ATPL · Technical · Principles Of Flight Atpl Ground Training… lesson — Page 89, Lesson 99

Let’s start with the big picture, because this page is really the hinge between two ideas. Up to now we’ve been studying airflow over a single aerofoil section — that’s two-dimensional airflow. But a real wing is a three-dimensional surface, and the moment you put a wing into the air, even tiny pressure differences will make the air flow sideways, towards any region of lower pressure. That’s the key physical fact. Three-dimensional airflow modifies the effective angle of attack, it increases drag, it alters the stalling characteristics, and it can influence the control and stability of the aircraft. So from now on, we stop thinking about just an aerofoil section and we consider the entire wing. Before we can talk about that wing properly, we need a precise vocabulary for its geometry. Let me walk you through each term, because every one of them will come back later. First, Wing Area, which we write as S. It’s the plan surface area of the wing — that is, the area you’d see looking straight down from above. Now here’s a subtle point: even if part of that area is covered by the fuselage or by engine nacelles, we still legitimately count the whole plan area. Why? Because the pressure carryover on those surfaces means the wing effectively still works across that region. So S is the full plan area. Next, Wingspan, written b. That’s simply the distance from wing tip to wing tip. Then Average Chord, written c. This is the mean geometric chord. And here’s the neat relationship: the product of the span and the average chord gives you the wing area. So b times c equals S. From those two we get Aspect Ratio, written AR. It’s the proportion of the span to the average chord — AR equals b divided by c. If the planform has curvature and the average chord isn’t easy to determine, there’s an alternative expression: b squared divided by S. The aspect ratio is important because it determines the aerodynamic characteristics and the structural weight of the wing. To give you a feel for the range: a high performance sailplane has an aspect ratio of about 35, a jet fighter about 3, and a modern high speed jet transport about 12. Now the chord at specific stations. Root Chord, written C sub R, is the chord length at the wing centre line. Tip Chord, written C sub T, is the chord length at the wing tip. From those two we get Taper Ratio, which is C sub T divided by C sub R — the ratio of tip chord to root chord. The taper ratio affects the lift distribution and the structural weight of the wing. Two extremes to anchor you: a rectangular wing has a taper ratio of 1.0, because tip and root are the same. A pointed tip delta wing has a taper ratio of 0.0, because the tip chord is zero. Next, Sweep Angle. It’s usually measured as the angle between the line of 25% chords and a perpendicular to the root chord. The sweep of a wing causes definite changes in compressibility, in maximum lift, and in stall characteristics. And finally, the Mean Aerodynamic Chord, the MAC. A rectangular wing of this chord and the same span would have broadly similar pitching moment characteristics. The MAC is located on the reference axis of the aircraft, and it’s a primary reference for longitudinal stability considerations. That’s why it matters so much — it’s the chord we use when we talk about where the centre of gravity sits relative to the wing. Now, before we leave this page, I want to bring in the two figures that sit just before it, because they tie directly into what we’ve just defined. The first is Figure 5.8, which shows take-off lift coefficient curves. It plots lift coefficient, C sub L, against angle of attack. There are three curves on it. The basic smooth wing gives you a certain C sub L max. A wing with ice gives you a lower maximum. And a wing with frost, dirt, water or slush gives you an even lower one. The point of that figure is contamination: anything on the wing surface degrades the maximum lift coefficient you can achieve. The second figure, Figure 5.9, shows the effect of flaps. In the clean configuration — that’s flaps up — you get one C sub L max. With flaps down, the whole curve shifts up, and C sub L max is higher. So flaps increase the maximum lift coefficient you can generate. So here’s how it all hangs together. The wing geometry we just defined — area, span, chord, aspect ratio, taper, sweep, MAC — sets the baseline aerodynamic behaviour. Then the operational factors, contamination and flap setting, modify the lift coefficient you can actually achieve at take-off. That’s the bridge between the geometry and the performance.