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Old Nov 17, 2012, 03:46 AM
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EDF duct reduction

Hey guys. been trying to find a thread I saw a while back that gave a good bit of math on reducing the diameter of a duct for an EDF. I've been searching the forum and even google. I think im just too tired to word the search right.

Also, I've got a couple of 10 year old GWS 55mm EDFs and am wanting to convert them over to brushless, but not sure what a good option would be for these EDFs.

I've also seen a few (larger) edfs that have the wireing come out the front of the system. This type of setup would work best on my designs that I am working on but im only finding them in 90mm+ sizes. Anyone have an idea of if anyone makes this type of EDF in the smaller sizes? Google searches just show tons and tons of EDFs, not sure if there is a special classification for a style with the wires coming out the front.

thanks in advance for any help
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Old Nov 17, 2012, 05:40 PM
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I believe you want an 85% fan swept area at your exhaust outlet. You can calculate the area of the whole fan then subtract the area covered by the hub/motor to get your "FSA."

As for your brushless motors, I believe those will accommodate a 22mm somewhere on the 3000kv range for 3s...GWS offered motors specifically for that application.
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Old Nov 17, 2012, 10:24 PM
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thanks MRGTX
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Old Nov 18, 2012, 11:53 PM
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So I just did the math for the reduction. A 55mm EDF with a 28mm center has a fan area of 1760^2mm. down to 85% its 1496^2mm. so a 55mm edf reduced down to 85% is about 44mm diameter circular outlet.
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Old Nov 19, 2012, 10:09 AM
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Quote:
Originally Posted by OutcastZeroOne View Post
So I just did the math for the reduction. A 55mm EDF with a 28mm center has a fan area of 1760^2mm. down to 85% its 1496^2mm. so a 55mm edf reduced down to 85% is about 44mm diameter circular outlet.
That sounds about right to me.

You might want to experiment a bit with this. Sometimes it's beneficial to have a larger exhaust diameter (which tends to make more thrust) and sometimes it's beneficial to have a smaller one (which tends to increase the velocity of the exhaust and hopefully speed of the plane). Your particular airframe, fan, motor combo may benefit from different dimensions....and that's the fun of playing with EDFs.

It's possible to choke down too far and not have enough thrust on hand to take advantage the exhaust velocity.

If you're not commited to your fans (and assuming you still have yet to buy motors), check out www.donsrc.com for some excellent small fan stuff (usually 64mm and smaller. iirc). Don offers dynamically balanced combos which run as smooth as silk. Everything I have bought from him works as advertised (or better). He also has templates for exhaust cones which are a great help.

No affiliation, just a happy customer.
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