Lecture on Drag

Introduction

• Previous lecture was about the lift form.
• This lecture focuses on drag.

Formula for Drag

• Simple formula for drag.
• Differences with lift:
  • Replace lift force L with drag force D.
  • Replace lift coefficient with drag coefficient.
• Coefficient is dimensionless.
• Variables have the same meaning as in the lift formula.

Components of Drag

• Drag consists of different contributions.
• Split into profile drag and parasitic drag:
  • Profile drag: Concentrated on wings.
  • Parasitic drag: Concerns other aircraft parts like the fuselage.

Breakdown of Profile Drag

• Pressure drag: Resultant of integral pressure over the profile.
• Friction drag: Arises from air contact with the body shape.

Types of Drag

• Skin friction: Air slides over the aircraft surface.
• Pressure drag: Creates turbulent flight and vortices.
• Wave drag: Arises from shock waves at high speeds.

Parasitic Drag

• Components that do not contribute to lift.
• Cd0: Drag coefficient of component independent of lift.

Difference between Friction and Pressure

• Pressure drag: Acts perpendicular to the surface.
• Friction drag: Acts parallel to the surface.

Three-dimensional Effects

• Air pressure decreases above the wing.
• Pressure increase below the wing.
• Vortex formation at wingtips.
• Winglets reduce drag by 3-6%.
• Fuel savings and greater range.

Lift and Drag Curve Analysis

• Lift curve: Almost linear up to maximum.
• Drag curve: Exponential, depending on lift coefficient².
• Lift-drag polar: Lift coefficient versus drag coefficient.
• Maximum CL/CD ratio: Glide ratio important for efficiency.

Glide Ratios

• High glide ratios for slender aircraft and birds like the albatross.
• Low glide ratio for Space Shuttle.

Conclusion

• Focus on minimizing drag.
• Drag comes from multiple sources.
• Next lecture: weight and cargo forms.