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
The inlet lip design on those older turbofans didn’t allow for great efficiency at low speed. Those extra inlets around the edge open at low speed to get more air into the compressor, and then close when the aircraft speeds up.
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