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This article is all about interference drag in airplanes. Let’s dive in!
Interference Drag: What It Is and How It Happens
Interference drag is caused by the mixing of airflow streams between different parts of the aircraft, like the wing and the fuselage, or the landing gear strut and the fuselage. When the air flows around these components and has to merge, it has to speed up to pass through the restricted area. This speeding up requires extra energy, and it also creates turbulence, which leads to an increase in drag. The sharper the angle, the more interference drag is generated.
Where to Find Interference Drag
You can typically find interference drag wherever there are sharp angles on your plane. For example, take a look at where the fuselage and wing meet. Behind the trailing edge of the wing adjacent to the fuselage, interference drag forms. The airflow above and below the wing mixes with the airflow around the fuselage, creating this drag. But if the wing were flying without the fuselage attached, there wouldn’t be interference drag at that location.
To reduce drag, designers use fairings to make the transition of airflow between aircraft components smoother. Take a look at the picture below to see how fairings can ease the airflow.
Interference Drag Isn’t Just at Wing-Fuselage Junctions
Interference drag isn’t limited to just where the wings and fuselage meet. A Cessna 172 wing strut, for instance, has fairings around the base and top where it connects to the fuselage and wing. Without fairings, these connections form very acute angles, which greatly increase interference drag.
Why Don’t They Just Design Planes Without Struts?
Well, they have. The Cessna 177 Cardinal was developed as a replacement for the Cessna 172 Skyhawk. The C177 doesn’t have wing struts; instead, it relies on a cantilever wing for structural support. Generally, cantilever wings are heavier and more expensive than adding struts to the outside of the airplane.
Retracting the Gear and Its Impact on Interference Drag
If you’ve flown airplanes with retractable gear, you might have wondered why retracting the gear seems to increase drag instead of decreasing it. As the landing gear retracts into the fuselage, a progressively acute angle is created between the fuselage and the landing gear strut. And since tight, acute angles cause more interference drag than wide angles, you’ll experience a momentary increase in drag as the gear retracts. The moment just before the gear enters the fuselage is when the most interference drag is created.
This is something to keep in mind especially when flying out of hot, high-density altitude airports. If you’re just above the ground and starting your climb, bringing the gear up will reduce your climb performance. On hot days, wait a few extra seconds and climb to a higher altitude before retracting the gear. Fortunately, the performance loss is only temporary until the gear fully retracts.
The Relationship between Airframe Components and Interference Drag
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, interference drag is just one of three major forms of parasite drag. The other two are form drag and skin friction drag.
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