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Old Aug 04, 2005, 09:30 AM
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der Oberflügel's Avatar
Schertz, TX
Joined Jun 2004
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I am a bit confused about ducted fan efficiency

Hi All,
When I see efficiency numbers, like the ones for the GWS motors, it seems that they are a lot less efficient than a regular prop setup. I guess I'm confused because I thought ducted fans are supposed to be MORE efficient. I am talking about thrust per unit power consumed.

Don't get me wrong, I can defenitely see the coolness factor of having a ducted fan for a jet model. Any other advantages? No prop strikes of course... any others? How loud are they?

Also, what would be a good first ducted fan model, assuming someone has got passed basic crashing 101 and basic aerobatics 201? Not nessicarily concerned about scale appearance, I just want it to be a good flyer!
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Old Aug 04, 2005, 10:11 AM
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Part of the reason DF's measure out less efficient is the relatively high pitch and small dia of the unit and testing it at "static" condition (no ram air from the model's airspeed) DF's are very inefficient when the model is not moving. You can easilly "stall the fan" and just be whipping air instead of making thrust.

An example (very old) which I observed:: A pilot was trying to fly his first DF model.. a Byron F-14 with twin .91 DF units and was barely able to horse it off the ground and couldn't get it to fly right. An experienced DF flyer watched and waited till he had some altitude... reached in and pushed the nose of the plane down... it suddenly took off like a rocket. for the next take-off attempt the newbie to DF followed the istructions of the experienced guy using the full runway to take off and immediately on pulling the wheels up he went to a vertical ascent accellerating as he went up.

while my example story is about a glow model... its still applicable to the e-power ones. While "cheater holes" in the belly would have reduced the problems the above guy had... you need to remember that a DF needs the ram air from having the model moving to work correctly.
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Old Aug 04, 2005, 10:45 AM
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Naw.. Ducted fans.. at least of a design that our little electric motors can spin are grossly less efficient than the identical motor spinning a propellor... no if ands or buts.
Prop equipped model jets fly very well indeed.. just don't have the same coolness factor.
Try the errrr.. 'New' prop driven 'Jet forum' for examples.
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Old Aug 04, 2005, 10:59 AM
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meteor's Avatar
Ontario, Canada
Joined Nov 2002
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Quote:
Originally Posted by der Oberflügel
I guess I'm confused because I thought ducted fans are supposed to be MORE efficient. I am talking about thrust per unit power consumed.
Be careful when trying to compare a figure like "thrust"...


Thrust is only half the equation in any power system. The other half is efflux velocity, or pitch speed in a propeller.

Certainly a big geared 11x6 prop will generate far more static thrust per watt with the same motor than a ducted fan, but the 11x6 prop will be moving a lot of air "back" at quite a slow rate. Whereas the fan will move a little air at a very high rate.


Your general observation is correct, give the same size and power input parameters, a ducted fan will give you more static thrust than a propeller, but the DF benefits disappear as speed increases. (this has to do with propeller tip vorticies at zero airspeed, and the inefficiencies therein...)


Also, contrary to the observation about EDF's needing "ram" air. A perfectly designed inlet and outlet system will be almost as efficient from zero airspeed all the way to rotor pitch speed. (assuming the rotor is not stalled at static, not actually very likely at modest power levels)



Hope this helps.
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Last edited by meteor; Aug 04, 2005 at 11:05 AM.
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Old Aug 04, 2005, 01:11 PM
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I had tried many times to fly my pusher jet against
another EDF jet, in real time, not in this forum.
Both jets had the same airframe, motor, battery, ESC...
the only different between the two is the Direct drive prop in one and
the EDF in the other. I would bet my life in the Pusher configuration as...
you know what?

I know EDF people are very proud folks and I can't help it.
The above is only one of my experiences in my leisure time.
Please understand that I tried not to offend anyone.
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Old Aug 04, 2005, 01:25 PM
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Also with the fast rate of latest advance technology...
EDF is, in a way, approaching the turbine performance level.
May those tandem fans and other advance fans, batteries, motors come
to play then we will soon be in EDF heaven

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Old Aug 04, 2005, 03:15 PM
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der Oberflügel's Avatar
Schertz, TX
Joined Jun 2004
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Thanks for all the replies. I see how a DF would "stall". I guess it basically is riding inside its own "cyclone" without providing much thrust.

I know a real jet needs ram air. You can feel this when taking off in an airliner. The pilot advances the throttle, you start to roll, and at a certain speed, you start accelerating much more rapidly.

Oh, now I'm getting myself more confused... With an electric motor, no need to worry about combustion. I wonder if some advantages of a DF is lost when switching to electric?
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Old Aug 05, 2005, 12:46 AM
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Quote:
Originally Posted by goonybird
Also with the fast rate of latest advance technology...
EDF is, in a way, approaching the turbine performance level.
May those tandem fans and other advance fans, batteries, motors come
to play then we will soon be in EDF heaven

EDF's make there thrust by using a fan for pushing air out at an acelerated velocity using a motor as the power sorce, and turbines make there thrust from pushing air out by burning fuel which is in a combustor chamber under high pressure thanks to the compressor section of the turbine, (don't make me go into details). Basicaly an EDF operates kind of like a Turbine engine with a Bypass Fan, only the power section is an electric motor, hence the fan.

To compare the EDF fan to the turbine, is like comparing a prop to an EDF, The spool up speeds of each is much different, with the prop having the fastest spool up speed, then the EDF and the turbine dead last. but then again you don't need to spin an EDF up to 120k RPM in order to get the same power that an EDF puts out at 40k RPM, (comparing the 90mm size fan to a wren 44 closely) EDF technology has improved much over the years, but i don't think that EFD's will be able to fly at the turbine speeds yet,,, and i stress yet, cus i know the day will come when they will. just need one of those CC 85 HV esc's, 12s4p 130mm EDF and a kick ass motor. By then the AMA will have the EDF guys take safety corses and get special ratings just like the turbine guys... Maybe i'll do that once i finish off my A-10 next year. So the day that EDF's that fly like Turbines really is not to far off.

Gene
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Old Sep 03, 2005, 12:56 PM
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Germany
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Quote:
Originally Posted by kriptonic
So the day that EDF's that fly like Turbines really is not to far off.
maybe compared to todays turbines.
But RC turbines are currently were the big turbines were in the 1940..1950ys.
They still use radial compressors that were only used on the very first turbines.

I personally belive EDF and turbines will never be completely at the same level. They simply are targeted for different use regions.
There is simply no replacement for the power/weight ratio you get for turbines (excluding a portable warp generator )

I've heared rumors that they are going to use EDF on airliners powered by fuel cells. Basically airliners use already today TDFs (turbine-powered ducted fans ). The idea is to use the turbines only on take off and switch over to fuel cells on cruise speed.

Fly save ...

Peter
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