In the quest for lower drag it is easy to forget the importance of the stall. An airfoil may have a very low drag coefficient, but if the airplane abruptly tip stalls while thermalling, far more altitude will be lost trying to recover than might have been gained by the marginally lower drag.
There are two phases to most stalls. The first occurs when the lift developed by the wing no longer increases with angle of attack—the lift curve rounds over. The second phase occurs when the lift suddenly falls back to a lower level as the angle of attack increases—the true stall.
What are the desirable stall characteristics that one should look for in the shape of the lift curve near stall? First of all, for a typical tapered wing (which closely approximates the “ideal” elliptic planform), the three-dimensional stall will be quite similar to the two-dimensional one. Thus the two-dimensional lift data shown in Chapter 12 provides a good indication of what will be experienced in flight. To illustrate both good and bad stall characteristics, the MB253515 and NACA 2.5411 respectively, serve as examples. The S2091 is an example of the intermediate case.
The stall characteristics of the MB253515 at Rn of 100k are excellent (see Fig. 12.55). “Phase 1”—the rounding of the lift curve—occurs at approximately 10°, but the actual stall which marks “phase 2” does not occur until at least 18° (the limit of the measurement). The 8° difference provides a broad, easily flown plateau with only a slight loss of lift from the maximum, even though flight in this region is inefficient because the drag is very high. For the MB253515 this angle of attack margin is quite large in comparison to most other airfoils.
On the other hand, the angle of attack margin of the NACA 2.5411 shown in Fig. 12.63 exemplifies the opposite end of the spectrum. This airfoil loses lift with no warning whatever (essentially zero angle of attack margin). Many airfoils show intermediate behavior, and some exhibit hysteresis as well at the stall. A typical example is the S2091B-PT at Rn = 100k, Fig. 12.84. While this airfoil exhibits a reasonable rounding of the lift curve, the sudden drop in C is over 0.3. What is more, there is hysteresis of almost 4°.
Left uncorrected, poor stall characteristics seriously compromise the controllability at low speed. Of course these problems are not unique to low-Rn airfoils; many “full-size” airfoils have similar problems. Over the years several remedies
have been developed to improve poor stall characteristics. Progressive spanwise twist, change of section (e. g. drooped leading edges) and stall strips are common, and two of these methods are often used together. For model sailplanes the most common approach is to use spanwise twist; however, it is not always effective. Flight tests on a model sailplane using a NACA 2512 (which is very similar to the NACA 2.5411) showed almost no improvement with changes in spanwise twist. Stall strips, on the other hand, were quite effective in alleviating the adverse stall characteristics.
In summary, many low-drag airfoils that might otherwise be reluctantly discarded because a deficient stall can be made usable, given the ability to improve the stall with these techniques.