JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III AIRCRAFT DESIGN Previous Question Paper (August/September - 2021 - R18)

The aircraft design process is iterative and typically divided into three distinct phases:

1. Conceptual Design: This initial phase focuses on defining the aircraft configuration, size, and performance requirements. Designers perform trade studies, select the propulsion system, and estimate the take-off gross weight to ensure the mission requirements are met.
2. Preliminary Design: Once the configuration is frozen, this phase involves detailed aerodynamic analysis, structural layout, and stability and control assessments. Wind tunnel testing and high-fidelity computational fluid dynamics (CFD) are utilized to refine the geometry and ensure compliance with design specifications.
3. Detail Design: This final phase focuses on the manufacturing aspects. Every component is designed for production, including structural joints, systems integration, and material selection. This phase results in the final engineering drawings and specifications required for assembly and certification.

The design procedure follows a loop where requirements lead to an initial sizing, followed by configuration layout, performance analysis, and optimization. If the design fails to meet performance targets or weight constraints, the process returns to the conceptual phase for adjustment.

The take-off gross weight (W₀) is calculated using an iterative approach based on the mission profile, which breaks the flight into segments (e.g., warm-up, taxi, take-off, climb, cruise, loiter, and landing). The total weight is the sum of the crew, payload, fuel, and empty weight.

The calculation is based on the weight fraction method:

Since the fuel weight depends on the mission segments, we use the fuel fraction (Mff) for each segment:

The procedure involves:

1. Estimating the mission fuel fraction (Mff) by multiplying the individual segment fuel fractions (e.g., M₁ × M₂ × ... × Mₙ).
2. Estimating the empty weight fraction (W_empty/W₀) based on historical data for similar aircraft types (e.g., using the Raymer or Roskam statistical methods).
3. Iteratively solving for W₀ until the calculated value converges, as the empty weight fraction is itself a function of the gross weight.