A centrifugal fan

[Simon Chaddock]

A normal EDF is to all intents and purposes equivalent to a turbofan but without the high velocity jet.

For most planes this means the ducts have to sized accordingly even though the original may have been a pure jet.

 

In model sizes particularly a centrifugal fan can generate much higher pressures than an axial flowunit so in theory can develop the same thrust from a much smaller volume of air although it would require rather more power to do it.

This 5" diam fan from a wet and dry vacuum is rated at 1200W yet only weighs 1.6oz

The fan inlet diameter is just 1.9" (47mm) so compared to the size and power of the fan the ducts would indeed be relatively small.

It appears it can generate as much as 2psi so a 1.5"diam (38mm) nozzle might produce over 2lbs thrust.

The overall diameter of the fan outlet annulus would be no more than 5.5" which could be fitted in say a 60" Vampire.

 

Of course there are lots of problems to be solved but of the face of it it might not be quite as daft as it sounds.

[Steve W-O]

2psi static (probably blocked), but insufficient volume of air to give any useful thrust with an open outlet.

 

 

It will continue to "suck" when the inlet is closed, better than a normal propeller.

 

Different design centrifugal fans would give a better result, but I think would have to spin too fast to simply replace a propeller, even if they could be as efficient.

Edited By Steve W-O on 01/02/2012 05:53:07

[Frank Skilbeck]

Don't forget that you would have to allow also for any ducting losses as the air would have to change direction on exiting the fan, and the diameter of the unit would have to take this into account.

[Steve Hargreaves - Moderator]

Definitely not an expert but aren't centrifugal fans better at sucking than blowing?? Isn't that why they use them in vacuum cleaners...?

[Simon Chaddock]

Most of the early turbo jets (and all turbo super chargers for that matter) use centrifugal compressors to 'blow'.

 

The way I look at it is to ask how much power could a 47mm axial fan absorb?

I doubt much more than 200W yet for the same inlet diameter even a simple centrifugal one can absorb 1200W.

 

It would be a way of cramming a lot of power into a relatively small volume of air.

 

[Steve W-O]

They use a different design.

 

Theone you showed must have an outside diameter of at least 150mm, not 47 mm, whict is only the size of the inlet.

 

It can be small, as it needs negative pressure at low flow.

 

The thrust would be related to the weight of air you could move, as well as pressure

[Simon Chaddock]

Steve

Yes, the fan is 5" (125mm) diam but unlike an axial fan the ducts are much smaller, Inlet 47mm and the outlet nozzle quite a bit less than that.

It would mean a Vampire could have scale size inlets and exhaust despite the big fan.

 

However as you say the question is whether the overall mass flow acceleration would generate sufficient thrust to fly.

[Olly P]

Simon - rig it up on a frame and test it!

 

Real results are always good....

[001]

In an Aeromodeller Annual from the 1960's there ia an illstration of a diesel powered centrifugial df. I wondered if, now we have very powerful brushless elecric motors that a large diameter horizontal fan, perhaps built into the fuselage and continuing into the wing roots of a delta (Vulcan etc.) might give good results.

After all one of the limitations of axial ducted fans is the diameter of the fuselage and the size and shape of the motor.

[flytilbroke]

Mind something chaps, input power used does not mean usable thrust. The early jets and the modern ones use air/fuel mix to get thrust. A Fanjet/Turbojet does not directly relate to either.

 

I doubt very much that an centrifugal fan would a practical, efficient drive unit for a model aircraft.

[Erfolg]

I seem to remember that the late PE Norman used a centrifugal fan on one of his models, possibly a Boulton Paul Delta. Although I thought most were axial flow.

 

I understood that axial flow fans were better suited to high volumes, generating medium pressure.

 

With respect to Centrifugal fans, they used to come with forward and backward curved blades. One way gave high static pressure (low flow), but at the cost of very high power requirements if the outlet was restricted. I think backwards bent blades could still generate high pressure head, but is more tolerant of a blocked outlet, not drawing the same high current.

 

I have only been involved with industrial fans systems. I used an axial flow fan, for an air curtain, where very high velocities were required, partly to stop spreading of the airstream and also to entrain fumes. The second, for space ventilation, where by default, only centrifugal fans were commercially available, to move modest volumes, against a very high pressure drop.

[Daithi O Buitigh]

Centrifugal fans were covered in Ron Moulton's 'Flying Scale Models' (and it wasJohn Coatsworth who developed them - P.E. Norman's models in the book used a standard axial fan).

 

Drawback was it had to sit horizontally with respect to the airflow - it couldn't use the scale intakes as the air intake came from the hub which meant a hole on top.

 

Ron gave thrust figures for both types of fan (for a 0.5cc diesel) an 8 blade axial with a 2 1/2 inch tailpipe gave 5 ounces of thrust, while a 10 blade centrifugal with a 7/8 inch tailpipe only gave 3 ounces thrust)

Edited By Daithi O Buitigh on 02/02/2012 16:20:54

[Erfolg]

The picture I remember did show a very large inlet on the top of the body, which fits with Daithi information. I think the picture I saw was of a Dehavilland 108 Swallow.

[Daithi O Buitigh]

The photos in the book were of the BP III and also a Chance Vought Cutlass - both show the humungous air intake on the top, but the main point is there was a 40 % reduction in thrust from the same size infernal confusion engine (smaller tailpipe notwithstanding)

[Erfolg]

Daithi

 

Am I correct in thinking that you are saying, that the centrifugal experiments were not as successful as axial flow?

 

It seems that the Boulton Paul was not a PE Norman model?

[Daithi O Buitigh]

It looks that way (and according to Ron Moulton, the Boulton Paul was by John Coatsworth)

 

If you can lay hands on a copy of 'Flying Scale Models' it's on page 95. The Cutlass (not the DH Swallow) is on page 89 and was also by John

[Erfolg]

I am working from memory Daithi.

 

I have never read Ron's book as far as I know. I assume I saw the pictures in Aeromodeller at some time. The irony is if I saw a picture today in a mag, by tomorrow I doubt if I would remember at all.

 

 

[Simon Chaddock]

I would certainly not expect a centrifugal fan to produce the same thrust as an axial unit with the same power input.

Axial passes a greater volume whereas centrifugal gives a higher pressure.

Apart from any duct losses which both will have the centrifugal unit has additional losses from the compression and also from the nozzle required to turn the pressure into thrust.

 

It certainly has been done before. This is the only picture I can find of the ducted centrifugal fan RTP Vampire from 1946.

 

Built by the Aeromodeller staff it had a double sided impeller made of solder brass and driven at 10,000rpm by a 60W wound field motor.

Mounted as in the full size with scale inlets and exhaust. It apparently gave 2oz thrust.

 

The question is would brushless motors and LiPo batteries enable this to be repeated for RC?

Edited By Simon Chaddock on 02/02/2012 23:20:53

[Daithi O Buitigh]

It was built by Sqdn Ldr Peter Hunt (again from 'Flying Scale Models') - power was a 24 volt supply on 14 foot lines, but again you are going to run into the efficiency problem - axial flow seems to have taken over as a superior thrust/weight ratio.

 

What you need to watch out for is that the fan isn't overdriven as it can shear a blade - and a lump of sharp flying material coming through the fuselage side can be very nasty. Ron mentioned that "On the basis of area, actual loading on a blade root of a 3" fan will be up to 24 lbs" (that was for an i/c driven fan running at 16 - 18,000 rpm).

 

As centrifugal flow is (approximetaly) 40% less powerful than axial, safety factors will come into the equation

[Peter Miller]

With the fan mounted flat you got an incredible swing on the model when the engine stopped. All that rotating force was transfered instantly to the airframe.

 

You would probably get significant swing as the rpm changed with speed variation.

[Daithi O Buitigh]

True - it is also mentioned that with the motor stopped the model rotated around the axis of the fan - flat spins are hardly great for a dead stick landing

[Erfolg]

It seems that practical experience demonstrates what many would have argued from a theoretical viewpoint. Practice demonstrates if there are any issues, and demonstrates the level of any issues.

 

I was thinking, why centrifugal a fan?

 

In the case of early jets, it was as much about building a robust fan. Could that have been part of the issues with early models? I guess they were easier to build with strength than an axial blade set?

 

The revs quoted are probably lower than many current electric DF, Are you thinking of even higher revs Simon?

 

Being an advocate of low speed fan units Simon, I would be surprised if you are changing your position. If so why?

 

 

 

[Martin Harris - Moderator]

Posted by Daithi O Buitigh on 03/02/2012 08:52:17:
True - it is also mentioned that with the motor stopped the model rotated around the axis of the fan - flat spins are hardly great for a dead stick landing

I'd have thought that the problems would be on opening the throttle from low speed rather than it being stopped, when it's just dead weight...

 

I suppose the answer might revolve (pun slightly intended) around providing opposite rotating fans?

[Erfolg]

Martin

 

Your solution would certainly work. At the expense of additional complication.

 

What I am not clear about is why a centrifugal fan in the first place. Axial flow fans are freely available at some very attractive prices.

 

Not knowing above the superficial level the merits of axial and centrifical fans, the pros and cons of each type, I am curious to understand why.

[Daithi O Buitigh]

Not just that (well allowing for the fact that a pair of smaller motors produce more push than a larger one) - you are ghoing to have to have TWO air inlets (top and bottom) which, apart from the expense in two ESCs, bigger LiPos, etc isn't going to do much to help with the flight (not to mention the odd look of a plane with a whacking big hole right through the fuselage which won't help the structural strength either)