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Principles of flight

5.5 The 4 forces during flight (Flyveprincipper 3 - 2 videoer)

During level flight at constant speed, the following forces act on the aircraft:

- 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 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.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 476