Angle of Attack

Angle of Attack
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Angle of attack (AOA) is an important concept in aerodynamics and aviation. It is one of the first concepts taught to all pilots, both civilian and military, since it plays a large role in most flight maneuvers and in keeping the aircraft under control.

Angle of attack is defined as the angle between the chord line of the aircraft's wing and the direction of the wind flow striking the leading edge of the wing. In non-technical terms, this basically means that it is the difference between the aircraft's pitch angle and the vertical angle that the aircraft is actually flying.

AOA and Lift

The most important thing to remember about AOA is that the amount of lift the aircraft is producing is roughly proportional to the angle of attack times the square of your airspeed. What this means is simple - if you are flying slow, you will need to maintain a higher angle of attack in order to keep the airplane in level flight. If you maintain a constant angle of attack while slowing down, the aircraft will begin to descend. Likewise, if you maintain an angle of attack while speeding up, the aircraft will begin to climb.

All wings have what is known as a "critical angle of attack". If the critical AOA is exceeded, the aircraft will "stall" - the airflow will separate from the wing and become turbulent, preventing the wing from producing lift, and the aircraft will begin to fall (as in "fall like a rock", not "descend like an airplane"). The ONLY way to recover from a stall is to reduce the angle of attack by increasing power and pushing the aircraft's nose down. If you pull back on the stick or allow the stall to continue, the angle of attack will continue to increase, deepening the stall and making it more difficult to recover. Eventually, you will end up in a "deep stall" where the aircraft is essentially falling straight down. These are extremely difficult to recover from.

The important point here is: It is easy to recover from a stall, as long as prompt action is taken to recover. Increase power and press the nose down.

Unlike most civilian airplanes, the F-16 is designed for high speed and maneuverability. One of the effects of this is that it has small, narrow wings, which are not efficient at producing lift. If you are a real life pilot, or have played civilian flight simulators, you will notice that the F-16 requires a relatively high angle of attack in order to maintain level flight. This is also why the aircraft has such high takeoff and landing speeds - at lower speeds, the aircraft would be unable to maintain enough lift while staying underneath the critical angle of attack.

AOA and Drag

Another important point is that the aircraft produces more drag at higher angles of attack. This is due to several factors - firstly, at a high angle of attack, the wing presents a larger cross-section to the airflow, resulting in more drag (this is referred to as "parasite drag"). Secondly (and more importantly), when the aircraft is at a high angle of attack, some of the lift it produces is directed backwards, against the motion of the airplane. This is because lift is always produced straight up from the chord line of the wing. This is referred to as "induced drag".

Because of this, your angle of attack is an important consideration when determining fuel efficiency and aircraft range. In order to keep the aircraft in level flight, the amount of lift you produce has to be equal to the weight of the aircraft. Therefore, an F-16 heavily loaded with bombs and fuel tanks will need to produce more lift than an F-16 that's only carrying a few missiles. As stated above, there are only two options for achieving this - fly at a higher speed, or fly at a higher angle of attack. Taking the first option requires a higher throttle setting (i.e., increased fuel usage), as does the second option (increased drag requires a higher throttle setting to maintain a specified airspeed). This is why aircraft on bombing missions, especially long bombing missions, often take fuel tanks.

Determining Your AOA

AoA Gauge

Unlike many civilian aircraft, angle of attack is actually quite easy to determine in the F-16. There are multiple instruments which directly tell you your AoA, and several indirect indications as well. The angle of attack gauge (located underneath the analog altimeter and directly next to your left knee) tells you your angle of attack, in degrees. It also has color bands which correspond to the angle of attack indexer next to the HUD.

F-16 Angle of Attack Gauge
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Cockpit art courtesy of Cockpits.nl

AoA Indexer

The angle of attack indexer is used during final approach to landing. In order to land the aircraft correctly, your angle of attack should be between 11 and 15 degrees. At these AOAs, the green circle on the indexer will be lit up. If your AOA is below 11 degrees, the amber symbol will be lit up, and if it is above 15 degrees, the red symbol will be lit up (and you are getting close to stalling - see below). The indexer can also be used as a "rough" reference for maintaining optimal turn speeds.

AoA Indexer
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Cockpit art courtesy of Cockpits.nl

Head-Up Display

The HUD can also be used to approximate your AOA. The gun reticle shows where the nose of the aircraft is pointing, and the flight path marker shows the actual vertical track of the aircraft. The difference between these is your angle of attack.

AoA using the HUD
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Cockpit art courtesy of Cockpits.nl

External links

Wikipedia: Angle of attack

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