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PTKay
12-03-2005, 04:51 AM
I always told you...
Ducted fan is the future:

http://www.aurora.aero/TacticalSystems/GoldenEye80.aspx

Our next type of VTOL recreational vehicle. ;)

krew
12-03-2005, 10:25 AM
I always told you...
Ducted fan is the future:

http://www.aurora.aero/tactical/GoldenEye-100.html

Our next type of VTOL recreational vehicle. ;)
It looks like a giant cup.

Ga6riel
12-25-2005, 09:03 AM
converging and prior to this design are the offerings of
Alexander Lippisch (Me 163 and GEV fame) the Aerodyne concept
and the Hiller VZ1 flying platform

but being that this particular form is a 'tail sitter' the risks are broadly well known, as the transition from vertical flight to horizontal flight is easily percieved, it is less so when landing. But ok for a UAV i guess.

gyro
12-25-2005, 02:29 PM
speaking of ducted fans....does anybody remember the movie "Slipstream" it had Mark Hamill (Luke Skywalker) in it from Star Wars. Anyway he flew a pusher powered aircraft with some sort of ducted fan I would like to know what aircraft that was and get some info on the ducted set-up they were using. This was not some Hollywood make believe craft as my Dad seen one years ago at Oskosh.

Cheers
Paul

gyro
12-25-2005, 02:53 PM
Ok, a little research and I found it...a bit to much HP though.

http://www.utility-aircraft.com/catalog/forsaleoa7.htm

Heather Poe
01-25-2006, 10:29 AM
http://www.pegasusrotorcraft.com/pegasuspresentation.pdf
This is a three seat gyro with jump-takeoff capability.

Udi
01-25-2006, 10:45 AM
Heather - as far as I know, the Pegasus was never manufactured and the company bankrupt many years ago.

Too bad though... Nice gyro.

Udi

Ga6riel
01-25-2006, 10:56 AM
there are indeed some useful advantages with ducted fans
and they are appropriate to gyros because

* they exhibit better static thrust for the same power

* diameters are usually given to engine rpm and are therefore somewhat narrower, the thrust stream has a wider footprint than a prop therefore a comparable footprint means a smaller ductfan.

* in fact, you can dispence with the redrive, and run the fan at engine rpm
this saves the weight, maintenance and expense of the redrive, also a lot of other engines become more viable.

* performance usually exceeds that of a free prop untill around 100mph, where the drag of the duct begins to impact on performance. The speed regime matches the gyro quite well.

* noise is abated, and the sounds are stronger at vectors about 45 degrees out

* safety in ground handling, the path of the fan being protected by the shroud and being more obvious to see and avoid while on the apron.

the downsides are considerable too:

* they are more complex to engineer, and its not like you can go out and choose one off the rack.

* tollerances like tip to shroud clearances need to be quite tight, by thou

* mounting the duct and fan in some sort of stable union can be difficult too

Cobra Doc
01-25-2006, 11:14 AM
If ducted fans were truly effective, radio control pilots wouldn't be shelling out $1500 for a true turbine.

Ga6riel
01-25-2006, 11:24 AM
i dont know much about model radio guys doc but the physics of the technology is irrefutable, its just more difficult than bolting on a prop to execute

ducted applications are common where diameters are restrained, high performance and ground safety is required, take a look at many of the UAVs that have limited airspeeds, or torpedo technology some day

Al_Hammer
01-25-2006, 11:56 AM
This is a design by Piasecki, I believe.

Here's a Long-EZ with ducted fan
www.ductedfan.com

davreich
01-25-2006, 12:20 PM
Hi,
Nice plane. Saw one down at Sun N Fun years ago. Sounds just like in the movie. More of a woosh when overhead. Not in production.

Cobra Doc
01-25-2006, 02:14 PM
Let's try this a different way.
Based on resounding success of Moeller Skycar...uh...
The unusual 700 mph "Flying Saucer" with it's twin turbine engines being tested by the US Air Force...never mind, that one became the basis for every hovercraft in the world.
Bell's development of the X-22... forget that one, it couldn't lift enough fuel to clear the airport.
Other than the Optica, I can't think of a single manned ducted or ringed fan that was successful. The Optica flys well enough, but based on production numbers, you can't really call it "successful". Even the "Flying Jeep" was abandoned.

Jazzenjohn
01-25-2006, 03:50 PM
Even the guy who put the ducted fan on his long EZ eventually took it off because it didn't perform well. If ducted fans were so good they would be everywhere. They have many drawbacks. I think by far the biggest reasons it's on that uav are for noise reduction and to reduce radar signature.

Ga6riel
01-25-2006, 07:08 PM
Cobra Doc said
"I can't think of a single manned ducted or ringed fan that was successful."

Doc manufacturers always prefer to be able to take components 'off the rack', and their pre-requisite is build economy, not complication, and small manufacturers are even less likely to pursue it. Axial fans, although efficient when correctly done, have a narrow f'up margin, and that would add significantly to development costs if they tried to do it inhouse. Add to that, the market for sub 100mph aircraft is what exactly? Its not hard to figure out why duct fan applications havnt worked out. I could fill the page with aircraft the usaf hasnt been able to make meet the specification, but that would be dumber than a broken bell. Based on that ideology one would quit every method of propulsion known to man.

jazzenjohn said
"Even the guy who put the ducted fan on his long EZ eventually took it off because it didn't perform well"

They are really only ever going to be any benefit to sub 100mph aircraft
the long eze was never going to be a good choice, even on paper he may as well have never started the modifications, hell if he couldnt figure that out, what hope would he have with the rest. I believe he had 3 attempts before he cancelled the cheque which supports my previous fudge/difficulty factor.

I believe this is the long eze mentioned

Vance
01-25-2006, 07:25 PM
Stanley Hiller built a flying platform with a ducted fan that worked very well. It is not an easy thing to pull off. The duct must be designed very carefully and the clearances are difficult to maintain.

A friend of mine, who worked at Hiller at that time, did a lot of testing and his conclusion was a 10 percent bigger prop worked just as well.

Thank you, Vance

Al_Hammer
01-25-2006, 07:56 PM
According to this site, some of the ducted fan designs don't have enough blade density. Otherwise, its just a shrouded prop.
http://www.gdsys.net/WWWmembers/unicorn/DUCTED_FAN/ducted_fan.htm

And that's why the turbine powered Rhein-Flugzeugbau Fantrainer 400 isn't a good performer. Needs more blade density. ;)
http://www.jetphotos.net/viewphoto.php?id=40476

Ga6riel
01-25-2006, 08:23 PM
yes I remember that too Al
you would think that surprising after RFB's experience with duct fan design, thats how difficult it is. Ofcourse there is nothing like the experience available for propellor design, which even yet is considered something of a black art.

here is the X-114 IGE also by RFB

Al_Hammer
01-25-2006, 09:07 PM
I like the flying boat, Ga6riel.

Ga6riel
01-25-2006, 11:13 PM
The 7 seat X-114 designed by Alexander Lippisch (Me 163) was the ultimate development of the ground effect machine program. At 3307lbs and powered by a single IO 360 Lycombing of 200 hp, it had a range of 631 miles on just 220lbs of fuel. The machine was wrecked when a test pilot put one of the sponsons into a wave and turned it over.

Jazzenjohn
01-26-2006, 03:55 AM
That truly is one sweet looking machine. Seeing the people next to it in the second photo really gives you a better impression of its size. Nevertheless, there are quite a few people who would very much like to have a great ducted fan machine and few that do.

Ga6riel
01-26-2006, 04:11 AM
they would be very useful for gyros
the idea that u could run without a redrive (recovers the weight)
the narrower diameter, would mean a return to straight keels
and focus the thrust line on the CG more easily
the higher order of safety in ground handling would be welcome too

i did hear of an ultralight aircraft that used a proprietry air conditioning fan for thrust, i will see if i can dig it up, trouble is, i have rather a lot of 'stuff' here :)

Brent Drake
01-26-2006, 04:33 AM
It looks like to me that a ducted fan is nothing more than an OSHA approved prop guard haha

Cobra Doc
01-26-2006, 05:57 AM
OK, you got me. I forgot the ground effect machines. The problem with ducted fans is the aspect ratio of the impellor. NASA and Boeing gave up on the fan-prop for the same reason and it wasn't even ducted.

PTKay
01-26-2006, 08:12 AM
Non of you have mentioned the biggest field of application for ducted fans,
that is hovercrafts.

Almost all of them (even the biggest) use ducted fans.

The speed regime of hovercrafts is below 100 mph,
ergo in the range of ducted fan highest efficiency.

They are also ground vehicles, so safety of operation is crucial.

Look for the off-shelf solutions by the hovercraft manufacturers.
I found some in Poland.

Ga6riel
01-26-2006, 08:15 AM
found yet more failures Doc, thats nice
Ligeti Stratos was the user of the air conditioner fan, power was by Konig, 32HP i think.

Lippisch had some history with ducted arrangements too. the Aerodyne concept understandably created quite a lot of interest, and proved that the duct itself could be a satisfactory wing/control surface, in some of the gyros with ducts, indeed the stators (which are necessary to maintain thrust and NOT apparent on fanprops or ringed props) double as control devices.

Cobra Doc
01-26-2006, 10:22 AM
Paul, I did mention hovercraft and alluded to the craft that gave birth to the technology.

Rob, those are some good failures!

Ga6riel
01-26-2006, 11:20 AM
LOL Cody, well sorta
the Ligeti b/se the designer was tragicly killed in the 2 seater while being his flying outside the envelope, and its believed someone in the US has the patent on join wing technology (i got that from someone at Orion ac)

The aerodyne was only ever and investigation into the idea, there was a toy that sorta uses the idea, u can throw it as long as a football field Im told.

Cobra Doc
01-26-2006, 11:54 AM
I'm sure there is place for ducted fans, as opposed to ringed props, somewhere outside the r/c or hovercraft market. So far all aother attemps at flying fans could be done better with a prop or a turbine. Hiller an Piasecki both had flying platforms that were beat out by the lowly helicopter or the fatasticly successful M-151 jeep. With the price of the r/c engines that are fast enough for ducted fans (30,000 -60,000 rpm) and the price of the impellor, or ducted-fan unit, both going up and small turbine prices on the decline, it's becoming cost effective to go turbine. Add in the "cool factor" and ducted fan is on the way out.
Take all the guts out of the big silver one, tie a cable to the front and it could replace aerostats on windy days!! Something like a radar kite.

mceagle
01-26-2006, 03:46 PM
Ga6riel, this topic has been done to death but I will still add a bit more. Most of the points you make in your post #8 are all valid but they must have been written by a politician because they fail to mention certain provios that drastically change the outcome. I will try to relate a few to you;

* they exhibit better static thrust for the same power - yes, given the same diameter, more efficient through cleaning up the airflow around the tips.
* diameters are usually given to engine rpm and are therefore somewhat narrower, the thrust stream has a wider footprint than a prop therefore a comparable footprint means a smaller ductfan. Also true but wider footprint at lower speed equals the same thrust as a narrower footprint at higher speed.
* in fact, you can dispence with the redrive, and run the fan at engine rpm
this saves the weight, maintenance and expense of the redrive, also a lot of other engines become more viable. You cannot have it both ways. doing away with redrive means higher propeller speeds which in turn means a smaller diameter less efficient prop.
* performance usually exceeds that of a free prop untill around 100mph, where the drag of the duct begins to impact on performance. The speed regime matches the gyro quite well. Once again, given equal diameters. It would be pretty hard to efficiently duct a 72" prop on a gyro
* noise is abated, and the sounds are stronger at vectors about 45 degrees out True once again, given the same diameter, Smaller high rpm duct fans are notoriously noisy
* safety in ground handling, the path of the fan being protected by the shroud and being more obvious to see and avoid while on the apron. Can't argue with this one - true
Charles Ligetti was killed in the Ligetti Stratos when undergiong stall testing. The aircraft, having no tail feathers, could not recover after stalling and fluttered like a falling leaf to the ground. If I remember correctly the 2 IC in the factory barely survived a crash doing the same thing. We had to push one over a cow pat once because the small duct fan did not develop enough static thrust to do it. Made a hell of a racket trying though. Seemed to perform OK at higher airspeeds once in the air (after long and noisy take off roll)

Ga6riel
01-26-2006, 04:31 PM
Yep no worries Tim, that reads ok to me. I guess I can be accused of having it both ways, but thats more like a used car salesman not a politician who would straight out lie his ass off and take a cut in the process. It jumps around a bit because I did all that off the top of my head.

As I recall it Ligeti was very close to the ground during this stall testing though, and that part of the flight regime wasnt all that well checked out, is that right ?

Cobra Doc
01-26-2006, 09:26 PM
I just about bailed on this thread with out another comment. Maybe I still should. There is probably a secret buried somewhere in the ducted fan concept. I doubt the "big money" will find it. It will be some garage tinkerer that will stumble across the secret. Ducted fans offer a lot of promise, but they have yet to make good on that promise. One thing that always bothered about ducted fan r/c models is that all the builders dump the engine exhaust out the side or bottom of the fuselage. Seems like a waste of good energy to me. I had considered building an F-86 model, but I was going to route the hot exhaust to dump back in to the fan stream. I figured it would give at least a marginal performance improvement adding the nice hot air to the cold stream in the jet tube. It's kind of based on the fact that 30% of the P-51s thrust at maximum speed came from the radiator. I've talked to several Mustang combat pilots and a few of the guys at Reno. It's not an urban legend. Maybe the ducted fan secret is to NOT waste available energy. Just a thought.

Mike G
01-26-2006, 11:40 PM
Try this,
www89.pair.com/techinfo/MassFlow/ductbook.htm

I bought this book some time ago, it gives a reasonable introduction without too much maths.

Mike G

PTKay
01-27-2006, 12:03 AM
Remember the PAV project by NASA ?

For noise abatment and safety reasons they chose ducted fan.

PTKay
01-27-2006, 12:12 AM
Another intersting proposal from the same PAV study.

Rather ring wing than ducted fan.

PTKay
01-27-2006, 12:16 AM
And one more...

Spiral duct.

PTKay
01-27-2006, 12:19 AM
Do you remember the sucessful Marvel project ?

http://shrike.erc.msstate.edu/raspet/raspet/pages/marvel.html

Cobra Doc
01-27-2006, 05:33 AM
The Marvel was an OK flyer. It's also more ringed-prop than ducted fan. The ducted fan is also not a new concept. If I remeber my aviation history correctly, the first attempt at a D/F flying machine was in 1918. It may be a Coanda design that I'm thinking about. Some historians mistakenly refer to it as "the first jet". Coanda my have had more success if he han't tried to fly out of his back yard. I believe the airplane was destroyed when it hit the garden wall.

Cobra Doc
02-04-2006, 05:51 PM
OK Ducted-fan Fans, check out the February issue of Popular Science, page 64 or www.popsci.com/aeroscraft
OK, its more blimp or dirigible than d/f aircraft. If it takes Almost-lighter-than-air to make ducted fans or ringed props work well, so be it. Actually, most of the smaller airchips do quite well with ducted fans. Most of them even have electric motors. Living in Phoenix does have one small advantage, a lot of airships winter here. MetLife is the most common, but Fuji is also a regular visitor. During events Like the PBR (formerly Phoenix) Open golf tournament, the Barret-Jackson Auction and the Thunderbird Ballon Classic I get to see a lot of airships from my office window.

Victor Duarte
02-05-2006, 04:17 AM
Paul,
the ring wing you show comes from Willard Custer channel wing.
http://www.custerchannelwing.com/index.html

Crash one
02-27-2006, 10:13 AM
Hi guys
I am a new member to these brilliant forums, the best I’ve seen, keep up the good work.
I am a retired toolmaker/special purpose machine tool designer, I do not profess to be an aeronautical engineer by any means. Though I do believe I think “outside the box”.
I have had only a couple of hours instruction on gyros, enough for the bug to bite, before that I was into gliding & went solo power fixed wing, my interest in gyros is from the fun & versatility point of view. To be able to chase a motor cycle round the airfield at 80 knots then to fly alongside a combine harvester cutting the corn, all at considerably less than helicopter prices was very impressive.
I have had an interest in ducted fans for a while but know not a lot about their performance.
I am in the process of designing a gyro in an attempt to address some of the problems relating to thrust line / cg & I think a D/F system could be an answer.
Suppose the thing consisted of say 4 fan elements each having 12 blades mounted in a semi-circular (bottom half only) venturi tube rather like a jet engine with the top sliced off. I saw something on TV about this half venturi system. A model a/c engine & prop mounted in a half venturi generated lift, the top half of the pressure decrease having nothing to “pull” on except the atmosphere above.
The fans would be contra-rotating & driven by hydraulic turbines via an engine driven pump.
This would form the rear “fuselage” of the gyro, twin rudders mounted at the rear canted inwards.
The mast would be a 4 sided pylon mounted at the rear & canted forwards
to the required hang point. The rotor head would be mounted on shock absorbing bushes. The thing would look something like a chopped up F16 fighter with the top rear fuselage sliced off.
Retractable gear of course & fully enclosed cockpit (the weather here in Scotland bears no resemblance to that in California)
I have also designed a cyclic pitch control system with an offset swash-plate gimbal pivot that would produce no movement of the blade pitch at the rear of the disc, equal 50% movement at either side & full movement at the front.
The reason for this is to prevent rotor/tail strikes; this complication may be unnecessary.
I have not yet built any of this (did someone say thank God?) except an experimental hydraulic turbine, this thing is 4” diameter & runs at 8500 rpm on 6 bar of air pressure, what it will do on hydraulics & swinging a fan remains to be seen, fluid temperature may be a problem.
I do not think all this crap will weigh too much though I think I may be better informed once I can afford the money & time to build something.
Any comments regarding this lot would be very welcome pos or neg.
Have a nice day guys
Trevor

rtfm
02-27-2006, 08:41 PM
G'day mate,
Hi from down in Auckland, New Zealand. Some of your ideas sound "mentally energetic" - just the way I like them... Well done. There is altogether too little outside-thinking, and it takes a brave (or thick-skinned) man to voice them on what is, for the most part, a fairly conservative forum.

But, hey - we're out here - fellow outside-thinkers.

I'm particularly interested in your cyclic pitch control system. Care to share a few drawings? I'm currently designing/building a 4130 tube frame pusher gyro with wings. And I am completely committed to jump takeoff capability. So I need a head which allows for pitch control. My gyro will have elevators, rather than relying on fore/aft rotor movement, so again, your head design sounds right on the money.

Drop me a line.

Kind regards,
Duncan
PS Do you follow rugby?

Crash one
03-01-2006, 03:43 AM
Hi Duncan
Thanks for your reply & interest, I'll get dwgs done in some readable format as soon as pos. In the meantime I'll describe it if pos, give you something to think about.
The rotor head bar would be 2 lengths alumin 1" X 3" machined to a wide "U" shape 2 feet long bolted together & bored centre vertical to suit rotor bearings & ends horizontal to suit split bush bearings & blade "pitch pins" Pitch pins pass thru bronze split bush bearings. Outer ends of pins have cross bore for attachment of blade (helicopter fashion) inboard end of the pins machined flat. Plates fitted extending out 90 deg to blade, hinges to plates down to swash plate are cut away to accept ball pivot to swash plate this will put ball pivots across centre line of rotor disc. The swash plate has its pivot on its Odia on the left side of the mast, directly under the blade control rod ball pivot
Fixed (non-rotating) swash plate control points at right side of mast.
Blades rotating anti clock controlled at trailing edge.
Pull swash plate down, blade at front of disc increases pitch, blade to rear doesn’t move, move stick to left, swash plate tilts forward, blade on right increases pitch, left side decreases equally, move stick forward, blade at front decreases to below “mean” pitch, blade at rear doesn’t move.
The swash plate is moved by “closed circuit” hydraulic pistons from identical units at joystick. Side control stick may be viable?
If the swash plate mounting is on a short stubby hydraulic cylinder this could provide collective pitch, extra cylinders added to lift/lower cyclic pistons move at the same time, I’ve figured out the plumbing but at this moment I’m trying to see it in my head & its getting crowded in there!!!
I also had the idea that if a high pressure water pump (gear type) could be engaged by a clutch & a couple of gallons of water pumped along the blades via a swivel pipe fitting & out thru nozzles at semi span this might spin up the rotors a bit more for jump take off. Needs a test.
I’ve read this back to myself & if you can understand it you’re a better man than I am, my wife tried & screamed in pain!!
Sorry I'm not a blood sports fan, I spend my time tinkering with things.
Good luck with the build, your interest has inspired me to pull my finger out a bi further.
Have a nice day mate
Trevor

rtfm
03-03-2006, 06:09 PM
G'day,
It's a splendid Sat afternoon here in Auckland, and I'm sitting in my workshop figuring out exactly how to bend my tube frame. Ican ben one side OK, but I can't get the second to match up perfectly. It is only my prototype made from mild steel tubing, but I want it to be almost right, so that I can use it usefully to make final adjustments before I commit to 4130 tubing, and professional benders/welders.

To help me along, I've cracked a few beers, but for some reason, while this has made the afternoon very pleasant, it hasn't helped with the bending. Or with your explanation of how your head works...

I really thinbk it is about time gyros grew vertical takeoff capabilities, so I'm looking forward to some sketches of your system. I'm working on a solution also, but until one actually makes a prototype, it is all in the head, and it's damned difficult to think in 3-D.

Looking forward to hearing from you again,
Regards,
Duncan

Crash one
03-04-2006, 04:00 AM
Hi mate
you're lucky, it's freezing & snowing up here.
is it possible to make up bending forming rollers with two grooves on them to bend both tubes at the same time?
dwgs getting done time permitting, yes thinking in3D is painful I must be a masochist, wife says I could bore for Britain at the Olympics.
I thought of building a scale model to check dimensions & concept using a Cindy doll as a template. Our daughter is 41 this year so I dont think I'd get away with that!!!
Enjoy the beer mate
All the best
Trevor

Vance
03-04-2006, 04:32 PM
Hello Duncan, I have considerable experance bending tubes for motorcycle frames. Take you time and learn how much spring back there is. This takes a little more study than you would think. Making stops on a bender makes repeating very easy. Don't be afraid with the mild steel to use a little heat to un-bend, or add bend. When you get to the 4130 there will be a lot more spring back and you have to be carefull with heat. To get a changing radius bend we used to pack the tube with sand and bend it with heat. We would stress relieve everything after the structure was complete. For production it is best to stay with standard radius bends with enough room between them to get a good grip. I find joy in working 4130, I hope you find it too. Thank you, Vance

NoWingsAttached
09-17-2006, 08:47 AM
One last item to add to this old thread, as I am a DF enthusiast. I have seen postings on RWF that espouse the notion the tip clearances are critical. The idea of tip clearances needing to be thousandths of an inch is theoretically correct, but technically wrong. Look inside the intake of a jumbo jet engine. One way to get around the problem of tip end recirculation of thrust air is taken up by Rolls-Royce, et. al. The design of the modern high-volumn blow-by fans is one that incorporates something that can best be described as a pipe bent in a circle, then cut radially - not in half, but more like 2/3, with the inner portion being the 1/3 that is discarded - to produce a "C" cross section. The cut edge is mounted flush with the inside of the duct, and the fan blade tips extend beyond the circumference of the duct surface so that the blade diameter is actually larger than the duct. The intake side of the "C" is slightly smaller than the output side, and the result is that the tip flow is intentionally recycled at a controlled rate that is a fractional percentage less than the total tip output flow.

No matter what, all the designers agree on this: even a shrouded prop is more efficient at gyrocopter speeds that an unshrouded one, as is a true DF. The trick is proper design and execution. Which brings me to think....

...I have a static dynomometer. I've heard of guys rigging trailers with dynos for thrust #'s for their buddies at fly-ins. Does anyone have contact info on who's done this set up? No sense in me redesigning the wheel. I will definitely be wanting to do this over the winter.

Dean_Dolph
09-17-2006, 09:58 AM
.......
No matter what, all the designers agree on this: even a shrouded prop is more efficient at gyrocopter speeds that an unshrouded one, as is a true DF. The trick is proper design and execution. Greg, I'm not sure all designers agree but enough do that this makes an intriguing topic for discussion.

In a gyro there are other considerations besides the tip clearance. Try weight for one! If any increased prop thrust isn't more than what is needed to lift the extra weight then why bother.

So, the trick is to build a duct or shroud and support system that is light while still being able to survive the forces and vibration. I'm not saying it can't be done but it is going to be one heck of a challenge....I have a static dynomometer. I've heard of guys rigging trailers with dynos for thrust #'s for their buddies at fly-ins. Does anyone have contact info on who's done this set up? No sense in me redesigning the wheel. I will definitely be wanting to do this over the winter. I'm not sure what you mean when you say you have a static dynomometer. I associate dynos with engine testing.

If you are referring to thrust tests, then yes these are occasionally done at fly-ins. There isn't any thrust test standard so there have been a hodge podge of test set ups.

They are simple in that a pound measuring device is anchored and the tail end of the gyro is hooked to it by chain or cable. The measuring device is either a spring scale (think big fish!) or hydraulic cylinder with a guage that reads in pounds. Since no one has proposed, or accepted, a thrust test standard; comparing the results of thrust tests has to be done with the idea that what is recorded is just close approximations.

bwhughey
09-20-2006, 05:20 PM
Fellow Aviation Enthusiasts,

Sorry if this is a rudimentary physics lesson for some of you, but for others, it may help clarify the issues. I do not intend for this to sound condescending.

Newton articulated the laws we must live with, namely:

Every action has an equal, but opposite, reaction.

Force = Mass * Acceleration

And a third important one (not Newton, but I forget who)
Energy = 1/2 Mass * Velocity ^2

Since we want to move a body against the force of gravity (fly), we must accelerate some other mass toward gravity. When it come to propellers, fans, or rotors, that mass consists of air. We can take a lot of air and accelerate it a little (helicopter rotor), or take a little and accelerate it a lot (ducted jet - Harrier).

However, since the energy required to create this opposition-to-gravity force with air (or any mass) increases exponentially with the resulting velocity of the mass, taking the big air mass and accelerating it a little will always win the efficiency race.

Not just most of the time, or maybe not when your fan is ducted, but always. Newton was a very sharp cookie.

The best a duct can do for you aerodynamically is dampen the tip vortex phenomena, which accounts for quite a bit of drag. Also, it needs to be said that increasing the fan blade chord actually INCREASES this drag component (also known as aspect ratio drag for wings, or sometimes expressed as "solidarity ratio" for helicopter rotors).

As another poster pointed out, you can get the same (or better) benefit using a larger disk area and turning it slower - this reduces drag too, because drag force increases exponentially with velocity, and the volume of air accelerated is obviously in direct proportion to disk area. Go look at some of the human-powered helicopter web sites and you'll see enormous disks with low 2-digit RPM rates.

So, considering all the fabrication and and airframe drag headaches with ducted fans, the simple (and most often selected) solution for increasing efficiency is to just make the fan blade longer. That said, ducts have obvious safety improvement and noise abatement potential. But there have been ducted fan efficiency "snipe hunts" going on since the 1920's to no avail.

If you want a fan to be more efficient, you have to take in MORE air and accelerate it LESS. There's no other way. A ring around your fan disk, no matter how exotic, won't alter the laws of physics.

See "Newton", above.

Regards,

Brad

NoWingsAttached
09-22-2006, 03:03 PM
Dean: I guess a GIANT fish scale is how you could describe the instrument I have. We used it to adjust the maximum disengagement setting on the torque limiters on our fiber optic cable pullers. Nifty gadget, fits in something the size of a shoe box, and accurate to 1/2 ft.lb. up to a maximum of 1200 ft lbs. So 500-700 lbs is right in the midrange of the dial, perfect for Gyro testing I'd say. Just hook one end up to something stationary and solid, and the other end to the tail of the gyro with a strap or cable. Rev 'er up, and the max torque attained is recorded by a static needle that is pushed into position with the dynamic needle that rotates as torque "dials" up.

dragonflyerthom
09-22-2006, 04:27 PM
Sounds like you could get some really interesting readings. Sounds like a precision instrument. I know I would be trying it out.


Thom

mceagle
09-22-2006, 06:41 PM
Brad, agree totally with every thing you said, except the following statement "ducts have obvious... noise abatement potential".
The only duct fans on aircraft that I have heard have been considerably louder and far more annoying than any conventional free propeller.

OzyRuss
09-22-2006, 07:31 PM
Brad.........thankyou, excellent info

Please.........please get onboard with the ozy forum "Rotorcraft.com.au"

The guys down here would appreciate your input.

cheers............

kc0iv
09-23-2006, 04:09 AM
So, considering all the fabrication and and airframe drag headaches with ducted fans, the simple (and most often selected) solution for increasing efficiency is to just make the fan blade longer. That said, ducts have obvious safety improvement and noise abatement potential. But there have been ducted fan efficiency "snipe hunts" going on since the 1920's to no avail.

If you want a fan to be more efficient, you have to take in MORE air and accelerate it LESS. There's no other way. A ring around your fan disk, no matter how exotic, won't alter the laws of physics.

See "Newton", above.

Regards,

Brad


Brad,

I've got a question if you don't mind.

Why is it that on hovercraft lift fans it shows a improvement when a duct is added?

Second point in the discussion would be the fact that there is a limit to the size of prop.

Reading several articles the biggest problem seems to be using the wrong duct design.

Leon
kc0iv

dragonflyerthom
09-23-2006, 04:46 AM
I think you have missed the point. First question is how many blades are you trying to duct? Second question is How much power are you using to duct the air? Turbines use ducting in order to create thrust. They don't use 2,3,or 4 props but many smaller props in order to take just a little with each blade. They are over powered also but it seems to work very well. Third question is the application of the duct. What is it goin to be used for., I know that my Air conditioning unit uses 2 squirel cages that are about 14 inches in dia. each and they cool and heat a 3000 sq ft home. The air coming from the ducts move a lot of volume of air. I personnely believe the ducted fan approach should be investigated further. I would be interested in the difference using the same power plant what the thrust would be between a say 68 inch three bladed prop and a 60 inch 35 bladed ducted fan would be using the same motor and ducted.


Thom

Ga6riel
09-23-2006, 08:39 PM
Hovercraft fans operate in near static thrust conditions, thats a benefit to ducted fans where for the same size as a prop they put out more thrust. Also, there is less vena-contrata constriction of the wake of the thrust. All things being equal if the flow of the thrust has less speed for the same thrust, that spells higher efficiency in near static conditions. Safety is improved too, where ground handling has the benefit of the fan shroud.

Another type of compressor, not often seen these days is highly recommended for hovercraft. The centrifugal type compressor is considerably more robust, a dam site easier to make being of flat surfaces. Yes centrifugals are less efficient than axials, but axials are enormously more difficult to design and construct, and have a narrower efficiency band. New axial fan designs are almost certainly wrong and fail on the first pass. Tis better in my view, to have an efficiency of 80% near guaranteed, against an uncertain output.

Centrifugals went out of fashion for jets because the front profile is greater in area, and the max efficiency is less. But in circumstances were a craft operates close to a surface of unreliable cohesion, with rocks sticks and other potential debris that would break a fan blade, centrifugals are the answer.

Noise signatures are directly related to diameter and tip speed. Mostly, fans are of very limited diameter or at least less diameter than the prop they replace and hence have extroadinary tip speeds. With the tips at or near supersonic speeds, yes its noisy. Where diameters are the same, the ducted apparatus exhibits less noise and, the noise is directed within a 45 degree cone aft of the fan. That zone in particular can be very noisy, thats your torpedo design 101!

kc0iv
09-24-2006, 08:16 AM
Hovercraft fans operate in near static thrust conditions, thats a benefit to ducted fans where for the same size as a prop they put out more thrust. Also, there is less vena-contrata constriction of the wake of the thrust. All things being equal if the flow of the thrust has less speed for the same thrust, that spells higher efficiency in near static conditions. Safety is improved too, where ground handling has the benefit of the fan shroud.

I am seeing more and more hovercraft that are being built where the main drive is also using duct fans. Most are using 8 or more blades with shorter lenght blades. With a speed of 2500 RPM. Well below sonic speed. A couple of these can be see at: http://www.hoverhawk.com/


Noise signatures are directly related to diameter and tip speed. Mostly, fans are of very limited diameter or at least less diameter than the prop they replace and hence have extroadinary tip speeds. With the tips at or near supersonic speeds, yes its noisy. Where diameters are the same, the ducted apparatus exhibits less noise and, the noise is directed within a 45 degree cone aft of the fan. That zone in particular can be very noisy, thats your torpedo design 101!

So to me it would appear with less tip speed and an enclosed duct there would be a reduction in noise. Plus as you say the noise would be directed in one direction.

As Tom said its an area that needs looking at.

Leon
kc0iv

bwhughey
09-25-2006, 09:24 AM
Rotary Friends,

First off, you should know that I'm an electronics engineer, entrepreneur, and inventor, not an aeronautical engineer or fluid dynamics expert. In fact, I have retained enough humility to not count myself as "an expert" in any particular field (I learn more that way). Like the old saying goes, if you put 5 experts in a room you get 6 disagreements.

That said, let's be clear that the physics behind free-field air propulsion versus air compression are two totally different things. It's analogous to the differences between classical mechanics and quantum mechanics - the same rules do not apply.

For example, high solidarity ratio (blade area as a percentage of disk area) is not a good thing for a helicopter rotor, but is a virtual necessity for high-pressure differential applications (jet turbine compressors, heater blowers, and yes, hovercraft pumps, etc.). In fact, I have seen solidarity ratios greater than 1.0 (the blade leading edge actually overlaps the trailing edge of the preceding blade) in rotary air compressors. Also note that "reverse taper" is quite often employed in the latter, where the tip actually has a longer chord than the root. Mount that same blade on your gyro, and you'll have a very expensive (albeit interesting looking) noisemaker.

Also note that the efficiency of air compressors is more often expressed as a ratio of temperature increase in the compressed air mass, not as a function of induced versus "ideal" power.

In short, an airplane propeller or helicopter rotor's job is not to compress air, but to provide free-field thrust - a cylinder of accelerated air. The resulting physics model is simply not comparable to a furnace blower. Any attempt to make the propeller look like a blower will be counter-productive from an efficiency standpoint.

I do agree that ducted fans look so cool...

Regards,

Brad

Ga6riel
09-25-2006, 11:26 AM
kc0iv
most of them would be those umm Danish I think air conditioner fans of about 860mm diameter. Yes they are popular, that dont make em right. Low rpm is probably to hold off catastrophic destruction. Small diameter compressors should run at high rpms to be efficient. That they dont means low efficiency to me. And if those plastic blades get hit by a decent rock its all over.

You might like to check out the work of Dr. William R. Bertelsen who had some interesting notions and one of note. The use of outboard engines and horizontal props in ducts. All the air went into the cushion, the props were gimballed to direct the driving force. it was the only hovercraft I know of that could traverse a slope in complete control. Control being the feature lacking in modern hovercraft design.

Brad
its entirely legitimate to describe centrifugals as compressors, free jet engines are described as axial or centrifugal compressors in engineering. I never met anyone who wanted to pump their tyres up with an RB211 jet engine before today.
Centrifugals have been used succesfully, though not as popularly on hovercraft for over 30 years.
Blade solidity in axials is related to power. RPM to diameter. Low power low rpm axials therefore have nothing like a solidity ratio of 1. High power high rpm compressors have multiple spools so as to provide enough blade area.
Reverse taper succeeds in this environment because of the presence of the duct, but it requires feeler guage type clearances to work.

Rotor Rooter
09-25-2006, 11:27 AM
When considering the pros and cons of ducted fans, perhaps a couple of advantages are;-

- The elimination of the 3% tip loss.

- The increased diameter of the duct's exit may reduce the speed at which the stream tube reduces its diameter and increases it velocity.

Just speculative guesses.

Dave

Ga6riel
09-25-2006, 11:56 AM
rather more too it than that dave
the absence of tip vortices has the unique effect on the downstream flow of maintaining the diameter of the duct. hence less speed within the wake, its clear you understand what that means.

the entry to the duct has a critical effect on the static thrust, with a bellmouth being de riguer. widening the duct aperture at the rear accelerates a vacuous flow more concerned with the behaviour of the air in the stream, from a few diameters of the duct upstream to many diameters downstream. While the existing speed of the flow behind the duct is essentially the same, it is the difference in the speed throughout the system that has been affected, and therfore incrementaly the difference in pressure. But that only works in near static condition. Ducts for speed are the other way around, having more of a straight through hole through the middle and the camber on the outside. The drag of the duct is said to make the system less efficient than a prop it replaces beyond 100 knots.

Jens
11-07-2008, 11:12 AM
Where can I find more about this concept plane? (See picture)
I have searched with NASA, PAV, Personal Air Vehicle etc.

Nice concept - compact, low speed etc.
Anyone who know the disadvantage with using the 'channel wing' concept?
Could be used on a gyro - I think.

Resasi
11-07-2008, 12:41 PM
Paul probably found out about his movie aircraft by now but it was the Edgley Optica, designed by John Edgley and built by Brooklands Aerospace.

It had a fully-glazed forward cabin seating three across similar to the Alouette helicopter. Powered by a Lycoming flat-six engine driving a ducted fan. Twin boom cantilever tailplane with twin rudders and a high-mounted single elevator. Fixed tricycle undercarriage with offset nosewheel. Standard all-metal construction with stressed aluminium skin.

Initialy powered by a 150 hp Lycoming O-320 engine the Optica went to the Lycoming O-540. Production began 1983, certified 1985. A crash in May 1985 suspected to be a stall led to the bankruptcy of Edgley. Optica Industries formed in October 1985 continued production. 25 built before fire destroyed the factory and all but one flying Optica. Company reformed as Brooklands Aircraft and the Optica returned to production but production ceased in March 1990, when Brooklands Aircraft went bankrupt.

One flew in a 1989 movie called Slipstream. Grounding order by the FAA due to cracks developed in the wing spars is no longer in force. Two examples flying in the U.S. two in Australia, two in storage in the UK.

Design of the Optica recently bought again by John Edgley who hopes to put it back into production. Optica 300 Series s/n 021 G-BOPO is being restored as a UK type demonstrator.

Jens
11-08-2008, 01:01 AM
..... Edgley Optica, designed by John Edgley and built by Brooklands Aerospace.

The Edgley Optica seams to have got maximum benefit from the ducted fan.

The one I am interested in, and to find more information about, is the concept machine pictured in my earlier post. Maybe Paul knows?
It combines the ducted fan & the channel wing!
What I consider most important (with most toys), is that it has to be handy, so to speak. Otherwise it will not be used!
Easy to assambly and disasambly, easy transportation, easy storage, easy to find start and landing areas etc.

That concept plane look like it could fulfill most of these things, but right, it is only a concept machine - so far....

Even the gyro (more difficult, I know) might be able to use the idears from this concept machine. Gyros have powerful engines and props - this could be turned into some lift, and give shorter starts, less load (half?) on landing gear at start etc..

tyc
11-08-2008, 09:49 AM
... Every action has an equal, but opposite, reaction.
... Force = Mass * Acceleration
... Energy = 1/2 Mass * Velocity ^2
... Since we want to move a body against the force of gravity (fly), we must accelerate some other mass toward gravity.
... taking the big air mass and accelerating it a little will always win the efficiency race.
... always.
... using a larger disk area and turning it slower
... solution for increasing efficiency is to just make the fan blade longer.
... you have to take in MORE air and accelerate it LESS.


Bingo!

... but make the fan blades too long and the potential for catastrophic flexing is increased. See the Delackner experiments. Also see the VZ-1 experiments.

tyc

Jens
11-09-2008, 03:18 AM
Where can I find more about this concept plane?

I am not seaking lift 'for free'.
The channel wing might require more power, but it might give shorter starts and a more compact plane.
Maybe one disadvantage would bee purer landing capabilities... :ohwell:

Using the combination of duted fan and channel wing on a gyro, might 'just' be interesting design speculations - but.... might be possible.