First, the lift curve itself — Page 83, Lesson 96

Let’s pick up right where the lift curve leaves off, because Figure 5.6 and 5.7 are really the heart of what I want you to carry forward. I’m going to walk you through the angle-of-attack story, then the L/D ratio, then the things that change stall speed — weight, surface condition, and high-lift devices. First, the lift curve itself. On the vertical axis we plot the lift coefficient, which we write as C sub L. On the horizontal axis we plot angle of attack in degrees. As angle of attack increases from zero, C sub L rises along a straight, linear portion — that’s the lift curve slope. But it doesn’t rise forever. It reaches a peak value, and that peak is labelled C sub L MAX — the maximum lift coefficient. The angle of attack at which that peak occurs is the stalling angle, and the point itself is marked STALL on the figure. Beyond that angle, a further increase in angle of attack produces stall, and the lift coefficient then decreases — the curve falls away. So the stall is not a sudden event at one point; it’s the region where the curve has peaked and begun to drop. Now, Figure 5.7 introduces the drag side. Along with lift, we have drag coefficient, C sub D. And the key ratio is L over D — lift to drag. On that figure you’ll see a point marked L/D MAX — the maximum lift-to-drag ratio. That occurs at a specific angle of attack, and that angle is labelled the OPTIMUM ANGLE OF ATTACK. Below that angle, and above it, the L/D ratio is lower. So the optimum angle of attack is the one that gives you the best aerodynamic efficiency — the most lift for the least drag. That’s the angle you’d want to cruise at for maximum range. Now, the design of the aircraft has a great effect on the L/D ratio. Let me give you the typical values. A high performance sailplane has an L/D max from 25 to 60. A modern jet transport is from 12 to 20. A propeller powered trainer is from 10 to 15. So you see, the sleek sailplane is far more efficient than the transport or the trainer. Next, the effect of aircraft weight on minimum flight speed. Here’s the key principle: a given aerofoil section will always stall at the same angle of attack. That’s fixed by the shape. But aircraft weight will influence the indicated airspeed, the IAS, at which that stall occurs. Why? Because to support more weight, you need more lift, and you get more lift by flying faster at the same angle of attack. So a heavier aircraft stalls at a higher IAS. And here’s the practical point: a modern large jet transport may have just over half its maximum gross take-off weight made up of fuel. So as fuel burns off, the weight drops dramatically, and the stall speed can vary considerably throughout the flight. That’s why your V-speeds change with weight. Now, the condition of the surface. Surface irregularities, especially near the leading edge, have a considerable effect on the characteristics of the aerofoil. C sub L MAX, in particular, is sensitive to leading edge roughness. Figure 5.8 illustrates the effect of a rough leading edge compared to a smooth surface. In general, C sub L MAX decreases progressively with increasing roughness of the leading edge. But here’s the useful limit: roughness further downstream than about 20 percent of the chord from the leading edge has little effect on C sub L MAX or on the lift curve slope. So it’s the first 20 percent of chord that matters. Now, what causes that roughness? Frost, snow, and even rainwater can significantly increase surface roughness. Dirt or slush picked up from contaminated parking areas, taxiways, and runways can also have a serious effect. And in-flight icing — that usually accumulates at the leading edge of aerofoils, and it will severely increase surface roughness, causing a significant decrease in C sub L MAX. That’s why de-icing and anti-icing are so critical. Finally, flight at high lift conditions and the effect of high lift devices. The aerodynamic lift characteristics of an aircraft are shown by the curve of lift coefficient versus angle of attack in Figure 5.9, for a specific aircraft in the clean and flap down configurations. A given aerodynamic configuration experiences increases in lift coefficient with increases in angle of attack until the maximum lift coefficient is obtained. A further increase in angle of attack produces stall, and the lift coefficient then decreases — same story as before, but now we compare configurations. The primary purpose of high lift devices — flaps, slots, slats, and so on — is to reduce take-off and landing distance by increasing the C sub L MAX of the aerofoil section, and so reduce the minimum speed. The effect of a typical high lift device is shown by the lift curves of Figure 5.9. The principal effect of the extension of flaps is to increase C sub L MAX and reduce the angle of attack for any given lift coefficient. And the increase in C sub L MAX afforded by flap deflection reduces the stall speed by a certain proportion. So flaps don’t just add lift — they shift the whole curve up and to the left, giving you a higher maximum lift coefficient at a lower angle of attack, which means a lower stall speed, which means shorter take-off and landing distances. High lift devices will be fully covered later in the book, so for now just hold onto that core idea. That’s the full picture from this section: the lift curve and its peak at C sub L MAX, the stall, the optimum angle of attack at L/D max, how weight shifts the stall speed, how leading-edge roughness degrades C sub L MAX, and how flaps raise C sub L MAX to lower the stall speed.