# ATAT101 Theory of Flight > # [[T101 Week 2| ◀️ ]] &nbsp;[[T101 Home| Home ]] &nbsp;[[T101 Week 4| ▶️ ]] &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; [[QR T101 Week 3| 🌐 ]] ># [[T101 Week 3#ATAT101 Theory of Flight|Week3]] >- [[T101 Week 3#Wing Shapes|Wing Shapes]] >- [[T101 Week 3#Aerodynamic Loads|Aerodynamic Loads]] >- [[T101 Week 3#Speed of Sound|Speed of Sound]] >- [[T101 Week 3#High Speed Flight|High Speed Flight]] >- [[T101 Week 3#Aircraft Axes|Aircraft Axes]] >- [[T101 Week 3#Primary Controls|Primary Controls]] >- [[T101 Week 3#Secondary Lift Devices|Secondary Lift Devices]] > [!jbPlus|c-blue]- Lesson Intro >### What > >In this lesson you will learn some fundamental concepts about the shapes of wings, and the effects of high speed on aerodynamics. Also, in last week's lesson you learned about the forces in play around an aircraft. This week we will learn how these forces are controlled by the pilot. > >### Why > >A technician must understand the physics and technology of how an aircraft works in order to understand how to maintain and repair it. When we are discussing flight controls, understanding how they work will help you to avoid mistakes that could have deadly consequences. You will use this knowledge throughout your aviation career. > >### Testing > >You will be tested on this material on 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. CLO 5. Explain flight controls including primary, secondary, and auxiliary controls ## Wing Shapes ### Planforms %%==[[Master QB1#Q00635|Q]]==%% There are a number of different shapes, known as planforms that a wing can have. A wing in the shape of a rectangle is very common on small general aviation airplanes. For airplanes that operate at high subsonic speeds, sweptback wings are common, and for supersonic flight, a delta shape might be used.  [[T101_3_001.png|➡]] ### Types of Wings  [[T101_3_002.png|➡]] #### Straight Wing %%==[[Master QB1#Q00636|Q]]==%% Straight wings are easiest to manufacture generally, and have good stall behaviours. #### Tapered Wing %%==[[Master QB1#Q00637|Q]]==%% %%==[[Master QB1#Q00638|Q]]==%% An elliptical shape or tapered wing can also be used, but these are harder to make and do not have as desirable a stall characteristic. Tapered wings do, however, have the highest theoretical structural efficiency. The taper refers to the [[chord]] at the root, and offers more strength and stiffness. This increases low speed drag slightly, but increases high speed drag considerably, especially in the transonic range. This thickness also provides an opportunity to store more fuel in the wing itself. #### Elliptical Wing The elliptical wing has the highest theoretical aerodynamic efficiency, but it is more difficult to manufacture. #### Swept Wing %%==[[Master QB1#Q00639|Q]]==%% Swept wings are used for aircraft that fly at higher speeds. One drawback is that they are harder to manufacture because they require high stiffness, and another is that they can handle poorly when near or in stalled flight. However, they do have significantly lower drag at high speed, and delay the formation of shock waves to increase the critical Mach number, more of which shortly in this lesson. ##### Increased directional stability Swept wings have a natural tendency to support directional stability. When yaw is encountered, the amount of wing facing the wind and its angle is different for each wing. The forward wing experiences more drag because it presents more frontal area to the wind. This then naturally counterbalances the yaw forces. ![[T101_3_003.png|350]] [[T101_3_003.png|➡]] ##### Can be swept forward A swept wing can be forward or backward sweeping, but large forward sweeping wings are very problematic for high speed flight. The stall characteristics inherent in this design makes it very unstable, requiring computer control to fly safely. At high speed a forward wing will also display dangerous flex effects if not built strong enough. #### Swing Wing A swing wing is a moveable wing that can adjust its angle back from perpendicular to the aircraft. Most often seen in high performance fighter jets, it combines the advantages of tapered wings and swept back wings by adopting these shapes as required in flight. #### Delta Wing Delta wings are swept back, but are more triangular in shape. They maintain the advantages of a swept wing, but have the advantage of higher structural efficiency and strength. ### Aspect Ratio %%==[[Master QB1#Q00640|Q]]==%% %%==[[Master QB1#Q00641|Q]]==%% The aspect ratio of a wing is the relationship between its [[T101 Week 2#Span|span]], or a wingtip to wingtip measurement, and the [[chord]] of the wing. If a wing has a long span and a very narrow [[chord]], it is said to have a high aspect ratio. A higher aspect ratio produces less drag for a given flight speed, and is typically found on glider type aircraft. This is another area of aviation where if you look a little deeper it becomes more complicated. In actual fact, the shape of the wing matters, and the math becomes more complicated. The area of the wing is important, and comes into the calculation of the Standard Mean Chord, which is the wing area divided by the wing span. So the more accurate description of the Aspect Ratio is the ratio of the wing span to the standard mean chord (and not just the chord, i.e. the straight measurement from leading edge to trailing edge). We do not need to concern ourselves with these distinctions to understand the broad differences between a high aspect ratio and low aspect ratio wing. [[Frequently Asked Questions]] At a given speed higher aspect ratios have lower drag and stalling speed. However, higher aspect ratio wings have structural challenges, higher parasitic drag, and poor maneuverability.  [[T101_3_004.png|➡]] The highest aspect ratio man-made wings are aircraft propellers, in their most extreme form as helicopter rotors. ### Effect of Aspect Ratio on Stall Characteristics %%==[[Master QB1#Q00642|Q]]==%% The aspect ratio contributes to stall characteristics. Drag produced by the wing is of the two types we have looked at: induced and parasitic. Vortices generated by the wingtips have a large impact on induced drag, because the vortices increase the velocity of the downwash aft of the wing, and thereby change the effective angle of attack. This causes more induced drag. By making the vortices less efficient, there is less induced drag. Two key components contribute here. By making the chord of the wing shorter (higher aspect) the vortex off the wingtip is less powerful. Secondly, the longer wingspan ensures that the effect of the vortices affect less of the wing's surface, as these vortices begin and are more concentrated at the tips. The dimensions of a high aspect wing contribute to a lower stall speed, and therefore a higher critical angle of attack.  [[T101_3_005.png|➡]] ### Angle of Incidence %%==[[Master QB1#Q00643|Q]]==%% %%==[[Master QB1#Q00644|Q]]==%% The angle of incidence of a wing is the angle formed by the intersection of the wing chord line and the horizontal plane passing through the longitudinal axis of the aircraft. Many airplanes are designed with a greater angle of incidence at the root of the wing than at the tip, and this is referred to as… #### Washout  [[T101_3_006.png|➡]] %%==[[Master QB1#Q00645|Q]]==%% This feature causes the inboard part of the wing to stall before the outboard part, which helps maintain aileron control during the initial stages of a wing stall. This makes entry into the stall condition quite gentle and controllable. It has a similar effect as the stall strips we saw last week. ### Dihedral and Anhedral %%==[[Master QB1#Q00646|Q]]==%% %%==[[Master QB1#Q00647|Q]]==%% %%==[[Master QB1#Q00648|Q]]==%% %%==[[Master QB1#Q00649|Q]]==%% %%==[[Master QB1#Q00650|Q]]==%% We looked at aircraft stability in the previous lesson. Most wings are angled up from the fuselage to the wingtip. This is known as dihedral. This helps lateral stability, also known as roll stability. Here's how it works. If the wings were completely level and flat out from the fuselage, in a turn, the aircraft would want to slip towards the lower wing, or the wing that dipped in the turn. There is nothing that would move the aircraft back to level flight, and so this design would make it a lot of work for the pilot to keep the aircraft steady and level. When the wings have dihedral, the air meets the low wing at a higher level of attack in a turn. This produces more lift, causing the low wing to want to lift the aircraft back to level flight. So dihedral provides a self-levelling effect. Some aircraft are actually too stable, and it is too much work to make the aircraft roll. In these cases, [[anhedral]], or downwards angled wings, are used to make the aircraft more maneuverable.  [[T101_3_007.png|➡]] ### T Tails %%==[[Master QB1#Q00651|Q]]==%% The tail assembly of an aircraft, including the horizontal and vertical stabilizers, the elevators and the rudder are known as the empennage. The vertical stabilizer can come in different variants. The T tail is one common variation, and it offers the advantage of being out of the slipstream of the wings, thus avoiding the turbulence produced by them. This gives the T tail better pitch control, especially useful on landing, as well as a better glide ratio. One disadvantage of this configuration is that the pitch control on propeller powered aircraft may be reduced because the elevators no longer have the propeller slipstream hitting them, reducing their effectivity. Another disadvantage is that this arrangement requires more strength and so it tends to be of heavier construction.  [[T101_3_008.png|➡]] ### Airfoil #### Cross Section The term [[airfoil]] often refers to the cross section of the wing. The different curves of the upper and lower camber have everything to do with lift and drag, and finding the proper balance has been a pursuit since the dawn of flight. These historical airfoils show you some differences, but all have the same basic shape. All attempt to disrupt the airflow in such a way as to leverage Bernoulli's principle to create lift.  [[T101_3_009.png|➡]] #### Symmetry in airfoils %%==[[Master QB1#Q00652|Q]]==%% An [[airfoil]] is considered to be symmetrical when the top camber and the bottom camber are the same. We will see in high speed flight that these types of airfoils experience the least movement of the centre of pressure and are thus useful at very high speeds. Symmetrical airfoils are used extensively in helicopters. #### Symmetry in aircraft ##### Most aircraft are symmetrical To widen our consideration for a moment from the wing shape to the overall aircraft shape, note the importance of symmetry. An aircraft viewed from above will most often have exactly the same profile on the left and right, that is, it will demonstrate symmetry on the longitudinal axis. This is primarily for the purposes of balanced handling, so that the aircraft behaves the same way in left and right turns. Sometimes the symmetry of an aircraft needs to be verified after a very hard landing, where measurements will be taken using precision devices to ensure the aircraft is not bent. ##### Don't forget the control surfaces When performing maintenance, it is critically important to follow your maintenance manuals. Control surfaces sometimes require rigging, and their range of motion must be correct to preserve the symmetry of the aircraft. For small single engine aircraft for instance, the rudder travel right and left may be the same, but multi-engine aircraft often have different requirements.  [[T101_3_010.png|➡]] ##### Problems when aircraft are not symmetrical Aircraft do exist that are not symmetrical, but they tend to pose difficult design challenges. You now have seen how the shape of the aircraft must be changed in relationship to its speed, and with these types of aircraft, every one of these issues is more complicated. ![[Pasted image 20210919114440.png|350]] ![[T101_3_011.png|350]] [[T101_3_011.png|➡]]  [[T101_3_012.png|➡]] ## Aerodynamic Loads ### Forces acting on an aircraft Dynamic refers to motion, and aerodynamic loads are forces acting on an aircraft when in flight. Normal flight can be broken into two categories. First is stable, level flight, that is, the aircraft is flying in a straight line, and is not accelerating, decelerating, ascending or descending. This is known as rectilinear flight. Remember we have described it as the aircraft having equilibrium, that is, the four forces of flight are balanced. The second, called curvilinear flight, is when an aircraft is travelling in a curve. When doing so, the balance of forces becomes much more complicated by the effects of centrifugal force, drag, thrust and all the other forces acting on an aircraft. #### Load Factor and "g" %%==[[Master QB1#Q00653|Q]]==%% %%==[[Master QB1#Q00654|Q]]==%% %%==[[Master QB1#Q00655|Q]]==%% Load factor is the summing and reduction all of these forces into a ratio of lift compared to weight. Typically, this is described using the term "g" for gravity. This indicates the perceived or apparent effects of gravity in a given situation. If an aircraft is travelling straight and level with no acceleration, a person inside would experience gravity as normal, and this situation would be described as 1g, or the normal effect of gravity. However, if an aircraft is turning, the load factor of the aircraft changes, and if this results in a ratio that is more lift than weight, a person inside would feel heavier, that is, would sense that there is more gravity. This would be indicated by a higher g number, with 2 being twice the effect of normal gravity. Put another way, if you held a one pound weight in your hand at 2g, you would perceive that the weight was two pounds. This is known as positive g. However, if the aircraft were to nose down, everything else being equal, a person inside would feel lighter, or would feel less of a gravitational pull. The load factor ratio here would be more weight than lift, and is known as negative g. It is interesting to note that the load factor also depends on the orientation of the vertical axis of the aircraft. If an aircraft is flying perfectly level, the g force, or load factor would be 1. However, if the aircraft were inverted and was flying upside down, but still in a straight level line, the load factor would be -1g. Passengers would sense this negative g as a sensation of being pulled up, opposite of gravity, i.e. towards the ceiling of the aircraft. #### Wing Loading The loads that a wing undergoes in flight is a critical design consideration. The 1g wing loading can be calculated as the aircraft gross weight divided by the total wing area. This tells how much work the wing is doing just to carry the aircraft. Wing loading can be a good indicator of how fast the aircraft will be, as high wing load aircraft must have higher takeoff and landing speeds. #### Fuselage You should also consider the fuselage in your thinking about load factors. The cabin in a high-flying aircraft is pressurized. This is another load that is felt by the fuselage in addition to the loads presented by centrifugal force, lift and normal gravity. Landing forces can be significantly higher than flight loads, and so the designed strength of the fuselage must take this into account. ## Speed of Sound ### Pressure changes %%==[[Master QB1#Q00656|Q]]==%% For the purposes of our aviation discussion, sound is pressure disturbances in the air. The common analogy of a rock being dropped in a pond and the resultant ripples in the water demonstrate the fluctuating differences in pressure. When an aircraft is in flight, every surface that causes a disturbance creates sound in the form of pressure waves that emanate or spread out from the surface of the aircraft. Being sound waves, these pressure waves travel at the speed of sound. ### Factors affecting the speed of sound The density of the air, its temperature, and impurities all affect the speed at which sound travels. In terms of temperature, the speed of sound increases as temperature rises. If you recall the bands of temperature in our [[atmosphere]], you can conclude that the speed of sound is not the same at all altitudes. #### Relationship between temperature and SoS  [[T101_3_013.png|➡]] %%==[[Master QB1#Q00657|Q]]==%% %%==[[Master QB1#Q00658|Q]]==%% Notice on this chart that the speed of sound stabilizes at around 36,000 ft. Remember we learned about the [[atmosphere]] that temperature also stabilizes at about 7 miles above the earth. Connect the dots to make sense of the chart: 1 mile = 5280 ft, 7 miles = 36,960 ft. Following the chart to higher altitudes shows what was explained earlier in our discussion about the [[atmosphere]], that the temperature begins to rise again somewhere between 16-20 miles above the earth. Note the effect on the speed of sound at these altitudes. We see that the relationship between supersonic speed and temperature is directly proportional.  [[T101_3_014.png|➡]] It is important to keep in mind then that the speed of sound is not a single speed, but is a dynamic descriptor, that is, its number as described in MPH or Km/hr will change with altitude or temperature. ### Mach Number %%==[[Master QB1#Q00659|Q]]==%% It is for this reason that Mach numbers are useful, in that they do give a single number for the speed of sound, even if that speed changes. The Mach number is a ratio of the true airspeed of an aircraft relative to the actual speed of sound in the given flight circumstance. So, Mach 1 represents the speed of sound (whatever that might be in the circumstances). Mach 2 represents twice the speed of sound. Fractions below 1 may also be used to indicate speeds approaching supersonic.  [[T101_3_015.png|➡]] ### Standard conditions speed of sound %%==[[Master QB1#Q00660|Q]]==%% Often when we refer to the speed of sound, it is assumed that we are referring to the speed of sound under standard conditions. This standard is an altitude of sea level and a temperature of 15 degrees celsius. Hopefully these two factors are familiar to you, and you thought immediately of a Standard Day, or Standard Atmosphere. Once again, we need something to refer to, hence the requirement for these standards. The speed of sound in this standard condition is taken to be: - 761 MPH - 661 Knots - 1224 km/hr ## High Speed Flight World War Two saw great leaps in aviation technology, and aircraft were capable of higher and higher speeds. By mid-war, P51s, Spitfires, and other types were reaching speeds close to that of sound, especially in dives. Pilots began to report control difficulties and unexpected problems which experts determined were due to flying too close to the speed of sound. In 1940, the National Advisory Committee for Aeronautics (NACA) commissioned Bell Aircraft to build a special research aircraft for the purpose of exploring the speed range near and beyond the speed of sound. It was considered better to do the research using an actual aircraft because the USA had no wind tunnels capable of operating at supersonic speed. The Germans did. The research aircraft Bell built was designated the X1. Two operational aircraft were finally built, although they did not fly until the war was over. By that time, the German research data had been captured and several of the questions the X1 was intended to answer were already known. Nevertheless, the X1 became the first aircraft to exceed the speed of sound in October 1947 when Chuck Yeager flew it to Mach 1.1  [[T101_3_016.png|➡]] ### Subsonic %%==[[Master QB1#Q00661|Q]]==%% An aircraft that is said to be in subsonic flight does not refer only to the speed of the aircraft. It means that all of the air flowing around the aircraft has a velocity of less than Mach 1. Remembering that air accelerates over certain surfaces, like the upper surface of a wing, note that if an aircraft is travelling at 500 mph, air travelling on the upper surface could be reaching 600 mph. To qualify as subsonic flight, none of these airspeeds would exceed Mach 1, and the speed of the aircraft will typically be around or a little over Mach 0.8. ### Transonic %%==[[Master QB1#Q00662|Q]]==%% %%==[[Master QB1#Q00663|Q]]==%% If an aircraft is flying such that some of the airflow around it is subsonic, and some is supersonic (exceeds the speed of sound), it is said to be in transonic flight. The air flowing on the upper surface of the wing can reach and exceed Mach 1, usually around halfway down the wing, and a shock wave will form that shoots out 90 degrees from the direction of airflow. It represents large changes in pressure, velocity and density of the airflow. This is called a normal shock wave, and it can cause stability problems in flight. It was the effect of these shock waves that surprised early pilots flirting with high speed flight. This shock wave can cause the boundary layer to be disrupted and interrupt the airflow along the wing. Because this is happening at some point between the leading and trailing edge, the centre of pressure can change, and this can cause the nose of the aircraft to pitch down. The speed at which this shock wave forms is known as the critical Mach number. As the shockwave forms, it moves from the front to the rear of the fuselage or [[airfoil]]. Because it is the aircraft itself that is creating the sound, once it reaches the trailing edge, the shockwave doesn't separate, but stays at the trailing edge. Transonic flight is normally between Mach 0.8 and 1.2. This shock wave can actually be seen if you know where to look. This video shows a shockwave on an airliner in transonic flight.[[V Shockwave Formation in Transonic Flight|🎞]] Here is an interesting video about airfoils as they approach transonic flight. [[V Transonic Flight|🎞]] #### Control surfaces Horizontal stabilizers also face the same effects when approaching supersonic speeds. They tend to lose their effectiveness under these conditions, and it was found that by making the angle of incidence adjustable, control could be maintained. The F86 was the first aircraft to use adjustable angle of incidence horizontal stabilizers to allow for control at supersonic speeds. The F86 Sabre was the backbone of the postwar jet propelled RCAF. We flew these jets in Europe in the 50's and 60's and even sent some to Vietnam. It is well known that the highest performing examples of this American designed aircraft were built in Canada. ### Supersonic %%==[[Master QB1#Q00664|Q]]==%% At supersonic speeds, all airflow around the aircraft exceeds Mach 1. The shock wave that was produced when entering transonic speeds moves all the way aft of the wing and stays at the trailing edge.  [[T101_3_017.png|➡]] Supersonic speed is the range from Mach 1.2 to Mach 5.0. You may wonder why speeds in excess of this are no longer considered supersonic, since they certainly are travelling faster than the speed of sound. This is because they have earned a new name, and speeds in excess of Mach 5 are known as… ### Hypersonic %%==[[Master QB1#Q00665|Q]]==%% An aircraft is going hypersonic when it exceeds Mach 5. ### What about Bernoulli? %%==[[Master QB1#Q00666|Q]]==%% %%==[[Master QB1#Q00667|Q]]==%% If we look at air very closely we find that the molecules are very far apart. When we compress those molecules the pressure goes up as the volume goes down. (You haven't forgotten [[T101 Week 1#Boyle's Law|Mr. Boyle]] already I hope. You have a test on this stuff coming up.) The spaces between the molecules get smaller. Therefore air is compressible. If we look at water very closely we find that the molecules are very close together. When we compress those molecules the pressure goes up, but the volume does not change because there is no space between the water molecules to allow them to get closer together. Therefore, water is not compressible. At speeds lower than Mach 0.3 air acts as if it's an incompressible fluid. Its velocity and pressure can change, but not its density. As long as it can still move, the space between molecules will effectively not change and therefore its density remains constant. So, at subsonic speed (less than Mach 1) air follows Bernoulli's principles as we have learned them.  [[T101_3_018.png|➡]] At supersonic speed (more than Mach 1) air can be compressed and its density increased. The space between the molecules can be and is reduced. So Bernoulli's principle is different for supersonic flows. At supersonic speed, the effects are the opposite.  [[T101_3_019.png|➡]] ### Shock Waves %%==[[Master QB1#Q00668|Q]]==%% A little more on the shock wave mentioned earlier. The sound coming from the aircraft surfaces are caused by the air being disturbed by the surface. Like the ripples in a pond, these sound waves radiate out from each source of disturbance. In subsonic flight, some of these waves radiate forward of the aircraft, and because they are travelling at the speed of sound, they travel ahead of the aircraft. However, when an aircraft starts to approach the speed of sound, these waves cannot escape the aircraft, and so they start to accumulate, first at the leading edges, and then further down the wing as described earlier. These accumulations of sound energy are what constitute a shock wave. When these waves reach the ground, you will hear the accumulation of sound energy as an explosive sound, known as a sonic boom.  [[T101_3_020.png|➡]] In this graphic, you can see that the sound waves radiate from each point where they are created. In image B, the aircraft is faster than the waves radiating out, and so they accumulate on the surfaces of the aircraft. In supersonic flight the aircraft is faster, so sound waves "pile up" at the nose of the [[airfoil]]. This creates shockwaves which are changes in the pressure and velocity of airflow. The shockwave concentrates the sound and increases its intensity. #### Three Types of Shockwaves There are three kinds of shockwaves. This graphic compares them, and explanations follow.  [[T101_3_021.png|➡]] #### Normal Shock Wave  [[T101_3_022.png|➡]] As speed increases, the shock wave changes. In transonic flight, the shock wave that forms and radiates out 90 degrees from the airflow is known as a normal shock wave. The velocity of air behind the shock wave is subsonic, and (remembering Bernoulli) its static pressure and density are higher. If such a wing were taken to supersonic speeds, the bluntness of the forward edge would also accumulate a shock wave, with resulting decreases in the lift of the whole wing. The air molecules pile up and create a wave detached and in front of the object, like a bow wave on a boat. Normal shock waves attach to curved areas of the wing where the airflow is just above Mach 1. They tend to form on the upper surface first at around Mach .75 and on the lower surface at around Mach .85. #### Oblique Shock Wave %%==[[Master QB1#Q00669|Q]]==%% %%==[[Master QB1#Q00670|Q]]==%% As we move into supersonic flight, we see differences in wing design. Wings that are designed for supersonic flight will typically have very sharp surfaces to reduce drag. When the aircraft is flying faster than the sound emanating from it, the leading edge and the trailing edge will have an accumulation of [[sound|sound energy]], and this energy radiates out. This is known as an oblique shock wave. It points downstream in the airflow, and the angle decreases as the speed increases, giving a flattening out effect.  [[T101_3_023.png|➡]] #### Expansion Wave  [[T101_3_024.png|➡]] %%==[[Master QB1#Q00671|Q]]==%% In this same graphic we see expansion waves. These are not sound shock waves. This is a different effect. Air at supersonic speeds acts like a compressible fluid. Being compressed, it seeks to expand wherever possible. As the shape of the wing changes from the leading edge to the trailing edge, it turns away from the airflow, and the supersonic air rushes to fill this gap and follow the new direction of airflow. At the point the direction of airflow changes, an expansion wave occurs. Behind the wave, the velocity increases and the static pressure and density decrease. We now know that this has a direct effect on the lift of the wing. In this case, the centre of lift is shifted towards the trailing edge. ### High Speed Airfoils #### Mach 0.6 Because the centre of lift changes as we pass through the speed of sound, transonic flight poses significant control and stability issues in flight, and therefore poses interesting design challenges as well. In a normal wing, the centre of lift, or the aerodynamic centre, is typically 25% of the way back from the leading edge. The effects of shock and expansion waves in transonic and supersonic flight the aerodynamic centre shifts back to 50% of the wing's chord. In these graphics, we see what would happen if an [[airfoil]] designed for subsonic flight were to go supersonic.  [[T101_3_025.png|➡]] At this fairly low Mach number, all the airflow over the wing is subsonic. #### Mach 0.82 (Critical Mach Number)  [[T101_3_026.png|➡]] %%==[[Master QB1#Q00672|Q]]==%% %%==[[Master QB1#Q00673|Q]]==%% Here we reach the critical Mach number, where some airflow is beginning to be supersonic, i.e. faster than Mach 1. This is where swept wings have an advantage in that they increase the critical Mach number, or in other words, they delay the separation of the airflow that is induced by the shock wave. It doesn't matter if the wing is swept forward or backward in regards to increasing the critical Mach number. However, an aft swept wing tends to have poor tip stall characteristics, but a forward swept wing has none of these problems. In theory, a forward swept wing has higher efficiency, but it is not seen frequently because it has other issues. A stall of this wing can cause a violent pitch up, and can be very unstable. Some of this instability can be addressed with computer flight control. It also needs to be extremely stiff because of dangerous effects when flexed. #### Mach 0.85  [[T101_3_027.png|➡]] Here we have surpassed the critical Mach number and a normal shock wave has formed on the top of the wing. Some airflow separation starts to occur behind the shock wave. #### Mach 0.88  [[T101_3_028.png|➡]] As we increase the speed, the normal shockwave moves aft and causes significant airflow separation. Also, the air travelling under the wing is now also at supersonic speeds, and so a normal shock wave starts to form under the wing as well. Again, behind this normal shock wave, the airflow is subsonic, with increased static pressure and density. #### Mach 0.95  [[T101_3_029.png|➡]] In this graphic both shock waves have continued to move aft and are now attached to the trailing edge. #### Mach 1.05  [[T101_3_030.png|➡]] Now the entire wing is moving faster than Mach 1 and a bow wave forms. The bluntness of the leading edge causes this accumulation of [[sound|sound energy]] , and causes a decrease in lift over the whole wing, demonstrating why this type of [[airfoil]] is not suitable for supersonic flight. If the leading edge were sharper, the bow wave would attach to the leading edge. #### Supersonic Airfoils %%==[[Master QB1#Q00674|Q]]==%%  [[T101_3_031.png|➡]] These two types of airfoils both have a sharp leading edge, reducing the lift decreasing properties of the bow wave. Notice particularly in the biconvex graphic how supersonic air still flows over the entire wing surface, unlike the conventional subsonic [[airfoil]] seen earlier. Also notice the symmetry of the supersonic [[airfoil]]. As Bernoulli's principle is reversed in supersonic flight, we must have a wing that will still deliver lift in both subsonic and supersonic flight. The shift in the centre of pressure must be minimized also, and because of this, supersonic airfoils are symmetrical. [[Supersonic Planes are Coming Back|🎞️]] ## Aircraft Axes ![[T101_3_032.png|350]] [[T101_3_032.png|➡]] ### Lateral, Longitudinal, and Vertical %%==[[Master QB1#Q00675|Q]]==%% %%==[[Master QB1#Q00676|Q]]==%% %%==[[Master QB1#Q00677|Q]]==%% %%==[[Master QB1#Q00678|Q]]==%% %%==[[Master QB1#Q00679|Q]]==%% %%==[[Master QB1#Q00680|Q]]==%% %%==[[Master QB1#Q00681|Q]]==%% %%==[[Master QB1#Q00682|Q]]==%% %%==[[Master QB1#Q00683|Q]]==%% %%==[[Master QB1#Q00684|Q]]==%% %%==[[Master QB1#Q00685|Q]]==%% %%==[[Master QB1#Q00686|Q]]==%% An aircraft in flight is controlled around one or more of three axes of rotation. The axes describe the rotational movement of the aircraft in three dimensions. These axes are the longitudinal, lateral, and vertical axes as shown in this graphic:  [[T101_3_033.png|➡]] #### Pitch, Roll and Yaw The movement around each of these axes has a specific aeronautical term. Motion around the lateral axis is called pitch. Rotation around the vertical axis is called yaw. Rotation around the longitudinal axis is called roll. ![[T101_3_034.png|350]] [[T101_3_034.png|➡]] ### Centre of Gravity >AKA Centre of Rotation %%==[[Master QB1#Q00687|Q]]==%% %%==[[Master QB1#Q00688|Q]]==%% These three axes of rotation intersect at the centre of gravity of the aircraft. For this reason, the centre of gravity is sometimes also referred to as the centre of rotation. An aircraft can be described as pitching up or down, which means that it is rotating around the centre of gravity on the lateral axis. ## Primary Controls ![[T101_3_035.png|350]] [[T101_3_035.png|➡]] ### Flight Control Surfaces #### Changing the shape of the aircraft …controls the aircraft %%==[[Master QB1#Q00689|Q]]==%% Flight controls allow the pilot to maneuver the aircraft and control it in all phases of a flight. Flight controls are most often associated with the wings, the vertical stabilizers, and the horizontal stabilizers, as these are where most flight controls are attached to the aircraft. These controls change the shape of the [[airfoil]] they are attached to, affecting its lift in predictable ways to allow the aircraft to be maneuvered. We saw the [[T101 Week 2#Four Forces of Flight|four forces of flight]] that act on an aircraft during flight. The primary control surfaces allow the pilot to change the direction and magnitude of these forces by changing their shape, and thereby control the speed, flight path and performance of the airplane. This control covers all three axes of rotation:  [[T101_3_036.png|➡]] ### Mechanical Control Systems %%==[[Master QB1#Q00690|Q]]==%% On smaller aircraft, the linkage from the flight control surface to the cockpit is often mechanical. These control systems give the pilot a lot of feedback from the air loads on the control surfaces, and the pilot will be able to feel the aircraft better, as he/she is directly connected to the aircraft. To route these control systems from the cockpit to the control systems sometimes takes a fair bit of creativity and ingenuity on the part of the designers. These systems are limited to smaller aircraft because as aircraft get larger, the forces on the control surfaces become larger as well. At a certain point, the limits of control of the aircraft would come down to the strength of the pilot. The two main systems are cables, and rods and tubes. #### Cable  [[T101_3_037.png|➡]] %%==[[Master QB1#Q00691|Q]]==%% %%==[[Master QB1#Q00692|Q]]==%% %%==[[Master QB1#Q00693|Q]]==%% %%==[[Master QB1#Q00691|Q]]==%% %%==[[Master QB1#Q00692|Q]]==%% %%==[[Master QB1#Q00693|Q]]==%% On smaller aircraft where the loads involved are not as much as larger aircraft, cables can be smaller and lighter while still being very strong. You may see some older galvanized cable that is coated in zinc for corrosion prevention on older aircraft, but any new installations will have stainless steel cables. Given the flexibility of cable, these systems are set up in a pull-pull configuration as in the above graphic to achieve control in both directions. Pushing a cable control would be as effective as pushing a rope, and thus pull-pull is employed. As with practically everything in aviation, cable systems can tend to get complicated. One factor for instance is the fact that cables are made of steel, as mentioned, but aircraft are made of aluminum. These two metals do not expand and shrink at the same rates given temperature changes. Small aircraft will tension the cables to allow for this, but larger aircraft will often have complicated tensioner systems to correct for differing expansion and contraction characteristics. Because of this, when you are doing maintenance action on a cable system, you will always set the cable tension specified corrected for the ambient temperature.  [[T101_3_038.png|➡]] Cable control systems use several devices, including main cables, terminals, and balance cables to close the loop required in a pull-pull system. Bell cranks and pulleys are used to go around corners, and bell cranks can also change the amount of motion in the turn. Often stops are employed to prevent overtravel of the controls. Maintenance of cable control systems includes checking the condition of the cables and connecting hardware and their security, and adjusting the tensions as required. Here is a tip: when cleaning these cables, make sure you use a cloth, so that if any strands of the cable are frayed, they will catch on the cloth and not on your hand. #### Rod and Tube  [[T101_3_039.png|➡]] %%==[[Master QB1#Q00694|Q]]==%% %%==[[Master QB1#Q00695|Q]]==%% Rod and tube systems are rigid, and therefore can employ a push-pull system. These systems tend to be heavier than cable control systems, tend to take up more space, and require other connections to go around corners. One advantage is that they are made of aluminum and thus expand and contract at the same rates as the fuselage, eliminating the need for tensioning systems. Torque tubes are more complex tubes, which can contain bell cranks to go around corners and may have friction devices included in their design. Some rod and tube systems can be adjusted by screwing the rod end in or out on threads. Some pushrods do not have this capability, and on others you may not be permitted to make adjustments. Maintenance of rod and tube systems consists of checking the condition and security of the rods and tubes and connecting hardware, and verifying freedom of movement, that is, checking that nothing is jammed. ### Powered Control Systems #### Hydraulic %%==[[Master QB1#Q00700|Q]]==%% %%==[[Master QB1#Q00696|Q]]==%% %%==[[Master QB1#Q00697|Q]]==%% %%==[[Master QB1#Q00698|Q]]==%% Much like the power steering in a car, hydraulic flight control systems offer control of large forces without requiring a lot of strength to operate. This makes the aircraft easier to control, but robs the pilot of the feedback from the control surfaces that help him to feel the aircraft respond to his/her inputs. There are several parts to a hydraulic flight control system. First is the mechanical linkage from the cockpit controls to the hydraulic controls. The hydraulic system has pumps driven by the engine, reservoirs, filters, pipes, accumulators, valves and actuators. The pumps generate hydraulic pressure for transmission via hydraulic lines to the actuators which convert this pressure into motion. This clearly separates the pilot from the aircraft from a feel perspective. This is not about pilot preferences for certain tactile sensations in flight, it has to do with them applying too much force to the controls because they cannot feel the resistance and great force being applied at the actual surface. This can result in the controls' force limits being exceeded and so sometimes artificial feel devices are installed which simulate the control forces for the pilot's benefit. These often use spring systems to give varying resistances at the control stick depending on airspeed. This creates a force gradient effect where the pilot feels greater pushback the farther they push the controls from neutral. It is artificial, but it helps greatly. If you have ever used a feedback wheel for sim racing or flying, you have seen this technology in action. Hydraulic systems are sometimes referred to as boosted systems, and they add complexity to an already complex system. They also add a significant amount of weight to the aircraft, due to the pumps and hydraulic lines primarily. This makes their use in smaller aircraft prohibitive. But the large control surfaces, and thus the large forces involved with larger aircraft makes them a necessity on larger aircraft. #### Fly by Wire %%==[[Master QB1#Q00699|Q]]==%% Fly by Wire can be considered a sub-type of hydraulic flight control systems. Rather than mechanical linkages from the cockpit to the hydraulic system, these systems use electrical signals. This can provide weight savings, and are very common on newer commercial and military aircraft. More advanced systems use computer control to deliver digital signals to actuate the hydraulic system. This enables the aircraft's computer systems to make complicated calculations based on information about airspeed, heading, altitude, attitude and more, to make optimal flight control movements. This becomes even more important for systems with combined controls (more of which later). This computer interpretation of the pilot's wishes expressed as control stick inputs allows for even a naturally unstable aircraft design to fly effectively. These systems must have redundant systems, as the computer control is the only path between the pilot and the flight control surfaces, and computer failure could be immediately catastrophic. These systems fly very complicated aircraft much better than any pilot ever could. They use all of the aircrafts computerized systems to make decisions that optimize the flight, including better fuel consumption and better reaction to flight events. They also prevent the pilot from inadvertently taking the aircraft out of a safe flight envelope, by limiting the motion of the flight controls to design specifications. Even more advanced are fly by light systems where fibre optics transmit light signals rather than electrical signals, typically enabling even more weight savings. #### Electrical Electrical flight controls are only used for secondary controls such as flaps, spoilers and trims. Electric motors typically need to be big and take a lot of power to move large loads quickly, thus they have so far been limited to slow and large loads, or small and quick ones. New motor technologies are emerging that may make primary flight control using electric actuation possible. Update: they're heeeeere…. ### Control Stops ...keep control surfaces within their range In order to ensure that the control surfaces do not exceed their designed range, control stops are used to constrain their motion. There are two types of controls stops. #### Primary %%==[[Master QB1#Q00701|Q]]==%% Primary control stops are installed at the control surface itself. They enjoy a mechanical advantage over secondary control stops. They are designed to prevent the control surface from traveling beyond its designed range, and potentially causing damage, or placing the aircraft outside of its safe envelope of flight. #### Secondary Secondary control stops are installed at the stick or pedals, that is, they operate at the pilot's end of the control system. These control stops are there to prevent the overloading of the control cables or push-pull rods in the control system, and to prevent the pilot's controls from damaging surrounding equipment.  [[T101_3_040.png|➡]] ### Ailerons %%==[[Master QB1#Q00702|Q]]==%% %%==[[Master QB1#Q00703|Q]]==%% %%==[[Master QB1#Q00704|Q]]==%% %%==[[Master QB1#Q00705|Q]]==%% %%==[[Master QB1#Q00706|Q]]==%% For all aircraft turning flight is achieved by use of differential lift. For example, more lift on one wing than the other results in a roll around the longitudinal axis.  [[T101_3_041.png|➡]] Ailerons control movement about the longitudinal axis which runs through the centre of gravity. This movement is known as roll. Ailerons are attached to the outboard trailing edge of each wing, and move opposite from each other. To increase their effectiveness by taking advantage of leverage, most small fixed wing aircraft have the ailerons located as close as possible to the wing tips. They are connected by cables, bellcranks, pulleys, tubes, or connected electrically or hydraulically to a control stick or wheel in the cockpit. Moving this control to the left will cause the left aileron to deflect upward and the right aileron to deflect downward. This changes the right wing's camber increasing the lift it produces while at the same time changing the left wing's camber to decrease its lift. The left wing drops, the right wing rises, and the aircraft rolls to the left. We saw control stops a moment ago. In the case of ailerons, controls stops would ensure that the upward motion of the aileron does not exceed 20°, and the downward motion does not exceed 14°. This is a typical range for ailerons.  [[T101_3_042.png|➡]] ### Rudder %%==[[Master QB1#Q00707|Q]]==%% %%==[[Master QB1#Q00708|Q]]==%% %%==[[Master QB1#Q00709|Q]]==%% The vertical axis of the aircraft runs from top to bottom through the middle of the airplane, passing through the centre of gravity. The rudder is attached to the vertical stabilizer and changes its shape in flight, causing rotation on the vertical axis, or yaw. It is attached via various means to the two pedals at the pilot's feet. Pushing the left or right pedal causes the aircraft to move its nose to the right or left.  [[T101_3_043.png|➡]] ### Elevator %%==[[Master QB1#Q00710|Q]]==%% %%==[[Master QB1#Q00711|Q]]==%% %%==[[Master QB1#Q00712|Q]]==%% %%==[[Master QB1#Q00713|Q]]==%% %%==[[Master QB1#Q00714|Q]]==%% %%==[[Master QB1#Q00715|Q]]==%% %%==[[Master QB1#Q00716|Q]]==%% %%==[[Master QB1#Q00717|Q]]==%% %%==[[Master QB1#Q00718|Q]]==%% %%==[[Master QB1#Q00719|Q]]==%% The elevator controls pitch, that is, movement about the lateral axis. Elevators are typically attached to the horizontal stabilizer, and are designed to change its shape in flight, precisely like the rudders and ailerons. Like the ailerons, the elevator is connected to the control in the cockpit. Pulling the control towards the pilot tilts the elevator up, decreasing the camber of the horizontal stabilizer, decreasing its lift, which creates a downward aerodynamic force. This causes the tail to drop and the nose to rise, rotating around the centre of gravity.  [[T101_3_044.png|➡]] ### Empennage #### Horizontal and Vertical Stability %%==[[Master QB1#Q00720|Q]]==%% %%==[[Master QB1#Q00721|Q]]==%% The empennage of an airplane is the assembly of the tail surfaces. Here you will find the vertical stabilizer and the horizontal stabilizer, or whatever compound controls may be employed. These surfaces all contribute to [[T101 Week 2#Aircraft Stability|aircraft stability]] in flight. #### Flight Controls A basic, and perhaps the most common configuration of an empennage is to have the rudder attached to the vertical stabilizer and the elevator connected to the horizontal stabilizer. Again, there are different configurations in use, but the functions of the empennage are always aircraft stability and control. ### Variations and Compound Flight Controls These primary control surfaces can appear with some variation. Some designs incorporate combination controls such as ruddervators, a combination of rudder and elevator controls, or involve the entire flight surface such as… #### Stabilators >Horizontal Stabilizer and Elevator …where the entire horizontal stabilizer can be moved for elevator functions. These are used on high speed fighter jets mostly, but also some civil patterns. It is also known as an all-flying tail. At higher Mach numbers, a stabilator has a higher efficiency, and can handle wide CG changes.  [[T101_3_045.png|➡]] #### Elevons >Elevator and Aileron In some high speed military aircraft, the stabilators will also perform aileron functions, and can then be called elevons. These types of compound controls require computer control as their movement is quite complex to accomplish the three functions assigned to them. #### Flaperons >Flap and Aileron %%==[[Master QB1#Q00721|Q]]==%% These compound flight controls are common on transport category aircraft. The range of motion of a flaperon is adjusted when more or less flap is required. They tend to be mounted towards the inboard.  [[T101_3_046.png|➡]] #### Ruddervators >Rudder and Elevator This combines the functions of a rudder and elevator. These will most often be seen (not that they are common) in a V Tail configuration. See the photograph of the Beechcraft Bonanza to see the angled configuration. In theory, this has less surface area than separate vertical and horizontal surfaces, but does not necessarily work that well in reality. The control systems for ruddervators are quite complicated.  [[T101_3_047.png|➡]] There are different designs for the primary surfaces themselves, where performance tweaks have been implemented in various ways. One example of this is differential ailerons, where the distance travelled by right and left ailerons is not the same so as to compensate for the differing lift of the right and left wings in a roll, and to reduce [[adverse yaw]]. ### Coordinated Turns While we have dealt with each axis and its control separately, most often these three axes are controlled in a coordinated fashion. For instance, using the rudder will point the nose from right to left, but this does not actually make the aircraft turn. It simply changes the angle the aircraft is facing, rather than its direction of flight. #### Rudder and Ailerons For a coordinated turn to occur, both rudder and ailerons are used together. If we take a right turn as our example, the ailerons are actuated by turning or pushing the control in the cockpit to the right, causing the right aileron to rise and the left aileron to drop. This is because the aileron action has changed the shape of each wing in a different way, affecting its lift characteristics. The aircraft begins to roll clockwise. The increased lift on the left wing also increases its induced drag, and this makes the aircraft yaw to the left. This is known as… #### Adverse Yaw %%==[[Master QB1#Q00722|Q]]==%% [[Adverse yaw]] is the natural and undesirable tendency for an aircraft to yaw in the opposite direction of a roll. Without any other help, the pilot would use rudder to correct this. The effect can be greatly minimized with ailerons deliberately designed to create drag when deflected upward such as a #### Frise Aileron  [[T101_3_048.png|➡]] %%==[[Master QB1#Q00723|Q]]==%% …which create parasitic drag on the upgoing aileron. Frise type ailerons are the most common design strategy to combat [[adverse yaw]], but there are mechanisms which automatically apply some amount of coordinated rudder as well. As the major causes of [[adverse yaw]] vary with lift, any fixed-ratio mechanism will fail to fully solve the problem across all flight conditions and thus any manually operated aircraft will require some amount of rudder input from the pilot in order to maintain coordinated flight. A small amount of right rudder is required to counteract this effect, and once the nose is pointed correctly in the right direction, the rudder can be recentred. #### Elevators %%==[[Master QB1#Q00724|Q]]==%% Because this is a very dynamic situation, with changing lift and drag parameters, the overall lift of the aircraft, or its centre of lift changing may cause the aircraft to pitch up or down. Elevator input by way of pushing or pulling on the control in the cockpit will ensure the aircraft stays in level flight while the other flight controls are moving. Typically however, the main force affected is the direction of lift as this diagram shows. This has increased the horizontal component of the lift, helping the aircraft to turn. But it has also decreased the vertical component of the lift, and so the aircraft drops. The pilot would have to pull back on the stick to increase the angle of attack to create more lift in compensation.  [[T101_3_049.png|➡]] ### Wing Loading %%==[[Master QB1#Q00725|Q]]==%% %%==[[Master QB1#Q00726|Q]]==%% Greater angles of bank require greater lift so that the vertical component of lift equals weight (to maintain altitude) and the horizontal component of lift equals centrifugal force for constant radius, coordinated turns. To turn, an aircraft must roll in the direction of the turn, increasing the aircraft's bank angle. Turning flight lowers the wing's lift component against gravity and hence causes a descent. To compensate, the lift force must be increased by increasing the angle of attack by use of up elevator deflection which increases drag. Turning can be described as 'climbing around a circle' (wing lift is diverted to turning the aircraft) so the increase in wing angle of attack creates even more drag. #### The tighter the turn, the more load on the wing The tighter the turn radius attempted, the more drag is induced. This requires that power (thrust) be added to overcome the drag. The maximum rate of turn possible for a given aircraft design is limited by its wing size and available engine power. The maximum turn the aircraft can achieve and hold is its sustained turn performance. As the bank angle increases so does the g-force applied to the aircraft, this having the effect of increasing the wing loading and also the stalling speed. The formula to determine wing loading is the aircraft's gross weight divided by the total wing area.  [[T101_3_050.png|➡]] The increased elevator input seen in this graphic cause much more downward force on the wings. This affects the stall speed as well, and so is an important design characteristic limitation for the pilot to know. In order to make a 3 degree per second turn at 500kts an aircraft would have to bank more than 50 degrees, which is uncomfortable due to the higher g's and risks pushing the wing loading past safe limits. This is why jet powered aircraft use a standard rate turn of 1.5 degrees per second. ## Secondary Lift Devices ![[T101_3_051.png|350]] [[T101_3_051.png|➡]] ### Moveable Secondary Controls #### Purpose of Secondary Controls - Improve performance - Relieve excessive control forces Secondary control surfaces include flaps, leading edge devices, spoilers, and trim systems. They serve two main functions, that of improving performance, and of relieving the pilot of excessive control forces. #### AKA - Also known as Auxiliary Controls - Also known as Auxiliary Lift Devices Secondary Controls are also known as auxiliary controls, or auxiliary lift devices and are typically moveable lift devices. To allow for safe take-offs and landings, an airplane would have to be designed to provide maximum lift at low speeds. This would limit the high-speed capabilities of the aircraft due to the high drag such a design would cause. Secondary controls allow the wing shape to be changed in flight, so that it can be optimized for low speed high drag flying during take-offs and landings, and also optimized for higher speed lower drag flight phases. #### Moveable and Fixed Lift Devices A lift device, also known as a high-lift device, or supplemental lift-modifying device, or lift augmentation device, or auxiliary lift device is a component or mechanism on an aircraft's wing that increases the amount of lift produced by the wing. These devices may be a moveable mechanism that is deployed when required. Other lift devices are fixed, and so are not controllable. We will look at some of these fixed devices next week. ### Flaps ##### Purpose of Flaps - Change camber of wing - Most common high lift device %%==[[Master QB1#Q00727|Q]]==%% Flaps are the most common high lift devices used on aircraft. They are attached to the trailing edge of the wing, and increase the lift as well as the induced drag when deployed. In effect, flaps allow us to fly with different wing shapes. When cruising at speed, the flaps are retracted, and the wing shape is optimized for high speed. To allow the aircraft to fly much slower for landing, the flaps can be deployed to change the camber of the wing, increasing its lift and the induced drag. If it were not for flaps, the wing would have to be designed with the safety of the landing as the first priority, which would be inefficient once cruising. We will look at a few different types of flaps. This graphic shows you some different characteristics of these types in terms of AOA and drag coefficient:  [[T101_3_052.png|➡]] Flaps are normally moved by an electric motor/jackscrew or hydraulically. #### Types of Flaps The four most popular types of flaps are the following: - Plain - Split - Slotted - Fowler Flaps #### Plain Flap  [[T101_3_053.png|➡]] - Increase lift by 50% %%==[[Master QB1#Q00728|Q]]==%% %%==[[Master QB1#Q00729|Q]]==%% %%==[[Master QB1#Q00730|Q]]==%% The plain flap is much as just described. When deployed, the camber of the wing changes, and so it not only provides greatly improved lift and drag but it also changes the centre of lift on the wing pitching the nose downward. Plain flaps typically increase lift by as much as 50%.  [[T101_3_054.png|➡]] #### Split flap  [[T101_3_055.png|➡]] Increase lift by 60% %%==[[Master QB1#Q00731|Q]]==%% %%==[[Master QB1#Q00732|Q]]==%% The split flap extends from the lower surface of the wing. It produces a slightly better increase in lift than a plain flap, capable of up to 60% more than the normal wing. However, it has considerably more drag because of the turbulence produced at the trailing edge. Both plain and split flaps when fully extended produce high drag with little additional lift. These flaps in particular cause large pitch changes, as they change the lift/drag characteristics drastically, changing the resultant lift vector. The split flap was invented by Orville Wright and James M.H.Jacobs in 1920.  [[T101_3_056.png|➡]] #### Slotted Flap  [[T101_3_057.png|➡]]  [[T101_3_058.png|➡]] ![[T101_3_059.png|350]] [[T101_3_059.png|➡]] - Increase lift by 65% %%==[[Master QB1#Q00733|Q]]==%% %%==[[Master QB1#Q00734|Q]]==%% The slotted flap is the most popular flap in use on aircraft today, both small and large. Slotted flaps increase lift more than plain or split flaps, often achieving 65% more lift. They change the camber so as to use boundary layer control to achieve their advantages. When the flap is deployed, it extends away from the wing, opening a slot between the wing and the flap. This allows high pressure air from the bottom of the wing to be directed to the top of the wing surface, increasing the speed of the boundary layer air, and delaying the airflow separation that decreases lift. ##### Slots and Slats? - slats make the slot We will see the terms slots and slats in this section. There is some flexibility in the use of these terms, but in general, a slat is an aerodynamic surface, either movable or fixed, which leaves an opening between itself and usually a main wing (or another slat). The opening is the slot. So, can you have movable slots? Yes, you will see the term used this way: a movable slat will create a movable slot.  [[T101_3_060.png|➡]] #### Fowler Flaps ##### Normal Fowler Flaps - 90% increase in lift  [[T101_3_061.png|➡]] %%==[[Master QB1#Q00735|Q]]==%% %%==[[Master QB1#Q00736|Q]]==%% Fowler flaps are a variation of the slotted flap. Its design changes the camber of the wing, as do the others, but it also increases the wing area. Instead of rotating down on a hinge like the others, it extends backwards on tracks. Because of this motion on deployment, the more general category for this type of flap is travelling flap. When first deployed, they add little drag, but by increasing the wing size, they increase lift significantly, up to 90% more than a normal wing. As it continues its deployment back and down, drag increases until at its maximum, like the other flaps discussed, it adds very little additional lift but maximum drag. ##### Double Slotted Fowler Flaps - 100% increase in lift %%==[[Master QB1#Q00737|Q]]==%% These flaps simply take the concept further, increasing the wing area more, and providing more boundary layer control. Double slotted Fowler flaps can increase the lift of a normal wing by up to 100%. ![[T101_3_062.png|350]] [[T101_3_062.png|➡]]  [[T101_3_063.png|➡]] ##### Triple slots - and more… Triple slotted flaps also exist, and there are other types of flaps in addition to the ones we have mentioned, although they are less common. They illustrate the constant drive to engineer more efficient aircraft and the ingenuity of those designing aircraft. #### Leading Edge Flaps/Slats - Even more lift - AKA Slatted Wing %%==[[Master QB1#Q00738|Q]]==%% The range of flight envelopes that flaps make possible, i.e. from as slow as possible to as fast as possible, is sometimes not enough. Leading edge devices also change the shape of the wing, changing its camber, and therefore increasing the lift. Sometimes the design is more to control the boundary layer. - Fixed or movable Sometimes leading edge flaps or slots are fixed. As we have seen with slotted flaps, they are a boundary layer control measure, redirecting air from the high pressure lower surface of the wing to the lower pressure higher surface, speeding up the boundary layer and promoting laminar flow. Note in the illustration that even though the slotted wing is a fixed device, because of the dynamic nature of flight, when we change the angle of attack, the effects of the leading edge flap change. Leading edge devices can provide an additional 40% lift compared to a normal wing. To help you get the terminology straight, leading edge slats create a slotted leading edge flap. They can be fixed or movable.  [[T101_3_064.png|➡]]  [[T101_3_065.png|➡]]  [[T101_3_066.png|➡]] #### Krueger Flaps ##### Lower camber, leading edge A Krueger flap is a flap that is hinged at the leading edge. Now that we are familiar with how an [[airfoil]] creates lift, we can easily see how a Krueger flap, by changing the lower camber, has a dramatic effect on the lift of the wing, even at small deflections. ![[T101_3_067.png|350]] [[T101_3_067.png|➡]] ![[T101_3_068.png|350]] [[T101_3_068.png|➡]] ### Trim Tabs #### Ground Adjustable Tabs %%==[[Master QB1#Q00739|Q]]==%% %%==[[Master QB1#Q00740|Q]]==%% %%==[[Master QB1#Q00741|Q]]==%% Aircraft are not always perfectly balanced, and this can be caused by uneven fuel burn, or passengers or cargo moving. This requires small movements of the control system to return the aircraft to balance. Other imbalances are less dynamic, and are simply the result of asymmetry of weight or load in the aircraft. Small, simple aircraft use fixed ground adjustable tabs to adjust for this. These are known as fixed tabs, or fixed trim tabs, or ground adjustable tabs.  [[T101_3_069.png|➡]] So these tabs adjust the shape of the flight surface, which affects the lift, in whichever direction. However, we know that speed is directly related to lift, and this is a shortcoming of ground adjustable tabs: they do not work the same at higher speeds. - Not effective at speed As aircraft go faster, fixed tabs no longer work well, and so trim tabs, pilot in-flight adjustable tabs, are required. #### Trim Tabs %%1%% %%==[[Master QB1#Q00742|Q]]==%% >Pilot adjustable in flight  [[T101_3_070.png|➡]] ### Balance Tabs >Counteract the forces of control surfaces %%==[[Master QB1#Q00743|Q]]==%% We said earlier that one of the purposes of secondary flight controls is to relieve the pilot of excessive control forces. As an aircraft flies faster, the forces on the primary control surfaces increase, and this makes them harder to move. Balance tabs address this by moving in the opposite direction of a primary control surface. They are coupled with a rod such the tab generates a force that reduces the force applied by the pilot, making the aircraft easier to fly. Balance tabs look at first glance to be much like trim tabs. They are not, however, controllable directly by the pilot. They simply act like a counterweight.  [[T101_3_071.png|➡]] ### Other Tabs #### Servo Tabs ##### Decrease control forces - Pilot control moves the tab, not the control surface %%==[[Master QB1#Q00744|Q]]==%% Servo tabs are similar in operation and appearance to "regular" trim tabs but only the servo tab moves in response to the pilot's controls, airflow forces on the servo tab move the primary control surface.  [[T101_3_072.png|➡]] Servo tabs are used on large aircraft such as the Boeing 727 to back up the hydraulic system.  [[T101_3_073.png|➡]] #### Anti-Servo Tabs ###### Increase control forces %%==[[Master QB1#Q00745|Q]]==%% Anti-servo tabs are similar in operation & appearance to servo tabs but they move in the same direction as the primary control and increase the control forces. All flying stabilizers are usually pivoted near the center of pressure, and have little control feel. Anti-servo tabs add this when required. These can also be known as Anti-balance tabs.  [[T101_3_074.png|➡]] ![[T101_3_075.png|350]] [[T101_3_075.png|➡]] #### Spring Tab ##### Like a servo tab >But with automatic changing for high speed Spring tabs are a form of servo tab, but with a spring added. This spring holds the tab in line with the control at low speeds, but gives under aerodynamic loads at high speeds and provides servo-like function.  [[T101_3_076.png|➡]] ### Control Surface Balances #### Static balanced for straight and level flight %%==[[Master QB1#Q00746|Q]]==%% %%==[[Master QB1#Q00747|Q]]==%% The control surfaces we have looked at may or may not tend to be balanced, depending on the design and their weight and hinging points. In order to prevent the control surfaces from deflecting in the presence of wind gusts, weights are added forward of the hinge point so that they tend to sit levelled naturally. This balancing of weight is called static balancing. Primary flight controls in their neutral positions are static balanced for cruising. In other words, when moveable surfaces are not being moved, the aircraft will proceed with straight and level flight. #### Counterweight >Prevents Flutter ![[T101_3_077.png|350]] [[T101_3_077.png|➡]]  [[T101_3_078.png|➡]] %%==[[Master QB1#Q00748|Q]]==%% %%==[[Master QB1#Q00749|Q]]==%% %%==[[Master QB1#Q00750|Q]]==%% If static balancing is not correct, the controls will tend to deflect when hit by gusts, and flutter can occur. This video clearly shows why this is an undesirable occurrence. [[V Flutter|🎞]] The static balance weights shown earlier are critical for preventing flutter. #### Dynamic Balances ##### Horn Balances Horns are extensions of the control surface at the leading edge which perform two functions. By providing an aerodynamic counterforce to the main control surface they reduce the force required by the pilot to move the surface, especially at high speeds. They also provide dynamic balancing by resisting the effects of gusts and reducing flutter. This function is often helped by the installation of weights in the horns. ![[T101_3_079.png|350]] [[T101_3_079.png|➡]]  [[T101_3_080.png|➡]] #### Balance and Stability To alleviate confusion between balance and stability, it may be helpful to see balance as the aircraft's natural tendency to do as asked, in straight and level flight, in turns, and while climbing and descending. Stability is the aircraft's tendency to return to this balanced condition once disturbed. ## Conclusion In this lesson we covered the following topics: - [[T101 Week 3#Wing Shapes|Wing Shapes]] - [[T101 Week 3#Aerodynamic Loads|Aerodynamic Loads]] - [[T101 Week 3#Speed of Sound|Speed of Sound]] - [[T101 Week 3#High Speed Flight|High Speed Flight]] - [[T101 Week 3#Aircraft Axes|Aircraft Axes]] - [[T101 Week 3#Primary Controls|Primary Controls]] - [[T101 Week 3#Secondary Lift Devices|Secondary Lift Devices]] You can demonstrate your understanding of the material in this lesson by answering the questions in the corresponding weekly practice quiz correctly. You have now covered all of the material to be tested on the [[T101 Intro#The questions on quizzes and tests are distributed as follows|midterm test]]. Feel free to practice with the quizzes until you are confident of your full understanding. And, as always, if the course supports are not helping you to understand, contact your prof right away. > # [[T101 Week 2| ◀️ ]] &nbsp;[[T101 Home| Home ]] &nbsp;[[T101 Week 4| ▶️ ]] &nbsp; &nbsp; [[QR T101 W3| 🌐 ]] &nbsp; &nbsp;[[FB T101|Please Help]]