Page 488 - PPL-engelsk 2025

Principles of flight

So, for the sake of stability around the lateral axis (longitudinal stability), it is

best to have the center of gravity in front of the center of lift (center of

pressure).

To this end, we need

a downward tail force
to keep the aircraft's

nose up during

horizontal straight

flight.

For the sake of the extra air resistance that a trim causes, it is important that the

tail force is as small as possible, preferably 0.

Therefore, it is important to load the aircraft correctly - especially in large

aircraft.

5.7.3.1 Design for longitudinal stability

If the aircraft is subjected to a gust in the wind,

the tailplane will experience a greater
percentage change in lift. The greater the

"arm" from the center of gravity to the center

of lift for the tailplane, the better the

longitudinal stability.

If the center of gravity is too far back and we

lift the nose of the aircraft, the centers of

pressure move forward, and the aircraft

becomes unstable.

Stability is achieved when the center of gravity

is in front of the center of pressure.

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Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 488