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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 08-24-2012, 10:26 AM   #1
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Default Two stage jet's ?

Now here's a question ... in the real world some jets use two stage engines ... in some cases multi-stage to get the required thrust / performance.

Has anyone experimented or considered two-stage EDF ? ie a low power EDF feeding a higher power EDF ? Or even two matched powerr EDF's in line ?

For example - an F16 has a long intake duct to it's EDF unit near the rear. Fitting a unit in the fwd section to overcome reduction of flow in the duct .. feeding the aft main propulsion unit to hopefully boost it's performance.

Crazy Idea ? Maybe ... but maybe it's not so bad ?

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Old 08-24-2012, 11:58 AM   #2
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It's been talked about a lot and tried a few times (including at least one commercially available two stage EDF) but basically it doesnt work very well, if at all. EDF's are fundamentally different to turbojets. In turbojets the fans are there to compress the air, not to make thrust. The thrust in a turbojet comes from the expansion of the gasses due to combustion. You will notice that in a turbojet the fans get progressivly smaller as the air is compressed into smaller volume. The multipe fan stages dont increase the volume or speed of air that the engine draws in.

On an EDF the idea isnt to compress the air but to move as large a volume as possble. Two fans doesnt really help much because the volume of air passing through the second fan is always the exact same as through the first fan (it has to be, no air is added between the fans and no expansion takes place) so it's impossible for the second fan to add any volume flow over and above what's going through the first fan.

I practice you could get some extra thrust by two series fans if they were very carefully designed, but the extra thrust would be less than if you just put more power through a single fan, and much less than if you also made the single fan bigger. Plus series fans would be much heavier.

Steve
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Old 08-24-2012, 06:39 PM   #3
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I think you're referring to a 2 spool jet engine. What's happening there is the low pressure compressor and turbine (At the very front and very back) are actually acting as a turbocharger, using the exhaust of the high pressure components to drive it. Sounds simplistic, but that's basically what it is.
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Old 08-24-2012, 07:09 PM   #4
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Nigel
As JPF says the only thing you can do with a second fan is speed up the airflow a bit but to keep the volume the same the second fan will have to be smaller and working how much smaller is quite difficult to avoid either (or both) loosing efficiency.

The only possible benefit would be a higher exhaust velocity but is it really necessary when airliners seem to be able to cruise efficiently at Mach 0.8 using a single stage fan?
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Old 08-24-2012, 07:24 PM   #5
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The idea was born out of the physical construction of the F16 model sitting in front of me ...

Total duct length from intake to exhaust : 85cms
Intake to front of EDF rotor : 62cms
Leaving only 23cms to the exhaust less the EDF unit of course.

That 62cms cannot be good .... So my thoughts were to assist air passage along it .. sort of creating a ram-air system similar to what my race-car had.

We use similar in the pipeline field - pre-booster pumps etc.

Oh well .. it'was a grab at straws idea .. trying to get this darned thrust up to reasonable level .. !

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Old 08-24-2012, 08:51 PM   #6
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Originally Posted by solentlife View Post
That 62cms cannot be good ....
You could try a cheater hole placed mid way between the inlet nozzle and the fan. If you cut out the foam neatly then you can always put if back if it doesn't work.
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Old 08-24-2012, 08:56 PM   #7
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Originally Posted by xmech2k View Post
I think you're referring to a 2 spool jet engine. What's happening there is the low pressure compressor and turbine (At the very front and very back) are actually acting as a turbocharger, using the exhaust of the high pressure components to drive it. Sounds simplistic, but that's basically what it is.
All turbojets work that way (i.e. a turbine in the exhaust driving a compressor in the inlet) What differentiates the two spool engines is that they have two separate turbines and two compressors each mounted on their own shaft and each rotating at different speeds... The principal is the same though.
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Old 08-25-2012, 09:02 AM   #8
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Originally Posted by JetPlaneFlyer View Post
You could try a cheater hole placed mid way between the inlet nozzle and the fan. If you cut out the foam neatly then you can always put if back if it doesn't work.
That crossed my mind actually ... I see that various other models have them ...

food for thought !

Nigel

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Old 11-08-2014, 02:23 PM   #9
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Nigel

Thought you might be interested in my Two Stage EDF development project photos.
Many guys have asked the same question as you as to whether it will work or not.
Well, it does work! I have the basic design configuration worked out and the test article has been flown twice. Much work ahead gathering data form test equipment for modifications.

This is my first post, however, I have been following most EDF Threads since 2011.
Go to the Photo Gallery, Members Gallery, Canned Fans. Click on My album "Two Stage EDF". For some reason all the photos are not initially loaded. It's best if you see them in proper order.

I will start a new Thread on my Two Stage EDF development adventure later this winter 2015! It's possible that it will generate new interest in the subject. Let me know what you think!

Jerry ( Canned Fans )
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Old 11-08-2014, 04:24 PM   #10
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Jerry,

I took a look at the photos, looks like a nice neat installation in the Dolphin, it would be nice to see the specs of the fan unit.
Also I'm very interested in what you mean by 'it does work'.. No one ever said it wouldn't 'work' as such. Of course with a 2 stage fan you will get thrust and the plane will fly, so in that respect it 'works'. What the physics say doesn't 'work' is the efficiency of the unit i.e. you have to put a lot more power and weight in, for a very small increase in thrust output. What I'd be interested in seeing is the thrust produced by the 2 stage fan vs. power consumed... Plus a comparison against the thrust produced by good a single stage fan of around the same power. Only if the 2 stage fan can demonstrate some advantage over a single fan would it be worth the extra complexity and cost.

As I said in the reply a couple of years ago, it has been tried before and they were sold as commercial units, but they were very inefficient in terms of thrust produced vs power consumed when compared to a good single fan unit. If you have managed to crack that problem then big kudos is deserved.

I lifted this photo from the album:
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