The Most Basic Parameter of Flight
Angle of attack
By Jim Sparks
Angle of Attack is the most basic parameter of an aircraft in flight. Depending on configuration of the aircraft, there are specific values of angle of attack that apply to stall, maximum glide angle, maximum climb angle, and maximum endurance. As AOA is such a vital indication, it needs to be presented to the flight crew in a way that can be quickly comprehended. A pilot controls aircraft speed with pitch commands and controls rate of climb or decent with power changes. An aircraft cruising at a constant speed can be considered as being in a state of equilibrium and for this state to exist: LIFT/DRAG = WEIGHT/THRUST
Anything placed in a relative wind is subject to an aerodynamic force called wind resistance, which depends on the shape of the object as well as the characteristics of the surrounding air — including speed and density. Mr. Bernoulli developed a principle based on fluid dynamics that defined the operation of a venturi, and is also the basis for aircraft wing design. A wing's Coefficient of Lift (CL) is dependent on the shape of the airfoil and Angle of Attack, where Angle of Attack (AOA) is the angle between the relative wind and the chord of the wing, and is an important component in the formation of lift. The factors that affect lift force are: air density, reference area of the wing, airspeed and coefficient of lift.
As the angle of attack increases, the lift produced by the wing also increases up to a certain point. Beyond this limit, high levels of drag result and the airflow separates from the wing's upper surface; resulting in a loss of lift. This condition is referred to as a stall.
At a high angle of attack, a slight airflow disturbance or a small change made by the pilot could lead to a balanced aircraft going beyond the point of recovery. In some situations, the wing's high angle of attack can result in a disrupted airflow to the horizontal stabilizer and can result in a "Locked in Deep Stall," which may render the aircraft unable to reduce deck angle due to loss of elevator control.
Aircraft manufacturers have different methods of giving the flight crew an advantage over the stall characteristics. On a swept wing aircraft, if the outboard wing should stall before the inboard, a sudden rotational force can cause an abrupt pitch up or pitch down. One method of prevention is to insure the inboard wing stalls first. Stall strips, triangular-shaped devices, are installed on the leading edge of the wing and result in a premature airflow separation on the upper surface of the wing at high angles of attack, causing a noticeable buffeting to the pilot.
A Stick Shaker is an electric motor with an eccentric mounted weight. When activated, it causes a vibration that can be felt through the elevator controls and is commonly used to provide the flight crew with an indication the aircraft is approaching the stall speed. Often a device of this type can be interfaced with the Automatic Flight Control System, which will either take action to avoid the high angle of attack or will disconnect the Auto Pilot to avoid exacerbating the situation. A stick pusher is another device that can be actuated as a result of a high deck angle. This is where the elevator is automatically pulled forward, resulting in a reduced angle of attack.
The aircraft's gross weight, pressure altitude, engine thrust, and outside air temperature all affect stall. An aircraft's performance is determined by a good optimization between speed, Angle of Attack, and weight. A higher than needed nose-up attitude will result in an increase of aircraft area exposed to the relative airflow, causing increased drag. Increasing engine power that in turn consumes fuel and decreases range, will help overcome this situation. By optimizing Angle of Attack, a substantial reduction in drag can result.
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