Modeling Propeller Blades

The SBD Dauntless used two types of the Hamilton Standard propellers:

• Hamilton Standard Constant Speed (counterweight propeller) used in the earlier Dauntless versions (SBD-1 … SBD-3). The blades of this propeller had smaller tips (Figure 52‑1a);
• Hamilton Standard Hydromatic used in the later Dauntless versions (SBD-4 … SBD-6). The blades of this propeller had larger tips (Figure 52‑1b):

Figure 52-1 The Hamilton Standard propeller blades, used in the SBD Dauntless

These two blades had different shapes. In this post I will recreate the earlier version, which was used in the SBD-1 .. -3 (Figure 52‑1a). Several posts later I will modify its copy to obtain the later model of the blade, as used in the SBD-4 .. -6 (Figure 52‑1b).

The main problem with recreating propeller blades of various historical aircraft is the lack of their precise drawings. In fact, I saw such a thing once, in the detailed plans of the Soviet WWII fighters. (Such a drawing contains the contour of the blade in the front and side view, as well as the set of subsequent airfoil sections, from the rotation axis to the tip). Nothing like this you can find for the typical Hamilton Standard blades! I looked for any trace of such drawings in the Internet. All what I found was a thread in one of the aviation forums. One of the participants of this discussion showed letters that he exchanged several years ago with the Hamilton Standard company. He asked for drawings of a blade that was designed in 1936. HS declined to reveal it, explaining that this design still remains their “business secret”.

In such a situation, all what we have are the photos of the real blades. (Until somebody makes a 3D scan of such a blade, and will publish it in the Internet — I hope that such a reverse engineering is legal). I used these references to draw the most possible blade contour in my scale plans. However, I had to rely on the photos for best estimation of the size and thickness of the airfoil sections of this blade, as well as the variation of their pitch along the span (i.e. propeller radius). Thus they may be less precise copies of the original than the rest of this model.

Now I see that I should draw the propeller blade vertically or horizontally on my reference drawings (Figure 52‑2). Well, I sketched them at the fancy angle of 120⁰, but never mind — it is still possible to use it. I will have to slide and scale the mesh vertices along the local axes of the blade object.

I started the work on the blade by creating a cylinder object. Then I rotated it by 120⁰, aligning to the reference drawing (Figure 52‑2a):

Figure 52-2 Initial stages of the propeller blade

Then I extruded its upper edge and flattened it at the tip (Figure 52‑2b).

In the next step I inserted a few additional edges in the middle of this blade (Figure 52‑3a), and shaped them so they resemble a flat-bottom airfoil (Figure 52‑3b):

Figure 52-3 Profiling the cross-sections of the blade

You can find such a flat-bottom airfoils in the most of the propeller blades. Why? Because their flat bottom edge creates a kind of technological base in this twisted, complex shape. (For example, it allows you to measure the local pitch).

I do not know what was the airfoil used in the Hamilton Standard blades. In one of the aviation forums I have found that it was RAF-6. It is not confirmed information. If it would be true, the leading edge of this blade should be sharper (RAF-6 had smaller radius of the leading edge).

When the cross-section shape of the blade was set, I started to form its contour in the front view (Figure 52‑4):

Figure 52-4 Shaping the blade contour in the front view

I stretched and shifted its airfoil edges until the blade contour fit the reference drawing. Figure 52‑4a) shows how the base (i.e. control) mesh of this contour looks like. In fact, I formed it directly, using the alternative display mode of the smooth resulting surface (as in Figure 52‑4b).

Finally I closed this mesh along the circular tip. Comparing the reference drawing with the photos, I decided that the contour of this tip was a perfect arc. What’s more, I decided that it was a little bit larger than on my reference drawings. Thus I created a reference object — a circle (Figure 52‑5a):

Figure 52-5 Shaping the tip of the blade

I modified the last edge of this mesh, shaping the resulting contour around the reference circle. Figure 52‑5b) shows the final shape of the mesh, while Figure 52‑5c) — the resulting tip surface.

Because I missed the information about pitch distribution along this blade, I decided to deform it in a dynamic way, using a curve via a Curve Deform modifier assigned to the blade object. In this case the deforming curve is a straight line, placed along the local Z axis of the blade (Figure 52‑6a):

Figure 52-6 Twisting the blade with a Curve Deform modifier

In such an arrangement I can control the pitch of this blade by changing the tilt value in the curve control points. At the tip the tilt is 0⁰, while at the last point (which lies on the propeller axis) it reaches the maximum value (25⁰). These values are just an estimation. The tilts in the middle points of the curve lie within this range (Figure 52‑6b). It dynamically deforms the control mesh of this blade (Figure 52‑6c). After a few trials I obtained the twisted shape that resembles the photos.

When you twist the shape using a modifier (like the Curve Deform in this case), you can easily switch into the original, untwisted shape of this blade. In this form you can easily introduce eventual modifications, like the sharper leading edge, or different shape of the tip. This feature will be useful when I have to create another version of this blade (for the SBD-5).

Figure 52‑7 shows the three clones of this blade, arranged as in the propeller:

Figure 52-7 The resulting blade objects

Their mesh is a copy of the original, with the “applied” (i.e. fixed) result of the Curve Deform modifier. Just in case, I preserved the original (not twisted) mesh of this blade together with the deformation curve in the References scene, among other auxiliary objects. It will be useful later, for the Hamilton Standard Hydromatic propeller, used in the SBD-5.

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

In the next post I will create the hub of this Hamilton Standard Constant Speed propeller.