JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV HELICOPTER ENGINEERING Previous Question Paper (September - 2020 - R16)
Answer any five questions
All questions carry equal marks
Main
How does a helicopter gain height?
Answer
A helicopter gains height by increasing the total thrust produced by the main rotor system to exceed the total weight of the aircraft. This is achieved primarily through the collective pitch control. When the pilot raises the collective lever, the pitch angle of all rotor blades is increased simultaneously. At a constant rotor RPM, increasing the blade pitch angle increases the angle of attack, which generates greater aerodynamic lift. Once the vertical component of the thrust vector exceeds the gravitational force acting on the helicopter, the aircraft begins to climb. The climb rate can be further adjusted by varying engine power to maintain rotor speed against the increased drag resulting from the higher pitch settings.
With neat diagrams point out the required forces direction and rotor plane of rotation for all possible flights of a helicopter.
Answer
The flight of a helicopter is governed by the manipulation of the thrust vector relative to the aircraft's center of gravity. The four primary forces acting on a helicopter are Weight (W), Thrust (T), Lift (L), and Drag (D).
- Hovering: The rotor plane is horizontal. Thrust acts vertically upward, exactly balancing the weight. Lift and drag are balanced by the rotor's rotation.
- Vertical Climb: The rotor plane remains horizontal, but collective pitch is increased so that the thrust vector magnitude exceeds the weight.
- Forward Flight: The rotor disk is tilted forward using cyclic control. This tilts the thrust vector forward, creating a horizontal thrust component that overcomes drag and a vertical component that supports weight.
- Sideward/Rearward Flight: The rotor disk is tilted in the desired direction of travel. The thrust vector tilts accordingly, providing a horizontal component of force in the direction of movement while maintaining enough vertical lift to counteract gravity.
In all cases, the rotor plane of rotation is perpendicular to the resultant thrust vector. By tilting the rotor disk, the pilot shifts the direction of the total thrust vector, allowing the helicopter to maneuver in three-dimensional space.
Explain in brief the action of Collective and cyclic pitch changes in vertical and forward flights.
Answer
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Explain the rotor arrangements including the path of the drive taken from main engine to rotor.
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If the helicopter is hovering at given altitude with forward velocity ‘V’ and velocity at the actuated disc ‘Vo’ then derive an expression for the Input power produced during hovering condition.
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Explain about the swash plate system for controlling cyclic pitch in a typical helicopter.
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Prove that power requirement decreases in Ground Effect. Also draw the ground effect diagram.
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Explain the procedure for solving the equations of motion of helicopter involving Drag to Lift ratio and Power to Lift ratio terms for estimating all non-dimensional coefficients.
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What do you mean by Figure of Merit? Derive an expression for figure of merit.
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