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Ducted fan blade design

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RonL

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Joined
Jan 9, 2007
Messages
502
Location
Texas
I did a search in the forum and came up with so many threads, I'm not sure how to narrow the search.

My question is based on using a prop calculator, I have obtained power and speed results for some recent post that I have made.
The input info was for a 3000 pound all up weight @ 200 MPH, air temperature 40*F @ 5000 Feet,
Prop speed 6000 RPM, The result showed around 32" prop and a power needed of 159 KW.:ponder:

Are there any calculators that have multi-blade options for input ? Three or five blades in a duct is what my mind sees as quieter and more efficient. (and yes I have a few crazy thoughts around this question, I will ease the minds and refrain from commenting, unless some really are interested:grin:)

Any help please, and thanks in advance.

Ron
 
Joined
Nov 28, 2009
Messages
11
Location
Sacramento CA
The Yahoo group referenced above has some greatdiscussion points, but the moderator is a bit draconian about that discussionas it is either HIS WAY--- or the high way.....

I have had discussion with him only a few times,to only be removed from the group for simply not falling into line under hisprogram ideas.

Bringing new ideas to the forum tends to getpeople belittled, or talked down too and even shot down or yelled at, as if youwere some sort of child.

I would suggest that you look into COMPRESSORBLADE theory rather than PROP THEORY.

Though many will ridicule me for saying so....... a compressor section (1st stage) in a high bypass compressor will giveyou a better theoretical calculation- rather than trying to build a fan basedoff of propeller theory..

High bypass compressors with a slight diffuser in the secondstage will net you a greater efficiency than any prop fan.

However the biggest issues to overcome afterreaching that forward momentum goal of 200mph (especially with an IO-320) is that a fan or compressor will have a much harder time PROCESSING theinflow of "FAST MOVING" static air of that airspeed which enters theduct, and because of the disc's inability to process it, which will inevitably start tooverspill the duct into the free air causing LOTS OF DRAG.....

That is why any consideration of DUCTED FAN propulsion must havesome sort of INLET SIZE manipulation to be able to process the proper amount ofair mass--- and allow the rest of it to go around without causing DRAG...

Otherwise....... you will be limited to about 180to 200 miles an hour TOPS----- and that will require the HP of an IO 540 min.

Keep posting....... I have been studyingthis Problem for about 18 years now, have a degree in aeronautics and anA&P license to boot.

I am actually starting work on a physical prototype todeal with these issues.... But it is a work in progress that will takesome time as I work as a design engineer in my day job, and a maintenanceDirector of a flight school on the weekends.

Cheers.
 
Joined
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Messages
11
Location
Sacramento CA
Also remember that the Duct Prop is not the only forward motion component in its 125mph forward movement-- as that rotor adds to that when the craft is pitched forward.

The Fan itself in the rear... is really inefficient considering the specs given.
 

LBarron

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Joined
Oct 11, 2006
Messages
122
Location
Irving, TX
Q.D. knows of what he speaks. Do your own research. Two of the best sources that I’ve found, and I’ve studied this for many years, are Axial Flow Fans and Ducts by Wallis (considered to be “the bible” for fan design by some folks who have designed, built and flown vehicles powered by fans), and a series of articles from the 60’s entitled Fans for Homebuilt Hovercraft. Another good article is The Internal Workings of a Ducted Fan by a chap named Klaus - it's in the archives of an RC forum and is very helpful. So I guess that's three good sources! Another good book is the Theory and Performance of Axial Flow Fans by Keller, but it might be difficult to find. There are many other sources of info; NACA technical papers, chapters of books about fluid mechanics and turbomachinery, etc, but the first three are relatively easy to understand and will allow you to design a fan with acceptable level of efficiency. These references are primarily for isolated airfoil fans, and only touch briefly on the effects and considerations of compressible flow. If you want to design a true compressor stage, get a copy of NACA SP-36 – Aerodynamic Design of Axial Flow Compressors – if you can understand it you can probably build a compressor stage.

Keep in mind that true ducted fans are different from shrouded propellers and are certainly not “propellers in a tube”. From what I’ve read there are many more design considerations for fans than propellers, so using a prop calculator may not be the best method of analysis. And it’s best not to guess about the number of blades, noise and efficiency. You really need to do the work, crunch a lot of numbers and then probably spend a bit of time and money testing what your spreadsheets come up with.

I'm a bit busy right now but if anyone is interested I can post links to some of the papers I mentioned above.

Good luck!
Regards,
Leland
 

RonL

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HBA Supporter
Joined
Jan 9, 2007
Messages
502
Location
Texas
Q.D. knows of what he speaks. Do your own research. Two of the best sources that I’ve found, and I’ve studied this for many years, are Axial Flow Fans and Ducts by Wallis (considered to be “the bible” for fan design by some folks who have designed, built and flown vehicles powered by fans), and a series of articles from the 60’s entitled Fans for Homebuilt Hovercraft. Another good article is The Internal Workings of a Ducted Fan by a chap named Klaus - it's in the archives of an RC forum and is very helpful. So I guess that's three good sources! Another good book is the Theory and Performance of Axial Flow Fans by Keller, but it might be difficult to find. There are many other sources of info; NACA technical papers, chapters of books about fluid mechanics and turbomachinery, etc, but the first three are relatively easy to understand and will allow you to design a fan with acceptable level of efficiency. These references are primarily for isolated airfoil fans, and only touch briefly on the effects and considerations of compressible flow. If you want to design a true compressor stage, get a copy of NACA SP-36 – Aerodynamic Design of Axial Flow Compressors – if you can understand it you can probably build a compressor stage.

Keep in mind that true ducted fans are different from shrouded propellers and are certainly not “propellers in a tube”. From what I’ve read there are many more design considerations for fans than propellers, so using a prop calculator may not be the best method of analysis. And it’s best not to guess about the number of blades, noise and efficiency. You really need to do the work, crunch a lot of numbers and then probably spend a bit of time and money testing what your spreadsheets come up with.

I'm a bit busy right now but if anyone is interested I can post links to some of the papers I mentioned above.

Good luck!
Regards,
Leland
Only have time for a thanks to you and QD. Might have time to start a little searching before Sunday evening.

Again, thanks

RonL
 

WurlyBird

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Joined
May 31, 2008
Messages
189
Location
Khowst City, Afghanistan
I have been reading a ton of threads on here lately about ducted fan design and application and I have a couple questions for people who have done a little more research then I have so far. My understanding is at low speeds you will produce more thrust for the power applied but will lose the ability to process the in flow at higher speeds and a lot more power is required. I have read several papers on line with a lot of theory and math but I have not found much in the way of observed numbers. So does any body have any observed values they can share, either their own or from another source? The things I have in mind are;
Engine size used
prop/fan/impeller diameter and RPMs
Static thrust
Top speed, obviously has a lot to do with design and drag but I am curious what speeds are realistic

Any time I start to think about the future when I will have the time to design a very personal plane it usually goes in one of only a few directions and ducted fans are in one of those directions. For what I think I will want I am okay with sacrificing high speed (100-120 kts cruise is fine by me) as long as I don't need 1000's of feet to take off.
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
13,575
Location
Port Townsend WA
I have been reading a ton of threads on here lately about ducted fan design and application and I have a couple questions for people who have done a little more research then I have so far. My understanding is at low speeds you will produce more thrust for the power applied but will lose the ability to process the in flow at higher speeds and a lot more power is required. I have read several papers on line with a lot of theory and math but I have not found much in the way of observed numbers. So does any body have any observed values they can share, either their own or from another source? The things I have in mind are;
Engine size used
prop/fan/impeller diameter and RPMs
Static thrust
Top speed, obviously has a lot to do with design and drag but I am curious what speeds are realistic

Any time I start to think about the future when I will have the time to design a very personal plane it usually goes in one of only a few directions and ducted fans are in one of those directions. For what I think I will want I am okay with sacrificing high speed (100-120 kts cruise is fine by me) as long as I don't need 1000's of feet to take off.
The larger the prop/fan the better( almost always)
Large inlet radius for slow speed. Small for high speed. Take your choice.
 

henryk

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Mar 8, 2010
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krakow,poland
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