Wing Approaching the Stall

Effects of control movements

Knowing what happens when the controls are operated is the most basic skill of piloting. It is also among the most misunderstood. When an airplane is flying, it has a good deal of forward speed and airflow over all of its surfaces. Control movements must be understood in terms of this airflow and its effects.

The Elevator

The elevator controls the Angle of Attack [AOA] of the wings, and subsequently the pitch. Pulling back on the stick results in a down force on the tail (the same thing is operating here that was operating on the wings, only in a different direction). If the controls are reversed, the opposite happens.

Effects of Back Stick Movement

Backward stick movement forces the tail down and the nose up. This rotation occurs around the center of gravity of the airplane. Initially the airplane, even though its nose is up, is still headed in the same direction - the only thing that has changed is the angle of attack. But an increase in the angle of attack results in an increase in lift, so now the airplane starts to go up. Then, like an arrow, it points into the wind, increasing its pitch. This process continues, viewed from the cockpit as an increase in pitch, until the pilot moves the stick forward to a neutral position and stabilizes the pitch.

The temptation to think that the stick directly raises or lowers the nose is very strong, and most of the time, roughly correct. But if the stick is moved back when the airplane is very close to the stall the aircraft will not pitch up much, if at all. This back stick movement and increase in AOA will stall the wing, causing a loss of lift and acceleration downward: now the pitch moves opposite the stick movement.

The Ailerons

The ailerons are a much simpler control than the elevator. Located near the wing tips on the trailing edge of the wing, they are used in unison to change the amount of lift each wing is producing and roll the airplane.

When the pilot moves the stick side-to-side from center, the ailerons move in opposite directions. In a roll to the right (as viewed from the cockpit), the right aileron goes up and the left aileron goes down. Each aileron serves to change how that part of the wing deflects the air and thus increases or decreases the amount of lift produced by each wing. The down aileron forces the air down harder, resulting in an increase in lift and the up aileron decreases the downward force, resulting in a decrease in lift. In the case of a right roll, the decreased lift on the right side and increased lift on the left side result in a roll to the right.

Aileron Effects

Operating the ailerons causes an effect called adverse yaw. Adverse yaw is the result of an increase in drag on the wing with the down aileron, or «upgoing» wing. This wing, since it forces air down harder than «downgoing» wing and producing more lift also produces more drag. The drag pulls the wing back and causes yaw. If this yaw is not corrected with rudder, the roll is said to be «uncoordinated».

The Rudder

The rudder is controlled by the «rudder pedals» located on the floor of the aircraft. They are both connected to the rudder so that when one or the other pedals is depressed, it moves the rudder in the desired direction. The rudder, connected to the vertical stabilizer, and then starts to deflect air much like a wing; only the resulting force is to the side. This force causes a change in yaw. As mentioned earlier, the rudder is not used very often, but when it is needed (e. g., in a crosswind), its presence is appreciated.

Engines

An engine produces a force which acts toward the rear of the aircraft which «thrusts» the aircraft forward. For this reason, the force produced by the engine is called thrust. Thrust is the most important force acting on an aircraft, because regardless of the type of aircraft, ALL need some type of thrust to propel them aloft. Even unpowered aircraft such as gliders need a tow plane to provide an external force to pull the aircraft into the air, where it can obtain airflow over the wings to provide the necessary lift to remain airborne. Hang gliders use foot power to initiate movement prior to «leaping» off a cliff. The most common means of developing thrust on powered airplanes comes from propellers or jets. Whether an aircraft has a propeller, a turbojet, or a turbofan, all of these produce thrust by accelerating a mass of air to the rear of the aircraft. The movement of this air to the rear creates an unbalanced force pushing the aircraft forward.

The Wright brothers made many important things come together for their historic first heavier-than-air flight. One of the most vital was an engine that efficiently produced thrust while not weighing too much. They used propellers – the only effective means available of transferring an internal combustion engine’s output into push or pull for the airplane. Propellers are essentially revolving wings situated so that the lift they produce is used to pull or push the airplane.

Most modern high-speed aircraft use a very different type of engine – the jet engine. Jet engines not only look different from propellers, they operate in a very different manner as well. More like rocket engines, jets produce thrust by burning propellant (jet fuel mixed with air) and forcing the rapidly expanding gases rearward. In order to operate from zero airspeed on up, jets use enclosed fans on a rotating shaft to compress the incoming air (and suck it in if the airplane is not going very fast) and send it into the combustion chamber where the fuel is added and ignited. The burning gases keep the shaft turning by rotating a fan before exiting the engine.

Turbojet Engine

Some other jet engines differ from this basic pattern by the way they compress the incoming air. Instead of forcing it down a restricting tube, the tweet’s centrifugal flow compressor literally flings the air outward into the compressor section exit, compressing it against the outside wall.