Stalling — Page 158, Lesson 192

I want to walk you through two stall-control devices now — the stall strip and the vortex generator. These are both ways of managing how and when the airflow separates from the wing, but they do it in opposite directions. Let's start with the stall strip. The idea here is to force the stall to begin at the wing root rather than at the tip. Why does that matter? Because when the root stalls first, the wing tips — where the ailerons live — keep flying longer, so you keep roll control right up to the stall. That's a much safer stall pattern. Now, one way to force a root-first stall would be to design the aerofoil section at the root with a smaller leading-edge radius. A smaller radius means the airflow has to turn more sharply around the nose of the wing, so it separates at a lower angle of attack. That promotes separation at the root, but it comes at a cost — it decreases overall wing efficiency. So instead of redesigning the wing, we attach stall strips. These are small triangular strips, and you see them in Figure 7.12, fitted to the wing leading edge. They do the same job — at higher angles of attack they promote separation at the root — but here's the key advantage: they will not affect the efficiency of the wing in the cruise. In normal flight the strip is essentially invisible to the airflow; it only does its work when you get up to high angles of attack. Now the second device — the vortex generator, Figure 7.13. These are rows of small, thin, aerofoil-shaped blades that project vertically into the airstream, about 2.5 centimetres. Each blade generates a small vortex. That vortex takes the free stream flow — the high-energy air outside the boundary layer — and mixes it down into the boundary layer, adding kinetic energy to it. This re-energizes the boundary layer, and a re-energized boundary layer tends to delay separation. So where the stall strip promotes separation early, the vortex generator does the opposite — it postpones separation, keeping the flow attached to higher angles of attack. So hold those two in contrast: stall strip — a small triangular strip on the leading edge that forces an early root stall without hurting cruise efficiency. Vortex generator — rows of small vertical blades that re-energize the boundary layer with vortices and delay separation. Both are tools for shaping the stall, just in opposite directions.