Making Supersonic Inlet Design Easier


New Member
Feb 16, 2009
Inlet design has been a black art since the invention of the jet engine, and designing an inlet that works well at subsonic and supersonic speeds has always been a challenge (ask the F-111 folks). Now Boeing has received a US patent for a multi-path inlet concept that promises to make aircraft-engine integration and optimization an easier task.

[Superficially it looks like the F-117's inlet, but that was an inlet screen for stealth and did not direct or compress the airflow.]

Boeing's multi-path inlet comprises a two-dimensional array of elements, or passages, each shaped to compress and deliver air to the engine. This tackles one of the challenges - designing an intake with sufficient length to provide the required compression that fits in the limited space available. The array inlet, according to the patent, can provide the same compression with one-tenth the length, making it easier to integrate the inlet in a wider variety of locations on the aircraft.

The array inlet can be attached to a movable mounting to tackle one of the other design challenges. Supersonic aircraft need to "start" the inlet by first swallowing external shock waves to establish stable flow conditions inside the duct. If the inlet is "unstarted" and the shock waves remain outside, engine thrust is reduced. Starting typically requires a variable-geometry inlet that adds weight and complexity, the patent says.

In Boeing's design, the movable array inlet would start in a position that provides a larger frontal area and airflow. When the aircraft accelerates to supersonic speed, the 2D array would move to a position that reduces frontal area until the shocks are swallowed, typically in less than a second, then move to a third position with greater frontal area but which could vary with speed and altitude. A flexible duct would link the movable array to the engine.

There are other aspects to the Boeing concept, including making the array conical or conformal to the aircraft's contours and using the outer elements of the array to remove boundary-layer air. And the individual passages don't need to be four-sided, the patent states: they could have three sides for compactness, or six for strength.

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