How Interference Drag Affects The Performance Of Your Airplane

To fly efficiently, it’s important that you understand how drag affects your airplane in different phases of flight. Here’s what you should know about one of the most common forms of drag created by your airframe…

MIXING AIRFLOW

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, 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 the angle, the greater the interference drag generated.

WHERE YOU’LL FIND INTERFERENCE DRAG

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. 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.

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, greatly increasing interference drag in these areas.

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 lacks wing struts, instead relying upon a cantilever wing for structural support. In general, cantilever wings weigh more and are more costly than adding wing struts to the exterior of the airplane.

RETRACTING THE GEAR

If you’ve flown airplanes with a 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.

If you’re flying a twin engine airplane out of a high-elevation field on a hot day and experience an engine failure, you should be careful about when you choose to retract the gear. If you’re just above the ground and already struggling to maintain a climb, you’ll likely reduce your performance even more in the process of bringing the gear up. The same is true for a takeoff or go-around in an single-engine retractible gear plane. The performance loss will only last as long as it takes for the gear to fully retract. Try climbing up to a higher altitude before bringing up the gear if your performance is marginal. That way, you won’t descend into the ground with the gear up.

Easy enough, right? As the angle between meeting airframe components shrinks, your interference drag will increase. Remember that interference drag is just one of three major forms of parasite drag; you should know them all.