Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech III Year HELICOPTER AERODYNAMICS Previous Question Paper (August - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
Explain feathering and ‘lead-lag’ with respect to helicopter blade.
Answer
- Feathering: This refers to the rotation of the helicopter blade about its spanwise axis (pitch change axis). It is the mechanism by which the pilot controls the blade pitch angle to vary the lift produced by the rotor. By changing the feathering angle, the pilot can adjust the thrust and control the aircraft's attitude. Feathering is achieved through the swashplate mechanism, which transmits control inputs from the cockpit to the blades via pitch links.
- Lead-Lag (Drag): This motion occurs in the plane of rotation, perpendicular to the flapping and feathering axes. It is caused by Coriolis acceleration, which arises when the blade flaps up or down. As the blade flaps up, its center of mass moves closer to the axis of rotation, causing it to speed up in its orbital path (lead). Conversely, when it flaps down, it moves away from the axis and slows down (lag). Lead-lag hinges (or drag hinges) are incorporated at the blade root to relieve the high in-plane bending stresses that would otherwise be imposed on the rotor hub and blade structure.
Describe the significance of flapping hinges.
Answer
- Flapping hinges are mechanical joints located near the rotor hub that allow the helicopter blade to move vertically in a plane perpendicular to the plane of rotation. Their primary significance lies in asymmetric lift compensation.
- In forward flight, the advancing blade experiences higher relative airspeed than the retreating blade. Without flapping, this would create a massive lift imbalance, causing the helicopter to roll toward the retreating side.
- The flapping hinge allows the advancing blade to flap upward, which increases the induced flow and decreases the effective angle of attack, thereby reducing its lift. Simultaneously, the retreating blade flaps downward, increasing its effective angle of attack and increasing its lift.
- This aerodynamic balancing ensures that the lift remains symmetric across the rotor disc, preventing structural failure of the hub due to excessive bending moments and maintaining lateral stability of the helicopter.
Briefly discuss the control requirements of a helicopter along with the four independent controls adequate for it.
Answer
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Explain the method to estimate hover ceiling.
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Write short notes on performance of ideally twisted, constant-chord blades.
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Starting from an expression for the figure of merit of an optimum hovering rotor, show that the figure of merit is maximum when the blade section are feathered to the highest value of (lift coefficient)^3/2 / (Drag Coefficient).
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Discuss the concept of rotor induced velocity, autorotation and Power-on flight.
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Describe the steps involved in calculation of climb performance of a helicopter.
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Compare aerodynamic forces and rotor inclination in hover, vertical flight and forward flight.
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With the help of neat diagrams, describe thrust-vectoring in VTOL airplane.
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Distinguish between VTOL and STOL aircraft.
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Explain the sources and consequences of drag of hovercraft on land along with various applications of hovercraft.
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