Airfoil Performance Concepts
Why bother with untidy unstable airfoils? The reason in a single word is “performance”. The shape of the lift and drag polar defines the airfoil performance or drag coefficient (cd) as a function of lift coefficient (cl). Generally, airfoils exhibit the lowest drag over a narrow range of angle of attack called the “drag bucket”. The airfoil shape solely determines the shape and position of the drag bucket.
Figure 3 is a generic performance plot for typical symmetrical airfoils. As expected, due to airfoil symmetry, the drag bucket is centered about the zero degree angle of attack and zero cl. This means that wing drag is the same for a given angle of attack or cl whether the plane is flying right side up or upside down. But since the drag bucket is centered around zero degrees, only half of the low drag angle of attack dynamic range is available for normal right side up (positive lift) steady state flight.
What about cambered airfoils? Figure 4 is a performance plot for a typical cambered or unstable airfoil. The drag bucket shown is about the same width as before but the center is shifted into the positive angle of attack region along with maximum attainable cl. Good news if we expect to fly right side up. The drag bucket now extends over a wider positive angle of attack than it did for the symmetrical airfoil. Does this mean that the airfoil performance continues to increase with increased camber? In general the answer is yes, but only up to a point. Camber can only be increased so far before drag takes over and overall performance takes a hit.
What kind of performance can be expected from stable airfoils? The transition from the symmetrical airfoil with no camber to the cambered (unstable) airfoil resulted in increased performance. Transitioning from symmetrical to stable airfoil, however, requires a change in camber line from straight to reflex. The center of the drag bucket moves toward the negative region of cl and, thus, really drags down both performance and maximum attainable cl as illustrated in Figure 5. This time performance really takes a nose dive.
Conclusion: Wings with cambered unstable airfoils perform best - and to stabilize that unstable wing without sacrificing performance requires active stabilization.
