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No Varable Area Fan Nozzles For CSeries...Why?  
User currently offlineJoeCanuck From Canada, joined Dec 2005, 6364 posts, RR: 33
Posted (3 years 5 months 1 week 7 hours ago) and read 3156 times:

I was digging through some CSeries news and I stumbled upon something I found both interesting and baffling; Pratt has dropped VAFN's for the CSeries engine but is keeping them for the NEO engine.

What exactly are VAFN's and what makes them advantageous for the NEO engines but not for the CSeries engine?

This pdf explains the change but not the technical aspects;


What the...?
2 replies: All unread, jump to last
User currently offlineCM From , joined Dec 1969, posts, RR:
Reply 1, posted (3 years 5 months 1 week 5 hours ago) and read 3120 times:

The VFN is a device to reduce bypass area at low thrust settings. You can think of it as a very small/simple thrust reverser which partially blocks the bypass air flow... effectively reducing the bypass ratio of the engine. The GTF architecture (and other very high bypass ratio engines) are designed for a very slow fan speed relative to the core. As a result, when the core is at very low speeds, the fan is at increased risk of stalling the LPT. Reducing the fan flow area and keeping some back side pressure on the fan helps prevent this.

User currently offlineJoeCanuck From Canada, joined Dec 2005, 6364 posts, RR: 33
Reply 2, posted (3 years 5 months 6 days 17 hours ago) and read 2962 times:

That makes sense. My blind guess was that it was a high speed thing. Under what conditions would this most likely be put to use; taxi, to/climb, cruise or descent?

What the...?
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