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 the basic forces in straight and level flight, and now I want to walk you through the consequences of that balancing act. Look at Figure 5.2, which shows the complete balance of forces in straight and level flight. You'll see the tailplane is producing a down force. Now, although that force is absolutely necessary for level flight, it has two adverse effects on aircraft performance, and I want you to remember both of them. The first adverse effect is this: notice that the tailplane down force acts in the same direction as weight. It's pushing down, just like gravity. So it effectively increases the weight of the aircraft. The aircraft has to support not just its own mass, but also this artificial downward force from the tail. 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 will produce induced drag, just like the main wing. 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 types of drag. Here's the key relationship: the greater the amount of balancing force produced by the tailplane, the greater the aeroplane's aerodynamic drag and effective weight. And we'll shortly understand that these two additional forces provided by the tailplane are detrimental to the aeroplane's en route performance in terms of range and endurance. More drag and more effective weight means you burn more fuel, so your range and endurance suffer. Now, here's the clever part. To some extent, the amount of tailplane down force required for level flight can be manipulated by moving the centre of gravity. In flight, this can be done in one of two ways. The first is by selective fuel consumption — choosing which tanks to burn from. The second is by fuel transfer — physically moving fuel between tanks. Let me give you the logic. If you consume fuel in the tail first, or transfer fuel out of the tail to other tanks, you're removing weight from the rear of the aircraft. That will move the centre of gravity position forwards. Conversely, if you use fuel in the centre or forward tanks first, or transfer fuel out of these tanks, you're removing weight from the front, and that will cause the centre of gravity to move 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 forces separated by a distance — that distance is the arm. If the centre of gravity moves forward, the distance between the lift force and the weight force gets longer, so the couple gets stronger. And here's the consequence: the greater strength from the 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 from Figure 5.2 and Figure 5.3. In Figure 5.3, with the more forward centre of gravity, those arrows are longer, showing the stronger nose-down moment. So the tailplane has to work harder to counteract that increased nose-down pitching moment — which brings us right back to where we started. A more forward centre of gravity means more tailplane down force, which means more effective weight and more drag. That's the trade-off you're managing when you think about fuel management in cruise.