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Electric Ducted Fan Jets Discuss electric ducted fan jets here including setup tips, power systems, flying techniques, etc.

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Old 07-23-2011, 03:38 PM   #1
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Exclamation Thrust tube draw more amps?

My current edf unit is a 5250kv 6 blade unit with 36 amp draw at load. Will the addition of a thrust tube with 80% FSA increase the amp draw?
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Old 07-23-2011, 04:39 PM   #2
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Maybe a little because it will increase the back pressure on the fan so increase loading on the motor.. I'd not expect to see a big difference though.
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Old 07-25-2011, 02:42 PM   #3
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I think you will find that for any given power setting an EDF will actually draw most current (and run at the slowest rpm) with no duct.
A short well designed duct can increase the net thrust even though the current will have dropped slightly compared to free air but generally the more the air flow is restricted the more the fan speeds up and the current falls.
Long ducts can reduce the maximum thrust of a fan quite significantly.
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Old 07-25-2011, 04:03 PM   #4
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Raitheslacker,

There you go.. One poster (yours truly) says a tapered thrust tube will slightly reduce static thrust and maybe, if anything, slightly increase amps...
Another (very experienced) poster seems to say the opposite...



So who is right?.....Guess you need to try it and see

Steve

PS Quorneng,
The question here is not 'duct or no duct'.. All commercial fan units come with a short duct or 'shroud' included. The question posed was the effect of adding a slightly tapered cone (a.k.a. 'thrust tube') to the outlet of the standard straight duct.
Does that change your answer?
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Old 07-26-2011, 04:53 PM   #5
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Hmmm both reasonings seem logical.

What would be considered a good length for a thrust tube? I currently made one that is 70mm in length.
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Old 07-26-2011, 07:28 PM   #6
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Well just for S&G's I ran an EDF set up on MotorCalc.
First off was just the fan unit 1"intake length and exhaust, diameters the same.
Second with a 70 mm exhaust length, no drop in size intake/exhaust diameter
Third same 70mm exhaust, exhaust dropped to 80% and intake still the same

Here's what M/C showed

Amps/volts stayed the same on all three.

#1 example Thrust x, speed x
#2 example Thrust dropped .4 %, speed stayed the same
#3 example, Thrust dropped 37% from #1 spec, but speed increased 36%

When I die, I want to go like my Grandfather did, in his sleep...... Not screaming like the passengers in his plane.
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Old 07-26-2011, 07:33 PM   #7
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My experience agrees with JetPlaneFlyer's. Adding any amount of duct to the bare fan unit increases the amp draw especially if its tapered down.

As far as the ideal length - less is generally better than more - but, thats more a function of the plane its installed in than anything else.

You usually dont have much if any choice on duct length. The most you can do is play with the exit diameter to maximise eflux velosity vrs thrust depending on whether or not you want max speed or max thrust or a compromise.

I think I need a signature.
Larry
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Old 07-26-2011, 07:57 PM   #8
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Originally Posted by Glacier Girl View Post
Well just for S&G's I ran an EDF set up on MotorCalc.
First off was just the fan unit 1"intake length and exhaust, diameters the same.
Second with a 70 mm exhaust length, no drop in size intake/exhaust diameter
Third same 70mm exhaust, exhaust dropped to 80% and intake still the same

Here's what M/C showed

Amps/volts stayed the same on all three.

#1 example Thrust x, speed x
#2 example Thrust dropped .4 %, speed stayed the same
#3 example, Thrust dropped 37% from #1 spec, but speed increased 36%

That is some good info. Are you able to run a test with 90% FSA with 70mm in length? Curious as to how much thrust is lost but speed gained.
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Old 07-26-2011, 08:00 PM   #9
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Originally Posted by Larry3215 View Post
My experience agrees with JetPlaneFlyer's. Adding any amount of duct to the bare fan unit increases the amp draw especially if its tapered down.

As far as the ideal length - less is generally better than more - but, thats more a function of the plane its installed in than anything else.

You usually dont have much if any choice on duct length. The most you can do is play with the exit diameter to maximise eflux velosity vrs thrust depending on whether or not you want max speed or max thrust or a compromise.
I'm actually making ducts for a F22 foamie im building. Trying to get the most speed out of it without losing too much thrust. Do you think 70mm length is ok or should I go shorter?
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Old 07-29-2011, 03:13 AM   #10
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70mm at 90% showed an approx 25% drop in thrust, and a 25% increase in speed, over a same diameter intake/exhaust, per MotoCalc.


You know you could always build an adjustable tulip style tail cone to give you the best of both worlds. Opened up and you get lots of thrust, and some speed, closed you loose thrust but you gain a bunch of speed.

One of my flying buds did that on his Multiplex Twister. Amazing bit of craftsmanship, using CF thread to pull the tulips closed via a servo, and letting the airflow blow the tulips open when the servo let tension in the line.
Even more amazing it actually worked. The plane gained speed when the nozzle closed up, vs how it flew open.

When I die, I want to go like my Grandfather did, in his sleep...... Not screaming like the passengers in his plane.
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Old 07-29-2011, 06:17 AM   #11
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Originally Posted by Glacier Girl View Post
70mm at 90% showed an approx 25% drop in thrust, and a 25% increase in speed, over a same diameter intake/exhaust, per MotoCalc.


You know you could always build an adjustable tulip style tail cone to give you the best of both worlds. Opened up and you get lots of thrust, and some speed, closed you loose thrust but you gain a bunch of speed.

One of my flying buds did that on his Multiplex Twister. Amazing bit of craftsmanship, using CF thread to pull the tulips closed via a servo, and letting the airflow blow the tulips open when the servo let tension in the line.
Even more amazing it actually worked. The plane gained speed when the nozzle closed up, vs how it flew open.
Sounds like an awesome but complicated project. Any schematics or pics for it? Im pretty crafty but im I dont think im that crafty to R&D something like that on my own.
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Old 07-29-2011, 09:10 AM   #12
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Originally Posted by Glacier Girl View Post
#3 example, Thrust dropped 37% from #1 spec, but speed increased 36%
Glacier,

Assuming that when you say 'speed' increased you dont mean speed of the plane in flight, you mean efflux speed?

I'm sure everyone understands that it's actual thrust that makes a plane move. The higher efflux speed will mean that thrust 'hold up' better at higher flying speed but if the actual thrust figure is reduced too much then the plane wont have enough thrust to ever attain those higher flying speeds in the first place!
IMHO, in most cases best performance will be found with zero or a very slight reduction of the 'thrust tube' FSA. The 80% figure is I believe used because on scale EDF's the outlet nozzle often has to be larger than scale to give reasonable performance. 80% FSA represents the best compomise between scale looks and thrust loss. Larger % will give more thrust but starts to look silly, anything smaller results in very significant loss of thrust.

But there is no 'one rule fits all' what works best one one model wont necassarily be best for another... It's a similar situation to the balance between static thrust and pitch speed with props.

Steve
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