Page 487 - PPL-engelsk 2025

Principles of flight

5.7.3 Longitudinal stability

As mentioned before, the aircraft has 3 axes.

Stability around the lateral axis is called longitudinal stability.

This is a very important form of stability, as it is very dependent on the pilot's

dispositions, first and foremost the load distribution, which determines the
location of the center of gravity.

There are 4 factors that affect longitudinal stability:

- center of pressure location

- center of gravity location
- tail plane size

- design and change in engine performance

Imagine a ball in a bowl. It will always return to the same

point. That is stability.

The same applies to the airplane if it is, for example, exposed to gusts of wind.

It must be able to return to its original condition by itself, without us having to

do anything.

This is called positive longitudinal stability.

Longitudinal stability depends on the location of the center of gravity and center

of pressure.

Imagine that the airplane is exposed to a gust of wind from the front. The

increased lift causes the airplane to rise.

However, this is counteracted by the aircraft being rotated around the center of

gravity, so that the nose is lowered – and this counteracts the increased lift.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 487