Originally Posted by
Check Airman
Your posts from the flight test world are an endless source of fascination. Thank you. When you talk about "separation", am I right in understanding that the airflow separates somehow? If so, I'm having trouble visualising how airflow could separate in front of a what' essentially a giant vacuum.
Like a wing stalling, but the top wing surface is the inside of the duct.
When going fast, the airflow is coming from straight ahead, actually expanding outward kind of as if the engine pod is just a blunt front end of a cigar and the air has to go around it. This is the design condition, and the cowl is designed for that with a slightly inward-turned lip to match the air expanding into it.
The slower you go, the more it sucks air from all around rather than straight ahead, so air comes at the lip inward from the outside. The most extreme of this, a standstill, has air coming in from all directions and is why you see engines on test stands (and some helicopters) have these big tuba horns put on the front to allow the air to flow in smoothly. A standard cowl can't do that, so the air has to make a nasty sharp turn around the lip, and it doesn't take much upset to stall it.
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