The 600 -seat jet transport aircraft proposed by Airbus...

The 600 -seat jet transport aircraft proposed by Airbus Industrie has a fuselage that is $240 \mathrm{ft}$ long and $25 \mathrm{ft}$ in diameter. The aircraft is to operate 14 hr per day, 6 days per week; it will cruise at $575 \mathrm{mph}(M=0.87)$ at 12 -km altitude. The engines consume fuel at the rate of $0.6 \mathrm{lbm} / \mathrm{hr}$ for each pound force of thrust produced. Estimate the skin friction drag force on the aircraft fuselage at cruise. Calculate the annual fuel savings for the aircraft if friction drag on the fuselage could be reduced by 1 percent by modifying the surface coating.

Key Concepts

Skin Friction Drag

Skin friction drag is the portion of aerodynamic drag that results from the friction of the air moving over an object’s surface. It is determined by the characteristics of the boundary layer of the fluid—whether it is laminar or turbulent—and depends on the surface area, roughness, and the velocity of the flow. This drag is a key consideration in the design of aircraft surfaces because even small reductions in friction can lead to significant performance and efficiency benefits in high-speed flight.

Aerodynamic Drag Calculation

Aerodynamic drag includes contributions from both pressure and friction forces acting on a moving body. The calculation of drag often involves using dynamic pressure, a drag coefficient, and an appropriate reference area. In many aerospace applications, engineers separately evaluate skin friction drag from the contributions due to shape and pressure differences to optimize the overall airframe performance during cruise conditions.

Boundary Layer Theory

Boundary layer theory describes the thin region of fluid close to the surface of a body in motion. Within this layer, viscous effects are significant and dictate the amount of skin friction experienced. The nature of the boundary layer—whether it is laminar or turbulent—plays a crucial role in determining the friction drag, which in turn influences overall aerodynamic efficiency and fuel consumption.

Fuel Consumption and Thrust-Fuel Relationship

Fuel consumption in jet engines is closely related to the thrust produced. Since aerodynamic drag directly influences the amount of thrust required to maintain cruising conditions, any reduction in drag can lead to lower fuel usage. This relationship connects the fields of aerodynamics and propulsion, demonstrating how improved surface characteristics and reduced friction translate to measurable decreases in fuel burn during operations.

Economic Impact of Drag Reduction

Modifying an aircraft to reduce drag, such as through improved surface coatings or aerodynamic refinements, can yield significant economic benefits over time. By quantifying the fuel savings associated with a percentage reduction in drag, engineers and operators can perform cost-benefit analyses to justify investments in design improvements. This concept underlines the importance of understanding and reducing aerodynamic drag to enhance overall operational efficiency and reduce lifetime fuel costs.

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