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.
Seriously, though, drag is simply air resistance to aircraft movement. It’s principally composed of parasite and induced drag.
In this article, you’ll gain an understanding of the meaning, influencing factors, calculation, and types of parasite drag. Plus a few ways to reduce it.
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 of Parasite Drag
There are three parasite drag 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.
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.
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.
The following equation is used to calculate parasite drag:
DP = ½ ρ V2 C,0 A
Where ρ = air density, V = velocity (airspeed), C,0 = zero-lift drag coefficient, and A = surface area.
It’s easy to see from the equation that parasite drag is directly proportional to a few variables, including air density, airspeed, and surface area. For example, it increases with the square of the airspeed.
Other factors play a role, though, which include 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.
Much effort goes into minimizing drag in designing and building aircraft because less of it means lower fuel consumption and better airplane performance.
Aircraft design employs streamlined components, fairings, and flush-mount rivets to reduce drag.
As a pilot, here are a few ways in which you can reduce parasite drag:
- Keeping airplane surfaces clean
- Closing doors, hatches, and windows
- Retracting landing gear and flaps
Flying at optimal altitudes and airspeeds also helps reduce drag.
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.
Parasite drag is a critical component of the total drag acting on an aircraft. While there’s no way to eliminate it completely, certain design elements and operational measures can substantially reduce it.
This type of drag isn’t all bad despite its name. For example, it plays a beneficial role in airspeed reduction during landing.
Having a solid grasp of parasite drag is essential for pilots because of its significance in flight operations, aircraft performance, and fuel efficiency.