Trimming

The weight of a whole airplane acts as if all of its mass were concentrated at one point. This point is called the center of gravity. If an airplane could hang from a wire at this point, it would hang perfect­ly level. A flying airplane is held up by lift. All the lift acts at one point called the center of pressure. Airplanes are designed so that the center of pressure and center of gravity are close to each other, but it is not possible to get them in exactly the same place. Both of them move during a flight as a plane’s speed and attitude change and as it uses up fuel. Passengers walking about in an air­liner move its center of gravity as well.

If the center of pressure is in front of the center of gravity, it pulls the plane’s nose up and the plane is said to be tail – heavy. If the center of pressure is behind the center of gravity, it pulls the plane’s nose down. Then the plane becomes nose-heavy. If a plane were consistently
nose-heavy or tail-heavy, the pilot would have to keep pulling or pushing the con­trol stick for long periods to stay level. To avoid this, planes have trim control. This is usually a wheel, lever, or group of switches in the cockpit. Pilots use trim control to move the center of pressure forward or backward until it lines up with the center of gravity, making the aircraft level. Small airplanes have panels in the tail, called trim tabs. They tilt up or down to move the tail up or down until the plane is level. Larger airliners trim their pitch by tilting the horizontal stabilizers in their tail. A level plane is called a trimmed plane.