Flight Control Systems — Page 156, Lesson 201

Let's start with the big picture. When you sit in the cockpit and move the controls, you're commanding the aircraft to change its flightpath or attitude. The question is: how does that movement actually get from your hands and feet to the flying control surfaces out on the wings and tail? There are three ways this can be achieved. First, mechanically — the control surfaces are connected directly to the cockpit controls by a system of cables, rods, levers and chains. That's the simplest, most direct link. Second, hydraulically — the control surfaces are moved by hydraulic power, though the control valve that directs that power may still be operated mechanically by your input. And third, electrically — movement of the cockpit control sends an electrical signal to the control surface, and the actual movement of the control may then be achieved hydraulically. So notice: electrical transmission of the command, hydraulic power to do the work. Now, let's look at a concrete example. Figure 7.2 shows a manually operated elevator control system for a light aircraft, with the main components laid out. The elevator is the control surface that pitches the aircraft nose up or down. Here's the key relationship: rearward movement of the control column causes upward movement of the elevator, which causes the aircraft to pitch nose upwards. And vice versa — push the column forward, the elevator goes down, and the nose pitches down. That's your pitch control. Control in roll is achieved by the ailerons. Turning the control wheel to the right causes the right aileron to move up and the left aileron to move down, giving roll to the right. So the wheel turn and the roll direction match — right wheel, right roll. Control in yaw is given by the rudder. Moving the right rudder pedal forward causes the rudder to move to the right and causes the aircraft to yaw to the right. Again, the input direction and the resulting motion match. And importantly, these movements are obtained by similar arrangements of cables, push-pull rods and chains for the elevator — so the same mechanical transmission principles apply across all three axes. Now, let's define what we mean by primary controls. Primary controls are controls which rotate the aircraft about its three axes and thereby cause a change in the aircraft's flightpath and/or attitude. That's the precise definition — they're the ones that actually rotate the aircraft about its three axes. And the primary controls consist of the elevator, the rudder and the ailerons, plus roll control spoilers. So the spoilers, when used for roll control, count as primary controls too. Let me make sure the three axes are clear, because everything hangs on this. Pitch is the nose up and down motion, controlled by the elevator. Roll is the banking motion around the longitudinal axis, controlled by the ailerons and roll control spoilers. Yaw is the nose left and right motion around the vertical axis, controlled by the rudder. Each primary control rotates the aircraft about one of those three axes, and that rotation changes the flightpath or attitude. So to summarise what we've covered: the three transmission methods — mechanical, hydraulic, electrical. The elevator and its pitch behaviour with the control column. The ailerons and roll with the control wheel. The rudder and yaw with the rudder pedals. And the definition of primary controls — elevator, rudder, ailerons, plus roll control spoilers — which rotate the aircraft about its three axes to change flightpath and/or attitude.