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Your plane creates interference drag every time you fly. But what exactly is it? Here's what you should know...

Mixing Airflow

Interference Drag is generated by the mixing of airflow streams 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, it needs to speed up in order to pass through the restricted area. As the air speeds up, it requires extra energy. At the same time, it creates turbulence, resulting in an increase in drag. The more acute (sharp) the angle, the greater the interference drag that's generated.

Where You'll Find Interference Drag

You can typically find interference drag anywhere you find a sharp angle on your plane.

For example, look at where the fuselage and wing meet. Interference drag forms behind the trailing edge of the wing adjacent to the fuselage. Airflow over top and underneath the wing mixes with airflow around the fuselage, creating interference drag. If the wing was flying without an attached fuselage, there wouldn't be interference drag at this location.

To reduce the drag, designers use fairings to ease the airflow transition between aircraft components, like what you see in the picture below.

But interference drag isn't just limited to where the wings and fuselage meet. 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. Without fairings, these connections form noticeably acute angles, significantly increasing interference drag.

So if there's drag produced by the strut connection points, why don't they just design the airplane without struts?

Well, they have. But it's not always ideal. The Cessna 177 Cardinal was developed as a replacement to the Cessna 172 Skyhawk. The C177 doesn't have wing struts. Instead, it relies on a cantilever wing for structural support. In general, cantilever wings weigh more, and are more expensive than adding wing struts to the exterior of the airplane.

Retracting The Gear

If you've flown airplanes with retractable gear, you should know that as you retract the gear, you significantly increase interference drag. You might be thinking, "Wait a second, I thought retracting the gear should decrease drag, not increase it?" Don't worry, you're not totally off.

As the landing gear retract into the fuselage, you're creating a progressively acute angle between the fuselage and landing gear strut. And since tight, acute angles cause more interference drag than wide angles, you'll be momentarily increasing drag as your gear retracts. The moment just before the gear move into the fuselage is where the most interference drag is created.

It's something to keep in mind when you're flying out of hot, high-density altitude airports.

If you're just above the ground and beginning your climb, you'll reduce your climb performance in the process of bringing the gear up. On hot days, wait a few extra seconds and climb up to a higher altitude before bringing up the gear.

Fortunately, the performance loss will only last as long as it takes for the gear to fully retract.

As the angle between airframe components shrinks, interference drag increases. That's why you see fairings placed around most airplanes where the sharp angles meet.

And remember that interference drag is just one of three major forms of parasite drag. The other two? Form drag and skin friction drag.

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