4

Wing Planform & Twist

             The chart to the shows the stall pattern for each of the various wing planforms. This information needs to be taken into account when picking a wing design for your plane.

             All plane design is a compromise. With research, it becomes known that the elliptical wing (Spitfire) is the most efficient in both drag and L/d. Unfortunately, it is difficult to manufacture. The tapered planform is a fair approximation of the elliptical wing. It is also far easier to manufacture. The famous “Hershey Bar” wing is the least efficient, but the easiest to build.

             It is clear, that except for the “Hershey Bar” wing, stall starts at the wing tips for all designs. However, even the “Hershey Bar” wing will stall at the tips when flaps are deployed.  The L-5 were are restoring has leading edge slots in front of the ailerons to correct this problem. Wing twist (washout) is the normal method used to create a safe, controllable plane under stall conditions.

 

             It should be obvious that reducing the stall problem by twisting the wing (washout) will reduce the effective lift of the airfoil. This brings to mind the for the questioning:

 

“A solution to a problem brings new problems, but do the new problems justify ignoring the benefits of the solution.”

 

             This is faced by every aircraft designer. There is a reason that almost all slow light planes use the rectangular “Hershey Bar” wing. It is easy to build and suffers very little drag impact. For high speed planes, this is not true. As the speed goes above 100 kts, the drag problem increases, assuming the aspect ratio (wingspan/chord) stays the same. Increasing the aspect ratio decreases the drag impact, but increases the structure weight due to mechanical structure.

 

             One of the fun things to do is search the Internet for the information for decision making. Are there modifications that can be made to make  any wing better. Yes, the previously addressed winglet helps, especially at low speeds (near max L/D). But adding a second wing can approximate an increased aspect ratio without the high structural weight gain. BUT, then one runs into the problem of airflow interference across one or both of the wings. Aren’t these research and decision making processes fun?