Modeling Hamilton Standard Hydromatic Propeller

In this post I start finishing the SBD-5 model. It differs in more details from the SBD-3 than the SBD1. One of the most prominent differences is the propeller. I will create it in this post.

In the later Dauntless versions (starting from the SBD-4) Douglas used the new propeller: Hamilton Standard Hydromatic. The SBD-1,-2,-3 used the older constant speed propellers, which used counterweights to oppose the force generated by the oil pressure in the control cylinder. (I created the model of this propeller in this post). The Hydromatic propeller used the oil pressure on both sides of the piston that controlled the pitch. It eliminated the massive counterweights, creating a lighter, smaller, and more precise pitch control unit. Hamilton Standard Hydromatic propellers has been widely used since 40’ (you can still encounter them in the various modern aircraft).

In the Dauntless, these Hydromatic propellers came with slightly modified blades (Figure 55‑1):

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Figure 55-1 The Hamilton Standard Hydromatic propeller (mounted in the SBD-4)

I used some photos to copy the contour of this new blade into the reference drawing. Then I copied the “old”, untwisted blade from the SBD-3 and modified its vertices so it fits the new shape (Figure 55‑2) (this is the view from the rear):

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Figure 55-2 Modification of the blade shape

(It was quite similar to the las stages of shaping the SBD-3 propeller blade, described in this post. Thus I will not elaborate about it here).

The hub (Hamilton also refers this part as the “barrel”) of this propeller had a quite complex shape (Figure 55‑3):

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Figure 55-3 Side view of the hydraulic hub (from a restored SBD-5)

This barrel splits into the front and rear halves. Because there is an oil under pressure inside, there are three bolts on each of the three flanges that keep these barrel halves together.

Beware: it seems that these classic Hydromatic propellers are rare, and some of the restored SBDs use different, non-original models. As a quick indicator you can use the number of the bolts around the barrel. The original propellers had a single bolt in the middle of each flange (as the propeller from Figure 55‑3).

The propeller from Figure 55‑3 was used in the flyable SBD-5 (“white 39”) from Chino Planes of Fame air museum. It seems OK, just misses a small detail: the cap on the tip of the dome.  Another example: in the flyable “white 5” from the Commemorative Air Force you can find a larger hub with two bolts in the middle of each barrel flange. What’s more, the blades of this aircraft have non-original shape. To further increase the confusion, there is a non-flyable SBD at the Palm Springs Air Museum, (“white 25”) which combines a non-original, larger Hydromatic barrel and the propeller blades from an earlier SBD version (SBD-3?).

In fact, the aircraft from Palm Springs is a real trap for the modelers:  its engine cowling combines panels from various Dauntless versions! (You can see in this photo that it has the carburetor air scoop from the SBD-3 and the side panels with narrow ventilation cutouts from the SBD-5).

The halves of this hub barrel were forged (or casted?), thus all of its edges and corners are rounded. It makes modeling of this element much more difficult, at least in Blender (you will see it in a moment).

To better understand this shape, I started with its conceptual model, without all these fillets and flanges (Figure 55‑4):

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Figure 55-4 Conceptual model of the HS Hydromatic hub

Studying the photos and available drawings of this control pitch mechanism, I decided that this “barrel” is a combination of three cylinders (the bases of the propeller blades) and a solid of revolution resembling a jug (Figure 55‑4). Using this conceptual model, I quickly determined the exact shape of this central “jug” that produces the same intersection edges as you can see in the photos.

In a CAD system the next steps would be easy: I would create the basic flange shape by adding some plates and small cylinders. Then I would rounded all their edges using various fillets, and the barrel would be ready.

Unfortunately, Blender has no such a powerful fillet feature: it only has a multi-segment Bevel command, which can create a fillet between two elementary faces. It is usually sufficient for architects. However, If I joined the conceptual model from Figure 55‑4 into a single mesh (using Boolean union operator), I would to be able to create the appropriate fillet along its edges. (Boolean operation produces in Blender a lot of small elementary faces along the intersection edge. Their size determine the maximum radius of a fillet).  I started to think about following pzzf7s’ suggestion about using the free AutoCAD 123D as an auxiliary tool for such parts. Ultimately I decided that before I do it, it is a good idea to create at least one of such difficult shapes using Blender tools. Later it will allow me to make a fair comparison between making complex mechanical parts in Blender and AutoCad 123D.

So I started modeling the propeller barrel in Blender. During this process I used the conceptual model as the reference object (I marked it in red — see Figure 55‑5):

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Figure 55-5 Initial stages of the hub barrel mesh

I decided to take the advantage of the internal symmetries of this shape, and prepared the mesh for 1/6th of the barrel — just half of a single blade base and one and half of the flange bolts. Thus I initially created two cylinders for these bolts (Figure 55‑5a). Then I joined these two cylinders into a single object, which is rotated along Y axis by 60⁰ (to create the local symmetry axis along the flange). I removed the half of the inner cylinder, because it is dynamically recreated by the Mirror modifier. In the next step I created the basic flange that connects these two cylinders (Figure 55‑5b). Then I added two inner edges, to bend the side faces of this mesh along the rounded sides of the reference surface (Figure 55‑5c).

Once I formed this flange, I started to shape the remaining part of this mesh. I added an arc that lies on the surface of the central solid of the barrel (Figure 55‑6a):

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Figure 55-6 Forming the forward part of the hub barrel mesh

The number of the arc vertices is extremely important here. It had to be similar to the distance between vertices of the flange edges that connects the bolt cylinders. In similar way I added another arc around the blade base cylinder, then extruded both these edges into two intersecting surfaces (Figure 55‑6b). Finally I generated in this mesh the intersection edge of these two surfaces (using my Intersection add-on). I used this edge as the base for forming two new rows of faces that replaced the original ones (Figure 55‑6c).

Now the shape of this object starts to resemble the barrel. I improved the shape of its fillet by adding additional edge (Figure 55‑7a):

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Figure 55-7 Finishing the forward part of the hub barrel mesh

Finally I shaped the inner part of the blade base (Figure 55‑7b) and filled the gap in the front of flange cylinders (Figure 55‑7c).

The rear half of the barrel was easier, because I started it from a mirror copy of the forward part (Figure 55‑8a):

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Figure 55-8 Forming the rear part of the barrel

Then I removed some of its faces and modified the shape of remaining key edges (Figure 55‑8b). Finally I connected these edges with new faces, and added additional edges along the fillets (Figure 55‑8c).

All in all, forming this element in Blender was not easy. On the next occasion I will try the AutoCAD 123. (I have to learn it).

In Figure 55‑9a) you can see the finished hub barrel. I also added the cap on the dome tip:

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Figure 55-9 Finishing touches — adding the cap to the dome

Figure 55‑9b) shows the finished assembly. I suppose that I will reuse this hub in many other models. A lot of the various aircraft which used the Hamilton Standard Hydromatic propellers. (At least those, which used the tree-blade model with the single bolt in the middle of their barrel flanges. I know that such a specific conditions sound strange, but it is a quite common model).

In this source *.blend file you can evaluate yourself the model from this post.

In the next post I will recreate other SBD-5 details that differ from the SBD-3.

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