| In Wil Schuemann's famous soaring magazine article, "A New Wing With Improved Low Speed Performance" he states that "Sailplane Designers have long known that changes in the sweep of sailplane wings of one half degree forward or aft are sufficient to cause a quite noticeable change in stall behavior." The article posts a theory claiming that the lateral motion of the airflow on the wings upper surface is modified by wing sweep so that it changes the stall behavior of the wing and therefore the low speed performance. Schumann admits that he did not have enough data to substantiate his theory however that did not stop the soaring community from accepting it. A quarter century later, it's time for the theory to be challenged. |
| The performance claim first: A "Schuemann wing" craze began based on the claims of improved performance and handling. The situation was hurried along by several research publications of bogus "crescent wing" induced drag results from aerodynamics programs that were known to be unreliable for calculation of induced drag. These computational results were understandably never realized in wind tunnel data, even though some researchers were working hard to torture their data into agreement with the bogus computational results. A third push for the swept tip frenzy was conclusions regarding the crescent plan form of bird wings and fish fins. The logic being that millions of years of evolution can't be wrong. This was completely disregarding the aerodynamic context that animal surfaces are very flexible. The actual benefit in the swept tip on animal wings and fins serves more to control the twist of their very flexible lifting surfaces. As the fin begins to lift, the load on the swept tip of the fin twists the tip in the washout sense. Sailplane wings are very stiff in comparison so the control of twist is much less of an issue (except maybe during launch and in high speed turns but that's another story for the next MSLNL issue). The overall crescent wing / swept tip performance benefit theory has been debunked many times over. This leaves us with the issue of stall behavior. |
| Unlike the performance effects, the stall behavior of crescent or swept tip wings is very obvious. A sail plane with unswept wings has a much more abrupt and less controllable stall than a "Schuemann" wing with the same airfoil. However the somewhat elaborate theory of stall control by lateral flow of the boundary layer on the wing's upper surface has a much simpler and more attractive alternative: Typical sail planes are designed to stall at the center of the wing first. A swept back wing has the area near the root located ahead of the center of gravity (CG) and the area near the tips is behind the CG. When the swept wing starts to stall the loss of lift is ahead of the CG while the tips continue to lift. This gives a nose down moment that reduces any tendency for the ship to go farther into a stall. An unswept wing does not have this nose down moment so the ship tends to pitch up into a full stall much quicker that the aft swept wing. This explanation makes much more sense than the lateral boundary layer flow theory, especially in light of the "long known" noticeable effect of wing sweep changes as small as one half degree. The conclusion then is that pitching behavior at the onset of stall can be controlled by varying the wing sweep. |
| The question to ask though is how do you determine the best amount of sweep. The best approach is probably to just find a sailplane that handles how you want at the onset of stall. After designing your new planform, add sweep to the wings so to duplicate the fore-aft distribution of area on the good handling ship that you found. You can sweep the whole wing, the tips or anything in between. The results will be similar as long as the fore-aft balance of area matches that of your good handling example ship. You should recheck the lift distribution of your wing with the sweep added and may find some small changes need to be made. Aft sweep tends to load the tips more. Generally, high aspect ratio wings will require fewer degrees of sweep than low AR wings. If your design is a has a much different AR that your example ship you could probably get close by scaling sweep inversely with AR. e.g.Use 2/3rds as much sweep angle when sweeping a new 12:1 AR wing in comparison to an 8:1 AR example wing. This will make the actual fore-aft distances of the root and tip equal between the example and new design. In summary: You can make your sailplane much easier to fly slowly by using the correct amount of wing sweep. |
| PS There was a stall handling problem during the development of the DC-3. Can you guess why the production model has swept back outer wing panels? |