Forces
Three principal forces act on aircraft and animals when gliding:
- weight – gravity acts in the downwards direction
- lift – acts perpendicularly to the vector representing airspeed
- drag – acts parallel to the vector representing the airspeed
As the aircraft or animal descends, the air moving over the wings generates lift. The lift force acts slightly forward of vertical because it is created at right angles to the airflow which comes from slightly below as the glider descends, see angle of attack. This horizontal component of lift is enough to overcome drag and allows the glider to accelerate forward. Even though the weight causes the aircraft to descend, if the air is rising faster than the sink rate, there will be a gain of altitude.
Read more about this topic: Gliding Flight
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