Forming the Main Landing Gear

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:

Figure 79-1 Main landing gear of a restored SBD-3

The upper part of the landing gear was an “L” – shaped tube, mounted between two wing spars. The lower part, visible below the wing, was a simple shock strut mounted to the wheel axle (see Figure 79‑2a). The axis of the landing gear retraction was parallel to the thrust line and perpendicular to the walls of the spars (Figure 79‑1).  The shock strut is deflected (by 6⁰) from the vertical axis, so that in the open position the wheel is directly below the axis of landing gear retraction (Figure 79‑2a):

Figure 79-2 The same landing gear from various SBDs

Figure 79‑2 also shows various treads of the SBD tires. The tires of the earlier versions (SBD-1, -2 and -3) had no tread pattern (Figure 79‑2b). The simple “straight grooves” treads appeared on the SBD-4 wheels (Figure 79‑2a), while in the SBD-5/6s we can find a more elaborate, “brick” (Figure 79‑2c) or “honeycomb” tread patterns.

Another interesting thing is the lack of the torque arms, that connect the cylinder and piston of the shock strut in most of the other aircraft (Figure 79‑3a):

Figure 79-3 A non-standard solution for blocking the eventual shock strut piston torsions

The SBD manual explains that the designers used splined cylinders and pistons in their shock strut (Figure 79‑3b). It looks like a quite elegant solution for the blocking random torsions of the shock strut piston (less outer parts that are prone to the eventual dust and jams). I did not find any complaints for this landing gear in the veteran memoirs and technical reports (usually they praise the “rugged structure” of the SBDs). However, all other aircraft designs use the torqe arms in their landing gear. Maybe they were just cheaper (i.e. easier to produce)?

Basically the work on the details means that you have recreate “in the mesh” all the parts you can see on the photos. Below you can see how I recreated the upper part of the landing gear leg (Figure 79‑4):

Figure 79-4 Forming the upper part of the landing gear leg

Recreation of such a die-cast part, with all of its additional walls, roundings, is a small challenge. To make it with as simple mesh as possible, I used several modifiers. First, I used the Mirror modifier to automatically generate the symmetric half of this object. (This symmetry was only possible because I decided to split this “L” – shaped part into two objects: this complex die-cast and a simple tube behind it. (This tube is not present in the picture above). Then I recreated all the rounded edges on this object using dynamically-generated fillets. Another modifier (Bevel) creates fillets along all the edges that I assigned a nonzero bevel weight. (The fillet radius is controlled by the value of this weight).

When this upper part of the landing gear leg was finished, I created the cylinder of the shock strut. It was just a tube with an octagonal flange ­– nothing difficult. Then I had to create the lower part of the leg (Figure 79‑5):

Figure 79-5 Forming the lower part of the landing gear leg

It was another die-cast, which shaping required some time. (As you can see in the figure above, I formed this mesh from two crossing tubes).

While creating such a detailed assembly, I prefer to model each of its parts as a separate object. It gives me the opportunity to take advantage of its local coordinate system, when I need it. For example – the shocking strut is a tube rotated by 6⁰. When I formed it, I often extruded its faces along this local axis. Another advantage of such a model structure is the possibility of quick, “natural” adjustments of various parts. (For example – piston movement along the cylinder. In my model it occurs along its local Y axis).

Building the landing gear, I tried to check its retracted position as early as possible. Figure 79‑6 shows first of these trials:

Figure 79-6 Checking the landing gear in the retracted position

As usual, a small fragment of the retracted landing gear leg protruded from the upper wing surface. I had to re-examine the photos, find which part has the wrong shape, and fix it.

I also used my photo references to recreate other landing gear elements, like the wheel brake disks (Figure 79‑7):

Figure 79-7 Using the photo reference in recreating the wheel brake disks

During this work I also found some differences between my model and the reference photo (see the notes in blue frames in the figure above). It seems that my landing gear is somewhat shifted forward.

Such a finding led to many rearrangements in the geometry of this assembly (Figure 79‑8):

Figure 79-8 Adjustments in the landing gear geometry

In the background of Figure 79‑8a) you can see my original drawing from 2015. After some deliberations, I decided to leave the center of the wheels at its current location, because it was dimensioned on the original general arrangement drawing. However, after measuring tire proportions on various photo, I decided that the SBD used slightly wider tires (30×7.5”) than the size (30×7”) specified in one of the comments placed on the original drawing from the SBD manual. (Sometimes draughtsman could make such a mistake). To fit the photo, I adjusted location of the shock strut, moving it slightly toward the fuselage. I also shifted downward the axis of retraction (rotation). Figure 79‑8b) shows how the updated model fits the reference photo. The tires on the photo still seems somewhat smaller than those from my model. However, I decided that this restored CAF aircraft could use a slightly smaller tires (29×7.5”). (This particular SBD-5 also uses at least another non-original part: a different version of its Hamilton Standard propeller).

Another element that requires some adjustments are landing gear covers. Although I created them during the modeling phase, now I have to compare their shape with the reference photo. Preparing for this test, I placed in my model a simple “stick” which I use as the hinge (I set it as the parent of the landing gear cover):

Figure 79-9 Fitting the landing gear cover and its hinge to the reference photo

The hard part was to determine the proper axis direction and the angle of this rotation. Surprisingly, the hinge was not lying directly on the aircraft skin (it would be the simplest solution). While it was relatively easy to find for a given hinge orientation a rotation which angle placed the left cover as on the photo, the same rotation applied to the right cover did not match the reference. It required some hours to find a combination that produced an acceptable (although not ideal!) match.

As you can see, I performed a lot of various checking, adjusting and matching while forming the key elements of the landing gear. All of this because it is still quite easy to correct the geometry of this assembly while it is relatively simple. It would be a nightmare, when I did such a thing on the final, detailed assembly, which you can see in Figure 79‑10:

Figure 79-10 The main landing gear (with all of its final details)

However, before I finished this landing gear in such a state depicted in the figure above, I had to create several dozen bolts of various size, as well as other details. I also discovered some small secrets of its retraction mechanism. You will find a short description of all of these findings in my next post (to be published next week).

I decided to not enclose the source Blender file to this post, because it would contain just these few basic landing gear components. I will add it to the next post, which describes this assembly in the finished state.

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