Electric Ducted Fan problem/question(s)

Me again. Sorry for being so dense... I've got a tiny problem. Friend wants a nice big foamie jet, but she prefers EDF instead of my usual pusher prop setup.
Now, the default layout has air ducts in there anyway, but they're only used to cool the electronics. They're basically like "Y", with the upper part of the "Y" being the intakes near the nose, and the stem of the "Y" being the main fuselage and out at the tail end.
I'm a bit clueless with EDF but did some testing - after reading a lot of information on the web. What I know so far is this:
- Ducted fans seem generally less efficient than an old fashioned prop. - Making the ducts as smooth as possible yields about 5% more output. I've only tried that by smoothing the inside of the test tube with normal tape, properly coating it should be better.
- The intake size need only be about the same diameter as the fan itself I've only tested this with static setup (i.e. not in the air). * Q: Does it make any sense to have bigger intakes in flight? Kinda "scooping up" air while zipping around the skies?
- The exit duct should be about 3/4 to 2/3 of the fan diameter. Decreasing it further increases current draw without providing significant additional thrust. * Q: What happens if I'd use two EDFs through the same exit pipe? i.e. one in each of the upper branches of the "Y"? Could I gain additional thrust that way, providing I mess with the exit to optimize efficiency (current draw vs. thrust) or will turbulence or whatever else make that kind of setup not feasible? * Q: On the other hand, would it be beneficial to increase compression by putting two fans in series, with the first EDF running slower than the second (closer to the exit) EDF? * Q: Are there any other ways to get higher output per watt?
My basic problem is simple: It will be a rather large model and EDF seems a bad choice for large models. Weight is fairly low, the whole plane without electronics weighs less than 2 pounds, with electronics and battery it's just slightly over that. Except that I'm used to having a little over 2 pounds of thrust in there, and I just don't see a way to get even near that with EDF. I'd like to use mostly standard EDF components, pretty much to keep it easy for her to get spare parts if she needs them.
Thanks for your help! Jenni

Jennifer Smith wrote:
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I have no idea what it weighs but have a look its called a Fantom
http://www.youtube.com/watch?v=3TBUzjEAzFc
http://www.ripmax.com/item.asp?itemid=A-PH008&Category
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On 01/17/2007 03:26 PM, funfly3 wrote:
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Thanks for the links :) The foamie I'm talking about is "a bit" bigger. That one looks to be about 3 feet wingspan. The one I'm building has a little over 6 feet wingspan... similar design though, but planning internal fans.
Jenni.

PS to previous post: Check Jet Hangar International and Bob Violett Models. They sell large scale EDF stuff. If you're a buyer, they're helpful! ;-)
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Jennifer Smith wrote:
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2.25LB thrust not sure of the size there is one with 7lb thrust mind you you do need 28 cells and a deep pocket http://www.rbckits.com/shop/index.php?act=viewProd&productId9

Jennifer Smith wrote:
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You will need a hell of a lot of power..a kilowatt at lest I'd say.

On 01/18/2007 06:11 AM, The Natural Philosopher wrote:
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I commonly power mine with two speed 480 brushed motors spinning 6x5.5 or 7x5.5 props, or two equivalent brushed motors (around 1500kv). I've found it flies with one single 2500kv brushless motor and a 7x6 prop, but without any reserve power. For unlimited vertical I need two ~2000kv motors spinning 7x6 or 8x6 props. The latter are only good for high altitude though. On low altitude (about 5000ft ASL) they pull too much power. For most of the power trains above I use 2x 2400mAh 3-cell lipos, one per motor, plus a smaller pack for the receiver+servos. That gives me about 20 minutes on full throttle, or about 30-40 on a more sedate flight speed. I've flown my first prototype for 45 minutes once, with lots of gliding and very little throttle.
Keep in mind that this is a delta wing, which gives it lots of wing area (roughly 1700 sq.in.).
Jenni

Jennifer Smith wrote:
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Also bear in mind that to achieve equivalent performance with a ducted fan you probably need about 4 times the power, which is what I was trying to say.
Food EDF starts at 200W/lb or thereabouts.

The Natural Philosopher wrote:
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Oh... I didn't think of that, now that you mention it, it does makes sense.
Jenni

Jennifer Smith wrote:
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You can expect that. Ducted fans get more efficient as speeds go way beyond model airplane speeds.
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Common sense would indicate that less drag inside the duct would do that, yes.
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No. Bell mouthed intakes make sense on the ground, but won't help in the air -- that "scooping up" action will create more drag than thrust.
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I suspect that you'll find that the exit duct _area_ should be the same as the minimum area of the air path between the duct and motor. You'd probably do well to have a fairing on the back of the motor & keep the overall area constant.
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All else being equal I would expect one bigger unit would be better than two smaller ones -- but if you take your two pipes & join them into a pipe with double the area it should be OK.
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I doubt it would be better than one bigger fan and bigger ducts.
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If you mean higher _thrust_ per watt, yes -- more area. That's why props are better, and geared props are good on slow planes. You just have to give that up for a fan.
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A 1:1 thrust/weight ratio is necessary for extreme airobatics, but you'll do just fine with a 1:3 thrust/weight ratio. With a fan this will still translate into a high top speed, but you'll see long (i.e. realistic) takeoff runs.

You can get lots of information from RCU:
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Also from RC Groups:
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And from Wattflyer:
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And you can ask an aeronautical engineer or three if you sign on to the jets e-mail list: Type snipped-for-privacy@lists.kidsource.com in the subject line, and "subscribe" in the message area. You'll be sent a message that you're signed in, then ask away!
Hope this helps!
Geoff

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This is about right, but they don't want to be bigger than the fan area either. Inlets are commonly around 95% of the fan area.
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No - there is no gain in thrust and the bigger inlets just spill air which leads to higher drag.
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I've heard 75% to 85% of fan area gives the highest thrust.
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I don't know if anyone has even tried this : If you have two inlets feeding a single outlet then normally you get more fan area by using a single big fan in the main duct than by using two fans one in each of the inlet ducts. Inlet ducts are nor often circular either which makes the problem worse.
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I think you'd want the air in a fixed area duct to be travelling at the same speed through both fans otherwise you'd waste energy compressing the air instead of accelerating it. The second fan would be working in the turbulent wake of the first so this is not likely to be very efficient.
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Higher output per Watt means higher efficiency which can be had at a price : better (usually brushless) motors and better fans. Schubeler spring to mind for more efficient fans, see <http://home2.vr-web.de/~schuebeler.impeller/home_e.html . You'll need deep pockets though...
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There are some electric ducted fan (EDF) units at <http://www.airpowernet.com/ which are reasonably priced and which promise 900 grammes (2 lbs) thrust or more. They are widely available, maybe your friend could use one of those ?
I would not personally attempt to modify a pusher plane to take EDF because the ducts will all basically be too small. Much better to sell the model she has and buy a package specifically designed for EDF.
Hth,

Jennifer Smith wrote:
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You can say that again.
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I think thats about right, yes.
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Intake and exhaust ducting is a very complex subject. I wouldn't want to say much about it except that i theory the duct OUGHT to start wide, narrow to the fan where its accelerated as it were, and then open up aain to reduce efflux speed. You get more thrust that way.
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I am not at all sure that is correct. See above. Better still hit the EDF forum in RCgroups, and ask the half a dozen people who REALLY know their onions. Amongst whom I do not count myself, for sure.
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Nots ure. Run te srisk of strangling them bith..waht counts it seems is smooth ductiong and teh ratio of intake to exahust to fan.. everything else is not a huge issue,.
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Often wondered about that myself.
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Use a propellor..or as large a diameter fan as you can.
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Art least with EDF you get almost no drop in thrust as the model gets up to speed.. the efflux velocities are typically well over 100mph. I'd research the 'widening the tailpipe' scenario..my gut feeling is that a proper venturi sculptured internally like the top surface of a wing such that the airflow is smooth, does.t break away, and slows down where it meets the outside to trafde speed for thrust..is what you want.
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The Natural Philosopher wrote:
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Widening the exit nozzle on a rocket engine is standard to get the most thrust. But with a rocket you are attempting to lower the pressure of the exit gas towards that of the surroundings at the end of the bell. In a ducted fan I do not think there is any significant excess pressure behind the fan unit as the exit velocities from the fan are way too low. They do not come anyplace close to sonic velocity. So while intuitively attractive I doubt widening the exit is of any benefit. I have never seen a ducted fan with such widening and I am sure people must have played with this factor to find optimum. Now if you built a fan that had behind the fan air speeds of mach 0.8 or more such widening should be a help but I do not think any fan approaches these duct velocities even in the air.

snipped-for-privacy@scn.org wrote:
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I never claimed to be an expert of ducted fans..I did hope to provoke a bit of discussion tho..and your reply was certainly interesting.