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Old Jul 06, 2007, 11:31 AM
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Belgium
Joined Sep 2006
545 Posts
Help!
Need help to improve EDF thrust.

Hi all,

I'm building an EDF F14 tomcat and am trying to optimise thrust from my 2 55mm EDF units.

teststand measurements for 1 EDF unit, not build in: 370g thrust @ 17A on 12V computer powersupply WITH intake ring, WITHOUT exhaust ducting.

When mounted in the plane I got only 200g of thrust @ 20A on 3s lipo. With intake ring, no special exhaust or intake ducting. So ok, as you can see there are some restrictions in the intake, and there is also a 4mm carbon rod sticking through the exhaust.



I made a thrusttube from overhead projector film, ending in 80% FSA. This already improved thrust up to 300g @ 20A per motor.



I also thought rounding the sharp edge around the lite ply wing spar box would increase smoothness of the intake flow and provide more thrust, but to my big surprise it didn't. I lost 20g of thrust, down to 280g of thrust per motor.




Intake area is about 110% FSA.

Can anyone tell me why the thrust went down when I rounded the sharp corner of the wingspar box ???
Would it be better if I put in a piece of thick paper, gently sloping around the wing spar box, in stead of the rounded piece ?
Is it normal to loose about 25% thrust on an EDF when installed into the model ?
Also any additional hints to improve efficiency / thrust would be much appreciated.

Thx !!!
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Old Jul 06, 2007, 12:03 PM
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Ed Waldrep's Avatar
Las Vegas, NV
Joined Dec 1996
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Yes a ramp of paper a couple of inches long would be better, it would be a more gradual change than the lip.

Did your lip go all the way to the top of the inlet? If not, if there was a gap above the lip (the lip shown doesn't extend up to the top of the ply spar box but installed I'm not sure how it would look), air may have been trapped above the lip. Even if the lip did go all the way to the top, a ramp of two inches or so makes a more gradual change than the lip, which makes a sharp curve. If the curve is too sharp air will seperate going around it and tumble, create tiny rotors, become turbulent. The more gradual the change the better.

Loss of thrust in a duct is normal. Once you are flying more air is coming into the ducting and the situation will be better. Maximising static thrust can lead to using cheater holes, and cheater holes hurt the top end speed unless they are designed really well to minimize the disturbance. It depends on the airplane, the F-14 inlets are long but at least they are straight, better situation than the F-16. Use cheaters if you absolutely have to but they will hurt the top end. If the power to weight ratio is low, cheaters may make hand launching easier and the top end speed wont be that high anyway, but for higher power levels a good javelin like heave is all you need for a good launch (and that can be a challenge, I've seen many girly throws and failed launches, usually because the launcher is holding the model with two hands and trying to run, it doesn't work very well).

Ducting improvements: use card stock (thick paper) to create a duct transition piece that is square at it's front and round at it's rear. The piece would end at the face of the fan and be four or five inches long. The front is sealed up against the inlet ducts with glue or tape, the rear would have be held in place with a balsa bulkhead with the material glued to an inside round opening. The balsa is there to form the piece as it's glued in. This "dog collar" or "lamp shade" could be formed and held to shape outside the model by maybe wetting the paper and wrapping it around a form but you'd have to carve the form from something. The front of the fan could be used to form the piece inside the ducting but the bell mouth/inlet lip gets in the way (I with the inlet lip was removable like on Wemotec fans). You can get buy without the transition piece, but it would smooth airflow some and reduce drag.

The other thing is rounding the inlets at the front of the inlet ducts, that'd improve static thrust.
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Old Jul 06, 2007, 07:24 PM
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Belgium
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Ed, thanks a lot for your clear explanations :-)

The lip did go all the way to the top of the inlet. The piece of the wingspar box you still see is normally covered by the top.
Since the lip is only 1cm wide it is probably indeed to sharp a curve. Didn't think of that.

Weight of the model should come out around 800 grams and with 300g thrust per motor I'd have 600 grams thrust as it is now. I will try the ramp idea.
Don't really like to cut cheaterholes in the plane and I kinda made it for (relatively) high top end speed (hence the 80% FSA on the exhaust duct). 600 grams thrust should be ok for a handlaunch on a 800g model, no ?

If thrust will increase with airflow then I think I'll leave it as it is for now and see where I end up with top speed. If I can end up at 70-80 mph I'll be a happy bloke :-) If not I'll give that inlet transition piece a try, but won't that piece get sucked flat and lower my inlet area ? Guess I'd have to kinda tension it when glueing it in so it can't bend inwards ?
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Old Jul 07, 2007, 04:48 AM
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Joined Nov 2006
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Quote:
Originally Posted by Ed Waldrep
The other thing is rounding the inlets at the front of the inlet ducts, that'd improve static thrust.
yes, that's the reason.
Have a look at a perfect intake rounding at Jörg Rehms F18:



Intakearea of F18 by Jörg Rehm.
Good to see is the diameter of the intakelip with a minimum of 3 to 4
millimeters to ensure, that the static thrust doesn't break down.
This works perfect on this model.

Another view:


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Old Jul 07, 2007, 08:29 PM
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Belgium
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I already rounded the inlets as good as I could, though they might still be a little sharp.
I tried the ramp (2 inches long, 1/3th inch high) in the inlet and although it looks much better it didn't really improve things. I also didn't loose thrust like with the lip though, so I might keep the ramp just for the looks. (it hides the wingsparbox and control rods)

I tried to standardise my thrustreadings at 230watts, cause I noticed that the charge condition of the battery can also make 20-30g of difference :

with lip : 280g thrust @230 watts
without any inlet improvements : 320g thrust @ 230 watts
with ramp : 317g thrust. @ 230 watts.

I don't think I'll be doing the transition piece. Seems to me that inlet improvements don't really improve things a lot and it won't be worth the effort. Maybe the fan already gets all the intake air it needs and my losses are in the exhaust with the carbon spar running through ??

Anyway 320g thrust x 2 should be enough to give reasonable performance on a 800g plane, no ?

One last question : 1 of the fans is vibrating rather heavily at high rpm. I should balance it, but it's the type that you press onto an adapter, so I can't stick a steel shaft through it to balance it with my magnetic balancer.
Any ideas how I can still balance the thing ?

Getting EDF's to work good sure is a challenge :-) But I like it !!!
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Old Jul 08, 2007, 01:53 PM
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Joined Dec 1996
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The rear of the transition piece would be held in place with a balsa or lite ply bulkhead. The bulkhead would be a square of material with a hole in the middle basically, and the transition piece/inlet would be glued to the inside of it at it's rear, that is what holds the piece in place and holds it's shape.

You already have the lip on the fan so how much improvement you'd see real world I don't know, probably not enough to worry about.
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Old Jul 08, 2007, 02:35 PM
Effumall
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Joined Nov 2005
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Have you experimented with a 3-bladed fan? Sometimes, you can produce more thrust by spinning fewer blades faster, especially in a small fan like this. Hobby-Lobby discovered just that with the Alfa jets.
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Old Jul 09, 2007, 04:25 AM
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Belgium
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I have experimented with a 2 bladed rotor when a piece of tape accidently got sucked in during a test and reduced the number of blades from 6 -> 2 :-)

The 2 bladed fan delivered about 200g of thrust @13 amps so it was only slightly more efficient but it produced less thrust. Also after a few tests the remaining 2 blades also flew off because, due to the very high RPM they elongated so much that they started rubbing the casing. (possible also balance was off, didn't check that as I don't know how to balance a press-on rotor.

One more modification I plan to do is to put a piece of heatshrink tube around the 3 motor wires where they are in the exhaust duct, so that they get squeezed nicely together and have less frontal area.
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