PPRuNe Forums - View Single Post - Theory on lift
Thread: Theory on lift
View Single Post
Old 26th Jul 2012, 08:30
  #41 (permalink)  
Owain Glyndwr
 
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes on 0 Posts
I wasn't talking about a turbojet, I was asking about the intake system that creates 63% of the thrust at cruise speed.
OK, you are asking about the bleed air, but it wasn't clear from the context of your text.

Regarding the multi-stage propeller bit: I understand that the blades act like a multi-stage propeller but I was saying their purpose is to compress the air and not to directly provide thrust like a propeller.
True of the compressor blades (on a turbojet) but not true of the turbine blades which do provide thrust (reaction to the acceleration of exhaust gases), just like a propeller produces a higher than freestream velocity behind it.

What is the "inlet air"? Where does that "inlet air" go? In the picture it appears that some air goes along the top, following the first arrow and bypassing the engine intake, through a tiny passage and then gets dumped at the rear of the engine right before the divergent exit. There is also another passage on the bottom of the engine where air bypasses the engine intake and meets at the rear of the engine right before the divergent exit. Does the air that exits out of those two passages contribute to the "inlet thrust"?
From what I know of the design, the "inlet air" is everything that went into the mouth of the intake. Some of that was bled off the roof of the intake (about where the flow went through Mach 1.0. That bleed air was, I believe, ducted to flow around the outside of the engine as cooling air. The top and bottom passages you refer to are two bits of the same passage - it was just split to get cooling air to the lower part of the engine.

This cooling air has to be dumped as efficiently as possible - on Concorde this was done by using it as 'secondary flow' in the final propelling nozzle. It therefore contributes to thrust eventually, but would not be considered to be part of intake thrust as being discussed here.

Edit: 8% of the thrust is from the engine in cruise - does that mean that the fuel burned is only related to that 8% that is from the engine? NONE of the thrust from the inlet is made by combustion?
Not sure what you are getting at, but certainly none of the intake thrust was made by combustion - unless you count that bit of the combustion energy that was used to drive "the inlet pumps" as described earlier!
Owain Glyndwr is offline