Mounting the Engine

In my previous post I have finished the second variant of the R-1820-52 “Cyclone” engine, which was used in the SBD-3 and -4. (It looks like the earlier R-1820-32 model, mounted in the SBD-1 and -2). In the resulting Blender file linked at the end of that post you will find two “Cyclone” versions: the R-1820-52 (for the earlier SBD versions, up to SBD-4) and the R-1820-60 (for the SBD-5 and -6). Each of these engines has its own “scene”.

To “mount” these engines into my SBD models, I imported both scenes to the main Blender file. I defined each engine variant as a group, to facilitate placing them in the aircraft models as the group instances. I also added the firewall bulkhead and updated the shape of the cowling behind the cylinder row. (I will refer to this piece as the “inner cowling”). So far I did not especially care for the shape of its central part, hidden below the NACA ring. Now I updated it for the real size and shape of the engine mounting ring (Figure 93‑1a):

Figure 93-1 Mounting frame details

On the photos I noticed a kind of bulges, extruding from the both sides of the inner cowling (Figure 93‑1b). I assumed that they are shaped around small triangle plates welded on the sides of the mounting ring. (I have no photo to proof this assumption). Anyway, I modified the shape of the inner cowling in the SBD-5 to match this feature. I assumed that the inner cowling in the earlier SBD versions (SBD-4, SBD-3,…) also had such “bulges”.

In Figure 93‑1b) you can see three openings for the intake air in the SBD-5 and SBD-6: a rectangular one in the middle and two round holes on the sides. These side openings are for the air filters, intended mainly for the takeoff and landing (Figure 93‑2):

Figure 93-2 Air intake system in the SBD-5 (and -6)

The idea is that the during the dusty conditions on the airfield the direct intake door (1) is closed, while the doors for the filtered air: (2) and (3) are open. When the aircraft climbs higher, its pilot flips positions of these doors, closing the filtered air input (2), (3) and opening the direct input (1).

There was no such a thing in the earlier SBD versions. It seems that the alternate filtered air input was introduced to many US aircraft in the same time: between 1942 and 1943. (You can also see the filter intakes in the P-40 starting from the M version, and in the P-51, starting from the B version). Maybe it was a general suggestion from the Army, after several months of the airfield war experience?

I added these two filters and their intakes to the R-1820-60 engine (Figure 93‑3):

Figure 93-3 Air filters in the R-1820-60 “Cyclone”

As you can see, these intakes are tightly fitted between cylinders 2-3 and 8-9 (Figure 93‑3a), so they have a quite complex shape (Figure 93‑3b). I do not have a photo for such an obscure detail, but the location of the filter determines, that the mixture intake pipes of Cylinder 3 and Cylinder 9 went through the corresponding intake body. (In principle, it is technically possible). I did not make holes in the deflectors between cylinders 2-3 and 8-9. (They would not be visible anyway, because both elements: the deflector and the intake are covered with black enamel).

The next aircraft-specific element is the exhaust collector. In the SBD-4 and earlier versions its outer contour had a circular shape (Figure 93‑4a). However, in the SBD-5 (and -6) it went around the air filters, so it had a slightly different shape around this area (Figure 93‑4b):

Figure 93-4 Differences in shapes of the exhaust collectors between the SBD-5 and the earlier SBD versions

I built these collectors from simple tubular segments. Each of these segments is first tapered by the Simple Deform modifier (1), then bent along its shaping curve by the Curve modifier (2) (Figure 93‑5):

Figure 93-5 Building the exhaust collector with tubular segments

The offset of the original tube object from the curve object determines the origin of the resulting shape on the curve. Small gaps between subsequent tubular segments are hidden under the joining rings (as in the real collector). 95% of this collector is closed inside the NACA ring, so I decided to not recreate the fillets along the edges of the individual outlet pipes. (Joining all these tubular meshes would be a time-consuming task).

I used some photos to compare proportions of the exhaust collector, circular reinforcement and the cross section behind the NACA cowling in the SBD-3. The findings led me to the conclusion that I should modify the bottom part of the engine cowling (Figure 93‑6):

Figure 93-6 Fitting the engine into the cowlings

Many months ago I found that the cross-section of the lower inner cowling in the SBD-5/SBD-6 had a non-elliptic shape (shown in Figure 93‑7b). I also assumed, that such a cross-section also occurs in the earlier SBD versions. Now I can see that I was wrong: the photo above shows that in the SBD-1.. SBD-4 it was a regular ellipse (as in Figure 93‑7a):

Figure 93-7 Different shapes of the inner cowling cross section in the SBD-4 and the SBD-5

It seems now that Douglas designers modified a little the bottom part of the engine cowling in the SBD-5, shaping its circular “chin” (Figure 93‑7b). Maybe they did it because of the larger oil cooler used in this version? (It was required by the more powerful engine). If in the SBD-5 they shifted whole engine 3.5” forward, such an additional modification is also possible. (There was no any bulkhead at this station, and this cowling piece was already shaped anew).

To determine the exact location of the engine along the fuselage centerline, I used the high-resolution reference photo of the SBD-5 (Figure 93‑8a):

Figure 93-8 Determining the exact position of the engine inside the NACA cowling

I shifted the engine along the fuselage centerline, until its crankcase matched the crankcase visible on the photo. Then I measured the f distance (Figure 93‑8a) and applied it to the SBD-3 and SBD-1 models. (In the SBD-5 the engine together with the NACA cowling was shifted forward, thus in the SBD-1 and SBD-3 I could not simply apply the absolute location of the engine origin).

Finally, I applied the materials to the engine models, copied the environments from the SBDs to the R-1820-52 and R-1820-60 scenes, and made test renders (Figure 93‑9):

Figure 93-9 Test renders of the engine models

On these renders I placed the engines “in the middle of the air” just to be able to evaluate all their materials in the full light conditions. Due to relatively small size of most of the engine elements, I used here only the procedural textures. I did not apply to this engine models any oils stains or other dirt. The historical photos show that the blue enamel on the crankcase was kept surprisingly clear, even in the worn-out aircraft. The other parts of the engine are obscured under the NACA cowling, so there is no need for additional “dirt” textures. You can see it in the test render of the R-1820-60 “Cyclone” inside the SBD-5 NACA cowling (Figure 93‑10):

Figure 93-10 Test renders of the SBD-5 model with the R-1820-60 engine

You can download the model presented in this post from this source *.blend file.

In the next two posts I will work on the details of the cowling behind the cylinder row.

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