Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 6
The Sample-1 rotor exhibits similar thrust to the Sample-3 aluminum rotor but requires
additional power. Comparing Figures 6.21 and 6.22, the aluminum rotor is closest to the
specified geometry, while the Sample-1 rotor has two blades at greatly reduced incidence
and one blade at increased incidence. The design code limits the section lift coefficients
to a prescribed value, typically one to two tenths below the maximum steady-state lift
coefficient, meaning that additional lift is possible.
Considering the drag polars of Chapter 3, near stall this lift would come at an ever
increasing cost in drag. The single blade of Sample-1 with increased incidence
compensates for the loss of lift from the two low incidence blades, but at a total increase
in power required as compared to the design and Sample-3. A similar argument may be
made for the Sample-2 rotor. In this case two blades have excessive incidence with
another slightly low. This creates another situation requiring excess power, but in this
case additional thrust is also generated. This is particularly evident between 35,000 and
45,000 RPM.
Incidence (deg.)
20
16
0
12
8
4
As Designed
Blade 1
Blade 2
Blade 3
Blade 4
0 2 4 6 8 10 12 14
r (mm)
FIGURE 6.21 Sample-1 blade incidence distributions based on quadratic fitting of laser-scan
data.
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