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Ducting: most importent factors?

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Old 02-25-2012, 09:24 AM
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wsmalley
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Default Ducting: most importent factors?

I'm working on my large scale (108" OAL), Su 27 Flanker using two 5" fans. I've built one 'bell mouthed' inlet and starting on the second. My outlet ducting is built with 1/64th ply, with a slight taper toward the rear. The inlet ducts are not 'lined' at this point..just the formers and outer sheeting. The question is: what features are the most important in achieving the maximum thrust?
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Old 02-27-2012, 01:20 PM
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DanSavage
 
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Default RE: Ducting: most importent factors?

No sharp inside corners. (Keep them rounded)
No double conversions. (Make sure the cross-sectional area doesn't change as the duct moves through the fuselage)
No bends greater than 15 degrees. (Self-explanatory)
Use a 2:1 elliptical inlet lip.
Keep inlet lip area to 90%-110% of the FSA.
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Old 02-28-2012, 11:36 AM
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Default RE: Ducting: most importent factors?

The Double conversion theory should not harm as long as it kept divergent (no more than 11%). Correct you do not want velocities accelerating (increase or decrease) rapidly during the transition. As long as the duct is kept divergent the entire path you can use a smaller inlet area down to 90% of FSA. Obviously if your inlet is above 100% then you can not be divergent. Anything above 100% FSA will cause turbulence at the inlet (air actually spilling out of the inlet) at some point in the speed envelope, limiting higher speed. There are many variables that affect this but bigger is not always better.

Great discussion.

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