smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

Thin Lipped Air Intakes

Thu May 20, 2010 1:45 am

What is the advantage of having a thin lipped air intake on jet engines especially ones that are planned to be flown at high supersonic speeds, if most air at design cruise speed is coming from head on what is the need for it? How exactly did this effect early jet engines in that they needed blow in air intake doors to make up the mass air flow at low speeds

Cheers
steve
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Thin Lipped Air Intakes

Thu May 20, 2010 2:59 am

Quoting smartt1982 (Thread starter):
early jet engines in that they needed blow in air intake doors to make up the mass air flow at low speeds

Those doors, called alternate air intake doors, on both the old and advanced cowl designs on B707 aircraft fitted with JT3D engines were there to provide additional fan air flow capabilities without enlarging the cowl air intake, to enable higher takeoff weights.

Example.

B707-320B (old cowl, small doors) MTOW, 327,000 pounds.
B707-320B advanced cowl, MTOW 333,600 pounds.
 
B727LVR
Posts: 233
Joined: Fri Jul 11, 2008 7:54 am

RE: Thin Lipped Air Intakes

Thu May 20, 2010 6:46 am

Quoting smartt1982 (Thread starter):
high supersonic speeds

Dont forget that at those speeds the air spped needs to be slowed down. For example, the F-14 (i know you meant pax, but this is just an example) the forward most part of the inlet narrows, and then the rest of the inlet opens back up slowing down the air into the engine. Its all part of bernuli's principle.
I'm like a kid in a candy store when it comes to planes!
 
GST
Posts: 808
Joined: Wed Jun 11, 2008 10:27 am

RE: Thin Lipped Air Intakes

Thu May 20, 2010 7:33 am

Shock waves are necessary to slow down the air to a speed that will not destroy your engine, and indeed to create lift on a supersonic wing. Unfortunately they also cause drag, and if they are not attached (a bow wave), they cause a huge amount more drag. What happens when air is forced round a sudden corner is that it is more or less instantaneously turned parallel to its new boundary through the shock wave. For any mach number, there is a critical maximum angle that it can be instantaneously turned by, and if the boundary (in this case an engine intake) is forcing it to turn a greater amount, the shock wave will be pushed in front of the body to lessen the angle that it must turn the airflow. Thus is created an extremely draggy bow wave. It is basically for this reason that a supersonic jet has sharp intakes so air going round the side does not have to turn a great distance, and the wave remains attached. Same for as well as sharp noses, leading edges etc, as it limits the shock wave drag.

There are also shock waves that are induced inside of intakes, as many are necessary to decelerate incoming airflow to subsonic speed where traditional diverging ducts and such can be used to decelerate it to a speed the engine is happy with. The sharp leading edges of the intake trigger the first shock waves in the intake airflow's deceleration.
 
Viscount724
Posts: 18832
Joined: Thu Oct 12, 2006 7:32 pm

RE: Thin Lipped Air Intakes

Fri May 21, 2010 1:27 am

Quoting 411A (Reply 1):
Quoting smartt1982 (Thread starter):
early jet engines in that they needed blow in air intake doors to make up the mass air flow at low speeds

Those doors, called alternate air intake doors, on both the old and advanced cowl designs on B707 aircraft fitted with JT3D engines were there to provide additional fan air flow capabilities without enlarging the cowl air intake, to enable higher takeoff weights.

JT8Ds on early 737-100/200s also had blow-in doors, visible in photos below.


View Large View Medium
Click here for bigger photo!

Photo © Ellis M. Chernoff
View Large View Medium
Click here for bigger photo!

Photo © Mel Lawrence


View Large View Medium
Click here for bigger photo!

Photo © Jan Olav Martinsen

Who is online

Users browsing this forum: No registered users and 26 guests