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Parasite Drag: What Is It?

Erik Brouwer

It's one of the least understood principles when it comes to discussing aircraft performance and aerodynamics: parasite drag. So what is it, and how does it affect your plane? Let's take a look.

The Basics Of Drag

If you want to understand parasite drag, you'll need a little background on what "drag" is in general. When two masses are in contact, they resist each other's motion. In the case of an airplane, air resists the forward motion of the airplane. So when it comes to flying, drag is the resistance of an aircraft's movements through air.

Total drag comes from different types of drag, and it has two major classifications. But today, we'll stick to parasite drag, and we'll talk about induced drag in a future article.

So What Is Parasite Drag?

Parasite drag is simply caused by the aircraft's shape, construction-type, and material. For instance, an airplane with a rough surface creates more parasite drag than one with a smooth surface. And there are three basic types of parasite drag:

1) Skin Friction Drag is the result of the aircraft's surface being rough. Olympic swimmers wear swim caps on their heads, so hair doesn't create extra drag and they can swim faster through the water. This same principle can be applied to most aircraft, where a smooth skin reduces skin friction drag, improving performance and fuel efficiency.

2) Form Drag is the result of an object's general shape in relation to the relative wind. Have you ever stuck your hand out the window of the car, first tilting it flat, and then vertical, into the wind? You might've noticed how when your hand is horizontal like an airfoil, that little effort is required to stick it outside. But when you open your hand into the wind, your hand has a tendency to fly backwards and requires more force to hold it position. That's the easiest way to understand form drag.

When it comes to airplanes, a Boeing 747 creates much more form drag than something small like your Cessna 172. And a thick Piper Cherokee wing produces more form drag than a Cirrus SR22 wing. Many aircraft are designed with retractable landing gear which eliminate or reduce the form drag created by the landing gear, once they're retracted. But the form drag from retractable landing gear can be used to your advantage when you're trying to slow down quickly, for instance. Finding a balance between form drag, performance, and aircraft size is one of the most tricky engineering aspects behind designing an airplane.

3) Interference Drag is generated by the mixing of airflow streamlines between airframe components such as the wing and the fuselage or the landing gear strut and the fuselage. As air flows around different aircraft components and mixes, a localized shock wave is formed, creating a drag sum greater than the drag that components would have by themselves.

For instance, look at where the fuselage and wing meet. Interference drag forms behind the trailing edge of the wing adjacent to the fuselage. Airflow overtop and underneath the wing mixes with airflow around the fuselage, creating interference drag. So if the wing was flying without an attached fuselage, there wouldn't be interference drag at this location. This type of interference drag can be minimized by the use of fairings to ease the airflow transition between aircraft components. A Cessna 172 wing strut for example has fairings around the base and top of the strut, where the strut meets the fuselage and wing.

Why It's Important To Understand Parasite Drag

It doesn't matter how big your plane is, how much thrust it has, or how many passengers it can carry. Understanding the basics of parasite drag helps you understand how aerodynamics work, and how it affects your airplane's performance.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and commercial pilot for Mokulele Airlines. In addition to multi-engine and instrument ratings, he holds a PIC Type Rating for Cessna Citation Jets (CE-525). He graduated as an aviation major from the University of North Dakota in 2018. He's the author of the articles, quizzes and lists you love to read every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures at http://www.swaynemartin.com.

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