Let’s pick this up right where the fin’s story gets interesting — Page 301, Lesson 358

Let’s pick this up right where the fin’s story gets interesting. I want to walk you through the factors that decide how much the fin actually contributes to directional stability, and then we’ll move on to the wing and nacelles. First, the fin doesn’t work in isolation. Its effectiveness depends on the dynamic pressure at the fin — that’s the pressure from the airflow hitting it. And that dynamic pressure can depend on the slipstream of a propeller, or on the boundary layer of the fuselage. The boundary layer is the slow-moving layer of air right next to the fuselage skin. So if the fin sits in that sluggish air, it feels less pressure and produces less force. But it’s not just the speed of the air. The local flow direction at the fin — the actual angle the air arrives at — is influenced by several things: the wing wake, the crossflow of air around the fuselage, the induced flow of the horizontal tail, and the direction of the slipstream from a propeller. Each of these has to be considered as possibly affecting the fin’s contribution to directional stability. So when we analyse the fin, we can’t just treat it as a simple vertical surface in clean air. Now, there’s a clever design trick. A high mounted tailplane — that’s the ‘T’-tail, where the horizontal tail sits on top of the fin — makes the fin more effective by acting as an “end plate”. An end plate blocks the air from spilling around the tip of the fin, which keeps the pressure difference across the fin stronger, so the fin generates more side force for the same sideslip. Let me explain the geometry of why the fin works at all. The side force on the fin may still be relatively small compared to that on the fuselage, which is destabilizing. But here’s the key: the fin’s line of action is far aft of the centre of gravity — the CG. Because the force acts at the end of a long lever arm behind the CG, the yawing moment it creates is relatively large. That large moment gives overall stability to the fuselage-fin combination. So even a modest force, placed far aft, can overcome a larger destabilizing force on the fuselage that acts closer to the CG. The principle behind the fin as a stabilizer is exactly the same as for the dorsal or ventral fin — those are the small vertical extensions, one on top and one underneath the rear fuselage. But the fin is much larger and, in particular, has a much higher aspect ratio. Aspect ratio is the span of a surface compared to its chord — a long, narrow fin has high aspect ratio. Because of that, the fin is effective at low angles of sideslip. It stays effective until the angle of sideslip is such that the fin’s angle of attack approaches its stalling angle. Above that value, the side force on the fin decreases as the sideslip angle increases, and the fin ceases to be effective as a stabilizer. That’s exactly the point where the dorsal or ventral fin becomes important. Because the dorsal or ventral fin stalls at a very much higher angle of attack, it takes over the stabilizing role of the fin at large angles of sideslip. So the big fin handles small sideslips, and the small dorsal or ventral fin handles the extreme sideslips where the big fin has stalled. Now let’s move to the wing and nacelles. The contribution of the wing to static directional stability is usually small. Let me break that down into three cases. First, a straight wing alone — its contribution is usually negligible. It just doesn’t produce much yawing moment in sideslip. Second, sweepback. Sweepback produces a stabilizing effect, and that effect increases with an increase in CL — that’s the lift coefficient. And since CL is higher at lower indicated airspeed — lower IAS — the stabilizing effect of sweepback is stronger at low speed. Third, engine nacelles on the wings. Their contribution depends on factors like their size, their position, and the shape of the wing planform. On a straight wing, nacelles usually produce a destabilizing effect. So to summarise the wing: a swept wing provides a stable contribution depending on the amount of sweepback, but that contribution is relatively weak when compared with other components. Let me explain why a swept wing is stable in sideslip. Consider a sideslipping swept wing, as illustrated in Figure 10.62. The sideslip means the airflow comes at the wing from an angle. On the swept wing, the leading wing — the one facing into the sideslip — presents a different effective sweep to the airflow than the trailing wing. That difference in effective sweep changes the lift and drag on each side, producing a yawing moment that tends to align the aeroplane back into the relative wind. That’s the stabilizing effect. But as I said, it’s weak compared to the fin. So the picture you should hold is this: the fin is the primary directional stabilizer, working through a long lever arm behind the CG, helped by the T-tail end plate, and backed up by the dorsal or ventral fin at extreme sideslip. The wing and nacelles add only small, and sometimes destabilizing, contributions that we still have to account for.