General Principles - Cruise — Page 234, Lesson 279

Let's start with the very foundation of cruise performance: the balance of forces in level flight. I want you to think of the aeroplane in the cruise as a rigid body being acted upon by four main forces, and here's the key idea — we don't treat them as four separate pushes. We group them into two couples. A couple is a pair of equal and opposite forces that don't act along the same line, and the net effect of a couple is to produce a rotation, a pitching moment, rather than a translation. So, 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, because these two forces are separated along the fuselage, they form a couple, and the effect of this couple on the aeroplane is to cause a nose-down pitching moment. And I want you to appreciate the scale here — this lift/weight couple is comparatively strong, so the nose-down pitching moment is large. Let me give you a concrete example from the 737-800 series. The maximum structural mass is 79,000 kilograms. At that mass, the aeroplane weighs about 770,000 Newtons. Obviously, in cruising flight, this weight is balanced by an equal and opposite lift force of 770,000 Newtons. So you have two enormous forces, separated by a lever arm, producing that big nose-down moment. Now, the second couple is produced by thrust and drag. The effect of this couple is to cause a nose-up pitching moment. But here's the crucial point — this couple is far weaker than the lift/weight couple. Again, using the 737-800, the maximum thrust produced by the engines is only 214,000 Newtons. 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. As a result, there is still a net nose-down tendency. The aeroplane wants to pitch down. So how do we maintain level flight? We need to generate an opposite moment that will balance this residual nose-down pitching tendency. And that is achieved by the tailplane, or horizontal stabilizer, on the aeroplane's tail assembly. The horizontal stabilizer 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. The down force generated at the tail is called the tailplane down force, or tail load. So with the addition of this tailplane down force, the nose-up and nose-down pitching moments are now in balance, and level flight is possible. Let me make sure you've got the full picture. We have the lift/weight couple producing a strong nose-down moment. We have the thrust/drag couple producing a weaker nose-up moment. The net is still nose-down. The tailplane produces a down force, which itself creates a nose-up moment, and that balances the residual nose-down tendency. That's the trim condition. That's what makes straight and level cruise possible.