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Old Nov 20, 2012, 11:28 PM
TEAM FUTABA TEAM DURALITE
mithrandir's Avatar
Joined Nov 2004
742 Posts
Discussion
General trends and 'splanations

can some of yuh'all comment on some of the basic configurations of an EDF power system and what the effects and reasons for said configurations might be?

For example... why would I choose a 65% vs a 85% exit area ratio?
What inlet areas would I choose for what reasons? Big inlet? Small inlet?
Round lips vs sharp lips? Diffusion (if any)???

I have heard the term "Eflux" used in the context of thrust....
Comments such as "Higher thrust with lower eflux".... is higher "Static" thrust what is meant?

thanks
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Old Nov 20, 2012, 11:37 PM
Do it Right, the first time!
CoolerByTheLake's Avatar
United States, MN, Hermantown
Joined Dec 2008
5,019 Posts
That might take a small book!
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Old Nov 20, 2012, 11:41 PM
right between the batteries
Justo23's Avatar
Farmington UT
Joined Jun 2010
143 Posts
Quote:
Originally Posted by CoolerByTheLake View Post
That might take a small book!
Agreed, but i have also said before that all this info should be consolidated into a "Sticky" at the top of this forum for ease of access and elimination of threads like this one as well as my own "Thrust Tube" thread...

Just a hint, if anyone gets EXTREMELY BORED in the wee hours of the morning lol
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Old Nov 20, 2012, 11:54 PM
Do it Right, the first time!
CoolerByTheLake's Avatar
United States, MN, Hermantown
Joined Dec 2008
5,019 Posts
Most of what I learned at first was just reading the threads and lots of searches. I don't think I made my first post for months after I joined. Just read.
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Old Nov 21, 2012, 12:15 AM
turbonut's Avatar
upland CA
Joined Dec 2004
7,204 Posts
Quick cover
smaller intake looks scale but wont feed the fan so makes for a long take off run
smaller exhaust is more efflux(faster jet stream) but less thrust so longer take off run but more top speed...make it too small and you loose both.
So if you have a light weight speedy design you can use the smaller holes
if its a bit heavy you will need more thrust so bigger holes...
very over simplified but that is the idea
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Old Nov 21, 2012, 12:30 AM
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LuvEvolution7's Avatar
St.Catharines, Ontario
Joined Jan 2009
10,377 Posts
Quote:
Originally Posted by mithrandir View Post
can some of yuh'all comment on some of the basic configurations of an EDF power system and what the effects and reasons for said configurations might be?

For example... why would I choose a 65% vs a 85% exit area ratio?
What inlet areas would I choose for what reasons? Big inlet? Small inlet?
Round lips vs sharp lips? Diffusion (if any)???

I have heard the term "Eflux" used in the context of thrust....
Comments such as "Higher thrust with lower eflux".... is higher "Static" thrust what is meant?

thanks
first of all, you wouldn't pick an exit area that small (65%). that's way too small and will stall the fan out. 85% is much more sensible for all around flying. if you want a speed model that's only good in that part of the envelope, I may push that to 70%, but that's about it. if you want max all around performance, you want around 100% FSA intakes and 85% exhaust. if you want to go fast at the expense of a bit of low end performance, you would go 85% intake and 85% exhaust. what you don't ever want to do, is go less on the intake than the exhaust. that's a no no and sure fire way to a smashed up plane.

you always want to strive for a rounded lip and not a sharp one. remember that fans pull air in from all directions, not just in front, so you want the smoothest transition as possible. rounded lips mean maximum thrust. sharp lips create vortices that rob power as they hit the fan face. you'll hear this at work too, cause the fan will be noisy as hell. your inlet ducts should be perfect too. they should never change at angles greater than 3 degrees and should transition as smooth as possible. if they diverge too fast, it will cause seperation, which again leads to eddies and vortices, which leads to lousy performance and noisy fans.

another thing to consider, is how the inlet(s) get the air to the fan(s). if it's a single duct to a single fan, you don't have any problems, as long as you stick to that 3 degree rule. if it's a two duct to a single fan, you've got to plan a little. for example, you want the two ducts to meet as close to the fan as possible, but not so close that it creates eddies and vortices that rob power. for that reason, you should allow the ducts to meet within about 2 inches of the fan face.

as for efflux and thrust, it's pretty simple. thrust is the oomph behind the plane, or the way the efflux columb interacts with ambient air. static thrust is at its highest when the fan moves as much air as is possible. the problem with this, is that the velocity of the air (efflux) is relatively low. if we want to go faster, we've got to increase the pressure. to increase the pressure, we neck down the exhaust from 100% FSA to 85% FSA, for example. what this does is back up the air a bit, which increases the pressure. this increase in pressure also increases the velocity of the air. imagine a hose pipe with your finger on it, as opposed to water just pouring out. same effect. we can only do this to a point though, before the air backs up so much, that it stalls out the fan. anything less than 70% will stall out the fan. so, high static thrust is good for applications such as airliners, but not so good for F-16 like performance.

hope this helps a bit, for now.

Rich
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Old Nov 21, 2012, 12:07 PM
Firecracker!
BillO's Avatar
San Francisco & Santa Cruz
Joined Oct 2004
2,352 Posts
Why should bifurcated intakes meet close to the fan? A single larger pipe has less drag then 2 smaller pipes of the same area. So wouldn't it be best for the two intakes to meet as soon as possible, subject to the smooth transitions rule?
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Old Nov 21, 2012, 01:01 PM
Air Assassin
RogueAce's Avatar
United States, FL, Pompano Beach
Joined Oct 2012
353 Posts
Rich..VERY nice explanation. Simple and concise. BillO...The bifurcated ducts meeting shorter would create vortices just like meeting too close to fan. Also the 2 ducts create the pressure that Rich had described. These things are all A BALANCE. To get more of 1 aspect you give up some of another. Short distances can create the same turbulence as long ones.
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Old Nov 21, 2012, 01:17 PM
turbonut's Avatar
upland CA
Joined Dec 2004
7,204 Posts
I like them to meet as far from the fan as possible but not to make large angle changes..if too close to the fan the air cant swirl to meet the blades
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Old Nov 21, 2012, 04:53 PM
Registered User
LuvEvolution7's Avatar
St.Catharines, Ontario
Joined Jan 2009
10,377 Posts
yeah, everyone's got their own oppinions and methods of doing things. it's what makes this hobby so great. we can always learn something off each other.
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Old Nov 22, 2012, 06:28 AM
Life begins at transition
Australia, VIC, Sale
Joined May 2007
3,593 Posts
I'm not gunna argue with Stu!
http://www.rcgroups.com/forums/showt...t#post16274544
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