Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 6
Recall that this rotor was designed using the Prandtl tip loss model with a maximum
section lift coefficient of 0.5 based on steady-state INS2d results. Although it does not
reduce the utility of the results or introduce any global metric discrepancies, analysis
with the contracted wake model indicates that outermost sections are operating
significantly past the prescribed design maximum lift coefficient, the veracity of this
prediction is supported by the OVERFLOW-D thrust distributions presented in Figure
6.32.
OVERFLOW-D predicts significantly higher sectional thrust near the tip, but even with
the contracted wake model active, the rapid analysis tool cannot model sections
operating beyond the steady-state limit imposed by construct of the two-dimensional
section database. Above this limit, a heuristic stall model is implemented, capping the
lift coefficient and increasing the section drag. In the case of rotor design, this
successfully bounds the geometry within the steady-state region, but the best that the
rapid analysis tool can do is indicate a potential problem in the tip region.
6.8 Modeling Effects on Design Configuration
6.8.1 Effect of Swirl Modeling on Design
The impact of the three different viscous swirl models on rotor design has been explored
by developing three different configurations using the rapid analysis and design method
described in Chapter 4. All the rotors are constrained to have four blades, a 5mm
diameter hub and a 2.5cm total diameter and all use the classical Prandtl tip loss
correction. This restricts the design variables to chord, incidence, and operating RPM.
The 5mm Smoovy 15 second duration motor data is used for all cases. These conditions
and constraints have been implemented to allow the as-built and tested four-blade rotor
to be used as a point of reference. Following the design process, the rapid analysis
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