This question is related to the answer I found on this particular thread, about the Frames in a Turbofan.

In a turbofan what holds the spinning axis?

I went on this thread after some research on Google, since recently I started working in the industry. I'm working on this particular part of the frame, and I would like to understand more about it. Also I am currently working on understanding more about jet engines in general and how they work, I find this absolutly fascinating since I found a lot of information and answers to my questions - and now for the curiosity :)

My question is about the Fan Frame here (GE CF6/CFM56 Turbofan on pics)

I would like to know the use of the 12 Circular Holes on the inside of the frame, between the 2 flows (Low-pressure compressor way, Right?), you can see here what i'm talking about on those pictures (Red arrows).

(I don't think this is to make the part lighter, because the outside edges of thoses holes, are re-inforced a lot with a big extra thickness.)

CF6 Fan Frame. enter image description here

Thank you.

  • 1
    $\begingroup$ A very nice first question! Welcome to Aviation Stack Exchange. If you haven’t already, can I suggest taking the tour and reading the help center so you can learn how this site works and continue contributing quality material like this. $\endgroup$ – Notts90 Dec 17 '17 at 9:24

The primary and secondary flow communicate through the mid-section of the fan frame, by the mean of the variable bleed valves (VBV). These 12 valves are located between the low pressure compressor and the high pressure compressor:

enter image description here

The pressure delivered by the compressor section is optimized for maximum engine power, but when the engine speed is low or the engine is decelerated, the LP compressor pressure can increase and clear the LP compressor stall margin. In such a case, the 12 VBV located in the fan frame are opened to restore a normal air flow.

enter image description here
CFM56-5A, source

enter image description here
VBV on a CFM56-5. Source: CFM56-5 Training Manual

The master valve (on top) is controlled directly by a motor, the other valves are linked to the master VSV by a flexible shaft.

  • 5: Fuel gear motor powered by HMU, controls position of the master VBV
  • 1: Master VBV
  • 2: Position feedback
  • 4: Position sensor
  • 7: Flexible shaft controlling the other valves

Bleed air is release in the secondary flow through vents:

enter image description here


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