An aircraft is a well-balanced machine. In straight and level flight, all forces are in equilibrium. To understand this concept, let's have a look at the moments of force. This is a balanced seesaw; however, with different weights on each end, it becomes imbalanced. A child's weight is essentially a force of gravity. If one child weighs ten kilograms and the other weighs five kilograms, and they are exactly the same distance away from the center, how can the seesaw balance again? Since the lighter child is half the weight of the heavier child, you must put her twice as far from the center to attain the perfect equilibrium.

Archimedes once said, "Give me a stick long enough and a pivot, and I shall move the world." What all this means is that little force can achieve bigger results given a long enough arm to create the moment of force that is needed. Mathematically, a moment is defined as force multiplied by the arm. Going back to our seesaw analogy, we can prove that the lighter child is able to balance the heavier child by using a longer arm to achieve the same moment of force.

These arrows represent the four main forces acting on an airplane in flight. For demonstration purposes, these forces are often shown to act from a single spot, but in reality, that's not the case. On a typical light aircraft, the average of weight is slightly forward of lift; thrust is above drag. Combining the moments of these forces together suggests then in this configuration, the airplane has a natural tendency to pitch down.

So how is this fixed? The stability takes care of it by generating an opposing downward force. This downward force does not need to be great, since the arm is long, which helps to create a moment of force that is strong enough for the airplane to remain in equilibrium.