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The angle of attack is one of the most important things to know during flight. This is done with an angle of attack indicator, here’s how they work.
Flying an airplane requires the proper balance of the four main components of flight, one of them being lift. Without lift, a plane would never leave the ground. One of the key aspects of generating lift is by having the necessary angle of attack. But what the heck is angle of attack and how is it determined?
Angle of attack indicators work by using either a pressure differential, transducers, or inertial references. With this, the indicator helps the pilot know what the plane’s current angle of attack is and provides feedback on how the pilot can achieve the perfect angle of attack for any situation.
Maybe you’re an aspiring pilot trying to learn as much about aviation as possible before getting your license, or maybe you’re a veteran of the skies that’s been using an angle of attack indicator for years. In either case, you might not actually know how they work and just how useful they can be. In this article, we’ll discuss angle of attack and why it’s so important, and then we’ll go over everything you want to know about how angle of attack indicators work.
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What is Angle of Attack?
When you’re flying, there are all sorts of things that you should know as a pilot in order to stay in the air and make a safe flight. One of these is the angle of attack (AOA) that you’re flying at, which is especially important when taking off and coming back in for a landing.
The angle of attack is the angle that the chord of the plane’s airfoil makes with the relative wing it’s coming in contact with. In normal terms, it’s the angle that the wings “attack” the wind with, or basically the angle that the wings make with the direction of the wind hitting the aircraft.
The AOA of an aircraft is directly correlated with the coefficient of lift (CL) that the aircraft experiences. This is so important because CL is one of the key variables in the overall formula for lift, meaning that it directly affects how much lift an airplane can produce. And as one of the four main components of flight, lift has a drastic impact on the ability of the airplane to even leave the ground, much less fly.
In general, as the AOA increases, the CL also increases. In other words, a higher angle of attack means more lift is created. However, this direct relationship only holds up to a certain point. This threshold is what’s known as the critical angle of attack.
What is the Critical Angle of Attack?
The critical angle of attack is the angle at which CL reaches its peak . In other words, once the critical AOA is passed, the airplane can no longer continue generating the necessary amount of lift and it will stall. This is arguably the single most important reason why angle of attack is so important.
You never want to pass the critical AOA because the airplane will stall, no matter the airspeed. This happens because above this angle, the airflow on top of the airfoil becomes separated. This is also known as boundary layer separation, and it means the laminar flow going over the wing has become entirely turbulent and lift cannot be generated any longer.
The critical AOA will vary from airplane to airplane, but it’s typically somewhere between 15 and 22 degrees. This is something you should know for every airplane that you’re flying, but in many cases it’s around 17 degrees.
What is an Angle of Attack Indicator?
As you can probably guess, an angle of attack indicator is a device or sensor that measures the plane’s angle of attack and indicates that measurement to the pilot. In a cockpit, it’s one of the many systems and indicators that a pilot must be accustomed to in order to fly an airplane.
Once you become a pilot and start getting some experience under your wings, using an AOA indicator will become a standard part of your routine. Without these indicators, it would be nearly impossible to know when you’re approaching a dangerous angle of attack, so they’re incredibly useful. But how do they work?
How do Angle of Attack Indicators Work?
I’ll expand on how AOA indicators work in much greater detail below when we go over the different types of indicators. In general, these indicators use probes and sensors to calculate the plane’s angle of attack. The indicators then pass this information along to the pilot so that the pilot can make any corrective adjustments needed to keep the AOA where it needs to be.
These indicators also take other things into account such as airspeed, since that will directly affect the optimal AOA. Most AOA indicators use colors (typically green, yellow, red, and blue) so that the pilot cna quickly glance at the indicator and know where they stand and where they need to be. While many of the AOA indicators in older airplanes use a dial, some modern ones have digital readouts, various colors, and even oral indicators.
Types of Angle of Attack Indicators and How They Work
There are three main types of AOA indicators that we’ll discuss below: transducers, differential pressure sensors, and inertial reference indicators. Each type calculates the angle of attack in its own way and has certain considerations to take into account. Let’s take a look.
Transducer AOA Indicators
The most common types of AOA indicator used in aviation today are the transducer types. These indicators are designed in such a way that they can move based on how the relative wind is hitting the plane and/or airfoil and measure the angle of attack directly. The two types of transducer AOA indicators most commonly used are the alpha vane and the stagnation point transducers.
Alpha Vane Transducer
Alpha vane transducer AOA indicators are the ones that are used by most commercial airplanes. These vanes are attached to the plane and are allowed to move and rotate freely based on how the wind is hitting the airplane (and subsequently, the vane). The vane accurately measures, in real time, what the angle of attack is based on how the wind is moving it around.
Stagnation Point Transducer
Stagnation point transducer AOA indicators are attached on the leading edge of the airfoil and are also allowed to move up and down depending on how the wind is flowing over the airfoil. Based on the angle that the transducer is pushed upwards or downwards, it can directly measure the plane’s angle of attack at any given moment.
Differential Pressure AOA Indicators
As the name suggests, differential pressure AOA indicators calculate the angle of attack by examining the pressure difference between two ports. These indicators are typically probes attached to the wing that have two holes at different angles. As the plane’s AOA changes, the holes experience different pressure relative to one another and the angle can be calculated.
Inertial Reference AOA Indicators
Of the three, inertial reference AOA indicators are the only ones that measure the angle of attack indirectly as opposed to directly. These work by measuring the angle of the pitch of the airplane and the angle of the plane’s flight path. By definition, the difference between these two angles is the angle of attack. But since it’s not measure the angle directly, it can have some slight accuracy errors in some cases.
Angle of Attack Considerations
When using an AOA indicator and thinking about angle of attack as a whole, there are a few additional factors that should be taken into consideration. This includes things such as icing, the use of flaps, and the accuracy of the indicators.
One of the biggest problems with AOA indicators is ice buildup on the airfoil and also the sensors itself. As ice builds up on the airfoil of the airplane, the critical AOA is affected and the necessary AOA to stay in the air changes. Additionally, some sensors can be frozen or get clogged with ice. Most indicators have heating capabilities built in to combat this buildup of ice.
Effect of Flaps on Angle of Attack
Recall from above that the angle of attack has to do with the angle of the chord of the airfoil, not just the wing itself. This is important because the chord of an airfoil is drastically different when the flaps are extended vs when they’re retracted. AOA indicators take this into account directly, but it’s important to understand this as a pilot so you know that the critical AOA is different depending on the flaps.
Calibration & Accuracy
Lastly, as with all probes and sensors, AOA indicators have some sort of accuracy issues inherently built-in. As mentioned above, this is more prevalent with inertial reference indicators, but they have some degree of inaccuracy. This is worsened if they're not properly calibrated as required. In the end, this is why the pilot should use the indicator as a reference, but should always refer to the plane’s operating manual on what the proper angle of attack should be.