An airplane flies because its wings and propulsion system interact with the air. The useful starting point is four forces: lift, weight, thrust and drag. In steady, level, unaccelerated flight, the upward and downward forces balance, as do the forward and backward forces. Climbing, descending and turning involve changes to that balance and to the flight path.
Lift and weight
Lift is an aerodynamic force acting perpendicular to the relative airflow. A wing changes the pressure field around it and turns airflow downward. Pressure and momentum explanations describe the same physical process; air does not have to meet at the trailing edge after traveling around opposite sides of the wing.
The amount of lift depends on air density, speed, wing area and the wing’s aerodynamic characteristics, including its angle of attack. Weight is the gravitational force on the airplane, including its fuel, occupants and cargo. Weight and balance limits therefore affect performance as well as handling.
Thrust and drag
A propeller accelerates air backward; a jet engine also produces thrust by accelerating a flow of air and exhaust. Thrust counters drag, the aerodynamic resistance to motion. Drag includes effects associated with the airplane’s shape and surface friction, as well as drag associated with producing lift.
Increasing speed does not simply remove drag. Different components change differently with speed, which is why each aircraft has specific operating procedures and performance data. A glider illustrates another point: an aircraft can generate lift without engine thrust by descending through the air.
Angle of attack and stalls
Angle of attack is the angle between a wing’s reference line and the approaching relative airflow. Beyond the critical angle, airflow separation reduces the wing’s ability to generate lift and the wing stalls. A stall is not the same thing as an engine stopping.
The airspeed at which a stall occurs changes with loading, configuration and maneuvering. Pilots learn this using the aircraft’s approved handbook and instruction, not a single universal speed. See NASA’s lift resource and the FAA’s Pilot’s Handbook of Aeronautical Knowledge for a fuller explanation.