JNTU B.Tech III CONCEPTUAL DESIGN OF FLIGHT VEHICLES Previous Question Paper (November/December - 2020 - R15)

The selection of an airfoil is a critical step in the conceptual design of a flight vehicle, as it directly dictates the lift-to-drag ratio, stall characteristics, and structural efficiency of the wing. The primary considerations include:

• Design Lift Coefficient (C_l): The airfoil must be selected to operate efficiently at the cruise lift coefficient. This involves matching the airfoil's pressure distribution to the expected flight regime to minimize wave drag at high speeds or profile drag at low speeds.
• Reynolds Number (Re): Airfoil performance is highly dependent on the Reynolds number. The designer must ensure the airfoil maintains attached flow and favorable boundary layer characteristics at the specific chord-based Re expected for the flight vehicle's operating envelope.
• Mach Number (M): For high-speed aircraft, the critical Mach number (M_crit) of the airfoil is paramount. Supercritical airfoils are often selected to delay the onset of wave drag by flattening the upper surface pressure distribution, thereby increasing the drag divergence Mach number.
• Stall Characteristics: The airfoil must exhibit docile stall behavior. A gradual stall with a large trailing-edge separation zone is preferred over a sudden leading-edge stall to ensure pilot control at low speeds. The pitching moment coefficient (C_m) at stall is also critical for longitudinal stability.
• Thickness-to-Chord Ratio (t/c): This parameter balances aerodynamic performance with structural requirements. A higher t/c allows for a deeper wing spar, reducing structural weight and providing volume for fuel or landing gear, but it may increase wave drag and reduce the critical Mach number.
• Pitching Moment (C_m,ac): The aerodynamic center pitching moment must be considered to minimize the trim drag of the horizontal tail. Airfoils with low absolute values of C_m are generally preferred to reduce the download required on the tail for longitudinal trim.
• High-Lift Requirements: If the aircraft requires significant lift augmentation, the airfoil must be compatible with high-lift devices such as flaps and slats. The ability to achieve high maximum lift coefficients (C_l,max) without excessive drag penalty is a key selection criterion.