Helicopter Dynamics of Flight PDF

BASIC HELICOPTER & PROPELLER DESIGN Helicopter dynamics of Flight Prepared By Engr. Shiara Denise M. Va...

BASIC HELICOPTER & PROPELLER DESIGN Helicopter dynamics of Flight Prepared By Engr. Shiara Denise M. Valencia Technical and Operational Terms Engr SDVM 2  Airflow in Forward Flight Airflow across the rotor disk in forward flight varies from airflow at a hover. In forward flight, air flows opposite the aircraft’s flightpath. The velocity of this air flow equals the helicopter’s forward speed. Because the rotor blades turn in a circular pattern, the velocity of airflow across a blade depends on the position of the blade in the plane of rotation at a given instant, its rotational velocity, and airspeed of the helicopter. Engr SDVM 3  Airflow in Forward Flight The highest velocity of airflow occurs over the right side (3 o’clock position) of the helicopter (advancing blade) in a rotor disk that turns (counterclockwise) and decreases to rotational velocity over the nose. It continues to decrease until the lowest velocity of airflow occurs over the left side (9 o’clock position) of the helicopter (retreating blade). Engr SDVM 4  Advancing Blade As the relative wind speed of the advancing blade increases, the blade gains lift and begins to flap up. It reaches its maximum upflap velocity at the 3 o’clock position, where the wind velocity is the greatest. This upflap creates a downward flow of air and has the same effect as increasing the induced flow velocity by imposing a downward vertical velocity vector to the relative wind which decreases the AOA. Engr SDVM 5  Retreating Blade As relative wind speed of the retreating blade decreases, the blade loses lift and begins to flap down. It reaches its maximum downflap velocity at the 9 o’clock position, where wind velocity is the least. This downflap creates an upward flow of air and has the same effect as decreasing the induced flow velocity by imposing an upward velocity vertical vector to the relative wind which increases the AOA. Engr SDVM 6 Engr SDVM 7  Dissymmetry of Lift is the differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half. This difference in lift would cause the helicopter to be uncontrollable in any situation other than hovering in a calm wind. Engr SDVM 8  Dissymmetry of Lift The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter, while the relative wind speed acting on the retreating blade is reduced by the The blade tip speed of this helicopter is helicopter’s forward airspeed. approximately 400 knots. If the helicopter is moving forward at 100 knots, the relative windspeed on the advancing side is 500 knots. On the retreating side, it is only 300 knots. This difference in speed causes a dissymmetry of lift. Engr SDVM 9  Translating Tendency (Drift) During hovering flight, a single main rotor helicopter tends to move in the direction of tail rotor thrust. This lateral (or sideward) movement is called translating tendency. Engr SDVM 10  Translating Tendency Engr SDVM Source: https://www.youtube.com/watch?v=17_FzAuoeWw&list=PL2tY7KqPLKuRoxnHXr827FUD3yFSF_o-2&index=6&ab_channel=FlightFirst 11  Pendular Action refers to the swinging motion of the fuselage side to side or back and forth. This refers to the swinging motion of a helicopter's fuselage due to changes in lift or thrust Engr SDVM 12  Pendular Action Engr SDVM Source: https://www.youtube.com/watch?v=0KlNcjEzbUU&ab_channel=FlightFirst 13  Coning The rotation of the rotor disk creates centrifugal force (inertia), which tends to pull the blades straight outward from the main rotor hub: The faster the rotation, the greater the centrifugal force, the slower the rotation, the smaller the centrifugal force. Engr SDVM 14  Coning Engr SDVM Source: https://www.youtube.com/watch?v=pT-zAuZpGXM&list=PL2tY7KqPLKuRoxnHXr827FUD3yFSF_o-2&index=8&ab_channel=FlightFirst 15  Coriolis Effect The Coriolis Effect is also referred to as the law of conservation of angular momentum. It states that the value of angular momentum of a rotating body does not change unless an external force is applied. In other words, a rotating body continues to rotate with the same rotational velocity until some external force is applied to change the speed of rotation. Angular momentum is the moment of inertia (mass times distance from the center of rotation squared) multiplied by the speed of rotation. Engr SDVM 16  Coriolis Effect Engr SDVM Source: https://www.youtube.com/watch?v=0KlNcjEzbUU&ab_channel=FlightFirst 17  Gyroscopic Precession The spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of gyroscopic action, one of which is precession. Gyroscopic precession is the resultant action or deflection of a spinning object when a force is applied to this object. This action occurs approximately 90° in the direction of rotation from the point where the force is applied (or 90° later in the rotation cycle). Engr SDVM 18  Coriolis Effect Engr SDVM Source: https://www.youtube.com/watch?v=0KlNcjEzbUU&ab_channel=FlightFirst 19  Gyroscopic Precession The spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of gyroscopic action, one of which is precession. Gyroscopic precession is the resultant action or deflection of a spinning object when a force is applied to this object. This action occurs approximately 90° in the direction of rotation from the point where the force is applied (or 90° later in the rotation cycle). Engr SDVM 20  Autorotation is the state of flight where the main rotor disk of a helicopter is being turned by the action of air moving up through the rotor rather than engine power driving the rotor. Engr SDVM 21  Autorotation Autorotation is permitted mechanically by a freewheeling unit, which is a special clutch mechanism that allows the main rotor to continue turning even if the engine is not running. If the engine fails, the freewheeling unit automatically disengages the engine from the main rotor allowing the main rotor to rotate freely. It is the means by which a helicopter can be landed safely in the event of an engine failure; consequently, all helicopters must demonstrate this capability in order to be certified. Engr SDVM 22  Autorotation The spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of gyroscopic action, one of which is precession. Gyroscopic precession is the resultant action or deflection of a spinning object when a force is During an autorotation, the upward flow of relative wind applied to thispermits object. the mainThis action rotor blades to rotateoccurs approximately at their normal speed. 90° in the direction of rotation from the point where the force In effect, the blades are “gliding” in their rotational plane. is applied (or 90° later in the rotation cycle). Engr SDVM 23  VIDEO REFERENCES HELICOPTER AERODYNAMICS https://www.youtube.com/playlist?list=PL2tY7KqPLKuRoxnHXr827FUD3yFSF_o-2 Engr SDVM 24