Drag Types

Why This Matters

Drag is the fundamental force opposing motion through a fluid, and understanding its various forms is critical for analyzing aircraft performance, fuel efficiency, and design optimization. You're being tested on your ability to distinguish between drag mechanisms—viscous effects, pressure differentials, lift penalties, and compressibility phenomena—and explain how each impacts flight at different speeds and configurations.

Don't just memorize a list of drag names. Know why each type occurs, when it dominates (low speed vs. high speed, high lift vs. cruise), and how designers minimize it. Exam questions often ask you to identify which drag type is most significant in a given flight scenario or to explain the trade-offs between lift generation and drag penalties.

---

Viscous Drag: Surface Friction Effects

These drag types arise from the viscosity of air and its interaction with surfaces. The no-slip condition at a surface creates a boundary layer where velocity gradients produce shear stress.

Skin Friction Drag

• Caused by viscous shear stress in the boundary layer as air molecules adhere to and slide past the surface
• Laminar flow produces less friction than turbulent flow, but turbulent boundary layers are more resistant to separation
• Surface roughness and wetted area directly increase this drag—polished surfaces and minimal exposed area reduce it

Profile Drag

• Combines skin friction and form drag acting specifically on an airfoil or wing section
• Represents the baseline drag of a wing at zero lift, determined by the airfoil's shape and surface quality
• Critical for airfoil selection—designers optimize profiles to minimize this drag while maintaining desired lift characteristics

---

Pressure Drag: Shape and Flow Separation

These types result from pressure imbalances around an object. When flow separates from a surface, the low-pressure wake behind the object creates a net rearward force.

Form Drag (Pressure Drag)

• Results from pressure differences between the high-pressure forward face and low-pressure rear wake
• Blunt shapes produce dramatically more form drag than streamlined shapes due to earlier flow separation
• Cross-sectional area matters—larger frontal areas facing the flow increase this drag component

Base Drag

• Occurs specifically at blunt trailing edges where flow cannot smoothly close behind the object
• Low-pressure wake region directly behind the base creates suction that pulls the object backward
• Boat-tailing and streamlined afterbodies reduce base drag by allowing gradual pressure recovery

---

Lift-Related Drag: The Cost of Flying

These drag types are direct consequences of generating lift. Creating lift requires deflecting airflow, and that deflection comes with an energy penalty.

Induced Drag

• Caused by wingtip vortices that create downwash, tilting the lift vector rearward
• Inversely proportional to airspeed squared—dominates at low speeds and high angles of attack
• Reduced by high aspect ratio wings and winglets that minimize vortex strength

Lift-Induced Drag

• Specifically tied to lift coefficient—mathematically expressed as $C_{D_i} = \frac{C_L^2}{\pi e AR}$ where $e$ is the Oswald efficiency factor
• Increases with the square of lift coefficient—doubling lift quadruples this drag component
• Represents the fundamental lift-drag trade-off that defines aircraft performance envelopes

Trim Drag

• Results from control surface deflections required to maintain balanced flight
• Tail surfaces generating downforce to balance the aircraft also generate their own induced drag
• Minimized through proper CG placement and aerodynamic balancing—poor trim costs fuel efficiency

---

Compressibility Drag: High-Speed Phenomena

This drag type emerges when airflow approaches sonic velocities. Shock waves form as the flow can no longer "communicate" pressure changes upstream.

Wave Drag

• Caused by shock wave formation at transonic and supersonic speeds, converting kinetic energy to heat
• Rises dramatically near Mach 1—the transonic drag rise is a critical design barrier
• Minimized through area ruling (Whitcomb bodies) and swept wings that delay shock formation

---

Composite Drag Categories

These classifications group multiple drag mechanisms for practical analysis. Engineers use these umbrella terms to simplify performance calculations.

Parasitic Drag

• Encompasses all non-lift-related drag—skin friction, form drag, and interference drag combined
• Proportional to velocity squared—doubles speed means quadruple parasitic drag
• Dominates at high speeds while induced drag dominates at low speeds; minimum total drag occurs where they're equal

Interference Drag

• Arises from component interactions—where wings meet fuselage, engines meet pylons, etc.
• Creates additional turbulence and pressure losses beyond what isolated components would produce
• Reduced through fairings and fillets that smooth the junction between intersecting surfaces

---

Quick Reference Table

---

Self-Check Questions

1. Which two drag types both increase with the square of velocity, and what physical mechanism do they share?

2. An aircraft is flying slowly at a high angle of attack during approach. Which drag type dominates, and why does increasing airspeed actually reduce it?

3. Compare and contrast form drag and wave drag: What do they have in common, and under what flight conditions does each become significant?

4. A designer adds winglets to an aircraft. Which specific drag type are they targeting, and what physical phenomenon (involving wingtip flow) are they disrupting?

5. If an FRQ asks you to explain why total drag has a minimum at a specific airspeed, which two drag categories must you discuss, and how do their speed dependencies differ?