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Somewhere Between a Propeller and a Ducted Fan

07/07/2011 1:47 AM

I fly a powered paraglider, and am wondering if there is any potential benefits to be had by having a more duct shaped shroud around the prop. As I understand it, a true ducted fan is more efficient, and quieter than an open propeller, however I have also come to know that ducted fans require fairly close tolerances between the blade tips and duct. So my question is, would something similar to the shroud in the below picture realistically provide any performance benefit over an open propeller (ignore safety and line tangling etc.) and if not, about how close and how deep would a shroud have to be to provide any benefit?

Thanks, as always any insight is appreciated

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#1

Re: Somewhere between a propeller and a ducted fan

07/07/2011 2:39 AM

Since I have been instructed to ignore safety and line tangling etc. I would like to point out that you could reduce weight and drag simply by eliminating the shroud altogether! While you're at it loose the wheelbarrow nose gear for the performance benefit and increased excitement of a small nose caster wheel! ;)

Perhaps safety should be considered after all?

Seriously, I believe that the increased weight, complexity, and expense of a ducted fan would make it impractical for civilian para-glider applications. But I like the way you think.

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#4
In reply to #1

Re: Somewhere between a propeller and a ducted fan

07/07/2011 8:25 AM

Perhaps I should state the ? differently - a typical ppg cage is made of tubing, and its sole purpose is to keep the pilot and lines from coming in contact with the prop (which is a very important purpose) being that this frame exists - is there any foreseeable advantage to creating a duct around the prop to improve thrust or reduce noise.

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#5
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Re: Somewhere between a propeller and a ducted fan

07/07/2011 8:53 AM

You would create a power to thrust advatage by using a proper ducted fan. They have more blades that are shorter but with a larger cross section than your propeller. Also the RPM needs to be much higher and with very close clearance to the blade tips. I suppose if you have Richard Bransons need for the exotic and similar funding it is worth trying.

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#8
In reply to #5

Re: Somewhere between a propeller and a ducted fan

07/07/2011 11:32 AM

So is there likely anything between a open propeller and "proper" ducted fan that provides a thrust or noise advantage, over an open propeller?

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#9
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Re: Somewhere between a propeller and a ducted fan

07/07/2011 11:50 AM

You ask us "... wondering if there is any potential benefits to be had by having a more duct shaped shroud around the prop." What benefit are you looking for? Faster? Quieter? More time in the air with same fuel amount? Safety? Appearance?

I have never participated in the sport, but it looks like a blast. I would think a desireable improvement would be noise reduction. If this is one of your goals, many advances are being made with electric propulsion for paramotors.

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#10
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Re: Somewhere between a propeller and a ducted fan

07/07/2011 12:21 PM

Looking for greater power to thrust ratio, and propeller noise reduction

Although electric paramotors are certainly improving, the main noise in paramotoring is actually from the propeller.

the noise from an electric paramotor isn't much if any better than that of a quiet two-stroke paramotor - They also now make very quiet 4 stroke paramotor engines as well.

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#13
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Re: Somewhere between a propeller and a ducted fan

07/07/2011 3:14 PM

Hmnmmm. Okay.

Just some thoughts...leave all in place as they are today, and add an effective duct. This will reduce the thrust losses from the tip vortices and thrust will (probably) increase. This seems undesireable to me... I would not want my paramotor to out-run the 'chute. We need to keep the RPM up (there is usually a vibration problem at low RPM in ducted fan) so we reduce the dia of the prop (this is better that reducing the pitch). Now, the inflow is and always was restricted by the harness, operator, etc. sitting right in front of the center of the prop, so all of the effective thrust was coming from the outer 40-50% (guess) of the prop arc. You can see where this is going...

In addition, duct profile must be CAREFULLY designed... it can create quite a bit of drag by stalling the airflow, this negating any gain that might have been had.

So, there may be a couple of reasons that I have never seen a paramotor with a ducted design. This does not mean someone has not made a successful effort, I simply have never seen one (successful or otherwise).

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#15
In reply to #13

Re: Somewhere between a propeller and a ducted fan

07/07/2011 8:44 PM

"outrunning the 'chute " isn't an issue - the more potential power the better for a few reasons -

-there are times when doing aerobatics when you want to be able to do just that

-when cruising - you will need less throttle percentage to maintain altitude

-you can fly a smaller wing which will fly faster and not be out-run

I think I now better understand the basics of why ducted fans are designed the way they are, and why it wouldn't be ideal for PPG , but smaller isn't necessarily a non-starter. See below:

Yes, the pilot obscures the smooth straight through flow of air, but that can be worked around.

I think by now I know what the answer is, but in the image at the top of the discussion (the blue quad) does the shroud / duct likely gain any thrust performance, or noise reduction over an open prop?

Thanks for the feedback, I do really appreciate it. I don't have any background in aero so I am not sure where to even start on this stuff.

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#17
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Re: Somewhere between a propeller and a ducted fan

07/08/2011 7:55 AM

While uncertain, I think the aerocar in your OP (original post) is using the apparent duct as noise reduction. The fit is not very close, and there is very little depth to take advantage of Bernoulli effect.

Ace had some good comments in #5, but you really have to watch out for too aggressive of a propeller pitch... to much torque (something not discussed here yet).

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#18
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Re: Somewhere between a propeller and a ducted fan

07/08/2011 12:27 PM

I agree. GA.

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#28
In reply to #15

Re: Somewhere between a propeller and a ducted fan

11/10/2014 8:13 PM

I've wondered about a ducted fan ppg for years.

Can anyone confirm or rebut these points for me?

1. My DK Whisper motor runs at +/- 6000rpm with a 2.54 to 1 reduction gear. A ducted fan wants to run 6000rpm or greater. My engine would run a direct-drive fan adequately and I could remove the reduction gear.

2. The duct has to be VERY aerodynamically designed (which I can't do yet), but the duct would eliminated the need for a cage making a smaller unit.

3. A 6 - 9 blade fan would only need to be 18 - 20 inches in diameter with a duct 16 - 20 inches long would provide the static thrust a paraglider needs.

4. A disrupted air inflow bothers a propeller more than a obstructed outflow (i.e. there are more tractor propeller airplanes than pushers). A ducted fan cares more about a clean outflow than inflow so the proximity to the pilot's back is less of an issue.

5. Blade tip clearance is critical, but what about blades with their own ring molded onto the tip? There would be zero clearance and the ring would spin in a recess in the duct.

Thoughts please?

Steve

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#29
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Re: Somewhere between a propeller and a ducted fan

11/10/2014 10:14 PM

Why have a duct at all if the ring is there?

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#30
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Re: Somewhere between a propeller and a ducted fan

11/10/2014 10:21 PM

the duct would still need to be designed with the correct airfoil shape. The ring would only eliminate the tip clearance issue.

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#31
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Re: Somewhere between a propeller and a ducted fan

11/10/2014 10:31 PM

Huh? Just design the ring with the correct airfoil shape.

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#32
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Re: Somewhere between a propeller and a ducted fan

11/10/2014 11:05 PM

I don't think the duct (8-10" long, 24" in diameter) would stand up to the centrifugal forces spinning 6000rpm (and still be light weight)

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#33
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Re: Somewhere between a propeller and a ducted fan

11/11/2014 3:10 AM

Hi Steve,

5. Blade tip clearance is critical, but what about blades with their own ring molded onto the tip?

I get your idea, and I have had the same idea too.

I dont have enough of an aero background to know where the break-even point for the two concepts are, but there are trade-offs between the ring-around-the-fanblades concept and a more traditional multi-blade fan.

The ring adds mass and manufacturing challenge, and there are still aerodynamically critical gaps at the leading and trailing edges of the ring.

Do you fly a PPG?

I put this project on the back-burner a while ago, but would love to get back into it at some point.

Cheers,

Jason

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#34
In reply to #33

Re: Somewhere between a propeller and a ducted fan

11/11/2014 3:17 AM

P.s. Took these photos from my PPG last weekend :)

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#35
In reply to #34

Re: Somewhere between a propeller and a ducted fan

11/11/2014 9:03 AM

Jason,

Your image of the blades with ring inside the duct is exactly what I mean. I've been flying ppgs since '96. Here's my DK Whisper GT.

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#14
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Re: Somewhere between a propeller and a ducted fan

07/07/2011 3:20 PM

I do not know of any.

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#12
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Re: Somewhere between a propeller and a ducted fan

07/07/2011 2:52 PM

I'm going to guess that increased RPM is not an option to the OP. I think he's inquiring for something retrofittable to his current set-up?

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#2

Re: Somewhere between a propeller and a ducted fan

07/07/2011 3:41 AM

That's a cute little craft, but where are the control surfaces? It looks as though forward motion would pitch it downward.

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#3
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Re: Somewhere between a propeller and a ducted fan

07/07/2011 8:17 AM

The way a powered paraglider works is by control of the paraglider surface, and the shifting of weight front-back and side-side between the risers which attach the fan and passenger to the wing

To give you a visual - watch this http://www.flickr.com/photos/jcs7928/5621226660/in/photostream

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#16
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Re: Somewhere between a propeller and a ducted fan

07/08/2011 7:43 AM

Good video. Not being a pilot there is a great deal I do not know. I was surprised in the video that the pilot took off toward the power lines. I figure you have to head into the wind to get started but...

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#6

Re: Somewhere Between a Propeller and a Ducted Fan

07/07/2011 8:59 AM

Would you confirm a few things for us:

1) I understand the picture in your original post (OP) is NOT your paraglider. It is a paradigm, which (it appears) has caused paradigm paralysis here.

2) What did the manufacturer say about this when you asked them?

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#7
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Re: Somewhere Between a Propeller and a Ducted Fan

07/07/2011 9:34 AM

1.) you are correct - I own a foot-launch paramotor. (backpack fan) here is a quick video if you aren't familiar:

http://www.flickr.com/photos/jcs7928/5621226660/in/photostream

2.) I do not know who built the pictured quad, but as far as backpack style units go- they "haven't tried it"

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#11

Re: Somewhere Between a Propeller and a Ducted Fan

07/07/2011 2:49 PM

If you study the tail rotor wreckage that crashed in bin Ladens courtyard, perhaps you'll get some ideas on designing a quieter rotor

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#19

Re: Somewhere Between a Propeller and a Ducted Fan

07/09/2011 7:00 AM

I am a soaring pilot and much of the noise our tow plane generates is from the tip velocities on prop. Many clubs mitigate this noise by replacing the two blade prop with a three or five bladed prop and reducing the rpms.

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#20

Re: Somewhere Between a Propeller and a Ducted Fan

07/09/2011 8:48 AM

As I inspect your confluence I NOTICED ,A pitch problem with your nose it should extend past your Rutter speed control module front end needs to come forward 13.2 in. If you don't mind is there a pitch problem at take-OFF?If so re-angle at 14 degrees at the start of the wing tip.[BACK WHEEL extender. Otherwise nice design

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#21
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Re: Somewhere Between a Propeller and a Ducted Fan

07/09/2011 10:03 AM

13.2"?

Not 13.1"?

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#22

Re: Somewhere Between a Propeller and a Ducted Fan

07/09/2011 10:04 AM

Any increase in efficiency with a full duct would probably be more than off-set by the additional weight.

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#23

Re: Somewhere Between a Propeller and a Ducted Fan

07/11/2011 1:09 AM

I suggest two improvements to increase thrust & reduce noise: 1. Minimize gap between blades & housing with a slightly conical exhaust end almost touching the blade tip; 2. Add FRP/other light material vanes to make the air intake stream smooth & even along fan surface overcoming obstruction of seat. Check GreenHeck fans for minimum clearances

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#24
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Re: Somewhere Between a Propeller and a Ducted Fan

07/11/2011 1:26 AM

I think this is a good answer. At first I misunderstood the thread. But this is a slow-moving craft, with thrust being more important than speed. The original picture showed a large gap between the prop and the shroud, and I agree this gap should be minimized. (The shroud and its supporting "spider" will also help to avoid entanglement in lines.)

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#25

Re: Somewhere Between a Propeller and a Ducted Fan

07/11/2011 2:44 AM

You essentially already have a ducted fan, or at least the critical surface.

What you don't have is the right propeller, nacelle airflow and totally lack a spinner.

I.e. the art is about here in airflow understanding;

I would think you first need to be more here in airflow thinking, streamlining wise;

Note also the square tip propeller. This tip, when supersonic, uses the surrounding air as a 'duct'.

A physical duct will provide the same effect.

Biggest thing to note above is propeller diameter to spinner diameter ratio.

"Spinners" principally prevent 'recirculation' through the center of a propeller.

With the body shape you have, plus the intake masking in the lower area especially - you will have very high recirculation losses without a spinner.

Summary

As most of the work is done by the outer third of a propeller, continuing the streamlining through to a quite large diameter spinner is no propulsion disadvantage, but a great gain through eliminating recirculation losses

A square tip prop will suit the duct you have. Factor in 'growth' with rpm, when considering tip/duct clearance

The duct you have can probably be improved without weight penalty, but % gain against my fee is probably uneconomic to other than the manufacturer.

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#26
In reply to #25

Re: Somewhere Between a Propeller and a Ducted Fan

07/11/2011 9:22 AM

pretty pictures...

chris

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#27

Re: Somewhere Between a Propeller and a Ducted Fan

01/01/2013 5:19 PM

Hi.. new to CR4. can you tell me where you found the pic of the paramotor in your post? many thanks..

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