Drag is the aerodynamic force that acts opposite an aircraft's motion through the air. Pilots and engineers must understand drag because it affects speed, fuel use, climb performance, range, and safety. In aviation, drag is usually grouped into parasite drag, induced drag, and wave drag.
Each type has a different cause, so each one changes differently with airspeed and aircraft shape.
Parasite drag comes from the airplane pushing through air and from air rubbing across surfaces, so it generally increases as speed increases. Induced drag is produced as a side effect of making lift, especially through wingtip vortices, and it is largest at low speeds and high angles of attack. Wave drag appears near the speed of sound when shock waves form and disturb the airflow.
The total drag curve combines these effects and helps identify the most efficient flying speed.
Key Facts
- Drag force can be modeled by D = 1/2 rho v^2 Cd A, where rho is air density, v is speed, Cd is drag coefficient, and A is reference area.
- Parasite drag increases roughly with v^2, so doubling speed can make parasite drag about four times larger.
- Parasite drag includes form drag, skin friction drag, and interference drag.
- Induced drag decreases as speed increases for the same aircraft weight because the wing can make lift at a lower angle of attack.
- Induced drag is strongly connected to wingtip vortices and can be reduced with high aspect ratio wings or winglets.
- Wave drag rises sharply near the critical Mach number when shock waves begin forming on parts of the aircraft.
Vocabulary
- Parasite drag
- Parasite drag is drag caused by the aircraft's shape, surface roughness, and parts interfering with airflow rather than by the production of lift.
- Induced drag
- Induced drag is drag created as a result of producing lift, mainly due to tilted lift forces and wingtip vortices.
- Wave drag
- Wave drag is drag caused by shock waves that form when airflow over parts of an aircraft approaches or exceeds the speed of sound.
- Angle of attack
- Angle of attack is the angle between the wing's chord line and the direction of the oncoming airflow.
- Critical Mach number
- Critical Mach number is the flight Mach number at which airflow first reaches Mach 1 somewhere on the aircraft.
Common Mistakes to Avoid
- Thinking all drag increases with speed is wrong because induced drag usually decreases as speed increases for a given aircraft weight.
- Ignoring air density in drag calculations is wrong because D = 1/2 rho v^2 Cd A shows that drag changes when altitude, temperature, or pressure changes.
- Confusing induced drag with parasite drag is wrong because induced drag comes from making lift, while parasite drag comes from shape, surface friction, and airflow interference.
- Assuming wave drag only happens after the whole airplane reaches Mach 1 is wrong because local airflow over curved surfaces can become supersonic before the aircraft itself is flying at Mach 1.
Practice Questions
- 1 An airplane has rho = 1.2 kg/m^3, v = 50 m/s, Cd = 0.030, and A = 20 m^2. Use D = 1/2 rho v^2 Cd A to calculate the drag force.
- 2 If parasite drag is 400 N at 40 m/s and increases with v^2, what is the parasite drag at 80 m/s?
- 3 A pilot slows an aircraft for landing while keeping the same weight. Explain why induced drag increases and describe one design feature that can reduce wingtip vortex effects.