Stalling — Page 179, Lesson 216

I want to walk you through the effect of wing contamination on stall speed. This is a critical safety topic, because it directly links what we've learned about the boundary layer and stall to real-world pre-flight decisions. First, the big picture. Any contamination on the wing — but particularly ice, frost, or snow — will drastically alter the aerodynamic contour and affect the nature of the boundary layer. That's the core warning. The wing's shape is what generates lift, and anything that changes that shape or the airflow next to it changes the stall characteristics. Let's start with ice. Ice forming on the leading edge of the wing produces two major effects. First, large changes in the local contour, leading to severe local adverse pressure gradients. Second, high surface friction and a considerable reduction of boundary layer kinetic energy. Let me unpack that. The leading edge is where the airflow first meets the wing. Ice there changes the shape dramatically, which disturbs the pressure distribution — that's the adverse pressure gradient. And the rough, icy surface creates high friction, which slows the boundary layer down, reducing its kinetic energy. A sluggish boundary layer separates from the surface much earlier. The result is a large decrease in CLMAX — that's the maximum lift coefficient, the peak lift the wing can produce before it stalls. And here's the striking number: this can increase stall speed by approximately 30%, with no change in angle of attack. That's important — the wing stalls at the same angle, but at a much higher speed. The added weight of the ice will also increase the stall speed, but the major factor is the reduction in CLMAX. So don't just think of ice as weight — the aerodynamic damage is the bigger problem. Now frost. The effect of frost is more subtle. A hard coat of frost on the wing upper surface produces a surface texture of considerable roughness. The basic shape and aerodynamic contour are unchanged — frost is thin — but the roughness increases skin friction and reduces the kinetic energy of the boundary layer. Separation will occur at an angle of attack and lift coefficients lower than for the clean smooth wing. So the wing stalls earlier, at a lower angle of attack. Tests have shown that ice, snow, or frost, with thickness and surface roughness similar to medium or coarse sandpaper on the leading edge and upper surface of a wing, can reduce lift by as much as 30% — that's a 10% to 15% increase in stall speed — and increases drag by 40%. So we're talking about a serious performance hit from something that looks like sandpaper. Now snow. The effect of snow can be similar to frost in that it increases surface roughness. But there's a critical operational point: if there's a coating of snow on the aircraft, it must be removed before flight. Not only will the snow itself increase skin friction drag, it may obscure airframe icing. And snow will NOT blow off during taxi or take-off. Don't assume it'll clear itself. Finally, the legal and operational bottom line. The pilot in command is legally required to ensure the aeroplane is aerodynamically clean at the time of take-off. It is very important that the holdover time of any de-icing or anti-icing fluid applied to the airframe is known. If this time will be exceeded before take-off, the aircraft must be treated again. So the chain is: contamination changes the contour and boundary layer → CLMAX drops → stall speed rises → and you're legally responsible for ensuring the aircraft is clean before you go. That's the whole picture.