Rotary Wing Aerodynamics PDF (AVIA-1035)

AVIA-1035 1  Airframe 1-45 Fully Articulated Rotor System  Various dampers and stops can be found on different designs to reduce shock and limit travel in certain directions.  Figure 1-98 shows a Fully Articulat...

AVIA-1035 1  Airframe 1-45 Fully Articulated Rotor System  Various dampers and stops can be found on different designs to reduce shock and limit travel in certain directions.  Figure 1-98 shows a Fully Articulated Main Rotor system with the features discussed. 2  Airframe 1-45 Fully Articulated Rotor System  Figure 1-98 3  Airframe 1-45 Fully Articulated Rotor System  Numerous designs and variations on the three types of main rotor systems exist.  Engineers continually search for ways to reduce vibration and noise caused by the rotating parts of the helicopter. 4  General 5-52 Main Rotor Systems  These hinges and their associated movement are shown in Figure 5-82.  The main rotor head of a Eurocopter model 725 is shown in Figure 5-83, with the drag hinge and pitch change rods visible.  The Semi-Rigid Rotor system is used with a two blade main rotor.  The blades are rigidly attached to the hub, with the hub and blades able to teeter like a seesaw. 5  General 5-52 Main Rotor Systems  Figure 5-82 6  General 5-52 Main Rotor Systems  Figure 3-83 7  Airframe 1-45 Main Rotor Systems  The use of elastomeric bearings in main rotor systems is increasing.  These polymer bearings have the ability to deform and return to their original shape.  As such, they can absorb vibration that would normally be transferred by steel bearings.  They also do not require regular lubrication, which reduces maintenance. 8  Airframe 1-45 Main Rotor Systems  Some modern helicopter main rotors have been designed with flextures.  These are hubs and hub components that are made out of advanced composite materials.  They are designed to take up the forces of blade hunting and dissymmetry of lift by flexing.  As such, many hinges and bearings can be eliminated from the traditional main rotor system.  The result is a simpler rotor mast with lower maintenance due to fewer moving parts. 9  Airframe 1-44 Main Rotor Systems  Often designs using flextures incorporate elastomeric bearings.  Figure 1-99 10  Airframe 1-44 Main Rotor Systems  Figure 1-99 11  Airframe 1-45 General 5-52 Anti-torque System  Ordinarily, helicopters have between two and seven main rotor blades.  These blades are usually made of a composite structure.  The large rotating mass of the main rotor blades of a helicopter produce torque.  Any time a force is applied to make an object rotate, there will be an equal force acting in the opposite direction. 12  Airframe 1-45 General 3-53 Anti-torque System  Newton’s third law states that for every action there is an opposite and equal reaction.  Therefore when power is applied to the rotor system the fuselage of the helicopter will tend to move in the opposite direction of the rotor.  This tendency is referred to as torque. 13  Airframe 1-45 General 3-53 Anti-torque System  This torque increases with engine power and tries to spin the fuselage in the opposite direction.  If the helicopter’s main rotor system rotates clockwise when viewed from the top, the helicopter will try to rotate counterclockwise.  For this reason, a helicopter uses what is called an anti-torque system to counteract the force trying to make it rotate.  The tail boom and tail rotor, or anti-torque rotor, counteract this torque effect. 14  General 5-53 Anti-Torque Systems  One method that is used on a helicopter to counteract torque is to place a spinning set of blades at the end of the tail boom.  These blades are called a tail rotor or anti-torque rotor, and their purpose is to create a force (thrust) that acts in the opposite direction of the way the helicopter is trying to rotate.  The tail rotor force, in pounds, multiplied by the distance from the tail rotor to the main rotor, in feet, creates a torque in pound-feet that counteracts the main rotor torque. 15  Airframe 1-45 Anti-torque System  Figure 1-100, Controlled with foot pedals, the counter-torque of the tail rotor must be modulated as engine power levels are changed.  This is done by changing the pitch of the tail rotor blades.  This, in turn, changes the amount of counter-torque, and the aircraft can be rotated about its vertical axis, allowing the pilot to control the direction the helicopter is facing. 16  Airframe 1-46 Anti-torque System 17  General 5-53 Anti-Torque Systems  Figure 5-86 shows a three bladed tail rotor on an Aerospatiale SA-315B helicopter.  This tail rotor has open tipped blades that are variable pitch, and the helicopter’s anti- torque pedals (positioned like rudder pedals on an airplane) control the amount of thrust they create. 18  General 3-53 Anti-Torque Systems  Aerospatiale SA-315B 19  General 3-53 Anti-Torque Systems  Figure 5-86 20  Airframe 1-46 Anti-torque System  Similar to a vertical stabilizer on the empennage of an airplane, a Vertical Fin or pylon is also a common feature on rotorcraft.  Normally, it supports the tail rotor assembly, although some tail rotors are mounted on the tail boom.  Additionally, a horizontal member called a horizontal stabilizer is often constructed at the tail cone or on the pylon. 21  General 5-53 Anti-Torque Systems  Some potential problems with this tail rotor system are as follows:  The spinning blades are deadly if someone walks into them.  When the helicopter is in forward flight and a vertical fin is in use to counteract torque, the tail rotor robs engine power and creates drag. 22  General 5-53 Anti-Torque Systems  An alternative to the tail rotor seen in Figure 5-86 is a type of anti-torque rotor known as a Fenestron®, or “fan-in-tail” design as seen in Figure 5-87.  Because the rotating blades in this design are enclosed in a shroud, they present less of a hazard to personnel on the ground and they create less drag in flight. 23  General 3-53 Anti-Torque Systems  Figure 5-87 24  Airframe 1-46 Anti-torque System  A Fenestron® is a unique tail rotor design which is actually a multibladed ducted fan mounted in the vertical pylon.  It works the same way as an ordinary tail rotor, providing sideways thrust to counter the torque produced by the main rotors.  Figure 1-101 25  Airframe 1-46 Anti-torque System  Figure 1-101 26  Airframe 1-46 General 5-53 Anti-torque System  A NOTAR® anti-torque system has no visible rotor mounted on the tail boom.  Instead, an engine-driven adjustable fan is located inside the tail boom.  This system uses a high volume of air at low pressure, which comes from a fan driven by the helicopter’s engine.  NOTAR® is an acronym that stands for “no tail rotor.” 27  Airframe 1-46 General 5-53 Anti-torque System  Air is vented out of two long slots on the right side of the tail boom, entraining main rotor wash to hug the right side of the tail boom, in turn causing laminar flow and a low pressure (Coanda Effect).  The air coming out of the slots on the right side of the boom causes a higher velocity, and therefore lower pressure, on that side of the boom.  The higher pressure on the left side of the boom creates the primary force that counteracts the torque of the main rotor. 28  Airframe 1-46 General 5-53 Anti-torque System  Coanda Effect  Additionally, the remainder of the air from the fan is sent through the tail boom to a vent on the aft left side of the boom where it is expelled.  The air exits the nozzle at a high velocity, and creates an additional force (thrust) that helps counteracts the torque of the main rotor.  A NOTAR system is shown in Figures 5-88 and 5-89.  The Coanda Effect (2:45)  NOTAR 3:57 29  Airframe 1-46 General 5-53 Anti-torque System  “The enclosed variable-pitch composite blade fan produces a low pressure, high volume of ambient air to pressurize the composite tailboom.  The air is expelled through two slots which run the length of the tailboom on the starboard (right) side, causing a boundary-layer control called the Coanda Effect. The result is that the tailboom becomes a "wing", flying in the downwash of the rotor system, producing up to 60 percent of the anti-torque required in a hover. The balance of the directional control is accomplished by a rotating direct jet thruster.  In forward flight, the vertical stabilizers provide the majority of the anti-torque, however directional control remains a function of the direct jet thruster. ”  (MD Helicopters.com) 30  General 5-54 Anti-Torque Systems  Figures 5-88 31  General 5-54 Anti-Torque Systems  Figures 5-89  NOTAR 32  Airframe 1-46 General 5-53 Anti-torque System  This action to the left causes an opposite reaction to the right, which is the direction needed to counter the main rotor torque.  Figures 1-102 33  Airframe 1-46 Anti-torque System  Figures 1-102 34  General 5-53 Anti-Torque Systems  For helicopters with two main rotor heads, such as the Chinook that has a main rotor at each end, no anti-torque rotor is needed.  For this type of helicopter, the two main rotors turn in opposite directions, and each one cancels out the torque of the other. 35  General 5-53 Anti-Torque Systems  Chinook 36  General 5-53 Anti-Torque Systems  Chinook 37  General 5-53 Anti-Torque Systems  Chinook 38  General 5-53 Anti-Torque Systems  Coaxial Rotors Kamov Ka-32A-12 39  General 5-53 Anti-Torque Systems  Coaxial Rotors Kamov Ka-52 40  General 5-53 Anti-Torque Systems  Coaxial Rotors Sikorsky S-69 41  General 5-53 Anti-Torque Systems  MV-22 Osprey tilt-rotor helicopter 42  General 5-53 Anti-Torque Systems  Eurocopter X3 43  General 5-53 Anti-Torque Systems  K-MAX 44  General 3-53 Anti-Torque Systems  Kaman K-MAX 45  Airframe 1-46 Controls - Rotor  The controls of a helicopter differ slightly from those found in an fixed wing aircraft.  The collective, operated by the pilot with the left hand, is pulled up or pushed down to increase or decrease the angle of attack on all of the rotor blades simultaneously.  This increases or decreases lift and moves the aircraft up or down.  The engine throttle control is located on the hand grip at the end of the collective. 46  Airframe 1-46 Controls - Rotor 47  Airframe 1-46 Controls - Rotor  The cyclic is the control “stick” located between the pilot’s legs.  It can be moved in any direction to tilt the plane of rotation of the rotor blades.  This causes the helicopter to move in the direction that the cyclic is moved. 48  Airframe 1-46 Controls - Rotor  As stated, the foot pedals control the pitch of the tail rotor blades thereby balancing main rotor torque.  Figures 1-103 and 1-104 illustrate the controls found in a typical helicopter. 49  Airframe 1-47 Controls - Rotor  Figures 1-103 50  Airframe 1-47 Controls - Rotor  Figures 1-104 51  Airframe 1-47 Controls - Rotor 52