What are the specific forces on an airplane when it is flying?

Answer 1

There are 4 forces acting on a flying plane : Lift, Thrust, Weight, and Drag.

Lift is the vertical force that keeps the plane airborne. It is generated by the difference in air pressure between the top of the wing (where air is moving faster) and the bottom of the wing (where it is moving more slowly). The higher pressure below pushes the wing upward.

Thrust is the forward motion of the aircraft, which provides velocity along its course and also generates the airflow over the wings that provides lift. Thrust is provided by the engines.

Weight is the force of gravity pulling down on the plane. It must continue to generate lift in excess of weight or it cannot retain its altitude.

Drag is the force opposing the forward motion, and is caused by friction with the air. Aircraft are designed to reduce their forward cross section, and with smooth surfaces, to minimize drag. At higher altitudes, there is less air and therefore less friction, so most long air routes will include a period at high altitude (over 30,000 ft / over 9000 m).

*Note that Inertia also limits the effect of the thrust of an aircraft, and larger planes need larger engines to accelerate to the necessary speeds. Gliders do not use thrust, but depend on rising air currents for their lift, and gravity for acceleration.