Page 477 - PPL-engelsk 2025

Principles of flight
- Lift - vertically upwards

- Gravity (weight) - vertically downwards towards the center of the earth

- Drag - horizontally backwards
- Thrust - horizontally forwards

5.5.1 Prerequisite for level straight-ahead flight

In order to fly level,

the opposing forces
must be equal to

each other; that is,

lift must be equal to

weight and drag must

be equal to thrust.

But that is not enough. The positive moment of the lift must also be equal to the

negative moments of drag and thrust if we want to fly level without using the

flight controls.

The weight acts at the center of gravity, therefore there is no torque from here.

If the above is fulfilled, we have an ideal situation where the 4 forces all act at
the same point: the center of pressure for lift (the red plus sign in the figure on

the previous page) which in this case is equal to the center of gravity.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 477