<h2 title="Stall Definition">Stall Definition</h2>
A sudden decrease in lift production when the wing exceeded the <a title="critical angle of attack" href="https://homapilot.com/general/principles-of-flight/angle-of-attack/" target="_blank" rel="noopener">critical angle of attack</a> is called a stall.
Increasing the <a title="angle of attack" href="https://homapilot.com/general/principles-of-flight/angle-of-attack/" target="_blank" rel="noopener">angle of attack</a> beyond the CLmax disrupts the progressive airflow from the upper wing surface. At first, the airflow starts to separate from the <a title="trailing edge" href="https://homapilot.com/general/principles-of-flight/four-forces-of-flight/#Trailing_Edge" target="_blank" rel="noopener">trailing edge</a>. As the angle of attack increases further, the airflow separation proceeds forward until the wing stalls completely.
When the stall occurs, the airflow on top of the wings breaks away, darg increases and the lift force significantly reduces, and it causes no longers to be able to support the weight of the airplane which results in loss of altitude.
A stall always occurs at the same angle for a given airplane, regardless of the airplane&acirc;&#128;&#153;s airspeed, flight attitude, or weight. Thus, an airplane can be stalled at any airspeed and in any flight attitude.
Another important thing you should know is a stall does not mean that the airplane&acirc;&#128;&#153;s engine(s) have stopped working or that the airplane is not moving.
<h2 title="Stall Recognition">Stall Recognition</h2>
A first the pilot must identify the flight conditions that caused the stall to occur and know how to take the necessary corrective action for stall recovery. This level of skill requires sufficient learning and experience that the pilot understands through sight, sound, and feeling during the flight.
1- Stall warning horn that can be easily heard in the cockpit if the airplane is equipped with this system
2- airplane buffeting
3- Noise reduction
Reducing of airflow&acirc;&#128;&#153;s sound when passing over the wings
4- High angle of attack (AOA) + Low airspeed
5- Mushy feeling in the flight controls (less effective controls)
<h2 title="Stall Recovery">Stall Recovery</h2>
1- pitch down the airplane&acirc;&#128;&#153;s nose to reduce the angle of attack.
2- If the airplane is in the bank, first level the airplane and then start to recovery.
3- add engine power as needed.
4- Return to the safe altitude and desire flight path.
During pilot training, pilot students practice two types of stall recovery to be prepared for stall recovery when dealing with a real stall.
<ul>
<li>Power-off stall: This type of exercise helps us to easily recover the airplane from the stall during an approach to the landing.</li>
<li>Power-on stall: also known as departure stalls, this type of exercise helps us to easily recover the airplane immediately after taking off.</li>
</ul>

Stall

<h2 title="What is Angle of Attack">What is Angle of Attack (AOA)</h2>
Angle of attack definition 
The angle of attack (AOA) or aerodynamic angle is the angle between the chord line of the airfoil (wing) and the direction of the <a href="https://homapilot.com/general/principles-of-flight/relative-wind/" target="_blank" rel="noopener">relative wind</a> which you can see in the image below.
<img class="alignnone size-full wp-image-838" src="/data/media/angle-of-attack.png" alt="angle of attack" width="750" height="422">
&nbsp;
<h3 title="Changing Angle of Attack">Changing Angle of Attack</h3>
If the angle of attack increases, the lifting force and induce drag will increase to a certain point. The excessive angle of attack (usually more than 17 degrees) causes the air flowing on the upper surface of the wing to separate from it and, leading to a loss of lift force, which is called a <a title="stall" href="https://homapilot.com/general/principles-of-flight/stall/" target="_blank" rel="noopener">stall</a>.
As you can see in the figure below, as the angle of attack increases, the lift force increases to a certain point, and then the lift force decreases and the wing reaches the stall. The point at which the wing enters the stall is called the critical angle of attack (Stalling Angle of Attack).
<img class="alignnone size-full wp-image-841" src="/data/media/critical-angle-of-attack-stall.png" title="critical angle of attack stall" alt="critical angle of attack stall" width="750" height="422">

Angle of Attack

<h2 title="Relative wind definition">Relative wind definition</h2>
Relative wind is created by the movement of an object through the air. To better understand the relative wind, we give an example:
Imagine a person sitting in a car on a no-wind day. If that person takes his hand out of the window, he will not feel the wind around his hand.
Now, if the car starts moving at a speed of 40 km/h (25 mph), that person will feel the wind at a speed of 40 km/h (25 mph) around his hand. This wind, which is created by the movement of a person&acirc;&#128;&#153;s hand in the air, is called relative wind.
<img class="alignnone size-full wp-image-812" title="Relative wind definition" src="/data/media/Relative-wind-definition.png" alt="Relative wind definition" width="750" height="422">In this lesson, we look at the relative wind in aviation.
In aeronautics, the relative wind is created by the movement of an aircraft or an airfoil through the air. In other words, the wind produced by the movement of the aircraft in the air is called relative wind. Relative wind is parallel to and opposite the flight path of the aircraft or airfoil.
<ul>
<li>If the flight path of the aircraft is forward, then the relative wind is backward</li>
</ul>
<img class="alignnone size-full wp-image-816" src="/data/media/What-is-relative-wind.png" alt="What is relative wind" width="750" height="422">
<ul>
<li>If the flight path of the aircraft is forward and upward, then the relative wind is backward and downward</li>
</ul>
<img class="alignnone size-full wp-image-817" src="/data/media/Relative-Wind.png" alt="Relative Wind" width="750" height="422">
<ul>
<li>If the flight path of the aircraft is forward and downward, the relative wind is backward and upward</li>
</ul>
<img class="alignnone size-full wp-image-818" src="/data/media/Relative-wind-and-flight-path.png" alt="Relative wind and flight path" width="750" height="422">
You should not confuse the actual flight path with the aircraft flight attitude. 
For example, the <a href="https://homapilot.com/general/principles-of-flight/main-structure-of-airplanes/#Fuselage" target="_blank" rel="noopener">fuselage</a> may be parallel to the horizon while the airplane is descending, which can be seen in the image below.
<img class="alignnone size-full wp-image-819" src="/data/media/flight-attitude-and-relative-wind.png" alt="flight attitude and relative wind" width="750" height="422">
As you can see in the picture, the airplane flies in level attitude, but its actual flight path is forward and downward so, the relative wind is backward and upward which is parallel to and opposite the airplane flight path.
Understanding what relative wind is in aviation is a prerequisite for being able to understand the <a title="angle of attack" href="https://homapilot.com/general/principles-of-flight/angle-of-attack/" target="_blank" rel="noopener">angle of attack</a> (AOA). In the next lesson, we will examine the angle of attack.

Relative Wind

<h2 title="what are the four forces of flight">what are the four forces of flight?</h2>
During the flight, the forces acting on a plane are lift, weight, thrust, and drag. Lift is an upward force formed by the movement of air on top and under the wing of an airplane and it supports the airplane in flight.
The weight is opposite to the lift force and is formed by the force of gravity.
Thrust is generated by the rotation of the airplane propeller in the air, which propels the airplane forward. It should also be noted that the airplane propeller is rotated by the force supplied by the engine.
&nbsp;
Drag is against the thrust force and is formed by the movement of the airplane, which limits the speed of the airplane.
<img class="alignnone size-full wp-image-770" title="four forces of flight" src="/data/media/Four-forces-of-flight.png" alt="four forces of flight" width="750" height="422">
&nbsp;
In the image above, the arrows that indicate the forces acting on the airplane are often called vectors. The size of the vector is indicated by the length of the vector and the direction of the vector is indicated by the direction of the arrow.
&nbsp;
<h3 title="when are the four forces that act on an airplane in equilibrium">when are the four forces that act on an airplane in equilibrium?</h3>
In straight-and-level, unaccelerated flight, The four forces are in equilibrium. The thrust is equal to and opposite to the drag force, and the lift is equal to and opposite to the weight.
&nbsp;
LIFT
As mentioned earlier, the lift force is opposite to the weight. When the airflow hits the airplane wing, this airflow is divided into two parts and passes through the upper surface of the wing and the lower surface of the wing, which due to the special shape of the wing causes low pressure at the top of the wing and high pressure at the lower surface of the wing, and eventually, due to this pressure difference, the lift force appears.
&nbsp;
Now to understand why the air at the upper surface of the wing is less pressurized than at the lower surface of the wing, we need to study Bernoulli's law to better understand the lift force.
&nbsp;
<h2 title="Bernoulli&acirc;&#128;&#153;s Principle">Bernoulli&acirc;&#128;&#153;s Principle</h2>
Bernoulli&acirc;&#128;&#153;s principle simply stated, says, &acirc;&#128;&#156; as the velocity of a fluid (air) increases, its internal pressure decreases.&acirc;&#128;&#157;
To better understand this law and prove it, we use the Venturi tube and test Bernoulli's law with it.
<img class="alignnone size-full wp-image-780" title="Venturi tube" src="/data/media/Venturi-tube.png" alt="Venturi tube" width="750" height="422">
As you can see in the picture above, the amount of velocity and air pressure is shown at the inlet of the venturi pipe. But when air enters the narrow part of the pipe, the air velocity increases and its pressure decreases, which is easily seen in the picture. Then, after the air passes through the narrow part of the pipe and enters the outlet, the speed and pressure return to the initial state.
&nbsp;
To apply the Bernoulli principle, air does not need to pass through a closed tube. Any surface that changes the airflow creates a venturi effect.
As you can see in the image below, the shape of the airplane wing is designed in such a way that the upper surface of the airplane wing is similar to the narrow part of the tube and the lower surface is similar to the wide part of the tube. Therefore, when the air hits the front of the airplane wing, the air velocity increases in the upper surface of the wing and its pressure decreases relative to the lower surface of the wing, which causes the lift force to be generated on the airplane wing.
The pressure difference between the upper and lower surface of the airplane wing is the main source of lift force.
<img class="alignnone size-full wp-image-784" src="/data/media/Four-forces-of-flight-1.png" alt="Four forces of flight" width="750" height="422">
Newton&acirc;&#128;&#153;s Third Law of Motion
The rest of the lift force is generated by the lower surface of the wing because the air deflects downwards when it strikes the lower surface of the wing.
&nbsp;
The third law of Newton states that for &acirc;&#128;&#156;every action (force) in nature there is an equal and opposite reaction&acirc;&#128;&#157;.
&nbsp;
Let's look at an example together to better understand.
If we take our hand out of the window of a moving car, our hand will be pulled back. As we know, due to the collision of air with our hand, the air is deflected forward and according to Newton's third law, it causes the opposite and equal force to be produced, which causes our hand to be pulled back. Watch the video for a better understanding.
&nbsp;
We examined two important factors in the production of lift force, the Bernoulli principle and Newton's third law. Many other factors determine the lifting capacity of a wing and you will learn about them in the future.
<h2 title="Airfoils">Airfoils</h2>
Airfoil is any surface or object, like a wing, fuselage, etc, that creates aerodynamic force when interacting with a moving airflow.
<h2 title="Leading Edge">Leading Edge</h2>
The front part of the aircraft wing, or in other words, the part of the wing where the air first hits when the aircraft is moving.
<h2 title="Trailing Edge">Trailing Edge</h2>
The rear part of the aircraft wing, or in other words, the part of the aircraft wing where the two air currents above the wing surface and below the wing surface reconnect and move away from the aircraft wing.
<h2 title="Chord Line">Chord Line</h2>
The chord line is an imaginary straight line drawn between the leading edge to the trailing edge of an airfoil.
<img class="alignnone size-full wp-image-786" src="/data/media/wing.png" alt="wing" title="wing" width="750" height="422">
<h2 title="Camber">Camber</h2>
The camber of an airfoil is the curved shape of their upper and lower surfaces. Usually, the upper camber is more curved, while the lower camber is comparatively flat.
&nbsp;

Four Forces of Flight

If you have ever been to a local airport for a fun flight or to travel to another airport, at first glance you will see a variety of airplanes in terms of shape and structure. But you will soon find that the main part of airplanes are almost the same. In this lesson, we will introduce the main structures of an airplane.
<h2 title="Fuselage">Fuselage</h2>
The fuselage is the most important part of the airplane, which has various functions. This is a place for passengers, pilots to sit and carry luggage and supplies. Also, airplane control commands are installed in the cockpit area, where the cockpit itself is installed in the fuselage.
Another role that the fuselage plays is that all other parts of the airplane, including wings, landing equipment, etc., are connected to the fuselage.
&nbsp;
<img class="alignnone wp-image-411 size-full" title="Fuselage" src="/data/media/Fuselage.jpg" alt="Fuselage" width="750" height="422">
<h2 title="Wings">Wings</h2>
Airplane wings are the part of the airplane that allows the airplane to take off. Due to the movement of the airplane on the runway, an air current is formed below and on top of the wing of the airplane, which leads to the production of force for the lift. The lift force causes the plane to fly, and we will study and explain this force in the following lessons to better understand it.
The position of the wings relative to the fuselage:
<ul>
<li>In terms of the position of the wings relative to the fuselage, there are three types of wings.</li>
<li>Wings that are attached to the upper part of the fuselage, this type of wing is called High Wing.</li>
<li>Wings that attach to the middle of the fuselage. The name of this group of wings is Mid Wing.</li>
<li>Wings that are attached to the lower part of the fuselage. This type of wing is called Low Wing.</li>
</ul>
It should be noted that airplanes that have one set of wings are called Monoplanes, and if they have two sets of wings, they are called Biplanes.
<img class="alignnone wp-image-413 size-full" title="Airplane wing" src="/data/media/Wing.jpg" alt="airplane wing" width="750" height="422">
<h2 title="aileron and flap">Aileron and Flap</h2>
Ailerons are used to allow the pilot to control and direct the wings during flight. These two moving parts are connected at Trailing-Edge or Rear and near the wing edge to the outside of the airplane. Ailerons are driven by the control lever in the pilot's hand. It should be noted that ailerons move against each other. For example, if the right aileron moves up, the left aileron moves down. This allows the pilot to steer the airplane around a horizontal axis, which we will explain in detail in later lessons. In this lesson, just remember that the aileron is a part of the airplane wing that gives the wings control.
The flaps are also attached to the wing of the airplane. The flaps attach at the trailing edge of the wing and near the fuselage. Flaps are used when the airplane takes off from the runway and lands at the airport because they increase the cross-section of the wing. Unlike ailerons, the flaps move in the same direction, For example, if the right flap moves down, the left flap moves down in the same direction, and vice versa. Flaps are controlled by the pilot from inside the cockpit.
In this lesson, just know that the flap is the part of the wing that is used during landing and taking off. We will examine the flaps in detail in the following lessons.
<img class="alignnone wp-image-414 size-full" title="Aileron" src="/data/media/Aileron.jpg" alt="Aileron" width="750" height="422">
&nbsp;
<img class="alignnone wp-image-421 size-full" title="Flap" src="/data/media/Flap.jpg" alt="Flap" width="750" height="422">
<h2 title="Empennage">Empennage</h2>
The whole end of the airplane is called the empennage, which you can see in the image below. Empennage consists of two parts, the vertical stabilizer or fin, and the horizontal stabilizer. We have all played darts and seen the blades at the end of the darts. The blades at the end of the dart make the dart stable at the time of throwing and do not change suddenly.
The vertical stabilizer and horizontal stabilizer on the plane play the role of stabilizing the airplane like darts. Simply keep the airplane in a certain direction and does not change direction suddenly during flight, because the airplane has a vertical stabilizer and a horizontal stabilizer.
<img class="alignnone wp-image-415 size-full" title="Empennage" src="/data/media/Empennage.jpg" alt="Empennage" width="750" height="422">
<img class="alignnone wp-image-433 size-full" title="Vertical Stabilizer" src="/data/media/Vertical-Stabilizer.jpg" alt="Vertical Stabilizer" width="750" height="422">
<img class="alignnone wp-image-434 size-full" title="Horizontal Stabilizer" src="/data/media/Horizontal-Stabilizer.jpg" alt="Horizontal Stabilizer" width="750" height="422">
<h3 title="Rudder and Elevator">Rudder and Elevator</h3>
The rudder is attached to the end of the vertical stabilizer. The rudder is controlled by two pedals located under the pilot's foot. Unlike the vertical stabilizer, this part is not fixed and is movable. It should be noted that the rudder allows the pilot to tilt the nose of the airplane to the left or right during flight.
Rudder and aileron are also used in combination when bypassing the airplane so that the airplane can experience a smooth lap. (Additional explanations will be provided in the relevant lesson).
In this lesson, just know that the radar is in the empennage section and is connected to the vertical stabilizer, allowing the pilot to tilt the nose of the airplane to the right and left. Let's take a look at how to use radar and additional explanations in the rudder lesson.
The elevator is attached to the end of the horizontal stabilizer, and unlike the horizontal stabilizer, it is not stationary and moves. The elevator is controlled by the control lever in the pilot's hand, allowing the pilot to move the nose of the airplane up and down. (We will provide additional explanations and a complete review of the elevator in the relevant lesson).
<img class="alignnone wp-image-423 size-full" title="Rudder" src="/data/media/Rudder.jpg" alt="Rudder" width="750" height="422">
<img class="alignnone wp-image-422 size-full" title="Elevator" src="/data/media/Elevator.jpg" alt="Elevator" width="750" height="422">
<h2 title="Landing Gear">Landing Gear</h2>
Like a car that needs wheels to move on the ground, the airplane uses the landing gear to move on the ground, and also the landing gear bears the weight of the airplane when landing. Light airplanes and Ultralight airplanes usually have three wheels. The main wheels are located on either side of the fuselage, known as the Main Wheel.
The third wheel can be installed both under the nose of the airplane and at the tail of the airplane. If the third wheel is installed in the nose of the airplane, it is called Nose Wheel and its design is known as Tricycle Gear.
If the third wheel is installed at the rear of the airplane, it is called the Tail Wheel and its design is known as the Conventional Landing Gear. It should be noted that in general, landing gears are divided into two types, Fixed Gear and Retractable Gear. Simply put, there are airplanes that the pilot cannot pick up landing gear during the flight, and the landing gear is constantly out, like Cessna 172. There are also airplanes that can pick up the landing gear after taking off from the runway, like heavy airplanes or Beechcraft Baron 58.
<img class="alignnone wp-image-416 size-full" title="Landing Gear" src="/data/media/Landing-Gear.jpg" alt="Landing Gear" width="750" height="422">
<h3 title="Shock Struts">Shock Struts</h3>
We all know that the car has springs and shock absorbers. The purpose of the spring and shock absorber design for the car is to absorb the force applied by the ground and bumps so that the car is not damaged in the long run.
Airplanes, like cars, use shock absorbers so that the airplane is not damaged by the force applied to the fuselage when moving on the ground and during landing. In aviation, these shock absorbers are known as Shock Struts.
&nbsp;
<h3 title="Brakes">Brakes</h3>
The brake of an airplane is almost the same as the brake structure of a car, but slightly different. In airplanes, brakes are usually mounted on two main wheels.
As you know, brakes are applied by one pedal in cars, but usually, in airplanes, brakes are applied by two pedals. This means that you can apply the brakes to the right wheel and the left wheel separately as desired, this system is known as Differential Braking.
Airplanes with differential braking systems can better bypass the runway or airport ramp because you can apply full braking to one wheel while the other wheel is completely free and can move easily. 
For example, if your plane has a differential braking system and you want to make a deep turn to the right, you can prevent it from moving by applying brakes to the right wheel, while the left wheel is free and can move easily. This will allow your plane to turn deep and comfortable to the right.
<img class="alignnone wp-image-418 size-full" title="Shock Strut and Brake" src="/data/media/Shock-Strut-and-Brake.jpg" alt="Shock Strut and Brake" width="750" height="422">
<h2 title="Power Plant">Power Plant</h2>
In small airplanes, the power plant consists of the engine and the propeller.
&nbsp;
<h3 title="Engine">Engine</h3>
The main function of the airplane engine is to power the propeller into circulation. Other functions of the plane engine include generating electrical power, vacuum sources for flight equipment, and heating energy for airplane heaters.
&nbsp;
<h3 title="Propeller">Propeller</h3>
The propeller is rotated by the force supplied by the airplane engine, and as a result of this rotation, the propeller creates the thrust force, which allows the airplane to move forward. We will talk about propeller and thrust in the next lessons.
&nbsp;
<h3 title="Firewall">Firewall</h3>
A firewall is the part of the power plant on which the airplane engine is mounted. Other functions of the firewall include removing the cockpit and protecting the pilots from the power plant.
<h3 title="Cowling">Cowling</h3>
The cowling protects the airplane engine from rain, dust, as well as a sudden drop in engine temperature.
<img class="alignnone wp-image-420 size-full" title="airplane engine" src="/data/media/airplane-engine.jpg" alt="airplane engine" width="750" height="422">

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Angle of Attack

Benyamin Amiri

Angle of Attack

Benyamin Amiri

In this lesson, you will learn about the angle of attack definition, critical angle of attack, the effects of increasing the angle of attack on the production of lift force, etc.What is Angle of Attack (AOA)

What is Angle of Attack (AOA)

Changing Angle of Attack