# ATAT101 Theory of Flight > # [[T101 Week 1| ◀️ ]]  [[T101 Home| Home ]]  [[T101 Week 3| ▶️ ]]               [[QR T101 Week 2| 🌐 ]] ># [[T101 Week 2#ATAT101 Theory of Flight|Week 2]] >- [[T101 Week 2#Elementary Aerodynamics|Elementary Aerodynamics]] >- [[T101 Week 2#Balanced Forces|Balanced Forces]] >- [[T101 Week 2#Unbalanced Forces|Unbalanced Forces]] >- [[T101 Week 2#How an Airfoil Generates Lift|How an Airfoil Generates Lift]] >- [[T101 Week 2#Aircraft Stability|Aircraft Stability]] >- [[T101 Week 2#Flight events|Flight events]] > [!jbPlus|c-blue]- Lesson Intro >### What > >In this lesson you will learn some fundamental concepts about aerodynamics. > >### Why > >A technician must understand the physics and technology of how an aircraft works in order to understand how to maintain and repair it. You will use this knowledge throughout your aviation career. > >### Testing > >You will be tested on this material on Assignment 2, the Midterm, and the Final Test, as per the [[T101 Intro#Testing and Grades|testing strategy]]. > >#### Approach and Objectives By understanding the following topics, you will have achieved the learning outcome for this lesson. Consult your course outline for the learning outcomes and other details of this course. > >##### Course Learning Objectives CLO 3. Explain theory of flight applicable to fixed and rotary wing aircraft. ## Elementary Aerodynamics ### Four Forces of Flight > [!aside]- Ref >[[AMT General Handbook Ch5_3#Four Forces of Flight|📘]] [[V T101 Elementary Aerodynamics|📺]] In flight, an aircraft is acted upon by four forces.  [[T101_2_001.png|➡]] #### Lift %%==[[Master QB1#Q00489|Q]]==%% Lift is upward force created by the wing. The science behind the effect of lift is interesting, and we will look at this in more detail shortly. For now, we will look at these forces in pairs. These pairs' forces oppose each other. Lift opposes the force of weight. #### Weight %%==[[Master QB1#Q00490|Q]]==%% Weight is downward force created by gravity. We are all familiar with the effects of gravity and weight. Aircraft are not exempt from these physical principles, and in fact, weight is a key consideration in aviation manufacture. As we will see, these forces act against each other, and the less an aircraft weighs, the less lift it requires. In other words, the force of weight opposes the force of lift. #### Thrust %%==[[Master QB1#Q00546|Q]]==%% Thrust is the force of the propeller or turbine engine. This is the motive force that causes the aircraft to move forward. You will learn about different types of propulsion in this program, but they all perform the same function: move the aircraft forward. Thrust directly opposes drag. #### Drag %%==[[Master QB1#Q00547|Q]]==%% %%==[[Master QB1#Q00548|Q]]==%% Drag is the friction of air flowing over the aircraft, also known as air resistance, and represents the force acting against the relative forward motion of an aircraft. A rough surface, or a malformed one, or rivets, or any imperfections could all contribute to increased air resistance of the aircraft as it moves through the air. Remember when we looked at humidity and the dew point, we learned that ice on the wings deforms their shape, and thus contribute more drag. These four forces that act on an aircraft are measured in pounds of pressure. ### Balanced Forces #### Equilibrium %%==[[Master QB1#Q00556|Q]]==%% %%==[[Master QB1#Q00557|Q]]==%% %%==[[Master QB1#Q00558|Q]]==%% In normal non-accelerating level straight line flight, the four forces acting on the aircraft are said to be in equilibrium. [[T101_2_003.png|➡]] Note that equilibrium does not indicate that the aircraft is standing still in the air. When forces are balanced, equilibrium indicates that velocity is constant. In regards to altitude, equilibrium would be indicated in an aircraft maintaining its altitude. [[T101_2_002.png|➡]] #### Thrust = Drag: Constant Velocity %%==[[Master QB1#Q00549|Q]]==%% If the aircraft is maintaining a constant height, direction and speed then the thrust force will balance the air's resistance (drag) to the aircraft's motion through it. #### Lift = Weight: Constant Altitude %%==[[Master QB1#Q00550|Q]]==%% So, if thrust and drag are equal, velocity is constant. On the vertical axis, if the lift and weight are equal, the aircraft remains at a constant altitude. ### Unbalanced Forces %%==[[Master QB1#Q00551|Q]]==%% [[V T101 Unbalanced Forces|📺]] Whenever these forces are not in perfect balance, something about the aircraft's condition is changing. It will change something about the flight, whether ascending, descending, accelerating, decelerating, turning etc. Here are the effects of unbalanced forces: #### Acceleration %%==[[Master QB1#Q00552|Q]]==%% There is more thrust than drag #### Deceleration %%==[[Master QB1#Q00553|Q]]==%% There is more drag than thrust #### Ascending %%==[[Master QB1#Q00554|Q]]==%% When the aircraft is climbing, or gaining altitude, there is more lift than weight #### Descending %%==[[Master QB1#Q00555|Q]]==%% When an aircraft is diving, or losing altitude, it has more weight than lift ### Direction of Forces %%==[[Master QB1#Q00559|Q]]==%% %%==[[Master QB1#Q00560|Q]]==%% %%==[[Master QB1#Q00561|Q]]==%% %%==[[Master QB1#Q00562|Q]]==%% %%==[[Master QB1#Q00563|Q]]==%% %%==[[Master QB1#Q00564|Q]]==%% %%==[[Master QB1#Q00565|Q]]==%% The direction of these forces is relative to the flight path, not the earth's surface. We previously described lift and weight as acting vertically, and thrust and drag as acting horizontally, but this is only true when the aircraft is in straight and level flight. In fact, lift acts perpendicular to both the flight path and the lateral axis of the aircraft, drag acts parallel to the flight path and thrust usually acts parallel to the longitudinal axis of the aircraft. Weight, however, is the effect of gravity which means that the force of weight is always towards the earth. [[T101_2_004.png|➡]] We will see later how the pilot is able to change the direction and magnitude of these forces and therefore control the speed, flight path and performance of an aircraft. ## How an [[Airfoil]] Generates Lift ### Bernoulli %%==[[Master QB1#Q00566|Q]]==%% %%==[[Master QB1#Q00567|Q]]==%% %%==[[Master QB1#Q00568|Q]]==%% [[V T101 How an Airfoil Generates Lift|📺]] An [[airfoil]] is a device that creates a force based on Bernouilli's principles when air is caused to flow over the surface of the device. Bernoulli's principle is one important foundation in understanding how the wing of an airplane generates lift. The wing on a slow-moving airplane (we will look at the differences for high speed later) has a curved top surface and a relatively flat bottom surface. The curved top surface acts like half of the converging shaped middle of a venturi. [[T101_2_005.png|➡]] Remember from the last lesson that Bernoulli showed that if you constrict the flow of air, its velocity increases and the static pressure decreases. By picturing a wing in the air as a venturi as per the graphic, we can see the air is constricted by the surrounding air, and the wing shape that interrupts the flow at the bottom of the venturi. If we now apply Bernoulli's principle, we see a drop in static pressure at the top of the wing. The static pressure on the bottom of the wing is now greater than the pressure on the top, and this pressure difference creates the lift on the wing. The faster an aircraft moves, the more these effects are felt. Therefore, everything else being equal, the faster a wing moves through the air, the more lift is generated. And, as we have seen, the force of lift directly opposes the force of weight. [[T101_2_006.png|➡]] ### And Newton Newton's third law explains that for every force, there is an equal and opposite reaction force. This is another way, in addition to Bernoulli's principle, to explain or understand the lift that is created by a wing. As you examine the airflow over a wing in flight, you see that the air that leaves the trailing edge is forced to move in a downward direction. The opposing force is upwards, and that is lift. [[Airplane Aerodynamics|🎞️]] ### Wing Dimensions [[V T101 Wing Dimensions|📺]] Every bit of a wing has a purpose. Its shape and dimensions all come together to provide the lift characteristics required of the aircraft designer. We will learn some of the terms surrounding wing dimensions. #### Span The wingspan of an aircraft is always measured in a straight line, from wingtip to wingtip, independently of wing shape or sweep. #### Thickness %%==[[Master QB1#Q00569|Q]]==%% %%==[[Master QB1#Q00570|Q]]==%% %%==[[Master QB1#Q00571|Q]]==%% %%==[[Master QB1#Q00572|Q]]==%% %%==[[Master QB1#Q00573|Q]]==%% %%==[[Master QB1#Q00574|Q]]==%% %%==[[Master QB1#Q00575|Q]]==%% A straight line from the top of the wing to the bottom of the wing at its thickest point is the thickness of the wing. A thick wing indicates a wing with a much larger upper camber than lower camber. [[T101_2_007.png|➡]] [[T101_2_008.png|➡]] #### Camber %%==[[Master QB1#Q00576|Q]]==%% Camber is the curvature of a surface. On a typical low speed wing, the camber of the upper surface is more pronounced, or more curved, than the bottom surface. The increased camber on top is what causes the velocity of the air to increase and the static pressure to decrease. The bottom of the wing has less velocity and more static pressure, which is why the wing generates lift. The air that hits the leading edge will meet at the trailing edge, but the upper and lower airflows have different distances to travel until they meet at the trailing edge, causing a pressure differential. Incidentally, this last point is not scientifically sound, as there is no obligation for air molecules to rejoin together after having been separated at the leading edge. It is one of those things that is not actually true, but it may help you to understand it at an appropriate level. #### Chord Line %%==[[Master QB1#Q00577|Q]]==%% %%==[[Master QB1#Q00578|Q]]==%%The chord line is an imaginary straight line running from the wing's leading edge to its trailing edge. The angle between the chord line and the longitudinal axis of the airplane is known as the [[angle of incidence]]. #### Chord %%==[[Master QB1#Q00579|Q]]==%% The length of the chord line is the chord of the wing. ### Relative Wind Relative wind is a relationship between the direction of airflow and the aircraft wing. In normal flight circumstances, the relative wind is the opposite direction of the aircraft flight path. If the flight path is forward then the relative wind is backward. If the flight path is forward and upward, then the relative wind is backward and downward. If the flight path is forward and downward, then the relative wind is backward and upward. Therefore, the relative wind is parallel to the flight path, and travels in the opposite direction. ### Angle of Attack %%==[[Master QB1#Q00580|Q]]==%% %%==[[Master QB1#Q00581|Q]]==%% The angle between the chord line and the relative wind is the angle of attack. We will soon see how important the AOA is to the lift and drag produced by a wing. Extreme AOA is also critical, causing a condition called stall. We will discuss stalls in more detail shortly. [[T101_2_009.png|➡]] It is important to keep in mind that when an aircraft is airborne, its velocity is relative to the surrounding air, not the earth's surface. This is a very dynamic environment. So if the relative wind changes, the AOA changes, and lift is affected. If AOA changes, relative wind may change, and lift is also affected. However, if the aircraft encounters a sudden change in the ambient air velocity — a transient gust —inertia comes into play and momentarily maintains the aircraft velocity relative to the Earth or - more correctly - relative to space. This momentarily changes airspeed and imparts other forces to the aircraft. The fact that inertia overrides the physics of aerodynamics is sometimes a cause of confusion. ### Stagnation Point %%ATAT101W2V080.mp4%% %%==[[Master QB1#Q00582|Q]]==%% As the air hits a forward moving [[airfoil]], it is split into upper and lower flows by the leading edge. If the leading edge is blunt, air pressure accumulates at the leading edge of the wing. This air is relatively still or stagnant, and the point at which these airflows separate is called the stagnation point. [[T101_2_010.png|➡]] ### Coefficient of Lift [[V T101 Coefficient of Lift|📺]] We saw that there are four forces acting on an aircraft in flight in combination. For many purposes, aerodynamicists have found it convenient to resolve that resultant force into just two components, lift and drag. First we will look at the forces that combine to give us our resultant lift. #### The Lift Equation The part acting perpendicular to the flight path is lift. The amount of lift is chiefly dependent on these factors, and can be calculated with the formula shown. CL is the [[coefficient of lift]], and helps us to understand the relationships between these factors. This is complicated stuff, and is well beyond the scope of this introductory course. In fact, as a technician, you may never have occasion to refer to this. However, seeing the formula allows us to see the relationship between various forces and factors at play when determining lift. If you don't believe me that this is complicated, have a look at the extras in the glossary. This formula takes into account: - the angle at which the wings meet the airflow or flight path - the shape of the wings particularly in cross section - the [[airfoil]], - the density (i.e. mass per unit volume) of the air, - the speed of the free stream airflow i.e. flight airspeed, - the wing plan-form surface area. > $ L = \frac{1}{2}\rho V^2S_{ref}C_L$ > where: $L$ denotes lift force. $V$ defines the velocity of aircraft expressed in m/s. $\rho$ is air density, affected by altitude. $S_{ref}$ is the reference area or the wing area of an aircraft measured in square metres. $C_L$ is the coefficient of lift, depending on the angle of attack and the type of aerofoil. In ATAT110 you learn about manipulation of formulas. One important, and not too difficult, lesson to learn is the relationships between elements of a formula, and the role of the equals sign. Review [[T110T SSGW05#Formula Relationships|here]]. This formula shows that lift is proportional to area and air density. In other words, more wing area normally means more lift, and denser air also creates more lift. [[Frequently Asked Questions#ATAT101 Theory of Flight#Coefficient of Lift|❓]] [[T101_2_012.png|➡]] #### Resultant Vector The resultant vector is all of the factors calculated into one total lift force. This graphic shows the angles and forces, known as vectors, of lift and drag combine to produce a resultant force. The angles indicated at the leading edge of the wing show how the resultant force changes depending on these angles. [[T101_2_013.png|➡]] #### Centre of Pressure %%==[[Master QB1#Q00583|Q]]==%% %%==[[Master QB1#Q00584|Q]]==%% %%==[[Master QB1#Q00585|Q]]==%% The centre of pressure is the point where the single force vector originates representing total lift produced. In other words, it indicates how much lift and in which direction. Another definition in slightly different words: The centre of pressure is the origin of the single force vector that represents the total lift produced and its direction. [[T101_2_014.png|➡]] You don't need to know the formulae behind this science, but it is useful for you to understand that the amount of lift and drag generated by a wing is dependent on the following factors: #### Speed and Lift >More speed = more lift The faster an aircraft moves, the more air is sent over and under the wings. This means the high pressure generated on the top of the wing is greater, and therefore produces more lift. Its coefficient of lift has increased. In fact, the formula you have just seen shows that lift is proportional to the square of speed, so traveling twice as fast increases the lift by a factor of 4, everything else being equal. #### Angle of Attack and Lift >More angle of attack = more lift %%==[[Master QB1#Q00586|Q]]==%% %%==[[Master QB1#Q00587|Q]]==%% %%==[[Master QB1#Q00588|Q]]==%% %%==[[Master QB1#Q00589|Q]]==%% An increase in angle of attack will produce more lift as well, until a failure point called a stall, which we will look at shortly. The pilot adjusts control pressures to apply an aerodynamic force to the aircraft's tailplane (or some other control surface). This has the effect of rotating the aircraft up about its lateral axis, pitching the aircraft nose up and increasing the AOA, increasing the lift of the wing, and thus causing the aircraft to climb. ### Coefficient of Drag [[V T101 Coefficient of Drag|📺]] The other side of the coin is the calculations for drag. > $F_D = C_D A \frac{\rho V^2}{2}$ where: $F_D$ is the drag force. $C_d$ is the drag coefficient, dependant on angle of attack. $\rho$ is air density, affected by altitude. $A$ is the reference area or the wing area of an aircraft measured in square metres. $V$ is flow velocity relative to the object. This formula reveals the relationship between the two most important factors in drag: #### AOA and drag More angle of attack = more drag As we just saw, a higher angle of attack results in more lift. The angle of attack also has a significant effect on the drag of the [[airfoil]]. The math is very complicated, and we do not need to go that deep. But you can try using your hand as an [[airfoil]] out your car window. We are familiar with the increased lift we get from increasing the angle of attack, but now notice just how much more drag is also produced. #### The shape of the wing and drag >significant effect on drag Later in this lesson we will look at the importance of the shape of a wing, but for now, know that the shape of a wing has a significant impact on the drag it produces. ### Types of Drag #### Induced Drag - By-product of lift - Increases with AOA %%==[[Master QB1#Q00590|Q]]==%% %%==[[Master QB1#Q00591|Q]]==%% %%==[[Master QB1#Q00592|Q]]==%% Induced drag is the unavoidable by-product of lift and increases as the angle of attack increases. Newtonian or dynamic drag is caused by the inertia of air. Pressure Induced drag occurs when the AOA is too large and the air flow becomes turbulent, especially on the top of the airfoil. We'll look at this in just a moment. #### Parasite Drag %%==[[Master QB1#Q00593|Q]]==%% %%==[[Master QB1#Q00594|Q]]==%% %%==[[Master QB1#Q00595|Q]]==%% Parasite drag is all drag created that is not involved in creating lift. There are several reasons for this drag, and therefore several types of parasite drag. #### Skin-friction drag %%==[[Master QB1#Q00596|Q]]==%% …is caused by the friction between outer surfaces of the aircraft and the air through which it moves. It will be found on all surfaces of the aircraft: wing, tail, engine, landing gear, and fuselage. #### Form drag %%==[[Master QB1#Q00597|Q]]==%% …is due to the shape of the object moving through the air. [[T101_2_016.png|➡]] #### Interference drag %%==[[Master QB1#Q00598|Q]]==%% …is generated by the mixing of airflow streamlines between airframe components. In the photograph below, the cowling between the fuselage and the wing helps to smooth the mixing of the airflows over the wing and around the fuselage, thereby reducing interference drag. [[T101_2_017.png|➡]] ### Lift/Drag Ratio - AKA Glide Ratio - Measures efficiency The ratio of lift to drag is an important measurement of aircraft performance. It is a measure of the aircraft's efficiency. It is also known as the glide ratio. >$\frac{L}{D} \text{ ratio} = \frac{C_L}{C_D}$ [[T101_2_019.png|➡]] The higher the glide ratio, or lift/drag ratio, the greater the distance that an aircraft can travel across the ground for a given change in height. In other words, it can glide for a longer distance. ### Airflow %%ATAT101W2V130.mp4%% #### Boundary Layer [[T101_2_020.png|➡]] %%==[[Master QB1#Q00600|Q]]==%% [[V T101 Airflow|📺]]The boundary layer is a very thin layer of air that covers and tends to adhere to the entire surface of an aircraft. It tends to flow smoothly as it moves over the most forward part of the wing. It promotes the generation of lift and measures are taken to increase this smooth flowing layer flow. This smooth flow is known as... #### Laminar flow However, once the boundary layer approaches the centre of the wing, it begins to lose speed due to skin friction and becomes turbulent. This is known as… #### Turbulent flow The [graphic above](T101_2_020.png) is an actual photograph taken inside of a wind tunnel, with smoke being blown over an [[airfoil]]. It clearly demonstrates the laminar flow near the wind surface, and the turbulence beginning at the trailing edge. This video shows a wing with streamers attached to show air movement. Observe the horizon in the video and how as the angle of attack changes, the air turbulence moves up the wing. [[Airflow During a Stall|🎞️]] ### Wingtip Vortices [[T101_2_021.png|➡]] [[T101_2_022.png|➡]] [[T101_2_023.png|➡]] %%==[[Master QB1#Q00601|Q]]==%% %%==[[Master QB1#Q00602|Q]]==%% %%==[[Master QB1#Q00603|Q]]==%% [[V T101 Wingtip Vortices|📺]] As the air passing over and under a wing meet again at the trailing edge, the higher pressure air from the underside of the wing flows upwards into the lower pressure air on the topside. At the wingtip, if there is nothing to prevent it, this will cause the higher pressure air to make its way to the top of the airfoil where the pressure is less. This creates a spiral or vortex of air that is very turbulent. They are strongest when the aircraft is flying slowly, and are larger on larger aircraft. These wingtip vortices are also known as wake turbulence, and are very dangerous for aircraft following, especially smaller ones, such as for a final landing. Imagine the wake behind a boat and trying to navigate a canoe in the resultant waves. In aviation, this turbulence can be enough to cause the complete loss of control of a following aircraft. [[Why aircraft say heavy|🎞️]] If you recall the boundary layer and the effects of turbulence, now consider that the wing has nothing but turbulent air around it in this circumstance. This then causes a serious decrease in the lift producing potential of the wing. ### Winglets %%==[[Master QB1#Q00604|Q]]==%% In a later lesson we will look at secondary flight controls, but here is an early example of a fixed lift device. To reduce the effects of wingtip vortices by guiding the airflow, winglets can be found on many models of aircraft. The surface of these wing extensions acts like a [[fence]] preventing air leakage over the outboard edge of the wing. Winglets are often angled or canted to redirect the vortex, and this can help to contribute to thrust. [[T101_2_024.png|➡]] #### Other devices %%==[[Master QB1#Q00605|Q]]==%% %%==[[Master QB1#Q00606|Q]]==%% %%==[[Master QB1#Q00607|Q]]==%% Other methods of reducing wing tip vortices include tip tanks, sharklets and dropped (Hoerner) wing tips. [[T101_2_025.png|➡]] ### Boundary Layer Control [[V T101 Boundary Layer Control|📺]] In order to reduce the turbulence over a wing's surface, a wing is generally as smooth as possible. This includes a smooth finish free of debris and dirt, and also clear of ice, which also disturbs this smoothness and decreases lift. #### Icing The effect of icing on the fundamental function of the wing is a serious issue, and measures are taken to ensure that ice does not form on the wing's surface, including heaters, expanding rubber bladders and deicing fluid. #### Ducting Besides ensuring the smooth surface of the wing, there are other methods to control the behaviour of the boundary layer, usually by controlling airflow. We will see how this is done in the lesson on flight controls. ## Aircraft Stability %%Ref A p.5-41%% [[V T101 Aircraft Stability|📺]] When an aircraft is in straight and level flight at a constant velocity, all of the forces acting on the aircraft are in equilibrium. The reaction of an aircraft to a disturbance to return to equilibrium is called stability. This motion is based on the actual action, and the time it takes. There are two types of aircraft stability: - Static - Dynamic ### Static Stability %%==[[Master QB1#Q00608|Q]]==%% Static stability refers to the aircraft's ability to return to its previous flight path. Picture an aircraft that is flying straight and level and is disturbed so that it pitches up. If the aircraft immediately returns to straight and level flight, it is said to demonstrate positive stability. The fact that the aircraft moves to correct this disturbance indicates that it has positive static stability. Static stability refers to the immediate response of the aircraft when disrupted. #### Positive %%==[[Master QB1#Q00609|Q]]==%% If static stability is positive, then the aircraft will return to its original position once the disruption is removed. #### Negative %%==[[Master QB1#Q00610|Q]]==%% If an aircraft has negative static stability, it will tend to increasingly continue off it course even when the disturbance is removed. It does not return to the original position, and it does not adopt the new position, it continues to behave as if its still being pushed by the disturbance and thus the situation only gets worse. #### Neutral %%==[[Master QB1#Q00611|Q]]==%% If an aircraft remains pointed in its new direction once the disturbance is removed, it is said to demonstrate neutral static stability. This graphic may help you to understand these three types of static stability. [[T101_2_026.png|➡]] ### Dynamic Stability %%==[[Master QB1#Q00612|Q]]==%% The time that it takes for an aircraft to regain equilibrium is called dynamic stability. Dynamic stability involves the oscillations that typically occur as an aircraft tries to return to its original position or altitude. So, it can be said that dynamic stability refers to how an aircraft responds to a disturbance over time. #### Positive %%==[[Master QB1#Q00613|Q]]==%% %%==[[Master QB1#Q00614|Q]]==%% %%==[[Master QB1#Q00615|Q]]==%% %%==[[Master QB1#Q00616|Q]]==%% If an aircraft in straight and level flight is disturbed and pitches up, and it immediately returns to straight and level flight, it is said to have positive dynamic stability. In this case, there is no hunting or oscillations as the aircraft goes past level to a nose down attitude and back past level to nose up until it finds level. #### Negative %%==[[Master QB1#Q00617|Q]]==%% %%==[[Master QB1#Q00618|Q]]==%% These oscillations as the aircraft pitches up or down until it finds equilibrium may increase over time. In other words, as it passes level, it goes farther nose down than the time before, and does the same when it oscillates to nose up. In this case, the aircraft is said to have negative dynamic stability. This effect is called porpoising because it resembles the motion of a porpoise in the sea. #### Neutral %%==[[Master QB1#Q00619|Q]]==%% %%==[[Master QB1#Q00620|Q]]==%% If these oscillations remain the same over time, the aircraft is said to have neutral dynamic stability. [[T101_2_027.png|➡]] In general, civil aircraft are at least neutrally stable. Aircraft that are not positively stable require much more pilot effort. In extreme cases, such as high performance military aircraft, computer control is required to present reasonable stability to the pilot. This allows for these aircraft to be built naturally unstable so that they can be extremely maneuverable, despite the large forces involved. [[T101_2_028.png|➡]] ### How Stability is Accomplished #### Longitudinally (Pitch) ##### Horizontal Stabilizers Also known as a tailplane, this airfoil helps with stability by providing lift on the tail, opposite to the lift of the wing when looked at as centred on the centre of gravity. When the aircraft pitches up due to a disturbance, the angle of attack of the stabilizer is also increased, and so it lifts the tail, countering the nose up attitude of the aircraft. It forces the aircraft to rotate around the centre of gravity back to level flight. [[T101_2_029.png|➡]] #### Laterally (Roll) ##### Anhedral and Dihedral When we look at wing shapes in the next lesson, we will discuss how [[anhedral]] and [[dihedral]] wings affect lateral stability. %% #JBTO change this after the move %% #### Directionally (Yaw) ##### Vertical Stabilizer The vertical stabilizer is responsible for maintaining directional stability. Any movement other than straight ahead causes a change in the stabilizers angle of attack, which causes it to produce lift opposite of the disturbance. ##### Keel Effect and Pendulum Effect [[T101_2_030.png|➡]] Another design feature that aids lateral stability is the keel effect provided by high wing aircraft. When wings are mounted high on the fuselage, most of the weight of the aircraft is below the wing. This makes the aircraft act like a pendulum below the wings, and provides the force necessary for the aircraft to tend to right itself. The larger mass of aircraft body below the wings also provides a similar effect as the vertical stabilizer, acting like the keel of a boat. [[T101_2_031.png|➡]] ##### Swept wings We will look at wings shortly, where it will be explained how the wing shape affects directional stability. %% #JBTO change this after the move %% ## Flight events ### Stall [[T101_2_032.png|➡]] %%==[[Master QB1#Q00621|Q]]==%% [[V T101 Flight Events|📺]]As the angle of attack increases, the turbulence affecting the boundary layer increases, and at a certain point, it will become so great that the air breaks away from the surface of the wing. This airflow separation is called a burble. The air no longer creates lift according to Bernoulli's principle, and the wing is said to be in a stall condition. This is known as the wing's… #### Critical Angle of Attack %%==[[Master QB1#Q00622|Q]]==%% %%==[[Master QB1#Q00623|Q]]==%% In the image of the wing boundary layer, view B, the angle of attack is such that the air has separated from the wing surface, the wing generates no lift, and is stalled. This is known as the critical angle of attack, or the stall angle. [[T101_2_033.png|➡]] This image gives you a better idea of the progressive nature of boundary layer turbulence and how it moves until it causes a stall. #### Stall Strips %%==[[Master QB1#Q00624|Q]]==%% Stall strips are protrusions fixed to the leading edge of a wing, usually inboard, to ensure that the wing root stalls before the wing tips. Stall strips are sometimes installed when deficiencies in the wing's stall characteristics are discovered, or are sometimes designed in at the outset. The stall strip changes the shape of the [[airfoil]] such that the wing root will stall before the wing tip, allowing the ailerons to continue to function and easing the wing into the stall condition rather than stalling over the whole wing simultaneously. [[T101_2_034.png|➡]] ### Stalling and Landing Speeds We now understand how a wing can be put into a stall condition. Many factors are at play when a wing stalls, such as the weight and lift of the aircraft, the forward speed, the angle of attack, and altitude and its effects on the density of the air travelling around the wing. #### Stall speed >slowest speed before stall %%==[[Master QB1#Q00|Q]]==%% In terms of the entire aircraft, stall speed is the slowest an aircraft can fly while maintaining level flight (not losing lift). When an aircraft slows down, it creates less lift. This can be compensated for by increasing the angle of attack. But we know that taken too far, the wing will stall. Because this condition is dependant on the aircraft's weight, speed, altitude, acceleration and other aerodynamic loads, calculations are done taking these into account when determining an aircraft's stall speed. Airspeed indicators and angle of attack sensors are used to predict stall conditions on aircraft. Note that stall speed can vary depending on the configuration of the aircraft, such as whether flaps are deployed or landing gear is down. [[Aircraft Stall Speeds|🎞️]] #### Landing speed >a safe margin above stall speed %%==[[Master QB1#Q00626|Q]]==%% Landing speed is then determined to be the lowest speed reasonably possible while avoiding the risk of stall. ### Stall Behaviour #### Twin engine The two wings on an aircraft do not necessarily behave the same way, as they may see different air. Therefore, stalls do not always happen with a strictly pitch down motion. Stalls in a twin with one engine inoperative can lead to roll or spin entry. The wing with the inoperative engine may stall, but the propeller slip stream delays the stall on the other wing, causing a strong departure from equilibrium. [[T101_2_035.png|➡]] ### Stall Warnings #### Pilot habits >Conditions change! Angle of attack, weight, speed and the surfaces of the aircraft all contribute to the onset of stall. It is important for pilots to not adopt bad habits in regards to their aircraft performance in relation to stalls. While a 60 knot approach may work very well on most occasions with a normal load, a flight with a heavier load, or with icing or gusting conditions may cause the aircraft to approach stall sooner, with obvious safety implications. We might assume that a pilot has simply to avoid a stall condition to be safe. But the dynamic nature of flight requires that if circumstances are about to conspire to cause a stall, the pilot must be informed about it immediately in order to rectify the situation. For this reason, stall warnings are a critical safety measure on any aircraft. #### Buffeting [[T101_2_036.png|➡]] When an aircrafts angle of attack is such that it is about to stall, the wake of the wing is very turbulent. The horizontal stabilizer is presented with only turbulent air, and this can be felt in the aircraft as a natural stall warning. #### Stick shakers Angle of attack sensors will, via a flight control computer, relay a high AOA condition to mechanical vibrators in the stick to give the pilot an unmistakable warning by vibrating the stick rapidly and noisily. #### Stick Pushers In some hydraulic flight control systems (more later), the stick will actually be pushed forward to guide the pilot to prevent a stall. #### Audible warnings Stall conditions as detected by sensors on the aircraft can also be used to activate audible warnings in the cockpit by activating electric or pneumatic horns. ### Spin %%==[[Master QB1#Q00627|Q]]==%% A spin is a special type of stall where one wing is more stalled than the other. The aircraft will yaw and/or roll quite quickly as the nose pitches down. This can cause a rapid loss of altitude. [[T101_2_037.png|➡]] ### Torque %%==[[Master QB1#Q00628|Q]]==%% Torque is a reactive effect to the rotation of a propeller. If you review what you know about [[T101 Week 1#Newton's Law of Action and Reaction|Newton's third law]], you can see that the force of the propeller turning to the right (clockwise as seen by the pilot), causes the aircraft to react by tending to bank to the left. [[T101_2_038.png|➡]] ### Corkscrew effect %%==[[Master QB1#Q00629|Q]]==%% %%==[[Master QB1#Q00630|Q]]==%% %%==[[Master QB1#Q00631|Q]]==%% The corkscrew effect affects directional stability, or yaw stability. The air pushed back by the propeller, known as propwash, spirals around the fuselage. The vertical stabilizer is on the top of the aircraft, and so the pressure from this air spiral causes a yaw motion to the left. To counter this, vertical stabilizers are often mounted on an angle. [[T101_2_039.png|➡]] ### Gyroscopic Precession [[T101_2_040.png|➡]] %%==[[Master QB1#Q00632|Q]]==%% %%==[[Master QB1#Q00633|Q]]==%% %%==[[Master QB1#Q00634|Q]]==%% A rotating disc shows an interesting reaction to forces applied to it. The resulting force is felt 90° in the direction of rotation. [[T101_2_041.png|➡]] #### Pitch causes yaw Since the propeller of an aircraft is effectively a spinning disc, forces applied to it affect the aircraft. This occurs most notably during pitch changes, e.g. rotation about the lateral axis. There is a tendency for the aircraft to yaw to the right when the nose is rising, and to the left when the nose is falling. [[T101_2_042.png|➡]] #### Taildraggers > Want to go left on takeoff Taildragger aircraft have a distinct left turning tendency during takeoff due to the effects of gyroscopic precession. ## Conclusion In this lesson we looked at the following topics: - [[T101 Week 2#Elementary Aerodynamics|Elementary Aerodynamics]] - [[T101 Week 2#How an Airfoil Generates Lift|How an Airfoil Generates Lift]] - [[T101 Week 2#Aircraft Stability|Aircraft Stability]] - [[T101 Week 2#Flight events|Flight Events]] You can demonstrate your understanding of the material in this lesson by answering the questions in the corresponding weekly practice quiz correctly. > # [[T101 Week 1| ◀️ ]]  [[T101 Home| Home ]]  [[T101 Week 3| ▶️ ]]     [[QR T101 W2| 🌐 ]]    [[FB T101|Please Help]]