Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 5
5.2 Rotor Manufacturing
5.2.1 Shape Deposition Manufacturing
Three different methods have been used in the manufacturing of the rotor prototypes.
The smaller rotors are fabricated by the Rapid Prototyping Laboratory at Stanford
University. The rotors constructed of epoxy resin have been built using the Shape
Deposition Manufacturing (SDM) process [31]. A pictorial summary of this method is
shown in Figure 5.1. This method can yield very accurate geometries during
manufacturing within certain limitations, but in the case of these micro-rotors, the
accuracy of the specified geometry after handling and use has proven to be a problem.
Each blade has a minimum thickness of only 20 mils, but the desired camberline is
accurately reproduced as seen in Figure 5.2. The blade airfoil geometry is the result of
the two-dimensional airfoil optimization at Re = 6000. The NACA four-digit thickness
distribution is clearly not reproduced in this case. The desire to minimize section
thickness is tempered by manufacturing minimum gage constraints. If the design
exceeds these minimums the tooling forces may deform or tear away the part. The trade
is then between a thicker airfoil section with a more conventional thickness distribution,
or a constant thickness section at minimum gage. The results of Chapter 3 indicate that
the latter is preferred. A second cross-section, in this case a NACA 4402 camberline, is
shown in Figure 5.3. In both cases the constant thickness regions are approximately 2%
chord, representing this minimum gage constraint. In this case, some shaping of the
leading and trailing edge regions is visible. This is achieved by reversing the
manufacturing orientation such that the kerf of the tool at the bounding tool paths
provides beneficial shaping. A comprehensive discussion of the manufacturing aspects
of the mesicopter project is provided by Cheng [32]. It is important to recall that, as
shown in Chapter 3, the camberline is the primary determinant of performance for very
thin sections.
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