Flight Controls — Page 180, Lesson 238

Let’s start with the problem this page is built around: excessive rudder deflection at high speed. With a simple control system, if you push the rudder pedal all the way, you get full rudder deflection. That’s fine at low speed, where you need big rudder authority. But at high speed, the same full deflection would put excessive loads on the structure — the airflow pressure on the rudder grows with speed, so a large deflection at high IAS can overstress the fin and rudder. So we need a way to limit rudder travel as speed increases. There are two main systems used to prevent excessive rudder deflection. The first is rudder ratio changing. In this system, the rudder pedals move through their full range at all speeds — your feet still travel the whole arc — but the rudder deflection itself reduces as indicated airspeed, IAS, increases. So the pedal-to-rudder ratio is being changed automatically with speed. The second is the variable stop system. Here, movement of the rudder is directly proportional to pedal travel — that relationship stays fixed. Instead, what changes is the physical limit of travel: the movement of both the pedals and the rudder is reduced with increasing IAS. The rudder pedal travel — the arc of movement — reduces with increasing IAS, and therefore the travel of the rudder is reduced. This reduction will usually begin at 165 knots. So the contrast is: ratio changing keeps pedal travel full but shrinks rudder deflection; variable stop shrinks the pedal travel itself, which then shrinks the rudder travel proportionally. Now, trimming. An aeroplane is trimmed when it will maintain its attitude and speed without the pilot having to apply any load to the cockpit controls. Think about what that means: if a control surface has to be deflected to keep the aircraft balanced — say, to hold the nose up — the pilot would have to hold a force on the control column to keep that surface deflected. That force can be reduced to zero by operating the trim controls. So trimming is about cancelling out a steady control force. When might you need to trim in pitch? The page lists three causes: changes of speed, changes of power, and varying centre of gravity positions. Each of those changes the balance of the aircraft and demands a new trim setting. Trimming in yaw is needed on a multi-engine aircraft if there is asymmetric power — that is, one engine producing more thrust than the other — and also as a result of changes in propeller torque. Both of those produce a yawing tendency you have to trim out. Trimming in roll is less likely to be needed, but it could be required if the configuration is asymmetric, or if there is a lateral displacement of the centre of gravity — the CG shifted sideways. Finally, the methods of trimming. The main ones in use are: the trimming tab, the variable incidence tailplane, spring bias, CG adjustment, and adjustment of the artificial feel unit. Each of these is a different physical way to achieve that zero-force condition — some move a small tab on the control surface, some change the angle of the whole tailplane, some use a spring to hold a force for you, some shift the centre of gravity, and some adjust the artificial feel unit that simulates control forces. We’ll go through each of those in detail as we continue.