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EDF efficiency

Can this be improved?

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Martin McIntosh25/09/2016 15:45:55
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Just a thought kicking around my normally empty head for you boffins to peruse.

Having built the TN Vulcan and reduced the crucial gap between the fan blades and the outer cage in order to improve efficiency it has struck me that it may be better if the fans were enclosed with a circle of material equal to the blade width, this possibly being extended to the eflux with a tube, shaft and bearing, maybe with a further coarser pitch fan at the rear.

Has any of this been tried before?

Dave Hopkin25/09/2016 16:27:56
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One word "Weight"...............

Geoff Sleath25/09/2016 17:48:32
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I spent a fair bit of time designing systems to measure blade untwist on turbo fan engines and others were equally involved measuring tip clearance. Engine designers obviously thought both were important for engine performance. On small EDFs I don't suppose untwist is all that important but I would have thought tip clearance should be as small as possible and surely the fan case extends the full size of the blade's chord already, doesn't it? Full size engines use a wear away coating so the tip clearance is absolutely as near zero as possible.

Not sure if adding a second stage to an EDF would significantly improve performance but I bet someone has tried it.

Geoff

Martin McIntosh25/09/2016 18:45:38
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Thanks for your replies guys.

I would have thought that totally enclosing the blade tips would eliminate any spillage, the gap between this and the main casing then being less important.

The weight of this would be negligible.

A 3D printer could easily produce this but I don`t have one.

Just trying to stimulate some ideas here because these things run at a very low efficiency in static mode compared with a normal prop. but come into their own at high airspeed.

Reducing the gaps on the Vulcan gave a very useful increase in static thrust. Cheap Chinese fans have a huge gap.

Denis Watkins25/09/2016 19:59:41
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If you have a friend in a local high school Martin, then most Technology Departments run a 3D printer nowadays as well as Laser CNC

Martin McIntosh25/09/2016 20:21:52
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Long time since I was at high school! My local 3D man is far too busy to bother with something like this since he designs bits for nuclear reactors. Maybe someone will take up the challenge.

Simon Chaddock25/09/2016 22:00:35
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"Cheap Chinese fans have a huge gap."

Yes but do they still have it when they are running?

The marks on the inside of the casing of my cheap Chinese fans suggest the blade gap tends to zero!

Martin McIntosh26/09/2016 08:55:33
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I have only used the 50mm ones which even with 0.5mm acetate glued inside did not touch. That is a huge gap for a fan of that size.

Dave Cooper 328/10/2018 09:21:27
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Just a thought :- When designing the radiators on my Le-Mans style sportscar, I went for a 'tight fit' between the radiator edges and the duct wall thinking that this would give the best result (it's a ram-air duct design). However, one day in the model wind-tunnel (10% scale), one of the radiators detached on one side and suddenly we were getting better mass-flow figures from the duct exit.

Next experiment was to fit smaller radiators with a small air gap between the rad's and the duct walls - result even better mass flow. No heat input to the rad's as all measurements were cold. This could, of course, just be reduced drag. Maybe a simple experiment with an EDF fan in a larger duct O/D to I/D could yield an optimal gap between the fan casing and the duct wall ?

Dave

Dave Hess28/10/2018 11:21:57
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Posted by Martin McIntosh on 25/09/2016 15:45:55:

maybe with a further coarser pitch fan at the rear.

Has any of this been tried before?

Flitetest did that on their A10. Each of their motors give about 1kg thrust on their own with a normal propeller, like they used, so it looks like they got some benefit from the second motors, but not much efficiency. They didn't really do it on a scientific basis. They had the ideas to use a higher cell count and coarser pitch on the rear motors, but they don't say that they actually tested that. You can see it here from about 3:00:

Piers Bowlan28/10/2018 13:00:50
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Interesting that you should mention this Martin. I have wondered for decades why rotating annular rings, for want of a better description, are not fixed to the circumference of some fans, doing away with a duct without losing efficiency. Dowry Rotol retro-fitted ducted fans or 'propulsors' as they called them, to a BN Islander back in 1976. This was to reduce noise as well as increasing efficiency. A fan without a duct suffers from horrendous tip losses, as we all know, due to the tip vortex. Running the fan inside a parallel duct reduces this and the smaller the gap between the fan tip and the duct, the more efficient it is. In the full sized world, keeping the duct light is usually a problem as it needs to be rigid, so is often made of composites and honeycomb. An angular rotating ring seems a logical alternative to a fixed duct; much lighter and more efficient?

I did think of experimenting with a model fan but considered it too difficult to make it with precision, in order to get it to balance with sufficient accuracy. 3D printing hadn't crossed my mind. Back in the mid-eighties there was a lot of research with unducted fans for short haul airliners. The concept was dropped as it was not considered efficient but I did wonder why a rotating ring wasn't considered to reduce tip loses.I have not found any images of an angular ring fixed to a fan on the net, either model or full sized. Has any one any ideas why it won't work?

Toni Reynaud28/10/2018 13:10:25
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What would be the drag on the rotating ring? Quite a lot, I would think.

Piers Bowlan28/10/2018 13:23:13
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Here is the Britten Norman Islander fitted with ducted fans, circa 1976 (for info). You may be right Toni but the fixed duct is not free from drag either. The rotating duct would have the same effect as winglets on a wing, to straighten up the airflow and delay the formation of the vortex. It would also prevent the blades from flexing so would make the fan altogether more rigid. 

 

Edited By Piers Bowlan on 28/10/2018 13:30:39

eflightray28/10/2018 14:29:42
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Back in the days of IC powered ducted fans, I remember many used 'flow straighteners' after the fan to improve the fans 'efficiency'.

Though 'efficiency' is a strange word, as propeller on model plane are generally still more 'efficient' than ducted fans.

What with getting the best inlet shape, 'bell mouth', and the rear duct shape and outlet size, 'percent of the swept area etc', add to that high current consumed compared to an exposed prop, it make me wonder why bother. Perhaps jet model looks comes before efficiency. (Flameproof suit already on).

But then I have always preferred props wink. Even my Vulcan uses props.

But have tried EDP, P=as in props, without worrying too much about the gap. They did the job I required of them.

.

PeterF28/10/2018 18:55:23
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If the rotating ring is rotating at 35,000 to 55,000rpm in a good ducted fan, the ring is going to be subjected to a high centrifugal force and it will not be a ring, it will be a dodecahedron or whatever based on the number of blades. A fan blade had most of the loading along the axis of the blade, so it inherently strong in tension. The ring will be subject to bending forces acting through its thickness and to keep small gaps will need to be very rigid. If the gap is too large you would run the risk of air from the higher pressure outlet side of the fan flowing back to the suction.

Piers Bowlan29/10/2018 07:28:53
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Yes I understand that it would be subjected to high centrifugal forces Peter and the unsupported sections of the ring would attempt to stretch outwards as a consequence. However for a multi blade fan (20 blades plus?) the distances along the rings circumference, between the blades, would be small. Clearly a material that was light and resisted stretching would be required (Carbon Fibre?). Perhaps on a full sized version titanium could be used, I wasn't proposing to build one of these myself, just trying to appreciate why it hasn't been tried. I appreciate that the blades of a fan are very strong in tension however they are subjected in very high twisting forces, the ring would resist that. A long ring (as in a tube) would not be viable for the reason that you gave but a narrow ring might increase efficiency of a simple un-ducted fan whilst reducing the noise produced by the tip vortex.

Not too sure when you refer to 'to keep small gaps (it) will need to be rigid'. Gaps between what exactly?

Edited By Piers Bowlan on 29/10/2018 07:36:16

Piers Bowlan29/10/2018 08:30:11
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Using a second (coarser pitch) fan at the rear is not particularly efficient. Fans (and propellers) work best in smooth streamline air, one of the reasons why tractor propellers are fitted to aircraft rather than pushers. Pusher propellers operate in the disturbed turbulent air from the airframe in front. Furthermore, with a twin tandem engine arrangement the rear propeller is operating within the rotating slipstream from the front propeller. A twin tandem will not suffer from the asymmetric engine-out problems of a conventional twin, hence the appeal of the configuration. Using a coarser pitch propeller on the rear engine is usually necessary and is fine as long as both engines are running. If the front engine fails then the rear engine will not be working efficiently as the pitch will now be too great, unless of course you have fitted a constant speed propeller. Having said that, there are several examples of successful twin tandems e.g. Cessna 377, Dornier Do.J Wal, Do 335 Pfeil, and Bert Rutan's round the world Voyager. With the Voyager the smaller rear engine was used for take off and the first few days of flight when it was heavy with fuel, after which it was shut down and feathered.

Running a second fan within the same duct will not be efficient for the same reason; turbulent air from the front fan. The flow straighteners that 'eflighttray' refers to are stators which improve the efficiency of the fan but if you need more thrust fit a bigger fan/bigger motor/more cells or, all of the above. Alternatively fit another fan in it's own duct.

 

Edited By Piers Bowlan on 29/10/2018 08:46:46

Simon Chaddock29/10/2018 09:41:02
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A ring around the tip of the fan blades is I believe called a 'shrouded' fan.

Whilst it will have no tip losses and apart from any structural issues what about the aerodynamic drag from the rings outer surface? It is a relatively large area that is travelling at blade tip speed.

It all comes down to which creates the biggest loss.

The fact that in full size turbo fan technology (the nearest thing to EDFs?) a shrouded fan has not been used suggests it is not a viable option.

Piers Bowlan30/10/2018 09:45:44
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I agree Simon, I googled 'shrouded fans' and it seems that they are a very rare beast indeed. The fact that I can find no evidence of shrouded fans being tried during research with Unducted fans back in the eighties, suggests that they were a non-starter. Curious, as I would have thought that there could have been a net benefit with the concept.

Dave Cooper 330/10/2018 21:06:59
60 forum posts

I wonder if anyone has investigated the number of fan blades and the rigidity of the fan housing as key variables to efficiency ?

I'm looking at installing one in a Jet Provost model next year (about 46" span), and, one supplier suggests that a twelve-bladed fan in a metal housing would be best. I think the recommended O/D was 80mm. On asking about the cheaper plastic Chinese fans, the advice was to walk away...

On reflection though, if the fan housing is tightly constrained by the duct walls - would it actually 'flex' that much ?

Dave

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