General Principles - Cruise — Page 234, Lesson 279

I want to walk you through the balance of forces in level flight — this is the foundation for everything we'll do on range and endurance later in the chapter. We're in the en route phase now, the cruise, and the whole performance story starts with the forces acting on the aeroplane. Let me set the scene. These forces are split into couples. A couple is two equal and opposite parallel forces that don't act along the same line — and because they're offset, they produce a rotation, a pitching moment. There are two main couples in level flight, and they're shown in Figure 5.1. The first couple is produced by lift and weight. Weight acts through the centre of gravity of the aeroplane, directly towards the centre of the earth. Lift balances weight, and it acts through the centre of pressure. Now here's the key point: because lift and weight act through two different points — the centre of pressure and the centre of gravity — they form a couple, and the effect of that couple is a nose-down pitching moment. And this couple is comparatively strong, so the nose-down pitching moment is large. You can see that in the figure as the large arrow pointing downwards to the right of the aeroplane. Let me give you a sense of scale, because these numbers matter. Take a 737-800 series. Its maximum structural mass is 79 000 kilograms. At that mass, the aeroplane weighs about 770 000 Newtons. And in cruising flight, that weight is balanced by an equal and opposite lift force of 770 000 Newtons. So the lift/weight couple is enormous. The second couple is produced by thrust and drag. And here's the contrast: the effect of this couple is a nose-up pitching moment — you see the upward pointing arrow to the right of the aeroplane. But notice, this couple is far weaker than the lift/weight couple. The maximum thrust produced by the engines of a 737-800 is only 214 000 Newtons. Compare that to 770 000 Newtons of lift and weight. So the nose-up pitching moment generated by the thrust/drag couple does not balance the stronger nose-down pitching moment of the lift/weight couple. The result is that there is still a nose-down tendency, as shown in Figure 5.1. So now the question becomes: how do we maintain level flight? We need to generate an opposite moment that will balance that residual nose-down pitching tendency. And that's achieved by the tailplane — also called the horizontal stabilizer — on the aeroplane's tail assembly. Here's the principle. The horizontal stabilizer, or tailplane, must be set at an angle which will cause a nose-up pitching moment to balance the aeroplane — or, more commonly expressed, to trim the aeroplane. Look at Figure 5.2 now. With the addition of the tailplane down force, the nose-up and nose-down pitching moments are now in balance, and level flight is possible. That down force generated at the tail is called the tailplane down force, or tail load. So let me pull the whole picture together. We have three forces at work in trimmed level flight: lift and weight forming a strong nose-down couple, thrust and drag forming a weaker nose-up couple, and the tailplane down force — the tail load — providing the extra nose-up moment needed to bring everything into balance. The tailplane isn't just sitting there; it's actively producing a downward force to trim the aeroplane against that residual nose-down tendency. One thing I want you to hold onto for later in the chapter: this balance is what we measure performance against. Range and endurance — how far and how long the aeroplane can fly — are the parameters we use to measure en route performance, and they all depend on this trim condition we've just established.