In one of the TAM 335 labs, an airfoil is rotated in the air duct until pressure is lost across the taps. Spoiler alert: the loss of pressure indicates that the airfoil has exceeded its critical angle of attack and lost its lift force. The angle of attack is the angle the chord line of the wing/airfoil makes with the wind passing over the wing (aka relative wind). For our purposes you can consider the relative wind to be horizontal.
Let’s backtrack a little bit. While in flight, planes experience four types of forces: lift, gravitational (opposing lift), thrust, and drag (opposing thrust). Air passing over the airplane travels in the direction of the drag forces (planes experience multiple types of drag). The wing is designed to create a pressure difference with higher pressure below the wing and lower pressure above, producing lift. In addition, the air that passes over and under the wing is often deflected downward upon leaving the wing’s surface, contributing to the lift force.
In level flight, the plane flies parallel to the ground, and to the oncoming air. When the plane’s nose aims upward so that the plane can climb, the angle of attack (in this case also called the climb angle) increases. Once the critical angle is surpassed, flow over the wing breaks down and lift is lost. It is necessary to push the plane into a descent in order to regain lift. After entering the descent, the pilot can ease the plane back to level flight with lift restored.
By Newton's third law, the forces a plane experiences come in action-reaction pairs.
Increasing the angle of attack puts a heavier load on the plane’s engine and decreases the airspeed. Without compensation or correction, this eventually leads to a stall. While it can be easy to stall a plane during take-off or landing, during which the plane is traveling at slower speeds with a higher angle of attack, it is much more difficult to stall a plane in level flight. If properly trimmed out, meaning that there needs to be very little pressure on the yoke to keep the plane on its desired path, the plane will want to fly straight. Pull it into a stall, and it should return to level flight when you let go.
Aerobatics planes enter stalls purposefully for maneuvers such as spins. In a fully developed spin, the inside wing experiences more drag and less lift and is considered “fully stalled,” while the outside wing experiences more lift and less drag, making it “not as stalled.” Completing a spin is basically the same idea as recovering from a stall— the nose must be pitched forward with both ailerons neutral (flat). Opposite rudder is applied to help correct from the direction of the spin (if leaving a counterclockwise spin, apply right rudder). Pilots often decrease the airspeed in a stall recovery to avoid putting excess stress on the plane.