Originally Posted by
Vessbot
That being said, I'm just an interested nerd, who's also waiting for Tdacer to expand on, and fix any inaccuracies. For one, I think it might be that the design condition has the air coming from straight ahead (the slug of air that enters the inlet has the same diameter as it does far ahead before any disturbance) while the expanding flow is at a speed faster than design. And the primary reason the inlet expands is to slow down the air and increase its pressure. And I'm wondering how this interacts with the "matching" of the airflow field at the lip I mentioned earlier.
Actually you explained it rather well in the previous post.
Inlets are designed to work very well and be very efficient at cruise - not takeoff. When sitting statically, as you explained the air is coming from all around - not just straight ahead - and the air to the side or rear has to turn a pretty sharp corner going around that inlet lip, that's where the inlet can separate (it can also happen if you over-rotate on takeoff - the bottom of the inlet lip can separate and cause an engine surge at a really bad time). Fans really dislike lots of distortion at the fan face - and in severe cases that distortion can even make it to the core compressor which likes it even less...
Now, this can all be made better by putting a bigger radius on the inlet lip (i.e. making the inlet thicker/fatter) -
but - that makes the inlet less efficient and increases cruise drag.
One solution that has been used is called 'blow-in doors' - spring loaded doors in the inlet that open inward due to the suction at low airspeeds so the air doesn't have to turn that sharp corner around the inlet. These are used on some military aircraft (I believe the Harrier had them) and were on the early 747s. The 747 was originally designed to cruise at Mach 0.87 and higher - so it had a very thin inlet lip - but that was prone to separation at low speed so they added blow-in doors. But that created a new problem - blow-in doors are noisy. Not only do they remove some of the acoustic treatment in the inlet, they create vortices that enter the fan and that creates a lot of noise. After the 747 entered service, it was determined that it seldom cruised that fast, so the later 747s got a redesigned inlet with a thicker inlet lip that didn't need blow-in doors.