Flight Controls — Page 173, Lesson 225

Right, let's get into the Flight Controls chapter. We're starting at the very beginning, with the purpose of the controls, and I want you to think of this as the foundation for everything else we'll cover. So, first, the fundamental purpose. For steady flight, the aircraft must be in a state of balance. That means zero moments around its axes. The controls are what enable us to achieve that balance for all possible configurations and centre of gravity positions. Then, secondly, the controls are required to manoeuvre the aircraft around its three axes. So we have two distinct jobs: trimming for balance, and manoeuvring. Now, let's define those moments around the axes, because this is the core vocabulary. We have three axes. First, the longitudinal axis. Rotation around the longitudinal axis is rolling, and it's controlled by the ailerons. For some aircraft, it's controlled by spoilers, or by a combination of the two. Second, the lateral axis. Rotation around the lateral axis is pitching, controlled by the elevators, or by a moving tailplane. Third, the normal axis. Rotation around the normal axis is yawing, controlled by the rudder. Now, on some aircraft, rotation around two of the axes may be achieved with one single control surface. Let's look at those. There's the elevon, which is a combination of elevator and aileron, used on tail-less aircraft. It gives both pitching and rolling. Then there's the ruddervator, used on a V tail, which gives both pitching and yawing. And there's the stabilator, which is a moveable tailplane combining the dual function of horizontal stabilizer and elevator. So it gives both longitudinal stability and control. Now, how is the moment around an axis actually produced? It's produced by changing the aerodynamic force on the appropriate aerofoil—that's the wing, the tail, or the fin. And this can be done in three ways. First, by changing the camber of the aerofoil. Second, by changing the angle of attack, which is the incidence, of the aerofoil. And third, by decreasing the aerodynamic force by "spoiling" the airflow. Let me unpack that first one, camber. Increasing the camber of an aerofoil will increase its lift. And deflecting a control surface down effectively increases its camber. So that's the principle: deflect the surface down, increase camber, increase lift, and that creates the moment. This principle can be applied to control about each of the axes—the elevator for pitch, the aileron for roll, and the rudder for yaw. Now, the second method: increasing the incidence, and hence the angle of attack, of an aerofoil will also increase its lift. The usual application of this system is for pitch control—that's the moving tail, the stabilator. And that's exactly what Figure 8.1 shows you. So to tie it together: we have three axes, three primary controls, and two fundamental ways to generate the aerodynamic force change—by camber change, which is your ailerons, elevator, and rudder, or by incidence change, which is your stabilator. And don't forget the third method, spoiling the airflow, which is how spoilers work for roll control. That's the complete picture of how we create moments around the axes.