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.
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RE: Ducted Fan
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
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
This would be a ducted fan that I was considering building.
RE: Ducted Fan
Jay
RE: Ducted Fan
I am always receptive to any info that I can obtain.
Thanks
RE: Ducted Fan
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
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
"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams
RE: Ducted Fan
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
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
"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams
RE: Ducted Fan
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 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
A ducted fan on a Rutan Long EZ
http://www.bridgingworlds.com/DUCKT.HTM
RE: Ducted Fan
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
'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
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 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
RE: Ducted Fan
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
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
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
I am looking for similar help. We are building a remote control car w/ a ducted
You may want to check out the Coanda Effect
http:
RE: Ducted Fan
RE: Ducted Fan
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
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
Maui
RE: Ducted Fan
is more than worth the cost. it has all you need to calculate that.
RE: Ducted Fan
RE: Ducted Fan
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
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
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
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
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
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
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
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Ducted Fan
RE: Ducted Fan
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
Check this site for ducted fans
http://www.jethangar.com/
RE: Ducted Fan
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