Flight Controls — Page 180, Lesson 241

I want to walk you through the rest of the trimming story, because we've just covered the main ways an aircraft is trimmed, and now we're looking at the finer points — the advantages, the power control system, and the artificial feel trim. First, let's recap the two big advantages of trimming by adjusting the tailplane incidence rather than by deflecting the elevator. The first advantage is that drag is less in the trimmed state, because the aerofoil is more streamlined. Think about it — when you trim with the elevator, the elevator stays deflected, which disturbs the airflow. But when you trim by moving the whole tailplane, the elevator can sit roughly neutral, so the airflow stays clean. The second advantage is that trimming does not reduce the range of pitch control, because the elevator is approximately neutral when the aircraft is trimmed. In other words, you haven't used up any of your elevator authority just to hold the nose up — you've got the full range available for manoeuvring. Now, here's an important point about power control systems. In a power control system, the load on the elevator is not felt on the cockpit control. The pilot doesn't physically feel the aerodynamic forces pushing back on the control surface, because the hydraulic or electric power does the work. But even so, trimming by adjusting the tailplane incidence may still be used, because the two advantages I just mentioned are still obtained. The amount of trim required will depend on the CG position — the centre of gravity — and recommended stabilizer settings will be given in the aircraft Flight Manual. And this is where I want to stress something critical for you as a professional pilot: it is important that these are correctly set before take-off. Incorrect settings could give either an excessive rate of pitch when the aircraft is rotated, leading to possible tail strikes, or very heavy stick forces on rotation, leading to increased take-off distances required. So a wrong stabilizer setting doesn't just make the aircraft feel odd — it can cause a tail strike on rotation, or it can make the stick so heavy that you need a longer runway to get airborne. That's why the pre-take-off check of the stabilizer setting is non-negotiable. Next, let's look at the spring bias trim system. In the spring bias trim system, an adjustable spring force is used to replace the pilot's holding load. So instead of the pilot having to hold a constant force on the stick to maintain the trimmed condition, a spring is adjusted to provide that force. And note this — no tab is required for this system. So it's a different approach from the trim tab systems we discussed earlier. Then we have CG adjustment. Here's the idea: if the flying controls are used for trimming, this results in an increase of drag due to the deflected surfaces. The out of balance pitching moment can be reduced by moving the CG nearer to the centre of pressure, thus reducing the balancing load required and therefore the drag associated with it. Let me unpack that. The centre of pressure is the point where the aerodynamic forces act on the wing. If your CG is far from the centre of pressure, the aircraft wants to pitch, and you need a balancing force — a deflected control surface — to hold it level. That deflected surface creates drag. But if you move the CG closer to the centre of pressure, the pitching moment is smaller, so you need less balancing load, and therefore less drag. This will give an increase of cruise range. And how do you move the CG in flight? CG movement is usually achieved by transferring fuel between tanks at the nose and tail of the aircraft. So you pump fuel forward or aft to shift the CG and reduce that trim drag. Finally, let's talk about artificial feel trim. This is crucial for power-operated controls. If the flying controls are power operated, there is no feedback of the load on the control surface to the cockpit control. The pilot can't feel the aerodynamic forces, because the power system is doing the work. So the feel on the controls has to be created artificially. When a control surface is moved, the artificial feel unit provides a force to resist the movement of the cockpit control. So it's a device that pushes back against the pilot's input, simulating the resistance you'd feel with direct mechanical controls. And to remove this force — that is, to trim — the datum of the feel unit can be adjusted so that it no longer gives any load. So trimming in a power control system means adjusting the reference point, the datum, of the feel unit, so that the artificial resistance disappears and the pilot doesn't have to hold any force. Let me tie this together. We've got four distinct trimming methods here: tailplane incidence adjustment, which is the cleanest and preserves elevator authority; spring bias, which uses an adjustable spring instead of a tab; CG adjustment by fuel transfer, which reduces trim drag and improves range; and artificial feel trim, which adjusts the datum of the feel unit in a power control system so the pilot feels no load. Each one solves the same problem — relieving the pilot of a constant holding force — but through a different mechanism, and each has its own operational implications, especially the stabilizer setting before take-off.