The scientific tourist #53 — Splitter plates and boundary layers

This week’s image (a bit delayed due to the holidays) is of an F-4 “Phantom” fighter, waiting for renovation at the Valiant Air Command Warbird Museum in Titusville, Florida:

Boundary layers and splitter plates

In particular, the starring attraction in this shot is a piece of hardware called a splitter plate — it’s the vertical, wedge-shaped thing just ahead of the engine intake.

Quite a few jet aircraft have either a splitter plate, or a lip on the engine inlet(s) that’s shaped to accomplish the same thing — maybe you’ve noticed this before, maybe not. At any rate, splitter plates exist for a very practical reason — they dramatically improve the performance of the plane. Every part of the aircraft in flight is surrounded by what’s called the boundary layer — a relatively thin layer of air next to the body of the plane, moving smoothly (but much more slowly than the main airstream) along the craft. If it weren’t for the splitter plates, the jet engines would be ingesting quite a bit of this slow air (as opposed to the faster air the plane is moving through, dramatically decreasing the jet’s thrust.

In the case of the F-4, the splitter plate has one additional trick up its mechanical sleeve. You can see that there’s a vertical seam in the middle of the plate — while the front part of the splitter plate is fixed, the rear part is hinged. In this way, the rear of the splitter plate can be used to reduce the amount of air entering the jet’s engines during high speed flight. That little post you can see jutting out into the inlet is an airflow sensor, used to control the position of the splitter plate’s ramp.

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