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Author Topic: Indoor ducted fan reality check?  (Read 210 times)
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Tiger Tim
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« on: February 16, 2017, 09:27:05 PM »

Hi all, long time lurker here but this may be my first post.

Lately I've been thinking about an indoor scale RC ducted fan jet.  Construction-wise I'm most comfortable with stick and tissue and there are a whole host of early JETEX plans that would go a long way towards a workable airframe.  The part I'm less certain about is the powerplant and that's why I started the thread.  I'm thinking of using a 1S system, probably from the E-Flite Champ, with a brushed 8mm motor driving the impeller from an Inductrix.  What I'm wondering is whether or not this little unit will have the power to fly a lightweight model, say in about the 30-35g range?

Further to that, are there any tips to building such a system?  I'm thinking the duct outlet should have the same cross-sectional area as the fan blades (subtracting for the spinner/motor cross section) and the intakes should be at least a little bigger than the outlet.  Correct me if I'm wrong, of course.  If I did try this, would I absolutely need an inlet duct or could the inside volume of the fuselage ahead of the fan act as a good enough plenum?  Airframe-wise I'm looking at the early jets and drawn towards the P-80 or DH Vampire/Venom, but maybe I'm better off with something that has a straight-through duct like a Yak-15 or something.  Maybe a Belphegor just to be weird.  Thoughts?
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MKelly
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« Reply #1 on: February 16, 2017, 11:04:44 PM »

Might take a look at this thread on RC Groups:

https://www.rcgroups.com/forums/showthread.php?1803743-P-80-SHOOTING-STAR-and-other-1S-JETS

Lots of interesting micro edf builds - more foam construction than balsa.

Hope this helps...
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Bill G
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« Reply #2 on: February 17, 2017, 01:22:19 AM »

If you haven't seen this, it looks like what you have in mind.  It uses an Inductrix rotor.
https://www.rcgroups.com/forums/showthread.php?2756964-MiG-15-1s-jet-finished/page2

I built a 30mm EDF Vampire years ago, with a remotored EDF30 fan using a 12mm Feigao motor.  With the inlets maybe a bit over scale, I think the short ducting makes up for the losses.  By the time I took the video, the old 10C lipo was ages old.  It was much faster when the lipo was new, and that was back around '07.
https://www.youtube.com/watch?v=7QFHPBempZE
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Robmoff
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« Reply #3 on: February 17, 2017, 05:48:11 AM »

There are a handful of flyers in my indoor club with EDF models, and between us have over 40 EDF models, some are twins and there are a smattering of four motor ones too.
For my money you could do worse than buy a kit, EDF unit and depron parts from: http://micromodeldrome.com/semi-profile-for-brushed-edf30-69-c.asp  I know its more expensive than roll your own, but you are paying for speed (buy today - fly tomorrow) and success (I can vouch for the Hunter and Sea Hawk), and I know that depron may not be the way you want to go either, but you will learn most of what you need to know quickly.
I don't think that the champ motor will be up to the job, the P51 motor can usually cope, unless you get a poor one or it's already well used, better motors can be had here : http://micro-motor-warehouse.com/collections/all-motors Oh and a duct can be a thrust killer in these small sizes, so good luck with that! You are spot on with your weight target, once you get to 50grams they can be a struggle to fly. Acceleration and climb rate will be low, and you can't do both at the same time, the walls come into play quite quickly and you need both speed and a little height to pull of the first turn, if you manage that it gets a little easier, but you will struggle a bit! You won't need three guesses as to how I know that.  Grin
Rob
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Tiger Tim
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« Reply #4 on: February 18, 2017, 08:15:54 PM »

Thanks guys, it's good to know I'm no totally out to lunch on this.  I see a lot of the depron jets just have very short ducts, about as long as their diameter.  If I were to build a jet with a full fuselage is there anything I should keep in mind for the duct design?  It seems to me that any pressure changes will rob power, especially downstream of the fan.
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Bill G
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« Reply #5 on: February 19, 2017, 01:46:55 AM »

My experience is that bifurcated inlets/outlets, undersized/restrictive inlets, and ducts with steep angle changes are where the greatest losses occur. Learned volumes with these two projects.
https://www.youtube.com/watch?v=rApFCTXHRUY
https://www.youtube.com/watch?v=Ly55BD4rsfw

I recently built a Yak23, as a long fuselage ducted example, with a small 12mm Hyperion 5300kv inrunner in a GWS EDF40 fan.  The fan is in the nose for balance and service reasons.  The ESC is in the long duct, along with a slight angle change toward the rear of the duct.  With the ducting and exit sized properly, the losses didn't seem to be much.  Most of the flight was at part throttle, with the launch at 1/2 power.
https://www.youtube.com/watch?v=3VkcS3HmL7I&t=19s
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