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Aerodynamic engineering term.

The balance of forces, and the computation thereof, determines the velocity vector of an aircraft at any point.

In English, this means you can work out how an aircraft is moving (directional component of velocity vector) and how fast (speed component).

In straight and level flight, the four major force vectors are equal. They are:
  • Thrust: moves the aircraft forward (usually). Drawn along the horizontal normal of the aircraft. The engine provides this force.
  • Drag: opposes Thrust, and is produced by aerodynamic drag from all parts of the aircraft. All drag vectors are summed and usually drawn as acting in the opposite direction to the relative wind.
  • Lift: Produced by any wing or wing-like surfaces. All lift vectors are summed and drawn as acting perpendicular to the direction of the relative wind. The Bernoulli Effect provides this force.
  • Weight: Produced by the act of gravitational attraction between the Earth and the aircraft. Always acts straight down (to the center of the Earth).

Here are some general rules.

When the forces are in balance (lift = weight, thrust = drag) the aircraft is in straight and level flight.

If you have more lift than weight (lift can be provided by changing the shape of the wing (using flaps) or changing the attitude of the aircraft so the relative wind has further to travel (ailerons), then the aircraft will climb.
If you have more thrust than drag, the aircraft will tend to increase speed - but eventually drag (which increases with the square of speed) will equal speed, and the aircraft will have zero acceleration.

As http://www.monmouth.com/~jsd/how/htm/4forces.html illustrates (with diagrammes), these rules do not hold for balistic flight (bombs, moon landers etc.).

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