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.
Angle of Attack Indicating systems have become very commonplace in high performance aircraft. Some aircraft manufacturers will use these instruments to provide the reference for activation of the aircraft's stall protection systems. Others will provide the flight crew with visual reference to deck angle that also displays how much margin exists before a stall occurs. These instruments are particularly useful during take off and approach to landing.
An AOA transmitter senses the direction of the airstream and sends the information to the indicator. Several types of transmitter are currently used and employ either a variable core transformer or a potentiometer.
In the case of the potentiometer, the wiper serves as the data transmitting element and is mechanically connected to the airstream-sensing probe. Two pressure-sensing slots are symmetrically located above and below the centerline of the probe leading edge and an equal pressure distribution across the slots keeps the sensor centered to the relative airflow.
When the aircraft makes a pitch change, the probe remains in constant alignment with the airstream, which results in a position change between the vane and the probe case.
The Angle of Attack transmitter is installed so that the probe extends through the fuselage, perpendicular to the airstream. Icing protection is accomplished by several internal electrically operated heaters. One heating element is incorporated in the case and is used to dissipate moisture that may accumulate under the probe. A second element is installed in the probe and is a high wattage device to eliminate any erroneous indications resulting from ice build-up during flight. Controls of the heating circuits are detailed by the airframe manufacturer or the facility accomplishing the AOA system installation. Frequently, the aircraft weight on wheels sensing system will control probe heat. Sometimes a temperature sensitive resistor is used to determine when heat is needed.
Probe attachment to the aircraft is accomplished by using a mount plate that is riveted to the airframe. When the mounting plate is installed at the specified position, the transmitter-mounting angle is established. and a final adjustment is made during a test flight. Thereafter, the transmitter may be removed and replaced without affecting the calibration of the AOA system.
The most frequently reported problems with this type system are damage to the probe. A prime consideration when washing the aircraft should be to avoid getting water directly in the area where the probe passes through the fuselage. This can result in moisture ingress that damages the internal electrical components.
Another source of damage results when the aircraft paint is removed by chemical means. If paint stripper is allowed to act on the fine bearings on which the AOA probe rotates, mechanical binding and subsequent erroneous indications will result. Another area of concern is unwanted activation of the heating system. Understanding what triggers probe heat is essential prior to conducting maintenance such as aircraft jacking or the application of external power for extended periods of time.
An interface unit receives the electrical signals from the probe and an internal computer then makes the computations needed to relate AOA vane angle to aircraft true angle of attack.
Other inputs to this computer may include flap and slat position as well as airbrakes or spoilers. In some cases, even landing gear extension and weight off wheels conditions are monitored. Once the information is processed, it is delivered to the various flight deck displays. Among these are the Angle of Attack Indicator, Fast - Slow Display on the Attitude Indicator, or the AOA Indexer. Failure of a flap position sensor is another frequent cause of erroneous indications.
Angle of Attack can be presented in units representing airflow direction relative to the fuselage where the AOA transmitter is located. The face of the indicator is graduated from zero to one, with zero representing zero lift and one being the point of stall. A green highlighted area begins at zero and goes up to .6 or sixty percent of stall. This is considered the area of normal aircraft operations. From .6 to .85, the face of the instrument is identified in amber. Operation here corresponds to the high angle of attack phases of flight such as initial climb or flight exercise. A red area beginning at .85 through 1 is considered a "Do Not Use In Normal Operation Zone," as the margin up to stall is lower than 15 percent.
In many indicators, a movable index can be set by the flight crew to establish a specific approach speed. This display is referred to as the V/VS Range. This relates to the stall speed of the aircraft relative to a safety margin. If the stall speed of an aircraft in a specific condition such as Full Flaps for landing is 100 knots and a 1.3 V/VS reference is set on the indexer, this means the aircraft will be flying at about 130 knots or about 30 percent better than stall speed if the pilot follows the speed reference. A movable index provides the flight crew with the ability to meet special conditions such as approach in turbulence or with an engine out where a higher stall margin is needed. A specific AOA can also be set to maintain an optimum cruise angle to enable the aircraft to achieve maximum range. This setting will change depending on aircraft type, but is frequently around .2 to .3. The indicator is an electromechanical device housed in a metal case with a glass face. Inappropriate cleaning techniques, such as dusting the face with a nylon-bristled brush can result in an electromagnetic charge that can have a significant effect on the display pointer. Mechanical shocks can also destroy the calibration of this type of instrument.
In addition to the indicator, several remote displays can be strategically located in the flight deck. These include a Fast - Slow scale on the Attitude Directional Indicator (ADI) or Flight Director Indicator (FDI). The ADI offers flight crew the ability to observe aircraft attitude, plus on many of these displays, other information can be viewed including Glide Slope and Localizer data used for an instrument landing. Having the Fast - Slow display enables the crew to constantly have an awareness regarding stall margin. Should the nose of the aircraft start coming up to where the stall margin is below what the pilot adjusted into the V/ VS reference, the Fast - Slow display pointer will start to show the pilot that the aircraft is a bit on the slow side. Likewise, if approach speed is to high, this display will instruct the crew to pull the nose up when they observe the pointer on the FAST side of the display. Many Fast - Slow displays will also work with selected airspeed compared to actual speed. This capability can be of great assistance when it comes to troubleshooting as a switch selection will provide a different information source without having to swap components.
An Angle of Attack Indexer is another remote display of the AOA indicator. A device of this type is generally installed on the top of the instrument panel or glare shield. This provides the crew with the ability to monitor stall margin while they are looking out the windshield. The Indexer consists of a Green Circle or "Doughnut," which is in the center of the display. There is a Chevron "V" installed above the Circle and another Chevron below the circle. The point of the "V" always aims to the circle. The lower chevron is amber while the upper one is red. As long as the aircraft is maintaining the preset V/VS on the AOA indicator, the Green Doughnut will be illuminated. Should the aircraft speed start to increase, the amber chevron, which points in an upward direction toward the doughnut, will illuminate, advising the crew to adjust the deck angle of the aircraft nose up. Should the red chevron illuminate, the flight crew should lower the nose as they have reduced their safety margin to stall below the preset point.
In many of the newer Electronic Flight Instrument Systems, AOA displays have been transformed into speed reference indicators. These Speed Queues typically appear next to the Airspeed display and can be automatically compensated when wing configuration is modified, or if the aircraft experiences an engine failure and best single engine rate of climb or decent needs to be maintained. Most of these later systems can also receive information regarding current aircraft gross weight from the Flight Management System (FMS) to provide a highly accurate reference to the point of stall. With newer technology systems, the air flow sensor is still an analog device that is either a potentiometer or a variable core transformer, and can generally be evaluated electrically by observing inputs from the sensor to a Data Acquisition Unit or air data computer. Many airframe manufacturers provide special tooling such as stall vane protractors that will enable precise troubleshooting as well as system calibrations.
As with most systems that have an effect on the flight environment, it is very important to read and understand the manufacturer's procedures prior to system testing or adjustment.
In short, a properly calibrated Angle of Attack indicating system will supply the flight crew with the ability to ensure a safe area above the stall margin, while giving a reference on the best deck angle for economy cruise or high speed conditions.