Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 3
Small improvements in lift and drag allow the R6 airfoil to outperform the NACA 4702.
The majority of the gains in lift and drag are connected to 5% less trailing edge
separation on the R6 compared to the NACA 4702. The optimizer is attempting to
exploit the benefit of limiting trailing edge separation. The maximum camber is moved
to the aft control point and this region is once again operating similarly to what has been
described as a separation ramp [22].
For the R6 airfoil at α=4.0, trailing edge separation occurs at 88% chord, growing to
86% at α=5.0. Beyond α=5.0, the separation point almost immediately moves forward
to 30% chord. Figure 3.21 shows the aft 60% pressure distributions at α=4.0 and α=5.0.
At α=4.0, there is a distinct inflection in the pressure distribution at 82% chord. The
reduced adverse gradients ahead of this location do not allow separation to move
forward, but the steeper gradients following the inflection point promote separation on
the aft portion of the airfoil. This is the primary source of the drag penalty seen with aft
camber at low angles of attack. At α=5.0, the inflection is no longer present. Beyond
α=5.0, separation moves far forward and performance rapidly degrades.
Cp
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
Beyond α= 5.0 deg., separation rapidly moves forward from 80% chord
4.0 Degrees
5.0 Degrees
0.4
0.4 0.5 0.6 0.7
x/c
0.8 0.9 1.0
FIGURE 3.21 C p distributions for the Re=6000 optimized airfoil.
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