as in the R-1820 F. I assume this value for the R-1820-52as as the most probable distance from the mounting lugs to the cylinders plane.

Thus the overall difference in the SBD-5 and SBD-4 horizontal lengths comes from the difference in their engine mount lengths (4”) and the difference in the distance from the mounting lugs to the cylinders plane between the R-1820-60 and -52 engines (8.3125– 7.875) = 0.4375”. This result can be rounded to 4.44” (or expressed precisely as 4 and 7/16”). According Douglas general arrangement drawing, the overall length of the SBD-5 was 33’ 1/4” (396.25”). Thus the overall length of the SBD-4 could be 32’  7  13/16” (391.81”).

What about the earlier Dauntless variants: SBD-1, -2 and -3s? The eventual difference in their lengths and the SBD-4 length comes from the different propellers. (They used “Hamilton Standard Constant Speed”, while the SBD-4 used newer “Hamilton Standard Hydromatic” propeller). In the NASM documentation I found a powerplant diagram (dwg no 5094793), which shows this older propeller variant and the contour of its spinner. Although the spinner shape a little bit different than in the archival photos, all other drawing elements seem quite precise. Using this picture I could make a more precise estimation of the few key dimensions:

Figure 108-7 Profile with the old propeller (as used in the SBD-1, -2, and -3)

In this blueprint (above) I identified vertical lines that mark the firewall and the engine mounting lugs planes, as well as the cylinders plane. I scaled the distance between firewall and engine mounting plane to the corresponding dimension (34.19”), then I read the B, C distances, and – just as additional check – the A distance. I received: B = 37.66”, C = 42.38”, A = 7.88” (which confirms the previous estimation: 7.875”). Thus the overall length of the SBD-1,-2, and -3 can be estimated as:

1. when the spinner was mounted: 32’ 6.3” (+/- 0.3”);
2. without the spinner (the case often observed in the archival photos): 32’ 1.5” (+/- 0.3”);

It is interesting that p. 2 agrees quite well with the SBD-1 and SBD-2 length (32’ 2”) listed in the BuAer performance data sheets from 30^th November 1942 (an repeated in many other later publications).

The aircraft dimensions listed in these BuAer data sheets are rounded (up?) to full inches. For example: the wing span is listed as 41’ 7”, while the exact value in Douglas arrangement drawing is 41’ 6.125” (41’ 6  1/8”).

Similar (single) BuAer sheet from 6^th August 1942 examined the SBD-3 and SBD-4. The horizontal length specified in this document (32’ 8”) agrees quite well with the length of the SBD-4. We can assume that in the sheet which examined both: the SBD-3 and SBD-4, BuAer engineers simply put the length of the latter (evidently they treated these two aircraft as a single variant).

Many publications cite SBD-3 length as 32’ 8”. This is wrong value, coming from overinterpretation of the BuAer data sheet from August 1942.