Parasite Drag and induced drag make up the total drag acting on an aircraft.

Plus wave drag near or at supersonic speeds.

How does it work, what causes it, and what are its types?

Read on to find out.

What is Parasite Drag?

Parasite drag, also called parasitic drag, is defined as the drag caused by any aircraft surface that disturbs, deflects, or interferes with the smooth airflow around the airplane. Parasitic drag is all drag created by aircraft motion through the air except for lift-induced drag.

Parasite Drag Equation

D_P = ½ ρ V² C_DP S

ρ = air density, V = velocity (airspeed), C_DP = coefficient, S = parasite area.

Parasite Drag Coefficient

The parasite drag coefficient, or more accurately, the zero-lift drag coefficient is an aerodynamic parameter used to quantify parasite drag on an aircraft. For example, the P-51 Mustang, like the one featured in Top Gun, has a zero-lift drag coefficient of 0.0163.

Types of Parasite Drag

Parasitic drag includes displacement, turbulence, and restriction of airflow around an airplane. There are 3 types of parasite drag: form drag, interference drag, and Skin Friction Drag.

Form Drag

Form drag, also called profile or pressure drag, is caused by the turbulent wake resulting from the separation of airflow around the aircraft and its components. It mainly depends on the shape and size of an aircraft structure. A flat surface produces more form drag than a streamlined airfoil.

A simple way to understand form drag is to stick your hand out of a car window and feel the air resistance. You’ll feel much less pressure when your hand is flat than when your palm is facing the airstream.

Streamlining airplane parts reduces form drag as you can see in the following illustration.

Interference Drag

Interference drag comes from the interaction of different air currents around adjacent airplane structures. It’s most apparent when components intersect at 90-degree angles. For example, the meeting point of the fuselage and wing produces significant interference drag.

Air flowing around the fuselage meets and mixes with air flowing around the wing at the wing root. This interaction creates a new, turbulent air current that restricts the smooth airflow and produces drag.

Aircraft design employs fairings and distance between components to decrease interference drag.

Skin Friction Drag

Skin friction drag, or simply friction drag, results from the friction between airflow and airplane surfaces because of their roughness. Friction drag is caused by the thin layer of air in direct contact with a surface slowing down and creating resistance to aircraft forward movement.

Skin friction increases with an increase in the area and roughness of a surface. Dirt, ice, and surface imperfections like portruding rivets make surfaces more coarse, which results in more friction drag.

Flush mount rivets, glossy finishes, and keeping the airplane clean are a few ways to minimize skin friction drag.

Factors Affecting Parasite Drag

Many variables influence parasitic drag on an aircraft. Let’s go through them one by one.

1. Airspeed: parasitic drag increases when airspeed increases, in fact, doubling the airspeed quadruples it. The correlation is clear when you look at the drag equation (D_P=½ρV²C_DPS) where V is the velocity or airspeed of the aircraft.
2. Air Density: air density decreases as the aircraft climbs, which means that there’s less air to cause drag at higher altitudes. Therefore, parasitic drag decreases with decreasing air density.
3. Shape: the shape of an airplane structure influences form drag, which is why airplane manufacturers streamline most components to reduce their drag.
4. Surface Area: the larger a surface is, the more drag it creates, which is evident from the drag equation (D_P=½ρV²C_DPS) where S is the parasite area.
5. Configuration: configuration is simply an aviation term for the position of its landing gear and flaps. Extending either will increase the area exposed to the airflow, which increases drag.
6. Contamination: dirt, snow, mud, and other contaminations increase surface roughness, and sometimes, area resulting in more drag.

Conclusion

Parasite drag is a component of the total drag that acts on an aircraft. The parasite drag types are form, interference, and skin friction.

While there’s no way to eliminate parasitic drag completely, certain design elements and measures can substantially reduce it.