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Principles of flight
5.3.3 Airflow around profiles
To understand how much difference there is in drag on different profiles, look
at the figure below. The frontal area is the same, but there is a significant
difference in drag.
When air hits a slanted plate, the airflow changes direction. If the front edge is
higher than the back edge, the airflow will be directed downward at an angle at
the rear.
Air consists of molecules, and they have a certain weight, and according to
Newton's law of action and reaction, they will exert a reaction force on the plate.
The arrow to the left
indicates the relative
wind direction.
The black plate is moving left - i.e., opposite the wind direction.
Three forces will now act on the plate:
- Lift: formed perpendicular to the relative wind.
- Drag: acts opposite the plate's direction of motion - i.e., to the right.
- Resultant: the combination of the vectors of the forces needed to create
lift and the forces needed to overcome drag.
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Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 449
