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Ducted Fan
6

Ducted Fan

Ducted Fan

(OP)
What is the easiest way to calculate potential thrust from a ducted fan?  I.E.  6 bladed 36 inch fan operating at 5000rpm.

RE: Ducted Fan

Howdy,
I am sure that there are a lot of other parameters that would need to be taken into consideration for an approximate answer.  Air flow, volume, velocity; blade pitch, blade chord, etc.   I have a Pratt&Wittney turbo fan calculation and quick reference book at the house.  I will see if there is any info in there that may help.

Jay

RE: Ducted Fan

Well, no good news to report.  The quick reference book is aimed more towards turbo fans.

Is this ducted fan one that you are making or is it a purchased item that could have some recorded data?


Jay

RE: Ducted Fan

(OP)
Jay,
This would be a ducted fan that I was considering building.

RE: Ducted Fan

This may or may not help, my friend next door had an ultralight prop cut down and placed on a Honda motor and now he has an outstanding bow fishing air boat.  My point is, his friend is into ultralights and may have some insight. Would you like for me to get some info from him.


Jay

RE: Ducted Fan

(OP)
Jay,

I am always receptive to any info that I can obtain.

Thanks

RE: Ducted Fan

AV8TOR,

One place to start looking is a basic book on Ducted Fan design by R. Hovey. Look it up on the web. If you don't have any luck, I can scan a few pages out of my copy and send them to you.

Dord2002

RE: Ducted Fan

There is no easy way to get it accurately.

You need to define: static thrust or at what speed do you want the thrust. The inlet lip radius has a huge static thrust effect; does the duct converge,diverge or is it straight? Those also have an effect.

Martin Hollman's book on ducted fans gets into this. pp. 98 -

Thrust in lbs= 9.35 x ( hp x D)2/3 power

hp = horsepower
D= rotor diameter, ft
 inside the partrentheses is raised to the 2/3 power

Optimum blade number, chord, angle of attack of the blades, and the tolerance between the blade tip and the shroud all have major effects. The there's compressibility effects as the tips get over 0.80 Mach. It's a bitch to design one of these suckers.

Chasbo

RE: Ducted Fan

As a layman, could someone explain to me the essential differences between using a ducted fan and a normal propeller? (Just the basics!)

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

RE: Ducted Fan

chasbo,
I assume that the Hollman equation gives static thrust. Is that correct? Is this equation a curve fit through a plot of existing designs, or a simplified formulation from a derivation?

RE: Ducted Fan

RobWard,

I will attempt a (partial) answer your question.

1. A fan usually has a higher pressure ratio across its face than a propeller, and a fan often has more blades.

2. A fan and shroud unit will have a smaller overall diameter than an ordinary airplane propeller for a given horsepower, mostly because the shrouded fan blades have a higher and more uniform loading along their span.

3. A ducted fan will be quieter for a given power level; because sideline noise, which is the worst part of a fan's or propeller's noise signature, is largely captured by the shroud.

4. A ducted fan usually runs at higher rpm than a bare propeller, which helps to make the ducted fan a good match for a high speed engine.

5. Static and low speed thrust for a ducted fan are typically higher than for a bare propeller at a given power level, because pressures induced on the shroud by the fan flowfield add to the thrust.  However, the static and low speed thrust advantage of the ducted fan is lost if its diameter is too small.

The thrust of an ordinary propeller at low speeds (under 50 knots)is generally poor.  Ultralights could gain both thrust and noise benefits from a well designed ducted fan, if the weight can be kept low - a major challenge.  

6. At higher speeds (above ~100 knots, depending on the details of the overall fan & duct design, the duct drag exceeds the duct thrust gain.

7. Duct weight is considerable, because the low internal and leading edge pressures can result in very high loads.

8. The weight and high speed drag penalties are the main reasons you don't often see ducted fans, except on jets.

9. Jet engines with bypass fans always use ducts, despite the weight and drag penalties.  Ducted fans are not essential for fanjets, however.  Douglas Aircraft did tests on a DC-9 with "unducted fan" engines.  Their fan blades were fewer in number and larger in diameter than the fan blades on an equivalent ducted fanjet engine.  The unducted fan engine was a somewhat more efficient propulsion system; but it had noise issues.  (On a typical fanjet engine, the duct helps contain the fan noise, plus the fan duct exhaust is mixed with the core jet exhaust to reduce jet noise as well.  The unducted fan can do neither of these.)  There may also have been certification issues due to extreme unbalanced forces resulting from loss of a large fan blade.

RE: Ducted Fan

Thank you Miper: that's very interesting. I remember a few years ago seeing some pictures of a plane with strange scimitar-shaped blades, I wonder if that was related at all.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

RE: Ducted Fan

To answer Miper:

The Hollman equations are for static thrust, and there are a number of simplifying assumptions to get to it. It's a simplified derivation from theory, but assumes a realistic figure of merit for conventional bellmouth designs.

Chasbo

RE: Ducted Fan

I am hoping DORD2002 sees this post.

I am in need of a copy of R. W. Hovey's book, "Ducted Fans for Light Aircraft" or at least copies of critical pages on how to calculate static thrust from a ducted fan.

If you could scan and e-mail to me the critical calculation pages, I'd be most appreciative.  I also desire the title page with ISBN.

I have done searches at amazon.com and bibliofind with no luck.  There must not have been many copies published.

Any help from anyone regarding this matter is appreciated!

Phil

RE: Ducted Fan

Flying Jeepster:

The EAA library has the Hovey book I'm sure, and will fax the relevant pages. I've seen the book advertised in Sport Aviation. Try it there!

Chasbo

RE: Ducted Fan

    A further very usefull reference fot those interested in designing ducted fans or shrouded propulsors is
'Aerodynamics of VSTOL flight', MxCormick,B.W.  Academick press. Library of congress no 66-30093  chapter 9
   A low pressure ducted fan, usually called a shrouded propulsor, can be designed in the same way as a free propellor with the adition of the flow from a series of ring vortices to simulate the duct as well as the flow field from a fuselage or nacelle.
     I designed one for a twin engined amphibian and compared it with a free propellor.  Both using the same high rpm  engine (suzuki 993 cc).  The smaller diameter fan had benefits in lower off set thrust line and higher low speed thrust at take off float hump speeds.  The weight of the ducts was more than compensated by the shorter propulsor supports and by the lack of need for a reduction drive for the shrouded propulsor.
  May i thank miper for his very succinct reply
   happy new year to everyone  
     motorglider

RE: Ducted Fan

I've been reading this thread with interest and I would like to know what the ideal duct shape would be, tapered or straight in front and behind the fan? Will a small (5%)reduction in cross section behind the fan increase thrust as I seem to recall an idea that it would? Can anyone help with the calculation for the ideal bellmouth radius and shape? I'm looking at an 1100mm fan (number of blades to be tried between 2 and 12)with a maximum tip speed of 165 m/s using 50HP - 80Hp. Maximum operating speed would be aroung 100km/h so I'm mainly interested in static/low speed thrust for acceleration. Keeping it as quiet as possible would be nice.

Motorglider, have you any further info on the twin engined amphibian as this sounds close to what I need? I also intend to use a Suzuki motor albeit 658cc and with a reduction drive.

RE: Ducted Fan

I have been looking into ducted fans for either an autogyro or compound helicopter application.
I purchased "Ducted Fan Design" by F. Marc Piolenc & George E. Wright.
Available here,
http://www89.pair.com/techinfo/MassFlow/ductbook.h...
Good book, cozy with the ducted fan mentioned earlier in this thread used thos book.
I have spoken with Marc on a number of occasions, hes a great guy, and will be more than happy to help with specific questions you may have.

RE: Ducted Fan

I am looking for similar help. We are building a remote control car w/ a ducted fan in the middle for the possibility of going up walls. The fan will suck air in at a high velocity from the undercarriage and spit it out the top creating a suction effect. i need a ducted fan that is approimately 5" in diameter. If anyone has any advice(on a type of ducted fan, or if we should use a reducing duct to increase the air velocity) I would greatly appreciate it.

RE: Ducted Fan

av8tor

For a good start on practical design of a ducted fan, check-out the following book: MODERN PROPELLER AND DUCT DESIGN by Martin Hollmann

This is available at: http://www.aircraftdesigns.com/books.html

Regards, Wil Taylor

RE: Ducted Fan

Hi aircushion,
the twin ducted fan amphibian was designed here. It is currently on hold but near completion. You can see more details at http://www.tiswildeair.com or e-mail me off line.
regards motorglider

RE: Ducted Fan

av8tor,


     Hey, I was reading the forums and boom it came to me on designing a duct fan, would it be a possibility if a duct fan housing(the circular housing)would be in a airfoil shape to help produce lift capability, so you can use less wing space? I just was curious of this thought.

RE: Ducted Fan

First I'd just like to thank everyone for all this valuable information. My question is concerning ducted fans. At this site: http://www.futurehorizons.net/jetpacks.htm there is an interesting use of ducted fans and I was wondering if anyone could tell me if this is bogus or practical. Thank you.

RE: Ducted Fan

Hi Jon8088,
I have seen the drawings for the Jetpack, it is quite alarming and a very amature design, tho I am sure someone will try to kill themselves with one probably quite successfully. The fans are chain driven from a single engine, failure of one chain would spin the occupant into the ground before he realised there was a problem. If the engine fails this machine cannot autorotate like a helicopter, it will simply fall to the ground. If anyone does intend to fly one of these machines I would recommend an explosively deployed parachute. Even with a parachute an engine failure between 20ft and 300ft altitude would almost certainly be fatal. Having seen footage of the test flights for the origional jetpack, I would imagine serious injury or death would result long before anyone managed to master the control of this thing.

Karl

RE: Ducted Fan

Mythbusters built one from those plans and it couldn't lift its own weight, never mind that of the pilot.


google for this paper for some performance data by NASA

Abrego2_AHS02.pdf

The figure of merit was around 50%, apparently

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Ducted Fan

I saw that Mythbusters episode - the vibration alone would shake your fillings loose. They concluded that it did not generate enough lift to actually allow a man to fly. And during testing the safety concerns were enough for them to constrain the device should it ever actually lift off of the ground.


Maui

RE: Ducted Fan

Sorry for not reading the entire thread first, guys.  Please disregard my last.

RE: Ducted Fan

regarding that "jet-back" design ...

notwithstanding that it doesn't work, and would probably kill you if it did, who'd want to strap a 200+lb back pack on, particularly when the cg is probably 2' behind you ?

RE: Ducted Fan

2 feet is a bit exaggerated, when you see him standing in profile he leans a bit forward, I 'd say a few inches.
Seems to me that, or the thing is actively stabilized, or you 'll need weeks of practice to learn how to fly it because it can not be naturally stable. Also a landing with more than 125 lb on your shoulders seems to me almost impossible to achieve. A chair or some other stand might have helped to take the weight of the pilot's shoulders.
Nevertheless, I think it looks nice and it would be fun to fly it although I would never do it.

Checking if it has enough power to lift 330 lb:

  
From the Rankine-Froude Momentum Theory of Propulsion :

1.  Power = (Thrust) * (Air Speed of the accelerated air mass) :
P = T * (V + v)
2. Thrust = [Flow of accelerated air Mass] * (Final Air Velocity Increase of this mass flow) :
T = [? * A * (V + v)] * (2 * v)       
Note:     a. The air mass being accelerated by the fan disk will increase speed before passing the fan disk,
when it reaches the disk the speed increase is v, then it continues to increase behind the disk until it reaches a total increase of = 2*v.
    b. In hovering conditions V = 0.
When we consider hovering conditions, the Thrust (T) equals the aircraft Weight (W), and using this in the 2 above equations by eliminating v you get:

For T = W,     P = SQrt((W^3)/(2*?*A))

In reality P will be higher (for a ducted fan, probably about 10%) because in the above equations there are no losses considered like for example the energy lost in rotational momentum.

Doing the calculation P necessary for hovering at sea level is about 51 HP. With all losses included let's say about 20% more is about 61 HP. In other words, he can not even lift 330 lbs.

Maybe it is a bit insane, but I like it anyway, and it could undoubtedly find useful utilities, maybe in the army or so.

RE: Ducted Fan

"Maybe it is a bit insane, but I like it anyway, and it could undoubtedly find useful utilities, maybe in the army or so."

The U.S. Army's combat VTOL weapon of choice is the AH-64 Apache.  It only carries a pilot, co-pilot-gunner, and their armaments.  Yet it still requires two 1900hp turboshaft engines. That's reality.  A 61hp one-man vehicle doesn't seem very practical in comparison.

RE: Ducted Fan

I’m sure the US Army does more than just fly around and shoot down enemies with AH-64 Apache Helicopters.

Also when I am saying that I like it, I really mean that I like it when people are trying to invent and make new things work. They may not be perfect and in desperate need of improvement, but I think you should not a priory condemn them.

RE: Ducted Fan

OK, but assuming losses of 10% when the DOCUMENTED losses are 50% seems a bit optimistic.

The fact is small ducted fans are inefficient. Asssuming that they aren't helps no-one.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Ducted Fan

OK GregLocock I was a bit fast with my assumption of 10% losses.
Realizing now that the fan has lots of blades, I must admit that the efficiency will be lower compared to a propeller with the same diameter and much less blades.

Sorry.

RE: Ducted Fan

GregLocock, where could I find information on fan efficiencies?  Specifically speaking fan blades/impellers.  I’ve purchased the book mentioned in this thread (Marc de Piolenc) and gives great information, but I would like to see graphs.

I have lots of info on DF’s since my master’s thesis was related to them.  But in it I’ve always treated impellers as another component that only reduced efficiency and never looked at it.  I was evaluating the losses due to obstructions inside high speed air ducts.  

It would be nice to see a graph with ‘x’ number of blades, ‘y’ fan swept area and ‘z’ fluid speed (air).  Maybe throw in ID and OD, RPM and or blade angle also.  Any combination would be interesting to see.


See ya.

BTW. Great forum!

RE: Ducted Fan

Look in the NACA LARC archive. Alternatively I found the NASA ducted fan paper with google.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: Ducted Fan

I suppose you don't have the index of the "Aditional reports" on NACA?

RE: Ducted Fan

samerprod,

those would be really great to see against each other.  if you find anything like that, let us know.

I have been thinking for some time about how to create a CAE tool to get quicker/easier answers for rough prototyping and then further refinement

RE: Ducted Fan

while I'm not as lofty as aero eng I do mech stuff. Here is the end of the line in small ducted fans for model airplanes. A few years ago they were the ultimate rage. Some got over 200 mph !! They are vastly complex models. However thay have been of late surpassed by the turbine engine. These are very fast. Easly 200 mph straight and level. Honest ingen.  You get a pretty nice car for the cost of these. And yes they do crash. Some even leave a smoking hole...literaly.
Check this site for ducted fans
http://www.jethangar.com/

RE: Ducted Fan

MrChristian (Military Background)
Information relating to ducted fans might best be sought by an Airship Manufacturer. (or Blimp)
Many of these use piston engine pods with a ducted fan drive.  Usually a small fan is used to enable turning to accommodate steering - low speed means a conventional rudder has little effect.
I believe Porsche engines were used by at least one .
Goodyeard might release some data - best of luck

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