# ATAT101 Theory of Flight > # [[T101 Week 4| ◀️ ]]  [[T101 Home| Home ]]  [[T101 Week 6| ▶️ ]]               [[QR T101 Week 5| 🌐 ]] ># [[T101 Week 5#ATAT101 Theory of Flight|Week 5]] >- [[T101 Week 5#Intro to Rotary Wing Flight|Intro to Rotary Wing Flight]] >- [[T101 Week 5#Rotary Wing Concepts and Terminology|Rotary Wing Concepts and Terminology]] >- [[T101 Week 5#Forces of Flight – Rotary|Forces of Flight – Rotary]] >- [[T101 Week 5#Rotary Wing Flight Controls|Rotary Wing Flight Controls]] >- [[T101 Week 5#Dissymmetry of Lift|Dissymmetry of Lift]] >- [[T101 Week 5#Anti-Torque|Anti-Torque]] >- [[T101 Week 5#Coriolis Effect|Coriolis Effect]] >- [[T101 Week 5#Rotary Wing Stability|Rotary Wing Stability]] >- [[T101 Week 5#Types of Rotor Heads|Types of Rotor Heads]] >- [[T101 Week 5#Auto-Rotation|Auto-Rotation]] > [!jbPlus|c-blue]- Lesson Intro >### What > >In this lesson we will shift to looking at the Theory of Flight as it applies to rotary wing aircraft. > >### Why > >The principles you have learned about flight do apply to rotary flight, but that application of theory is different in many ways. Your career may well take you to rotary wing outfits. Your knowledge of how these machines work will enable you to maintain and troubleshoot these aircraft more effectively. > >### Testing > >You will be tested on this material on Assignment 4 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 ## Intro to Rotary Wing Flight ![[T101_5_001.png|400]] [[T101_5_001.png|➡]] ### First Flights The word helicopter comes from the Greek words helix, which means spiral, and pteron which means wing. So the name itself indicates it most notable feature. #### Germany 1936 The first practical helicopter flew in Germany in 1936. It was capable of carrying its pilot, but not much more. ![[T101_5_002.png|400]] [[T101_5_002.png|➡]]  [[T101_5_003.png|➡]] #### America 1940 In the United States, the VS-300 flew in 1940. This was the first successful single rotor helicopter to use a single plane tail rotor.  [[T101_5_004.png|➡]] #### First Certification ##### USA 1946 %%==[[Master QB1#Q00777|Q]]==%% The Bell 47 was the first helicopter certified for civilian use in 1946. More than 5,600 Bell 47s were produced.  [[T101_5_005.png|➡]] You may find this 27 min. video about the history of Bell Helicopter interesting. It explains several rotary wing principles with vintage footage of early experiments. [[V History of Bell Helicopter|🎞]] ### Differences of Rotary Flight #### The theory is the same ![[T101_5_006.png|400]] [[T101_5_006.png|➡]] We recall from previous lessons chord, span, angle of incidence, relative wind, AOA, lift, drag, thrust, weight, stall, center of pressure, and [[airfoil]] shapes. We saw that understanding relative wind and its effects on fixed wing flight is crucial to understanding how the surfaces of an aircraft make it fly. These same principles and factors that affect fixed wing flight explain how helicopters fly. #### The application is different But rotary flight has some significant differences. ![[T101_5_007.png|400]] [[T101_5_007.png|➡]] In a helicopter, the wings move, while the aircraft itself may or may not. The theories that you learned about still apply, but it is implemented differently in a helicopter. Rotary flight is a complicated and dynamic process. Consider a simple example. We have a helicopter moving forward, while the wings rotate above. This means that the wings constantly see a different relative wind, depending on whether they are moving forward or backward as they rotate. #### Constant change of relative wind %%==[[Master QB1#Q00778|Q]]==%% %%==[[Master QB1#Q00779|Q]]==%% Just like on fixed wing aircraft, the relative wind is how the wind hits the lift creating surfaces. Helicopter blade position is constantly changing, and so therefore does the relative wind. Additionally, rotor heads move and bend, also affecting the relative wind. ## Rotary Wing Concepts and Terminology ![[T101_5_008.png|400]] [[T101_5_008.png|➡]] ### Basic Parts of a Helicopter ![[T101_5_009.png|400]] [[T101_5_009.png|➡]] ![[T101_5_010.png|400]] [[T101_5_010.png|➡]] These diagrams show common basic parts of a helicopter. You should know all of these terms to help in our discussion of rotary wing flight. ### Rotary Wing Concepts #### Reference Plane %%==[[Master QB1#Q00780|Q]]==%% You must understand the word plane in this context. In geometry, a plane is a surface generated by a straight line moving at a constant velocity with respect to a fixed point. So, when we talk about planes, we are talking about the paths taken by rotating objects. A hub is the central part of a circular object. A rotor in helicopter talk is a wing that moves. A rotor hub is the mechanism that joins the blades of the rotor. The reference plane of a main rotor hub is the path of rotation perpendicular to the hub. You may imagine a disc like shape that is 90° to the vertical shaft holding the hub. Technically it is different than the plane of rotation which can vary from perpendicular. In stable flight, these two planes are essentially the same. #### Pitch Angle %%==[[Master QB1#Q00781|Q]]==%% ![[T101_5_011.png|400]] [[T101_5_011.png|➡]] Pitch angle is the angle between the [[chord line]] of the [[airfoil]] or blade and the plane of rotation. Note that the plane of rotation and the reference plane, while parallel in this example, are two separate planes. #### Angle of Attack %%==[[Master QB1#Q00782|Q]]==%% ![[T101_5_012.png|400]] [[T101_5_012.png|➡]] When we talk about angle of attack for rotary wing aircraft, it is effectively the same definition as it was for fixed wing. The angle of attack is the angle between the [[chord line]] of the [[airfoil]] and the relative wind. We will see that this concept is complicated by the ever-changing relative wind situation. #### Tip Path Plane %%==[[Master QB1#Q00783|Q]]==%% ![[T101_5_013.png|400]] [[T101_5_013.png|➡]] The tip path plane is the path the tips of the blades travel in their rotation. It is also sometimes called the rotor disc, which is as descriptive. #### Track and Balance The plane of rotation on a balanced helicopter is flat. When the blades are out of track, usually due to differences in lift, the tips rise and fall in their rotation. ![[T101_5_014.png|400]] [[T101_5_014.png|➡]] #### Coning %%==[[Master QB1#Q00784|Q]]==%% ![[T101_5_015.png|400]] [[T101_5_015.png|➡]] Coning is the warping of the plane of rotation as a result of the weight of the aircraft. The blades bend under the load of the weight, and the more the weight, the more the bend. ### Rotor Blade Characteristics #### [[Airfoil]] Shape %%==[[Master QB1#Q00785|Q]]==%% %%==[[Master QB1#Q00786|Q]]==%% ![[T101_5_016.png|400]] [[T101_5_016.png|➡]] We have seen how the centre of pressure or centre of lift moves around as the angle of attack changes. Especially on asymmetrical airfoils, it changes dramatically. This is experienced most strongly as movement around the lateral axis, and so this moving centre of lift affects longitudinal stability. This can be addressed in several ways on a fixed wing aircraft as we have seen, but not so with helicopters. Changes in the centre of lift can cause instabilities in the rotor, and because this effect is less with symmetrical airfoils, most helicopters are usually this shape. #### Varying Speeds of the Blade = Asymmetric Lift ##### Different parts of the wing travel at different speeds %%==[[Master QB1#Q00787|Q]]==%% %%==[[Master QB1#Q00788|Q]]==%% So we have established that a helicopter rotor blade is an [[airfoil]], acting like the wing on an airplane being driven through the air to create lift. In the case of a fixed-wing aircraft in normal flight, the entire wing travels forward through the air at the same speed. In the case of helicopter rotor blades, the tips of the blade travel very fast through the air while the parts near the rotor hub travel much more slowly. Because lift increases with speed, the outermost sections of the rotor blades generate more lift than those parts closer to the rotor hub. So a rotor blade is said to be an asymmetric generator of lift, because of the difference in lift generated along its length. Helicopter manufacturers try to reduce this differential effect (that is, aim for more equality of lift along the blade length). This is done in two possible ways. ##### Geometric Twist %%==[[Master QB1#Q00789|Q]]==%% Twisting the blade so that the blade root near the hub presents a higher angle-of-attack and thus higher lift is called geometric twist. ##### Blade Taper %%==[[Master QB1#Q00790|Q]]==%% Another method to smooth out the effects of varying lift along a blade's length is to taper a blade toward its tip, which reduces its surface area, in turn reducing its lift as you move to the bladetip. When the helicopter is travelling forwards with respect to the [[atmosphere]], a further phenomenon comes into play, dissymmetry of lift, which we will discuss in more detail shortly. Do not confuse asymmetrical lift, the differing lift along a blade, with dissymmetry of lift. ## Forces of Flight - Rotary ![[T101_5_017.png|400]] [[T101_5_017.png|➡]] Even a quick consideration of the differences between rotary flight and fixed wing flight would reveal that thrust is developed differently with a helicopter. While fixed wing aircraft typically push air backwards along the longitudinal axis of the aircraft, a rotary wing aircraft typically does not. ### Forward Flight #### The four forces remain In forward level flight, the four forces you became familiar with in fixed wing theory are very similar. ![[T101_5_018.png|400]] [[T101_5_018.png|➡]] #### Pitch Axis / Feathering Axis %%==[[Master QB1#Q00791|Q]]==%% %%==[[Master QB1#Q00792|Q]]==%% ![[T101_5_019.png|400]] [[T101_5_019.png|➡]] We know that an increase of relative wind speed will create more lift. We also know that increasing the angle of attack of an [[airfoil]] increases lift. Now recall Newton's law of inertia. If we wanted to generate more lift by increasing the speeds of the blades, we would require a lot of power to overcome inertia to speed or slow the blades. Because of this, we typically want to keep rotor speed constant in a helicopter, and so to increase or decrease lift, we must adjust the angle of attack. For this reason, each blade on a helicopter main rotor will have the capability of rotating on the pitch axis, also known as the feathering axis, in order to change the pitch angle of the blade, and thus changing the angle of attack. It may be easy to confuse the terms pitch angle and angle of attack, but if you go back to the definitions you can be clear. To recap: pitch angle is the angle between the [[chord line]] and the plane of rotation, while angle of attack is the angle between the [[chord line]] and the relative wind. Since a helicopter can fly in any direction, rather than just forward with a fixed wing aircraft, these two terms may often be different. ### Hovering, climbing, and descending #### Forces along the vertical axis %%==[[Master QB1#Q00793|Q]]==%% When a helicopter is hovering, climbing or descending, the resultant force vectors are quite unlike fixed wing, as all forces are oriented vertically in relation to the aircraft. This graphic shows hover and vertical ascent vectors, while descending vectors would still be vertically oriented, but of varying force. ![[T101_5_020.png|400]] [[T101_5_020.png|➡]] ### Other Forces #### Weight The weight of the aircraft is transferred through the mast to the yoke and then the blades. #### Induced Drag %%==[[Master QB1#Q00794|Q]]==%% ![[T101_5_021.png|400]] [[T101_5_021.png|➡]] We know that induced drag is the byproduct of lift. This of course affects helicopter airfoils as well. And, as with any [[airfoil]], increased lift makes more induced drag. You may find it interesting to note that a helicopter in a hover basically has only induced drag. There is very little of the other types of drag associated with dragging a fuselage through the air, since it is stable. The only drag that affects the balance of forces is the induced drag from the creation of lift. #### Thrust On a helicopter there is no propeller or turbine to provide forward motion to bring air across the wings. To generate lift, the thrust is applied to the rotor blades. The thrust required depends on the lift the rotor is required to generate. There is some debate as to the nature of thrust and lift in a helicopter. Some insist that there are two forces here, and that they have different vectors. This method makes it easy to understand that when moving from a hover, where both vectors point up, to forward flight, the thrust vector moves forward, but at the expense of the upward lift vector. Others simplify the two vectors into one, and imagine the helicopter moving from hover as changing the lift/thrust resultant vector to something off vertical, causing forward motion, while still keeping the aircraft aloft. #### Centrifugal Force %%==[[Master QB1#Q00795|Q]]==%% ![[T101_5_022.png|400]] [[T101_5_022.png|➡]] The spinning motion of the rotor blades causes centrifugal force which pulls the blades away from the rotor head. Rotor blades will typically droop at rest, but once spinning, they will straighten as a response to this centrifugal force. #### Lift and Centrifugal Force %%==[[Master QB1#Q00796|Q]]==%% ![[T101_5_023.png|400]] [[T101_5_023.png|➡]] In flight, rotor blades are actually rarely straight and level, but the centrifugal forces are always in play. But as the blade spins, it also generates lift, and that force is directed at right angles to the blade. This pulls the blade in an upwards direction, and in conjunction with the centrifugal force leads to a resultant lift vector upwards and away from the blade. #### Ground Effect %%==[[Master QB1#Q00797|Q]]==%% %%==[[Master QB1#Q00798|Q]]==%% Another unique effect on lift is experienced by helicopters hovering within half of the rotor diameter from the ground. The rotors are moving more air down than can escape from under the helicopter which provides extra lift in a hover.  [[T101_5_024.png|➡]] Explained another way, the air underneath the aircraft becomes more dense and forms a cushion beneath the aircraft. Moving more than three to five miles per hour causes the loss of this effect, and because the aircraft is so low to the ground (or would not be under this effect) the pilot must immediately correct for this loss of lift. #### Precession %%==[[Master QB1#Q00799|Q]]==%% Gyroscopic precession is an important force in helicopter flight dynamics. Remember the effects of precession from [[T101 Week 2#Gyroscopic Precession|earlier in the course]]. We discussed the effects of force on a rotating disc, and how the resultant force vector of force applied will appear 90° from the force in the direction of rotation. The rotor path qualifies as a rotating disc, and thus displays the same properties. ![[T101_5_025.png|400]] [[T101_5_025.png|➡]] ![[T101_5_026.png|400]] [[T101_5_026.png|➡]] This means that when we apply forces to the rotor disc by adjusting the blades, the deflections that are caused occur 90° later than the inputs. This must be understood when looking at how the motion of a helicopter is controlled. Here are some videos that may help you to understand torque and precession. This one demonstrates precession nicely: [[V Gyroscopic Precession|🎞]]. This one is a fun video using tinkertoys and an RC model to explore a few related issues around gyroscopic precession: [[Are Helicopters Gyroscopes|🎞]]. This videographer also has a whole series on helicopters on [Youtube](https://www.youtube.com/watch?v=WdEWzqsfeHM&list=PL6CECC2E56B68A2C3). #### Vibration %%==[[Master QB1#Q00800|Q]]==%% ##### Caused by rotational forces While not a force per se, vibration is a major issue on all rotary aircraft. Anything that spins will generate vibration, and perfectly balanced machinery is very hard to achieve. The main rotor is the largest spinning component, but the tail rotor also contributes to overall vibration. Actually, anything rotating on the aircraft, including rotations in the engine, will contribute to vibration. Vibration in the airframe can create resonances or harmonics that can quickly fatigue components and cause their catastrophic failure. ##### Vibration control Various types of equipment are used to balance rotating components and reduce vibration, and more advanced systems are computer controlled.  [[T101_5_027.png|➡]] ## Rotary Wing Flight Controls ### Direct Rotor Head Tilt In some helicopters, the plane of rotation is adjusted by tilting the entire mast.  [[T101_5_028.png|➡]] ### Separate Blade Control #### Typical Flight Controls #### Swash Plate %%==[[Master QB1#Q00801|Q]]==%% %%==[[Master QB1#Q00803|Q]]==%% A swash plate allows for the adjustment of each blades pitch at different points in its rotation. This allows the pilot to control the direction of the rotor thrust vector. The swash plate is two concentric disks or plates. One plate rotates with the mast, connected by idle links, while the other does not rotate. The rotating plate is also connected to the individual blades through pitch links and pitch horns. The non-rotating plate is connected to links that are manipulated by pilot controls—specifically, the collective and cyclic controls. The swash plate can shift vertically and tilt. Through shifting and tilting, the non-rotating plate controls the rotating plate, which in turn controls the individual blade pitch. #### Cyclic %%==[[Master QB1#Q00802|Q]]==%% %%==[[Master QB1#Q00804|Q]]==%% %%==[[Master QB1#Q00805|Q]]==%% The cyclic is used to control the main rotor in order to change a helicopter's direction of movement. While the aircraft is hovering, the cyclic controls the movement of the helicopter forward, backward and laterally. When in forward flight however, the cyclic behaves much more like a typical fixed wing aircraft, with left and right motions producing roll, and forward and backward motions affecting pitch. #### Collective %%==[[Master QB1#Q00806|Q]]==%% %%==[[Master QB1#Q00807|Q]]==%% We have learned that it is better to adjust the angle of attack rather than the speed of the blades to increase or decrease lift. This is accomplished by the pilot by the use of the collective (pitch) control. Collective changes the pitch angle of all of the main blades collectively, i.e. together and at the same time, and independent of their position. Like cyclic, the collective behaves a bit differently depending on the state of the helicopter. In level flight, an increase of collective increases the lift and the helicopter will climb. If the helicopter is tilted forward, by the use of the cyclic, the collective will increase the lift, but this time in a more forward direction, causing forward acceleration, as well as some upward lift. Now recall that if we increase the lift, we also increase the induced drag. To overcome this, an increase in throttle is required to prevent the rotor blades from slowing down. #### Tail Rotor Pedals %%==[[Master QB1#Q00808|Q]]==%% Tail rotor pedals are used to adjust the pitch of the tail rotors to compensate for main rotor torque. We will look at these torque forces in a bit more detail shortly. #### Synchronized elevators %%==[[Master QB1#Q00809|Q]]==%% The purpose of synchronized elevators is to increase the fore/aft Centre of Gravity (CofG) range by lowering the tail as the aircraft accelerates. This reduces some of the induced drag caused by the increased lift of the main rotors. It also reduces the drag of putting most of the fuselage vertical to the relative wind. Most synchronized elevator systems are automatically controlled. #### Boosted controls %%==[[Master QB1#Q00810|Q]]==%% Like larger fixed wing aircraft, it is advantageous to reduce the forces the pilot has to deal with in larger helicopters by boosting the power applied to the control surfaces with the use of hydraulics. Boosted controls are also known as powered controls. #### Droop compensation %%==[[Master QB1#Q00811]]==%%  [[T101_5_029.png|➡]] We have seen that we want to keep rotor speed constant. But as we increase the collective, the torque required from the engine increases, and so the rotor speed drops. A droop compensator will automatically increase or decreases engine power as the collective is changed to reduce rotor speed changes. In smaller helicopters, this is simply accomplished by the pilot controlling the throttle on the collective control. This graphic puts all of these controls together to show their effects in flight. ![[T101_5_030.png|400]] [[T101_5_030.png|➡]] ## Dissymmetry of Lift %%==[[Master QB1#Q00812|Q]]==%% %%==[[Master QB1#Q00813|Q]]==%% %%==[[Master QB1#Q00814|Q]]==%% %%==[[Master QB1#Q00815|Q]]==%% %%==[[Master QB1#Q00816|Q]]==%% ![[T101_5_031.png|400]] [[T101_5_031.png|➡]] As a helicopter blade rotates, there is more relative wind over the blade as it advances, and less as it retreats. ![[T101_5_032.png|400]] [[T101_5_032.png|➡]] As the advancing blade makes more lift, it tends to rise up, which changes the angle of the resultant lift vector, which decreases its lift. The retreating blade creates less lift, which leaves it in a more horizontal position which effectively increases its lift, as the resultant force vector is more vertically oriented. This results in lift that is not symmetrical around the rotor disc, or the plane of rotation. This dissymmetry of lift is also caused by other factors. In the previous lesson we looked at how the cyclic and the collective change the pitch angle of the blades to control lift. In the case of the cyclic, this pitch is changed separately for each blade and so the angle of incidence is changed differentially. This creates a differential lift in the rotor system. This changes the attitude of the rotor system. Note that it doesn't change the overall lift produced by the rotor system. It simply produces more on one side than the other. Note that in a hover, there is no dissymmetry of lift, the rotor system lifts equally throughout the disc. #### Flapping ##### Response to differential lift %%==[[Master QB1#Q00817|Q]]==%% This dissymmetry of lift causes the blades to move up or down in response to this differential lift, trying to compensate for it. Remember that a single blade will, when acted upon by the cyclic control, produce more and less lift as it rotates. So, now imagine what the blade wants to do when it's angle of incidence is increased on one half of the rotation, and decreased on the other. This up and down motion is flapping. This difference in lift is known as dissymmetry of lift, and the up and down motion as the blade rotates is known as flapping. Remember that in a hover, there is no dissymmetry of lift, and therefore no flapping. ##### Flapping Hinge %%==[[Master QB1#Q00818|Q]]==%% ![[T101_5_033.png|400]] [[T101_5_033.png|➡]] Rather than transfer the upward and downward forces to the hub and the mast, a flapping hinge at the root of the blade is used to control this dissymmetry of lift by allowing the blade to move in response to these forces rather than transmitting them into the mast and aircraft. ### Translating Tendency #### Tail rotor causes drift %%==[[Master QB1#Q00819|Q]]==%% Another dissymmetry experienced by helicopters in flight is the thrust effect of the tail rotor. As we have seen, the tail rotor will produce thrust to the side, countering the torque effect of the main rotor. This will cause the helicopter to drift sideways in hovering flight. #### Solutions %%==[[Master QB1#Q00820|Q]]==%% ##### Offset mast Offsetting the mast is often the solution, as it changes the tip path plane of the rotor, countering the drift in hover. ##### Rigged cyclic In some helicopters the tip path plane is altered by rigging the cyclic system to give the required tilt when the cyclic control is at level. In a way, this is comparable to fixed trim on a fixed wing aircraft. ##### Inherent Sideslip Translating tendency does not stop when the helicopter is moving forward. There is a tendency for the helicopter to push to the side that the tail rotor is pushing. This is called inherent sideslip, and on larger helicopters this can be compensated for by mounting the tail rotor in a vertical fin or stabilizer. Additionally, mounting the tail rotor higher on this vertical stabilizer will reduce the tendency of the helicopter to lean, as the tail rotor is more in line with the torque plane of the main rotor, causing the fuselage to lean less. ![[T101_5_034.png|400]] [[T101_5_034.png|➡]] #### Translational Lift ##### Different than translating tendency As a helicopter gains speed, additional lift is generated due to the increased efficiency of the rotor system. Air flow increases into the rotors which increases lift. But a transverse flow also occurs, as the rotor disc is tilted, which means that the rear of the disc has a higher downwash angle. This increases lift at the rear, attempting to pitch the nose of the aircraft down. On a semi-rigid rotor head system, you will hear a distinct beat sound and the pilot will feel feedback in the cyclic control. The aircraft may want to roll as well, due to the gyroscopic precession on the rotor head. This effect can happen anytime the aircraft is accelerating, but is especially noticeable at an airspeed of 15 to 20 mph. The hinges of a fully articulated rotor head eliminate this effect. More on this shortly. ## Anti-Torque ### Newton's Third Law ![[T101_5_035.png|400]] [[T101_5_035.png|➡]] The power required to spin the blades of a rotary wing aircraft makes torque. Newton would remind us that this creates rotational motion in the opposite direction. This means that if we simply had a main rotor on an aircraft, the fuselage would turn in the opposite direction as the blades, and would thus reduce the speed of the blades as well as causing uncontrolled spinning of the fuselage. For this reason, anti-torque systems are required on helicopters. In fact, it has been a key design issue since the inception of the helicopter. But again, this is complicated on helicopters by other factors. As the main rotors adjust their pitch angle, and thus their lift and drag, the torque varies. And if we are to have directional control, we must be able to counter the torque of the main blades in such a way as to point the aircraft where we need it. So anti-torque systems must be adjustable. ### Tail Rotor %%==[[Master QB1#Q00821|Q]]==%% ![[T101_5_036.png|400]] [[T101_5_036.png|➡]] Tail rotors are the most common anti-torque configuration on helicopters. The power to drive the tail rotor is taken from the engine via the main gearbox. This would normally mean that its rotation is directly related to the main rotor. To adjust its anti-torque capabilities, its pitch is adjustable. ![[T101_5_037.png|400]] [[T101_5_037.png|➡]] #### Vertical Fins %%==[[Master QB1#Q00822|Q]]==%% Many conventional helicopters with tail rotors employ some method to reduce the power requirements of the tail rotor in flight. A vertical fin for example can be offset in order to help keep the fuselage straight during forward flight. This reduces the requirements for the power required by the tail rotor to do this job. ### Fenestron %%==[[Master QB1#Q00823|Q]]==%% The Fenestron is a technological advancement over the basic tail rotor. It is a protected tail rotor operating like a ducted fan. The term Fenestron is a trademark of multinational helicopter manufacturing consortium Airbus Helicopters (formerly known as Eurocopter). The Fenestron differs from a conventional tail rotor by being integrally housed within the tail unit of the rotorcraft and, like the conventional tail rotor it replaces, functions to counteract the torque of the main rotor. While conventional tail rotors typically have two or four blades, Fenestrons have between eight and eighteen blades; these may have variable angular spacing so that the noise is distributed over different frequencies. By placing the fan within a duct, this results in several distinct advantages over a conventional tail rotor, such as a reduction in tip vortex losses, the potential for substantial noise reduction, while also shielding both the tail rotor itself from collision damage and ground personnel from the hazard posed by a traditional spinning rotor. It was first developed by Airbus Helicopters who have installed it on several of their models, and is used by several manufacturers in current models. ![[T101_5_038.png|400]] [[T101_5_038.png|➡]]  [[T101_5_039.png|➡]] ### NOTAR %%==[[Master QB1#Q00824|Q]]==%% NOTAR is an acronym for No Tail Rotor. It is a helicopter system which uses a fan inside the tail boom to build a high volume of low-pressure air, which exits through two slots and creates a boundary layer flow of air along the tailboom which changes the direction of airflow around the tailboom, creating thrust opposite the motion imparted to the fuselage by the torque effect of the main rotor. Directional yaw control is gained through a vented, rotating drum at the end of the tailboom, called the direct jet thruster. Advocates of NOTAR believe the system offers quieter and safer operation over a traditional tail rotor. ![[T101_5_040.png|400]] [[T101_5_040.png|➡]]  [[T101_5_041.png|➡]] ### Dual Rotor %%==[[Master QB1#Q00825|Q]]==%% Some helicopters use two main rotors rotating in opposite directions to cancel their torque. This method is quite effective, but requires all of the mechanics for two main rotors with the consequential weight and space penalties. It does have very good lift characteristics though, and can often be seen on heavy lift helicopters.  [[T101_5_042.png|➡]] ![[T101_5_043.png|400]] [[T101_5_043.png|➡]]  [[T101_5_044.png|➡]] ![[T101_5_045.png|400]] [[T101_5_045.png|➡]] ### Co-axial Rotor %%==[[Master QB1#Q00826|Q]]==%% Co-axial rotor systems are yet another approach to countering the torque of the main rotor blades. A second rotor is placed below the main rotor but rotates in the opposite direction of the main, thus countering its torque. This design also produces more lift, as there are two main rotors. ![[T101_5_046.png|400]] [[T101_5_046.png|➡]]  [[T101_5_047.png|➡]]  [[T101_5_048.png|➡]] ### Intermeshed Rotor %%==[[Master QB1#Q00827|Q]]==%% Somewhere between dual and co-axial is the intermeshed rotor system. It also has two rotors rotating in opposite directions to counter each others' torque. But rather than being mounted directly under the main rotor, or completely separate from it, their rotation planes overlap. ![[T101_5_049.png|400]] [[T101_5_049.png|➡]]  [[T101_5_050.png|➡]] ## Coriolis Effect ![[T101_5_051.png|400]] [[T101_5_051.png|➡]] ### Blade speed changes due to centre of gravity changing %%==[[Master QB1#Q00828|Q]]==%% As a blade flaps up, its centre of gravity moves inwards, and the blade speeds up. As it flaps down, its centre of gravity moves outwards, and so the blade slows down. This effect can be seen when figure skaters hold their limbs close to their body, moving their centre of gravity inward to spin faster, and extending their limbs to move the centre of gravity outwards, slowing their spin. ### Lead-Lag Hinge %%==[[Master QB1#Q00829|Q]]==%% To relieve these forces, blades with a flapping hinge also have a lead-lag hinge, which allows the blade to change its sweep in response to the Coriolis effect. These hinges may also be called hunting and dragging hinges, using slightly different descriptors for the sped up and slowed down behaviours of the blades. As the advancing blade rises (because of the flapping hinge) its centre of gravity moves inwards and makes it spin faster. The lead-lag hinge allows the blade to lead without transferring all of this force back to the hub and mast. Similarly, when the blade is retreating, it goes slower, and the lead-lag hinge allows it to lag. If lead-lad hinges were not installed on blades with flapping hinges, vibrations and instability would result. Typically, at the extent of the limits of the lead-lag hinge there are dampers to limit their travel and smooth out the motion when maxed out. ## Rotary Wing Stability ![[T101_5_052.png|400]] [[T101_5_052.png|➡]] ### Causes of Instability Helicopters, like fixed wing aircraft, have static and dynamic stability requirements and issues #### Rotor following fuselage The rotor disc will follow the fuselage, this affects blade flapping and tilts the rotor in the direction of disturbance, causing pitch to accelerate in direction of disturbance. #### Change in speed Changes in speed will cause the rotor to tilt. As speed increases the rotor will tilt backwards, as speed decreases the rotor will tilt forwards. This will accelerate the movements of the helicopter. #### Changes tilt of rotors - Changes CofG Because the helicopter centre of gravity is below the rotor head, a change in rotor system speed will cause the CofG to oscillate the tilt of the fuselage, which will in turn aggravate the rotor disc tilt. #### Must be corrected ##### Negative dynamic stability Most of these issues must be corrected immediately, as their effects strengthen other effects, generally making everything worse. ### Stabilizing Measures To compensate for these instabilities, different methods have been devised. #### Bell stabilizer bar %%==[[Master QB1#Q00830|Q]]==%% Bell helicopters use a stabilizer bar, which acts as a gyroscope and has weights at both ends. The mass of this bar and its weights counteracts the tendency for pitch changes leading to instability.  [[T101_5_053.png|➡]] #### Paddles Paddles are a variation on the stabilizer bar, but rather than simply relying on the weight and gyroscopic properties of the bar, more control can be taken by changing the [[airfoil]] shape.  [[T101_5_054.png|➡]] #### Offset Mast Hinges Another method involves offsetting the flapping hinges from the rotational centre of the mast, causing movement to happen further away from the centre of gravity. #### Electronics Modern systems can employ computer control to adjust helicopter controls when flight instabilities occur. Often the computer will be able to sense the onset of instability before a pilot could. ### Ground Resonance %%==[[Master QB1#Q00831|Q]]==%% [[Ground resonance]] is a particular kind of instability which is a self-excited vibration which occurs on the ground. It may buildup gradually or it may appear very rapidly. If [[ground resonance]] is not corrected immediately it will often destroy the helicopter. This problem is usually associated with fully articulated rotor systems and is the result of geometric imbalance of the main rotor system, typically caused by a defect in the rotor system. The blades take abnormal positions around the lead/lag axis, one blade leads while the adjacent blade lags (out of phase). This unbalances the rotor and moves the center of gravity of the rotor off center. This imbalance of the rotor causes an oscillation (wobble) which is transmitted throughout the entire helicopter, giving movement from side to side as well as fore and aft. If this action becomes violent enough, it may roll the helicopter over or cause structural damage. This situation may be further aggravated by the reaction of the helicopter and the ground through the gear struts and wheels. During landing, a set of blades that are already out-of-phase may be further aggravated by the touchdown, especially on a one-wheel landing such as occurs on a slope landing or with a flat tire or strut. When this occurs, the forces of the vibration cannot be absorbed by the strut or tire and a counter-wave is sent back through the helicopter, usually resulting in further imbalance of the rotor. It will be further aggravated by incorrect shock strut pressure and/or tire inflation. Shock absorbers to reduce this effect are often included in helicopter landing gear design. When this situation occurs, immediate power application and takeoff will stop this condition by changing the natural frequencies of the helicopter. In recent years, manufacturers have reduced these occurrences considerably by redesigning the dampeners, oleo struts, tires, and the gears. These videos will give you a clear picture of the seriousness of ground resonance. In this clip, even without striking the ground, the aircraft is completely destroyed by the vibrations: [[V Explained AS350 Eurocopter Helicopter Self-Destructs|🎞]]. This one is a test/demonstration on a CH-47 in ground resonance that leads to catastrophic damage: [[V CH-47 Ground Resonance]]. Here is the same aircraft from a different angle. This larger aircraft has more power and mass, and you can clearly see the violent motion that destroys the aircraft: [[V CH-47 Rear View|🎞]]. This pilot knew what to do to correct for ground resonance, watch closely: [[Helicopter Ground Resonance|🎞️]]. He knew he couldn't wait until the passenger door was shut! ### Blade Stall %%==[[Master QB1#Q00832|Q]]==%%  [[T101_5_055.png|➡]] #### Causes Just like a fixed wing airfoil, a helicopter blade can stall. ##### Large Loads Applying the theory that we have already learned, we can understand that if weight overcomes the lift created, a wing will effectively stall ##### Surface Contamination The ability of an airfoil to generate lift depends on a strong boundary layer flow, and surface contamination will, just as it will on a fixed wing, diminish the lift created by the airfoil. ##### Stall AOA is exceeded The turbulence on the upper camber caused by boundary layer separation eventually destroys all lift creating capabilities of an airfoil. ##### Rotors too slow If relative wind is insufficient over the airfoil, it will cease generating lift. ### Retreating Blade Stall #### Relative wind again At high speeds, the speed of the retreating blade and the speed of the relative wind can be close to the same. Once again, our theory tells us that no relative wind means no lift. This happens in stages of course. As the retreating blade gets faster, more angle of attack is required, and eventually, the blade tip will stall.  [[T101_5_056.png|➡]] This effect can be hard to predict due to contributing factors such as wing loading, temperature, altitude, and which maneuvers are being done by the aircraft. This means that retreating blade stalls do not all necessarily happen at top speeds. ### Vortex Ring State %%==[[Master QB1#Q00833|Q]]==%% #### Getting sucked down into the downwash ![[T101_5_057.png|400]] [[T101_5_057.png|➡]] At low speeds and high rates of descent, a helicopter can fly into its own downwash, causing the blade roots to stall, and vortices to form at the hub, decreasing lift. If the pilot adds more power, the effect is only made worse, as the vortices become stronger, and lift decreases even more. The only solution to this is to unload the rotor and fly out of the downwash. ## Types of Rotor Heads ![[T101_5_058.png|400]] [[T101_5_058.png|➡]] #### Movements to Consider Every rotor head design needs to take into account the effects of these movements: ##### Pitching/Feathering This is the motion that we have discussed as controlled by the cyclic and the collective, that is, the adjustment of each blade's pitch angle. ##### Flapping Remember that flapping is how the blades compensate for the dissymmetry of lift in the rotor system. ##### Leading and lagging Leading and lagging is the horizontal movement of the rotor blades forwards and backwards along a vertical hinge. Review [[T101 Week 5#Coriolis Effect|Coriolis Effect]] from earlier. A lead lag hinge will allow these forces to equalize which removes undue stress of the system. Lead and lag are also known as hunting and dragging. ### Three main types - Rigid - Semi-rigid - Fully articulated There are three main ways that helicopter designers address these issues. ### Rigid %%==[[Master QB1#Q00834|Q]]==%% A rigid rotor head only uses a feathering axis. It does not have a flapping hinge, nor a lead-lag hinge. This means that it does not inherently correct for dissymmetry of lift, and therefore their use has been somewhat limited. However, the use of new materials such as fiberglass that make the blade more flexible can allow a rigid head to account for flapping and lead and lag. #### Rigid Eurocopter/MBB BO 105 ![[T101_5_059.png|400]] [[T101_5_059.png|➡]] ![[T101_5_060.png|400]] [[T101_5_060.png|➡]]  [[T101_5_061.png|➡]] ### Semi-Rigid %%==[[Master QB1#Q00835|Q]]==%% An important distinction for semi-rigid rotor heads is again the handling of flapping. In a semi-rigid system, opposing pairs of blades do flap, but they do so together, that is, one flaps up and the other flaps down (as one would expect) without a hinge for each. The semi-rigid rotor head is possibly the most popular design. It allows the entire rotor head to rock to allow flapping, and its underslung head design shifts the rotors centre of gravity to allow for the Coriolis effect when flapping. This underslung design means that a vertical hinge to allow for lead and lag is not necessary because the centre of mass doesn't change significantly. ![[T101_5_062.png|400]] [[T101_5_062.png|➡]] ![[T101_5_063.png|400]] [[T101_5_063.png|➡]] #### Semi-rigid Bell 47 ![[T101_5_064.png|400]] [[T101_5_064.png|➡]]  [[T101_5_065.png|➡]] #### Semi-rigid Bell 206 ![[T101_5_066.png|400]] [[T101_5_066.png|➡]]  [[T101_5_067.png|➡]] #### Semi-rigid Bell 204/412 ![[T101_5_068.png|400]] [[T101_5_068.png|➡]] ### Fully Articulated %%==[[Master QB1#Q00836|Q]]==%% ![[T101_5_069.png|400]] [[T101_5_069.png|➡]] Fully articulated rotor heads are the most complicated type. A fully articulated rotor head historically used hinges for each form of motion. However, more modern versions use elastomeric components and flexible composites to allow the same control. Because all the forces are larger in larger aircraft, the control these rotor heads provide are normally seen on heavy load bearing helicopters. This could include high aerodynamic loads as well, i.e. for high maneuverability. ![[T101_5_070.png|400]] [[T101_5_070.png|➡]] ![[T101_5_071.png|400]] [[T101_5_071.png|➡]] ![[T101_5_072.png|400]] [[T101_5_072.png|➡]] ##### Semi-rigid Eurocopter AS 350 ![[T101_5_073.png|400]] [[T101_5_073.png|➡]]  [[T101_5_074.png|➡]] ##### Fully Articulated Hughes 369 (500) ![[T101_5_075.png|400]] [[T101_5_075.png|➡]]  [[T101_5_076.png|➡]] ## Auto-Rotation %%==[[Master QB1#Q00837|Q]]==%% ### Engine failure If a helicopter engine fails, the pilot will perform auto rotation. This is the process of producing lift with the rotor blades as they freely rotate from the flow of air up through the rotor system. ### Disengage rotor from engine The rotor system will be disengaged from the engine with the use of a freewheel unit. ### Adjust pitch to catch upward wind The pitch of the blades is adjusted to cause the blades to be deflected and thus rotate faster as the aircraft descends. You may liken this to a child's pinwheel toy and the way it spins in reaction to air passing through it. This is helped by weights in the rotor blade that are intended to reduce the rate of fluctuation of rotations per minute. ### Close to ground, change pitch This rotor speed is enough that when, close to the ground, the pilot changes the pitch angle up, the blades momentum produces lift in the same way as if they were under power from the engine. ### Enough spinning momentum to generate enough lift for landing %%==[[Master QB1#Q00838|Q]]==%% The speed of the rotors must be kept within a safe rotational speed range, because if the blades, now spinning freely, spin too fast, they can suffer structural damage. If, on the other hand, the blades turn too slowly, they will not have enough momentum energy stored to slow the rate of descent when pitched up near the ground.  [[T101_5_077.png|➡]] The pilot must time the changing of pitch very carefully. This clip shows a pilot practicing autorotations, with some mixed results: [[AutoRotation Close Call|🎞️]] Of course, in order to do this maneuver successfully, a helicopter must be high enough or be moving forward fast enough to be able to generate the speeds required by the rotors to successfully regain lift and let the helicopter down safely. This information specific to the model is included in the Pilots' Operating Handbook (POH) and is known informally as the dead man's curve. The factors taken into account to generate this curve include the density altitude, the gross weight, the rotor rotational speed, the forward speed, the collective position, and the rigging. Does anyone have a calculator? This characteristic of helicopters distinguishes them from fixed wing aircraft in regards to safety. ### Additional Resources This 1969 video called "The Theory of Helicopter Flight" does a good job of explaining the theory of helicopter flight, including good footage and explanation of the cyclic control and swashplate mechanism: [[V The Theory of Helicopter Flight|🎞]] ### Conclusion In this lesson we covered the following topics: - Rotary Wing Flight - Rotary Wing Terminology and Concepts - Forces of Flight - Rotary - Rotary Wing Flight Controls - Dissymmetry of Lift - Anti-Torque - Coriolis Effect - Rotary Wing Stability - Types of Rotor Heads - Auto-Rotation You can demonstrate your understanding of the material in this lesson by answering the questions in the corresponding weekly practice quiz correctly. > # [[T101 Week 4| ◀️ ]]  [[T101 Home| Home ]]  [[T101 Week 6| ▶️ ]]     [[QR T101 W5| 🌐 ]]    [[FB T101|Please Help]]