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Sep 07, 2017, 05:57 AM
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Theory Ducted Fans

After reading the following a number of times I realised I might appear arrogant and a 'know it all' this is not the case I am very careful. For any theory to be used for a particular application it must be balanced I.E. in this case it must precisely follow Newtons Laws, Impulse and Thermodynamics. If there is any imbalance then either the theory is incomplete, plain wrong or not applicable to this situation. ALL of the theories that I have read, when examined by this criteria, fail - so does that make me arrogant?
This is basically a cry for help. I am an aerospace engineer with over forty years of experience including about twenty five years in gas turbines, I retired and looked around for something as an interest, I was an aeromodeller in my youth. So I looked at electric flight and in particular electric ducted fans. I want to design and build units from scratch, so I looked around the web for details about design, performance, efficiency etc. I thought this was well done and dusted, digging out my texts on thermodynamics I discovered this particular topic was skirted around, yes compressor blades, propellors and open fans but nothing on ducted fans that didn't reference ducted propellors*. Everything I looked at rang alarm bells in my head, there is nothing available from googling that made any sense, maybe there are books available with adequate explanations but so far all I have seen are scaled down explanations of fans that are based on high velocity units (as used in turbofans), clearly sub 100m/s ** units will not operate in the same way. Secondly there are the 'duct'*** only explanations that are based on work much more applicable to ram jets/pulse jets and not EDF's. I find the way so called experts 'talk down' to people not trained in the relavent disciples abhorant. People who are not trained in engineering or physics are not stupid, secondly any 'expert' who can't explain in plain language with little or no mathematics, in my experience, doesn't understand the process.
So finally to my request for help, I have found that people are much better observers than they are credited with, I always listen to what people say, they may not have the technical training but mostly their observations are correct. It's our job to fit the the theory to reallity. After considerable thought I have arrived at the beginnings of an 'Ideal' cycle for these units, It predicts many things about how they will perform. To start with I will list a small number of predictions from this, as yet incomplete, theory and see if those of you using them agree and if not I will have to start again!
1). They will have very slow initial accleration and frequently need hand launching.
2). The maximum air speed achieved will be about 60% of the static efflux velocity. (The speed in meters per second that the air is expelled from the fan when the unit is on the ground)
3). The run time at a particular throttle setting will be almost the same whether static or in flight.
Thats a simple start, if enough of you agree then I will continue developing this principle and attempt to complete the 'Ideal' cycle and then take it to a practical applicable solution with the minimum of mathematics and of course , if I succeed, will be available online.
Sorry, I don't usually add notes but felt, after I had written it, some explanation was due. A little more technical I'm afraid.
* Ducted propellors outward flow still operates in free stream so contraction and increased velocity can occur.
** At energy levels below 100m/s or 5,000 watts per killogram air is generally considered incompressible and therefore density is constant and pressure varies with temperature.
*** Duct theory requires the impulse to accelerate the flow/change density and in the case of low velocity fans they are pretty well constant velocity devices adding pressure (heat) not velocity. The velocity comes from the change in pressure at the entry to the fan and therefore is initially powered by atmospheric pressure/heat. I might add that nozzles as used with EDF's are NOT generally Convergent in the true sense, they are used to concentrate the flow NOT add pressure.
Hoping to get some serious replies that enable me to continue. Thanks for reading.
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Sep 07, 2017, 10:48 AM
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eagle60's Avatar
A guy, I do not remember who, on mid aug published in this forum his edf theory, which I enclose here. maybe it could help.
Anyway I did comment that if you want to get the same thrust of a prop you must spend more than double the power.
Sep 07, 2017, 01:50 PM
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Many thanks will look.
Sep 08, 2017, 03:26 AM
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Once again thanks Eagle60 , I checked you're comment about power required for a prop versus fan and yes same sink rate, level airspeed, mass and with both electric motors operating at the same efficiency I calculate you would need about 2.1 times the power!
Last edited by gryhrdoldfool; Sep 08, 2017 at 07:09 AM.
Sep 08, 2017, 04:54 AM
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eagle60's Avatar
I believe this is due to the fact that prop accelerates a lot of air at relative slow speed and the friction increment is just the speed difference with the flying speed outside the fusolage.
with the fan you accelerate lesser air at much higher speed and you must add the friction inside the fusolage plus you must consider also the efficiency loss due to the air intake friction.
Sep 08, 2017, 07:06 AM
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Hi eagle60, I will put all I have in my blog and then you will see how it all hangs together, in a loose sense I think you're right, however, because a prop operates open stream it can add acceleration to the air, but a fan causes acceleration to happen before the entry of the air into the fan, the fan sustains this motion giving rise to, in the 'IDEAL' case, a constant volume rise in pressure which manifests as a rise in temperature, consequently the power available is limited to the maximum forward velocity that can be achieved before a rapid rise in drag occurs. Sorry I've read this a number of times and I'm struggling to express this in understandable language. OK a little simple maths... the efficiency of an EDF is about 60% so that's 60 watts in every hundred is available to move air or move the plane, because of high drag onset at about 65% of the velocity of the air moving through the fan the amount of energy available for flight is 65% of 60 watts or 39 watts, an equivalent propeller can return over 80 watts . Probably as clear as mud but I'll work on my descriptions. Cheers.
Sep 08, 2017, 07:51 AM
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eagle60's Avatar
I am not really interested in edf: I stick on props; I just wanted to be helpful.
But if you want to understand edf you should split the problem in 2 :
1) the fan
2) the system in which it operates.
What is giving energy to the system is the spinnin blade and in that place you have acceleration; this is problably less efficient than a prop as operating speeds are higher ( and a phisical principle says the higher the speed the lower the efficiency). In case of no air movement you have the equivalent built up of a pression ( if you close the exaust) or depression ( if you close the intake). Any obstacle on both sides results in a loss of thrust.
Remember that it works like any other pump : any loss in the intake is found in the exaust multiplied per the compression ratio.
With this my considerations are finished as I am pretty sure I have nothing to teach you, eventually only to learn.
Sep 08, 2017, 09:43 AM
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You have been helpful and don't underestimate yourself, you realise its a whole system and not just a duct or a fan - many so called experts have failed at this first hurdle. Your comments have focused my attention on what is important to convey and what is not! Nothing wrong with props, in my youth I flew control line combat, and radio's that used tone and reeds! My greatest possession was an early 2 channel fully proportional FM set. I am also sure that you could teach me many things I don't know.
Sep 08, 2017, 01:09 PM
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eagle60's Avatar
Could be. Anyway visit my blog and let me know (with a message or a comment) if there is something of your interest .
I am particularly proud of my wing concept that you can find here :
Sep 09, 2017, 12:09 AM
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RickC_RCAV8R's Avatar
@gry :

I am very interested in this stuff for application on a quadcopter as a secondary thrust source . SEE the Armadillo project on my blog for all of the details . I built and sent this model to a mate in Australia recently . He posted hte maiden flight in hover mode before he went on his annual holiday . The EDF video should be posted in another week on his blog CYNR100 .

I can verify the hand launch aspect of using thse sources . Once the impulse of the launch getting the model up to an operating speed is key . Having EDFs as a secondary source on a quadcopter seems a natural step as they would be invoked whilst the model is already moving forward quickly . An augmented BOOST of thrust .
Latest blog entry: Filament Fridge ?
Sep 09, 2017, 04:55 AM
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Hi Rick and Eagle, in the next few weeks I am going to add my initial work to my blog, this will cover the 'Ideal' case for EDF's . Imagine a vertical and horizontal axis with the vertical about centre, to the right you have compressors ( a single stage) and to the left ducted fans, at the far left is the 'IDEAL' unit not possible to create but shows all the general characteristics of these units. This is the stage I'm at and I will create a write up and post on my blogg in the next week or two. After that I need to do a great deal of work to define the characteristics that enable the design of units moving from the extreme left to the vertical axis. At the moment I'm not sure how close you can get to the the vertical axis but I suspect there are very powerful units out there that get very close. Why does this matter, in the' IDEAL' case when the unit is static we have 'thrust' ( force) but no power, and as you move towards this axis you start to have a reserve of power to initiate acceleration. Thank you both for your interest.

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