Parasite drag. What an interesting name, right?
It may evoke an image of airborne worms living on an airplane and sucking its fuel.
Which is kind of true.
Let’s explore what it is and why it matters in flight.
What is Parasite Drag?
Parasite drag is a type of aerodynamic drag caused by any aircraft surface that disturbs, deflects, or interferes with the smooth airflow around the airplane. It’s all drag created by aircraft motion through the air, except for lift-induced drag.
It’s called “parasite” because it resembles an undesirable entity attached to the aircraft that mostly provides no benefit while reducing its performance.
However, this type of drag can be useful in certain situations where pilots need to lower speed or altitude quickly, such as during the approach to landing.
Types
1. 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.
It’s called so because it’s generated due to the aircraft or component shape (or form).
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.
2. 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.
3. 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 protruding 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.
Calculation
Aside from the drag equation, measuring parasitic drag involves computational fluid dynamics, wind tunnel testing, and actual flight data.
Combining the three offers useful information that helps optimize aircraft design to reduce drag and improve performance.
Coefficient
More accurately, the zero-lift drag coefficient is an aerodynamic parameter that reflects the drag acting on an aircraft when it generates zero lift.
For example, the P-51 Mustang, like the one featured in Top Gun, has a zero-lift drag coefficient of 0.0163.
Equation
- Dp – Drag force (Newtons)
- ρ – Air density (kg/m³)
- V – True airspeed (m/s)
- A – Reference area (m²)
- CD0 – Coefficient (zero-lift, dimensionless)
Factors
Aside from the variables in the equation, other factors play a role, including the following:
- Flight Conditions: air temperature, humidity, and altitude all affect air density, and therefore, the amount of parasitic drag experienced by an aircraft.
- Shape: the shape of the airplane or component influences form drag, which is why airplane manufacturers streamline most components to reduce drag.
- Configuration: configuration is simply an aviation term for, mainly, the position of aircraft landing gear and flaps. Extending either will increase the area exposed to the airflow, which increases form drag.
- Contamination: dirt, snow, mud, and other contaminants increase surface roughness, which increases skin friction drag.
Reduction
Streamlined components, fairings, and flush-mount rivets help reduce form, interference, and skin friction drag.
As a pilot, here are a few ways in which you can reduce parasitic drag:
- Keeping airplane surfaces clean
- Closing doors, hatches, and windows
- Retracting landing gear and flaps
- Flying at optimal altitudes and airspeeds
It’s worth mentioning that some high-performance aircraft use advanced drag reduction methods, such as the hybrid laminar flow control on the 787-9 Dreamliner.
Final Thoughts
Parasite drag is a significant component of the total drag acting on an aircraft, especially at higher airspeeds.
This type of drag isn’t all bad despite its name.
For example, it plays a beneficial role in airspeed reduction during approach and landing.
