The Impact of Ground Effect — Page 98, Lesson 109

Let’s start with the core idea: ground effect is all about how close the wing is to the ground, and its biggest effect is on induced drag. I want you to hold onto that phrase — induced drag coefficient, written \( C_{Di} \). That’s the drag that comes from generating lift, the drag created by the wing’s downwash and wingtip vortices. Ground effect reduces it, but only when the wing is genuinely close to the ground — within half the wingspan, to be precise. That’s the threshold: if the wing is higher than half its span above the ground, the effect is negligible. Let me give you real numbers so you feel the scale. Take a representative aircraft with a 40-metre span. At a height of 40 metres — that’s one full span — the reduction in \( C_{Di} \) is only 1.4%. Barely anything. Drop to 10 metres, which is a quarter of the span, and the reduction jumps to 23.5%. Now drop to 4 metres, just one-tenth of the span, and you get a 47.6% reduction. So you see the pattern: the last few metres make a huge difference. The graph in Figure 5.22 plots percent reduction in induced drag coefficient against the ratio of wing height to span, \( h/b \), at constant lift coefficient, \( C_L \). The curve is steep near the bottom — that’s why those final metres matter so much. Now, here’s a practical consequence. The height of the wing above the ground when the aircraft is in the landing attitude depends on where the wing is mounted on the fuselage. A low-wing aircraft sits its wing closer to the ground, so it experiences a greater degree of ground effect than a high-wing aircraft. That’s a general rule you can carry with you. Now let’s move to the tailplane, because ground effect doesn’t stop at the wing. Look at Figure 5.23. The wing’s downwash — the air deflected downward behind the wing — is decreased by ground effect. That changing downwash angle modifies the effective angle of attack of a low-mounted tailplane. A high-mounted tailplane may sit outside that changing downwash and avoid the disadvantage. So the key contrast: low tailplane gets its effective angle of attack altered by the downwash change; high tailplane may be clear of it. Now, Figure 5.24 brings in tailplane camber. Camber is the curvature of the aerofoil — positive camber curves up, negative camber curves down, and a symmetrical tailplane has no camber at all. Here’s the important finding: the type of tailplane camber does not influence the pitching moment generated when downwash from the wing changes. Decreased downwash will always result in a nose-down pitching moment. Increased downwash does the opposite — nose-up. So regardless of whether the tailplane is positive camber, negative camber, or symmetrical, the response to a downwash change is the same direction of pitching moment. And one last point to tie it together: downwash doesn’t change only because of ground effect. It also changes when you operate the flaps, and when a shock wave forms on the wing at speeds higher than \( M_{CRIT} \) — that’s the critical Mach number, the speed at which airflow over the wing first reaches Mach 1 locally. So understanding this downwash phenomenon is a key element of the whole subject. That’s the full picture of ground effect and its influence on the tailplane.