General Principles - Cruise — Page 234, Lesson 281

Right, let's pick this up at the cruise phase of the General Principles chapter. We've already established that in straight and level flight, the tailplane produces a down force to balance the aircraft. Now I want to look at the two adverse effects that this balancing force has on performance, and then how we can manipulate it by moving the centre of gravity. So, first, the adverse effects. The tailplane down force is necessary for level flight, but it costs us. The first effect is that this down force acts in the same direction as weight. Think about that — weight pulls the aircraft down, and the tailplane is also pushing down. So it effectively increases the weight of the aircraft. The aircraft has to support more than its actual mass because of this additional downward force. The second adverse effect is its contribution to drag. The tailplane is an aerodynamic surface designed to produce lift — in this case, negative lift, a down force. But because it's producing lift, it generates induced drag, just like the main wing does. And on top of that, it also produces parasite drag, which is the drag from the airframe itself, the skin friction and form drag. So the tailplane contributes both induced and parasite drag. Here's the key relationship: the greater the balancing force the tailplane has to produce, the greater the aerodynamic drag and the greater the effective weight of the aeroplane. And these two extra penalties — the drag and the effective weight — are detrimental to the aeroplane's en route performance, specifically in terms of range and endurance. More drag means more fuel burn, and higher effective weight means more lift required, which again means more drag. So it all compounds. Now, here's the interesting part. We can manipulate the amount of tailplane down force required by moving the centre of gravity. In flight, we can do this in one of two ways. The first is selective fuel consumption — choosing which tanks to burn from. The second is fuel transfer — physically moving fuel between tanks. Let's think about the direction of movement. If we consume fuel in the tail first, or transfer fuel out of the tail to other tanks, the centre of gravity moves forwards. Conversely, if we use fuel in the centre or forward tanks first, or transfer fuel out of these tanks, the centre of gravity moves aft. Now, what happens when the centre of gravity moves forwards? The magnitude of the lift/weight couple increases, because the arm of the two forces is now longer. Remember, a couple is two equal and opposite parallel forces. Lift acts upward at the centre of pressure, weight acts downward at the centre of gravity. If the centre of gravity moves forward, the distance between these two forces — the arm — gets longer. A longer arm with the same forces means a greater moment, a greater couple. And this greater lift/weight couple increases the nose-down pitching moment. The aircraft wants to pitch nose-down more strongly. You can see this by comparing the length of the lift/weight pitching down arrows in Figure 5.2 and Figure 5.3. In Figure 5.3, with the more forward centre of gravity, those arrows are longer, showing the increased nose-down moment. So to summarise what we've covered: the tailplane down force is necessary but it costs us in effective weight and drag. We can reduce that cost by moving the centre of gravity, and moving it forward increases the nose-down pitching moment, which we'll see requires more tailplane force to balance — and that's the trade-off we'll explore next.