In my previous posts (published in May and June) I focused on the cylinder. I think that it is the most difficult part of every air-cooled engine. Since that time I have made a significant progress, which I will report during nearest three weeks.
Let’s start with the rear section of the crankcase (behind the cylinders). Do you know how difficult is to find a decent photo of this area? The original pictures from the “Cyclone” manual are of moderate quality (Figure 88‑1a):
The modern photo (Figure 88‑1b) reveals more details. In general, it looks that the rear part of the crankcase is formed from two cylindrical segments. The intake pipes extend from the first (i.e. forward) of these segments. (There is a centrifugal supercharger inside). The upper part of the last segment contains rectangular air scoop, which also provides the mounting points for the carburetor (Figure 88‑1b). The rear wall of this segment forms the base for various auxiliary aggregates: magnetos, oil pump, starter, etc. As you can see in Figure 88‑1b), aggregates from the R-1820 exposed in the Pima Air Museum differ from the manual photo (Figure 88‑1a). I think that such equipment could be used in the B-17s. On this photo I also finally determined an important feature of the R-1820 geometry: its mounting points. (They are dimensioned on the installation drawings, but I had to find them among all these nuts and bolts that you can see on the crankcase).
In this post I will finish the first cylinder of the R-1820 “Cyclone”. It will be the “template” object, which I will clone eight times around the crankcase when I finish the other parts of this engine.
Although in my previous post the cylinder head received the full set of its cooling fins, it still lacks some details. One of them are the reinforcements of the valve covers:
As you can see, these reinforcements break the symmetry of the left and right valve covers. Both of them resemble a thick plate, but one is oblique, while the other is vertical. They are not the most prominent features of this cylinder head, and it took me some hours to determine their probable shape. Finally I classified them as the secondary features of the covers, which I have to recreate, for the assumed level of details.
The fins of the air-cooled cylinder heads are a state-of-art piece of metallurgy (Figure 86‑1):
At the first glance, it is hard to believe that they were cast as a single piece. But when you look closer, you will discover that these fins “grow up” from the solid parts of the head as naturally, as the hair from the head (Figure 86‑2):
Try to imagine the shape of molds used in the production of these parts, and the challenges faced by their manufactures! (There is an interesting post about this. It describes production of the R-1830 Twin Wasp cylinders). Basically, modern producers of the heads for the air-cooled aircraft engines use the same technology as eighty years ago.
In my model I will recreate these fins in a somewhat simplified form, as a few separate Blender objects. I will also skip some fine details of their shape (for example the small features that I marked in the figure above). Such a simplification conforms the moderate level of details that I assumed for this model. It is always possible to make a more detailed version of this object later.
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.
In this post I will recreate the main and the front sections of the crankcase, and the cylinder basic shape. Let’s start this model by forming the middle section of the crankcase (Figure 84‑1):
This section is always obscured by the cylinders, so you cannot see it clearly on any photo. That’s why I used here the original drawing from the manual. Generally, this barrel-like shape contains nine cylinder bases. It is formed by two steel castings, bolted to each other. (These bolts are hidden inside the crankcase, between the cylinder openings).
The engine is the heart of every powered aircraft. In the case of the SBD it was the Wright R-1820 “Cyclone 9” (the “G“ model). In fact, this engine was one of the “workhorses” of the 1930s: designed in 1931, it was used in many aircraft, especially in the legendary DC-3. “Cyclone” was a reliable, fuel-saving unit for the Navy basic scout type. (Remember that the “Dauntless” was not only the bomber: it was also a scout airplane). In general, the R-1820 is a classic nine-cylinder, single-row radial engine (Figure 83‑1):
The R-1820 G had been produced for over two decades, not only by the Curtiss-Wright, but also (under license) by Lycoming, Pratt & Whitney Canada, and Studebaker Corporation. Thus various less important details of this engine “evolved” during this period. In this post I would like to highlight some of these differences. I will focus on the forward part of this engine, because at this moment I am going to create a simpler model of the “Cyclone”, intended for the general, “outdoor” scenes. Inside the closed NACA cowling, you can see only its forward part. (Thanks to the air deflectors, placed between the cylinders – see Figure 83‑1). In such an arrangement, the visible elements are: the front section of the crankcase, cylinders, ignition harness, and the variable-pitch propeller governor. While the front section of the R-1820 crankcase remained practically unchanged in all versions, and the governor depends on the propeller model, I could focus on the cylinders and their ignition harness.
Identification of the version differences is the basic step, because otherwise you can build a model of non-existing object that incorporates features from different engine variants.
The Dauntless had fixed tail wheel of a typical design among the carrier-based aircraft. The tail wheel assembly consisted a fork connected to two solid-made beams, which movement was countered by a shock strut. The beams and the shock strut were attached to the last bulkhead of the fuselage (Figure 82‑1):
In previous post I discussed how the SBD landing gear retracts into its wing recess:
In principle, it is simple: the landing gear leg rotates by 90⁰. However, the parts responsible for shock strut shortening during this movement increase mechanical complexity of this assembly. The figure above does not even show the deformations of the brake cable, which follows the shock strut piston movements.
For some scenes I will need the landing gear extended, while for the others – retracted. In practice, moving/rotating each part individually to “pose” my model would be a quite time-consuming task. That’s why I created a kind of “virtual mechanism”, which allows me to retract/extend the landing gear with a single mouse movement. In the previous post I already presented its results in this short video sequence. In this post I will shortly describe how I did it.
The SBD shock absorbers had to disperse a lot of the kinetic energy of landing aircraft, minimizing the chance that the airplane accidentally “bounce” back into the air. (This is a key requirement for the carrier-based planes). For such a characteristics you need a relatively long working span between the free (i.e. unloaded) and the completely compressed (i.e. under max. load) strut piston positions. Indeed, you can observe that the Dauntless landing gear legs are much longer in the flight than in their static position on the ground (Figure 80‑1):
The working span of the SBD shock strut piston was about 10” long, while the difference between the static and the free (extended) piston positions was about 7.5”.
I published my previous post a month ago, but the current stage of this project – detailing – requires less frequent reports. (Otherwise the posts would become rather monotonous: week after week they would describe making similar things, using the same methods). I started this last phase of the Dauntless project by recreating its main landing gear. First, I had to finish it, then I am able to write about this process. Thus I will describe it in this and next two posts. (I will publish them in a short sequence, week after week).
The retractable main landing gear of the SBD was probably a direct descendant of an experimental solution used in the Northrop 3A fighter prototype. In general, it looks quite simple: