One of the most prominent features of the R-1820 engine cylinders are their rockers. More precisely – their covers, cast as the part of the cylinder head (Figure 85‑1):
The R-1820 was a classic four-stroke engine. Its cylinders had two valves: single intake valve, connected to the supercharger via a wide pipe, and single exhaust valve. Movements of these valves were controlled by cams, via pushrods and rocker arms mounted in the cylinder heads. The covers housing these valves and rocker mechanisms were placed on the right and left side of the cylinder head.
To simplify my model, I decided to separate the cylinder fins from its “solid” body (i.e. to create them as separate objects). However, because in the reality the cylinder head was cast as the single piece, it is very difficult to precisely determine its shape hidden between these fins (Figure 85‑2):
While you can see the upper parts of the rocker covers on the reference photos, you can only guess their contours below the “fin surface”.
There is a blueprint that provides some additional clues (Figure 85‑3):
However, I have some doubts about details of the contour that you can see on the rear view above. (I marked it with thick dashed lines in the picture). Look at the lowest part of this top contour: it should correspond to the upper (outer) surface of the combustion chamber. According other drawings, the shape of this chamber resembled a regular dome. If so, why the fragment of its contour visible in this drawing seems to be (a little) oblique? In the cutaway depicted in the first photo in this post (Figure 85‑1) I cannot see such an oblique shape. And why the side contours of this heads (the vertical dashed lines below the valve openings) are not symmetric? Thinking about it, I concluded that this drawing was not focused on the precise representation of the cylinder geometry: its main goal was to show the lubrication areas. Thus all these details, which we can see here, were drawn thanks to a “good will” of its draughtsman. They were hand-made, ink-traced drawings, and we can be just thankful to this technician for such a detailed piece of work. Still, I assumed that these lines can differ a little from the real contours – just because of the plain human error.
I formed the basic shape of the rocker cover using two clones of the same mesh: I placed one instance on the auxiliary drawing, while the second instance is located in its proper position on the cylinder (Figure 85‑4a):
Modeling this cover as a separate object allowed me to switch between its local (along the valve axis) and global coordinate systems. I could also modify this mesh switching between its clones. I used the instance, located on the cylinder, to fit its base into the combustion chamber dome. The other instance of this cover, placed over the auxiliary drawing, allowed me to follow the shape of this element. (In fact, I could also put another instance of this mesh over the top view of the rocker cover. However, I did not do it – just because I used this view only during the initial phases of the modeling, and it was relatively easy to rotate the modeled object and move it over the side view).
This is the initial, “conceptual” model of the cylinder head, so I split it into the key “solids” and formed the semi-spherical cover of the exhaust valve as another object (the red one in Figure 85‑4). Such an arrangement allows for easy manipulating of these parts. During this phase I have to determine their most probable sizes and locations. For example – following the precise location of the exhaust opening, I discovered that for the size as in the “Lubrication Chart”, it has to be placed in a slightly different position (as in Figure 85‑4b). Otherwise, the right-bottom corner of the rim around exhaust opening would “sink” into the combustion chamber dome. (Of course, I also checked multiple times the most probable radius of this dome!)
When the whole thing seemed to match the photos, I made the rocker cover asymmetric (by “applying” its Mirror modifier and modifying the resulting faces). Then I modeled the oblique pushrod base (Figure 85‑5a):
To avoid some potential errors in the future, I started with placing the pushrod (another object) in the proper position, then formed the base around it. Figure 85‑5b) shows the resulting mesh. Note the sharp edges in its upper part. In the next step I rounded them, using a multi-segment Bevel modifier (Figure 85‑6a):
To have more control over these fillets, I used the weight-based version of the Bevel. Figure 85‑6a) shows the mesh edges that have a non-zero bevel weight marked in yellow. However, even in such a case, I could not avoid an artificial sharp edge between two fillets that were too close to each other (Figure 85‑6b). Well, in this situation I had to “apply” this modifier, and manually introduce small fixes to the resulting faces (Figure 85‑6c). I also dynamically created a “rim” around the upper edge of this cover. It is generated by the Solidify modifier, assigned to the thin face strip around this edge. Figure 85‑6d) shows the final result of these modifications.
While working on these parts, I simultaneously “scanned” the Internet, searching for more reference photos. Sometimes they just expose details, which were obscured in the reference materials that I already have. In this case – it was a protrusion on the rocker cover around the first and the last bolt (Figure 85‑7a):
I just had missed this tiny detail while forming the upper part of the rocker cover! Now I had a headache, how to fix it in a quick way. Ultimately I prepared two reference “cylinders” (I marked them in red, as you can see in Figure 85‑7b). Fortunately, there were many faces around the area that I had to modify. I placed these faces on the corresponding reference cylinders using the Blender Sculpt tool. (It allows me to push/pull multiple faces at once in a gradual manner).
You can see the final result of this modification in Figure 85‑8a):
Frankly speaking, I can see now that this protrusion had somewhat smaller radius. Ultimately I decided that it is “good enough” for the assumed level of details.
In the next step I cloned the rocker and valve covers onto the opposite side of the cylinder head: over the intake valve (Figure 85‑8b). In this first approximation of these parts, I rotated the intake valve cover (marked in red in the picture above), trying to find the proper location and angle of the intake opening. To fit it better, I also placed in this model the intake pipe. I knew, that in the future I will adjust its shape multiple times. That’s why I crated it initially as a simple cylinder, smoothed by the Subdivision Surface and bent along a parent curve using Curve Deform modifier. By controlling the location, rotation and shape of the parent curve I had full control over this pipe.
The intake rocker cover had also a unique feature: two bolts on its front and rear walls (Figure 85‑9a). They were intended for mounting around the engine an eventual NACA cowling. (Wright added these bolts on the Army request). For this “conceptual” stage of the modeling, I decided to add the bases of these bolts as a separate part. (Because I expect that I will move/modify shape of this element many times, before I reach the result that matches the reference photos). I will eventually join it with the cover (and add appropriate fillets around its edges) when it fits well.
In the next step I transformed the clone of the intake cover into a completely separate object (marked in blue in Figure 85‑9b). I also added the bolt bases around the exhaust and intake openings. (As you can see in the figure above, there are four of them on the exhaust cover, and three on the intake cover). Initially I created these bases as separate objects.
Once I verified their location, I joined these bolt bases with the cover mesh (Figure 85‑10a):
I joined these objects by applying a Boolean (Union) modifier. However, after such an operation the resulting edges required some manual “cleaning” (removing doubled vertices and edges).
I also formed an initial approximation of the rocker upper cover (Figure 85‑10b). I just placed it over the left rocker. The front contour of this part had to fit the circular contour of this engine (dimensioned on the original installation drawing). I also rounded its upper edge using a multi-segment Bevel modifier, but I can see that this part will require further modifications.
While working with these rocker covers, I discovered that I made a mistake in reading the original blueprints! I thought that one of the exhaust rocker cover elements was a cross-section, while it was oblique view of one of its fins (Figure 85‑11a):
On the reference photos I can also see that the bottom pushrod base plane was bent, with sharp side edges (Figure 85‑11b). Thus I had to modify accordingly the bottom part of this cover (Figure 85‑11c).
Well, such “discoveries” slow down the overall progress of the work, but they are inevitable, if you want to build a close copy of the real object. They happen all the time, as I am collecting growing number of the reference photos. In fact, I have measured that I spend at least half of the overall time on analyzing the photos. (Sometimes I also sketch on a paper the most complex shapes, before I start to model them in Blender. These sketches help me to better “understand” the objects that I want to recreate). The complex details of the cylinder head are often obscured by the fins, which makes this element an extremely difficult case. I am sure that I will identify and fix many of similar mistakes in the nearest future. For example: I will shift and rotate the cover of the intake valve multiple times, and then have to adjust the intake pipe after each of these updates. That’s why I prefer keeping this cylinder as an assembly of multiple, relatively simple objects. It would be much more difficult to modify this head, if it was a single, complex mesh. (In such a case you would have to care about all of its intersection edges!).
You can check details of this model in the source *.blend file. In the next post I will model the cylinder head fins.