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Old Nov 23, 2004, 05:28 PM
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ducted fan pitch?

hi friends.

i heard that the ducted fan or any thing such as them can provides a LESS pitch and torque than common props we are using for palanes,is it correct? if yes so why? i saw some of them mounted on the hover crafts and r/c wings but i dont know why?

thanks for any input ,Mohammad
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Old Nov 23, 2004, 06:22 PM
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Well, ducted fan impellers need to be at a much lower pitch (typically 1630x3, 2030x3, 2030x5) than most aircraft props because they run at such high rpm.

They provide lower thrust (for the amount of power put in) than large slow revving aircraft props, because they are very inefficient. Electric motors like to run fast (30,000-50,000rpm), but props are most efficient when they are large and run slowly - hence the need for gearing (a 12x8 prop, on say, 6.6:1 running at 5000rpm - on 10A that will provide 10x the thrust of a 50,000rpm ducted fan or tiny direct drive prop spinning at 20,000rpm but using the same current).

Impellers and tiny DD props are basically boring holes in the sky - but if one needs speed or scale looks - then use a ducted fan or DD!

Cheers, Phil
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Old Nov 23, 2004, 06:51 PM
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thanks my friend Phil

but really i couldnt understand you

do you can talk another kind!?
thanks,Mohammad
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Old Nov 24, 2004, 09:50 AM
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Quote:
Well, ducted fan impellers need to be at a much lower pitch (typically 1630x3, 2030x3, 2030x5) than most aircraft props because they run at such high rpm.

They provide lower thrust (for the amount of power put in) than large slow revving aircraft props, because they are very inefficient. Electric motors like to run fast (30,000-50,000rpm), but props are most efficient when they are large and run slowly - hence the need for gearing (a 12x8 prop, on say, 6.6:1 running at 5000rpm - on 10A that will provide 10x the thrust of a 50,000rpm ducted fan or tiny direct drive prop spinning at 20,000rpm but using the same current).
These are all very interesting generalizations aplying mostly to GWS products. Fans are not ineffiicent. Static thrust alone is not an indicator of efficiency. Fan pitch is just a fan variable. Highly loaded fans will have much more "pitch" than most props.

Greg
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Old Nov 24, 2004, 10:13 AM
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Quote:
Originally Posted by gkamysz
These are all very interesting generalizations aplying mostly to GWS products. Fans are not ineffiicent. Static thrust alone is not an indicator of efficiency. Fan pitch is just a fan variable. Highly loaded fans will have much more "pitch" than most props.

Greg
One of the biggest problems which I keep seeing on these threads is that everyone wants to be a critic,myself included.
If someone makes an attempt to provide a simple answer to a complex question then someone desides to debate the issue then the issue becomes more cloudy for the thread starter. This is especially true when ther is a language barrier to start with.
Yes fan units can have high pitched blades however most of the ones used in small electrics are low pitched designed to operate in the 30,000 plus rpm range for speed not thrust.
Many of the models which use such fan unit have to be launched with a device which provides 20 mph plus air speed. The reason is simple air speed equals lift produced by wings.Ever wonder why aircraft carries have that big steam driven cylinder that they hook to jet aircraft to launch?

Charles
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Old Nov 24, 2004, 10:53 AM
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It also comes with people asking general questions and others answering with spacific examples. myself included. See attached pic of a EDF55 impeller, in this case its high pitched, roughly 45degrees at the base mount of the blade which is 55mm diameter and driven by a 350 class motor.

I've been under the impression, which could be wrong... that ducted fan impellers were typically higher pitched and get their high RPM from the small diameter, just as we do with small diameter props. IOW, DFs are designed for speed which sacrifices thrust. The duct is there to focus the thrust and improve efficency, same as a Kort nozzle on the wheel (prop) of larger boats. http://www.fairplay-tugs.com/main/fa...aq=Kort+Nozzle

Of Course that will depend on the fan diameter and the motor used to drive it. A smaller motor for the same fan will require a lower pitch to get the high RPMs, and vice versa.

mike
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Old Nov 24, 2004, 11:05 AM
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If someone makes an attempt to provide a simple answer to a complex question then someone desides to debate the issue then the issue becomes more cloudy for the thread starter.
Sure but if you like to answer questions because you can type rather than providing a proper answer that is your choice.

So to answer Mohammad's question. Pitch of a fan is dependent on may factors. It dictates how much torque it takes to turn the rotor. Matching the motor torque to the fan torque is the reason for having different pitches. In general the smaller motors used in GWS edf50 fans make little torque and must have low "pitch". Other fan units may have much greater torque requirement. It depends on which motor and what kind of performance the designer had in mind.

Greg
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Old Nov 25, 2004, 10:26 AM
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thanks all friends for answers,

before i start this thread i wanted to build a small hovercraft, i saw 2 kind of props for this task, one kind was a multi blades fan and other kind was a common 2 blades prop,i was woundered what kind of these props are better for lifting force part.
i couldnt get any idea from your inputs.

i couldnt understand your means about that pitch and torque!? for this job?
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Last edited by mohammad_sa39; Nov 25, 2004 at 10:29 AM.
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Old Nov 25, 2004, 11:09 AM
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I have no experience with hovercraft... but it seems to me that the design of a ducted fan will give more pressure for lift, the airs speed hitting the ground may also help with the lift. Bottom line is you'll need answers from experienced hovercraft guys.

mike
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