Flight Controls — Page 173, Lesson 229

Let’s pick up right where we left off — we’ve covered the set-back hinge and the horn balance, both of which work by letting aerodynamic pressure act forward of the hinge line. Now I want to walk you through the remaining balance types, and they’re a really important family because each one trades something different. First, Internal Balance. This works on exactly the same principle as the set-back hinge, but the balancing area is inside the wing. So instead of the balance area sticking out ahead of the hinge, it’s enclosed within the aerofoil structure. Here’s how it works: when you move the control surface, it causes pressure changes on the aerofoil, and those pressure changes are felt on the balance area. Let me give you the example from the text. If the control surface is moved down, the pressure above the aerofoil is reduced and the pressure below it is increased. That reduced pressure is felt on the upper surface of the balance, and the increased pressure on the lower surface. So the pressure difference across the balance produces a hinge moment that is opposite to the hinge moment on the main control surface. The result is that the overall hinge moment is reduced. So internal balance is neat because it does the same job as the set-back hinge, but it’s all tucked away inside the wing. Now, Balance Tab. All the types we’ve looked at so far — set-back hinge, horn, internal — they all provide balance by causing some of the pressures on the control surface to act forward of the hinge line. The balance tab is different. It causes a force to act on the control surface trailing edge, and that force is opposite to the force on the main control surface. The tab is geared to move in the opposite direction to the control surface whenever the control surface is deflected. So if the control surface goes down, the tab goes up. That tab force at the trailing edge helps push the surface, reducing the hinge moment. But here’s the trade-off, and it’s important: unlike the previous types of balance, the balance tab will give some reduction in control effectiveness, because the tab force is opposite to the control force. So you’re getting lighter stick forces, but you’re paying for it with a slightly less effective control. Next, the Anti-balance Tab. This is the opposite idea. The anti-balance tab is geared to move in the same direction as the control surface. So if the surface goes down, the tab goes down too. That means it will increase the control effectiveness — the tab adds to the control force. But of course, it will increase the hinge moment and give heavier stick forces. So this is used when you want more authority from the control, and you accept that the stick will feel heavier. Then we have the Spring Tab. This is a modification of the balance tab, and the key difference is that the tab movement is proportional to the applied stick force. So the harder you push, the more the tab moves. That means maximum assistance is obtained when the stick forces are greatest. This is achieved by putting a spring in the linkage to the tab. The spring tab is used mainly to reduce control loads at high airspeeds — because that’s exactly when aerodynamic forces are highest and stick forces get heaviest. So the spring tab gives you assistance exactly when you need it most. Finally, the Servo Tab. And this one is a completely different philosophy. The purpose of the servo tab is to enable the pilot to move the control surface easily. In this system, there is no direct movement of the control surface as a result of moving the cockpit control. Let me say that again, because it’s the key point: the pilot’s control input does not directly move the control surface. Instead, the pilot’s control input deflects the servo tab, and the force on the tab then deflects the control surface until an equilibrium position is reached. So the pilot moves the tab, the tab moves the surface. Now, there’s a very important consequence of this. If the aircraft is stationary on the ground, movement of the cockpit control will give no movement of the control surface — only of the tab. And it should be noted that if external control locks are fitted to the control surface, the cockpit control will still be free to move. That’s a critical point for ground operations — you can move the stick freely even though the surface is locked, because the stick is only moving the tab. So to summarise the family: set-back hinge and horn and internal balance all use pressure forward of the hinge. The balance tab uses a trailing-edge force opposite to the control, at the cost of some effectiveness. The anti-balance tab moves with the surface to boost effectiveness but stiffens the stick. The spring tab gives assistance proportional to stick force, mainly for high-speed load reduction. And the servo tab completely decouples the pilot from direct surface movement — the pilot moves the tab, and the tab moves the surface.