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Ducted Fan 6
- Thread starter av8tor
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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
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
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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
Jay
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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
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
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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.
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.
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
"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams
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
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
FlyingJeepster
New member
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
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
You might want to try this link.
A ducted fan on a Rutan Long EZ
A ducted fan on a Rutan Long EZ
motorglider
New member
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
'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
Aircushion
Mechanical
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.
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.
rotorworks
New member
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,
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.
I purchased "Ducted Fan Design" by F. Marc Piolenc & George E. Wright.
Available here,
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.
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.
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