JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year FLIGHT VEHICLE DESIGN Previous Question Paper (March - 2021 - R16)

The take-off weight build-up is an iterative process used to determine the initial gross weight (W_TO) required to meet mission requirements. It is based on the mission segment weight fraction method, where the total weight is broken down into constituent parts: crew, payload, fuel, and empty weight.

• Mission Segments: The mission is divided into phases (warm-up, taxi, take-off, climb, cruise, loiter, descent, landing). Each phase has a weight fraction (W_i/W_i-1) representing the ratio of final weight to initial weight for that segment.
• Weight Fractions: The product of these fractions gives the overall mission weight fraction (W_final/W_TO).
• Iterative Equation: The take-off weight is defined as W_TO = W_crew + W_payload + W_fuel + W_empty. Since W_fuel and W_empty are functions of W_TO, the equation is solved iteratively until convergence.

Empty weight estimation is typically performed using statistical regression models based on historical data of similar aircraft classes. The empty weight fraction (W_E/W_TO) is expressed as a function of W_TO, aspect ratio, wing loading, and thrust-to-weight ratio:

W_E/W_TO = A × W_TO^C × K_vs

Where A and C are empirical constants derived from existing aircraft databases, and K_vs is a variable geometry or material correction factor. As the design matures, this estimate is replaced by a component-based weight build-up, summing the weights of the wing, fuselage, empennage, landing gear, propulsion system, and systems.

Performance requirements at the concept design phase define the operational envelope and mission capabilities that the aircraft must achieve. These requirements act as the primary constraints for sizing the wing and propulsion system.

• Take-off and Landing Distance: Defines the minimum field length requirements (S_TO and S_L). This dictates the required thrust-to-weight ratio (T/W) and wing loading (W/S) for safe operation on specific runways.
• Climb Performance: Ensures the aircraft can clear obstacles and meet air traffic control requirements. It is expressed as the rate of climb (ROC) or climb gradient at various flight phases (e.g., all-engines-operative or one-engine-inoperative).
• Cruise Speed and Range: Defines the aerodynamic efficiency (L/D) and specific fuel consumption (SFC) targets. The Breguet Range Equation is typically used here to link aerodynamic and propulsion parameters to the mission range.
• Maneuverability: For combat aircraft, this involves turn rates and load factor (g) limits. For civil aircraft, it focuses on gust load alleviation and stability margins.

These requirements are visualized using a Matching Plot (or Constraint Analysis Plot), where T/W is plotted against W/S. Each performance requirement forms a curve on this graph. The region where all curves intersect represents the feasible design space. The designer selects a point within this region that minimizes W_TO while satisfying all constraints, effectively balancing aerodynamic performance with structural and propulsion limitations.