Seen on the JT3D on a 707 [here](https://www.youtube.com/watch?v=EI3owgIj6zs) and on the original TF39 on a C-5 Galaxy [here](https://www.youtube.com/watch?v=cpqOxpY2n2Y).
These open up when the engine is throttled up and close when throttled down – are these additional intakes? Why are they included in the design only on a few aircraft? I can only remember seeing them on one more; a variant of the B-52.
In: Engineering
It’s to do with how the aerodynamics of air capture change depending on velocity, which you can see [here.](https://qph.fs.quoracdn.net/main-qimg-df8df6c49ba63efdf58f054cd4ff594f) To gather enough air at low speed, a jet will have to gather it from a wide capture area, which will progressively decrease as velocity increases. At low speeds, the area is so large that it would cause flow separation and turbulence from the extremities of the suction area immediately after clearing the lip of the intake, which would massively impact compressor efficiency. The purpose of the secondary inlet doors is to smooth out this gradient and allow the flow to stay attached to the engine, generating more power with greater efficiency. These doors are usually spring-loaded and opened/closed automatically due to the suction of the air.
More modern jets have big thick intake lips to mitigate this very problem, because it allows air to stay attached even at very low speeds.
Source: Aeronautics Student
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