Flight Controls — Page 173, Lesson 227

Let’s start with the spoiler, because it sets up the whole idea of what a control surface is doing. A spoiler is a device for reducing the lift of an aerofoil, by disturbing the airflow over the upper surface. It is used to give lateral control by reducing the lift on one wing but not on the other. So when you want to roll the aircraft, you deploy the spoiler on one side, that wing loses lift, and the aircraft banks toward that wing. That’s the spoiler’s job. Now, the real heart of this section is the hinge moment. Imagine an aerodynamic force acting on a control surface. That force will tend to rotate the control around its hinge, in the direction of the force. The moment is the force multiplied by the distance from the hinge to the control surface centre of pressure. That product is called the hinge moment. The force itself can come from two sources: the angle of attack of the aerofoil, or the deflection of the control surface. And here’s a key assumption — the total hinge moment is taken as the sum of the separate effects of angle of attack and control surface deflection. So you treat them as additive. Why does this matter to the pilot? Because to hold the control in its position, the pilot has to balance that hinge moment by applying a load to the cockpit control. So the cockpit control load depends directly on the size of the hinge moment. Bigger hinge moment, bigger stick force. Now, the problem. The aerodynamic force on the control at a given deflection depends on two things: the size of the control surface, and the speed squared. Notice that — speed squared. So on large and fast aircraft, that force can get huge, and the resulting hinge moments and stick forces would be too high for easy operation of the controls. The pilot would need assistance to move the controls in those conditions. That assistance comes in two forms: power operation, or aerodynamic balance. Let’s look at aerodynamic balance. The idea is to use the aerodynamic forces on the control surface itself to reduce the hinge moment. There are several ways to do it, and the excerpt gives us two. First, the set back hinge line. The moment arm of the control surface force is the distance from the hinge to the centre of pressure on the control surface. If you move the hinge back into the control surface, that arm gets shorter, and the hinge moment is reduced. The crucial point here is that setting the hinge back does not reduce the effectiveness of the control — only the hinge moment of the force is reduced, not the force itself. So you keep the same control authority, but you cut the load the pilot has to fight. Second, the horn balance. The principle is similar to the set back hinge, in that part of the surface is forward of the hinge line. Forces on that forward part give hinge moments in the opposite direction to the moments on the main part of the surface. So the overall moment is reduced, but again, not the control effectiveness. So the takeaway for this whole section is the distinction between hinge moment and control effectiveness. Aerodynamic balance attacks the hinge moment, not the force, and therefore not the effectiveness. That’s the professional distinction you need to hold onto.