A centrifugal fan

[Martin Harris - Moderator]

They could always be laid side by side and extend into the wing roots, perhaps, in a suitable model. Not that I'm advocating the use of centrifugal fans, just making idle suggestions in a brainstorming manner on a quiet day at work!

[Simon Chaddock]

Erflog

What I had in mind would indeed be a low power fan, for 5" diameter, however it should be significantly more powerful than an axial fan with the same 47mm inlet diameter.

 

Of course an axial unit with its bigger ducts would be more efficient but then a prop would be even more so.

 

My intention is to fly something like a Vampire with scale inlets and exhaust and certainly no cheat holes!

The only question is whether say a 55mm axial fan, with slightly restrictive ducts, might actually produce as much thrust.

 

The fan I have is rated at 1200W, so provided I do not exceed that power input it should be safe enough.

[Daithi O Buitigh]

Simon - if you read what I posted earlier (and quoted from Ron Moulton's 'Flying Scale Models') - a centrifugal flow is about 40% LESS efficient than an axial flow, even with the narrower efflux.

 

It's all been tried (back as far as 1946) and that was the reported findings

[Steve W-O]

But if it didn't fly, it would make an interesting handivac for the workshop

[Erfolg]

My interpretation of what has been written is that the fan is mounted horizontally in the model, with a scale inlet and outlet.

 

I see the inlet as an issue, yet see the issues of turning the airstream through two 90 degrees bends as far more significant than any inlet issue. I would also suspect that the ducting will take some cramming in.

 

Seems a challenge Simon.

[Simon Chaddock]

Daithi

Agreed.

40% less efficient for the same power input but I am looking at a fixed scale size inlet.

For a given inlet area a centrifugal fan can deliver more thrust than an axial unit but requires quite a bit more power to do it.

 

Erfolg

With a 5"(125mm) fan suggesting a 6" (150mm) diameter fuselage the ducting required for its 1.8" (47mm) inlet will actually be relatively small.

 

By far the biggest problem will be efficiently collecting the air stream from the circumference of the fan and ducting it to a single 40mm(?) nozzle without loosing what little pressure the fan has generated!

 

A challenge indeed.

[PatMc]

Simon, I think you may be reading too much in the power rating that you quote. As has already been pointed out the input power doesn't necessarily give any indication of the output power (other than the theoretical max that can be achieved).

Note that this Hoover Multi Function has a 1400W rating but the performance rating is only 220 AirWatts. There's an explanation of AirWatts here in Wiki.

 

After all the convoluted ducting it would involve to install a centrifugal unit I doubt that the power output would make it anything near as useful as an axial fan using similar input power.

 

[Daithi O Buitigh]

Again - this is what Ron Moulton said about it

 

"Firstly, the centrifugal fan must, perforce, lie on its side horizintally in the model. This is because it will need a diameter upwards of 3 in. for a .5 c.c. and from the edge of that we still have to conduct the airflow back to the tailpipe."

 

You will NOT get away with a 'scale intake' (this has all been gone through years ago and the laws of physics are a constant (except in Ireland)

 

From the table on page 96 of FSM

 

                   Engine  Fan dia     No of blades Tip angle Tailpipe dia thrust model weight

 

axial           0.5 c.c   3" axial      8                     30 deg             2 1/2"     5 oz            10 oz

centr          0.5 c.c.   3 1/8"        10                   straight            7/8"        3 oz            8 oz

 

Now note - more blades, slightly larger fan and scale tailpipe - and STILL 40% less power  with the same power source

Edited By Daithi O Buitigh on 03/02/2012 22:51:55

Edited By Daithi O Buitigh on 03/02/2012 22:53:00

[Simon Chaddock]

I know a centrifugal fan is less efficient than an axial unit but that is not the point.

 

For a given inlet diameter a centrifugal fan can absorb more power and convert it into a higher pressure than a single axial unit can ever do.

This is exactly the reason why many early turbo jets used a single stage centrifugal compressor. Axial compressors need multiple stages to achieve the same effect and it took a good few years of development before they could achieve the same compressive efficiency as a centrifugal one.

 

And why must the centrifugal unit lie on its side? it didn't in full size hence the rather 'portly' nacelles or fuselages of the centrifugal turbojet planes.

 

I must say it is a very sweeping statement to say "you will NOT get away with a scale intake".

I think you will find it is not the law of physics but the power available and the weight of the plane that determine what is possible.

 

We shall see.

 

[Daithi O Buitigh]

You seem to miss over 50 years of research in this.

 

I wish you luck in proving everyone back to the 40s and 50s to be wrong. Let us know your results

 

The resaults quoted weren't mine - they came from other people who researched it thoroughly and it was Ron Moulton and John Coatsworth who said that it needed a 3 inch intake

[Erfolg]

Simon

 

I do not think it was an issue of converting shaft power into air flow that encouraged the use of centrifugal compressor. I think it was about the ease of obtaining a strong compressor stage and the simplicity.

 

With a jet engine compression ratio is important, hence the high fuel and low efficiency of model jet engines. Do you want or need any compression ratio as an ideal with a DF? Is it not about mass and acceleration to generate a force?

 

When you look at early jets, a lot had a dumpy look, not just because British early jets were centrifugal, although all German service jets were axial, it was because there was recognised that short ducts were required. Also as time went on and speeds increased, it became necessary to have a short duct length to slow up the air, feeding the engine, or a adjustable cone.

[Radge]

Hmm....Secondary axial compression, same shaft driven within a graduated venturi chamber...

[Simon Chaddock]

Erflog

A centrifugal fan (compressor) only becomes a possibility if the plane design is restricted to scale inlet size.

The mass flow is thus restricted so the only way to generate more thrust is to increase the air flow speed. An axial fan does this by a pressure differential across the blade and has a practical limit.

A single stage centrifugal compressor pressure ratio is not so limited and via a nozzle can achieve much higher exit velocities so in theory can give a higher thrust for a given mass flow.

The problem is increasing thrust by raising pressure is a squared power function whereas raising thrust by increasing mass flow is linear.

 

So you have to have a good reason to use pressure rather than mass flow to gain thrust!

[Daithi O Buitigh]

Simon, you overlook one basic principle in that. Sure you can increase the pressure but you cannot pump out more air molecules than you suck in. Matter cannot be created or destroyed (just converted) is what I was taught when I was a little sprog in schoo. Just increasing the pressure will force the engine to suck air from places it's not supposed to suck from (visions of the oozlum bird here flying in ever-decreasing circles until..(well you know the rest)

 

Everyone who has actually designed and built centrifugal flow fans has agreed the intake HAS to be big to get the required volume of air to hit the fan

[Erfolg]

I do not think there is a practical way around large inlets, relative to full size jets. In principle  there principle of operation was/is fundamentally different. It is the addition of fuel to the air stream and the burning it to generate gases etc, that is so different.

Very much as Daithi has pointed out, what goes out the back comes in the front. The inlet being responsible for some of the pressure drop (wasted power) and as the pressure drop is related to area and velocity, it makes sense to optimize the size as much as possible.

I do admire your tenacity in seeking scale fidelity, I personally think compromise can be a good thing.

Edited By Erfolg on 05/02/2012 12:24:38

[Simon Chaddock]

Still at the thinking stage.

I have a nice big 3 view of what I believe is an F1 Vampire.

The task is to work out how big it would have to be to give a scale wing root inlet area equal to the 47mm diam inlet of the 5 inch centrifugal impeller.

The answer comes out close to 48" span and the fuselage just about 6" diameter.

So far so good.

 

The next question is how much thrust would an ordinary EDF of 47mm diam give?

The problem is there isn't one.

A good 40mm on 4S can give 10oz (300g), a 55mm double this, so lets say for 47mm 15oz (450g).

 

Thus the task is to persuade the 5" centrifugal unit to give15oz (or more) thrust to equal or better an EDF. The risk of course is that the extra power required will add excessive weight.

 

As a aside I wonder what the thrust of a 55mm EDF would be if restricted to the scale inlets?

 

Finally, could I build a 48" Vampire to fly satisfactorily on 15oz thrust?

Of all the questions the last one troubles me least!

[Simon Chaddock]

First I am having to use IE as you cannot post an picture with Firefox and the images actually in the post are low quality unless you click on them.

The lash up test stand..

No shroud at the moment as I just want to get an idea of the motor performance. The impeller is mounted on a modified prop adapter but still uses the big steel washers used on the original vacuum installation.

An extraordinarily cheap 2700kV heli motor from Giant Cod. It certainly absorbs a lot of power so will need a bigger ESC probably 40 maybe 60A.

I have in mind to try an 'annular' nozzle at full fan diameter.

The air leaving the tip of the fan it is traveling very fast so it seems logical to simply turn it through 90 degrees rather than try to slow it down to gain pressure only to speed it up again at a normal exit nozzle.

ps The spell checker does strange things with the cursor if you try to manually correct spelling mistakes!

[Simon Chaddock]

In the best tradition of doing too many things at once I have decided to approach my goal of an EDF with scale inlets in two ways.

1. Develop a centrifugal unit with 47 mm diam inlet to see how much thrust it will deliver and at what weight. The 'engineering' approach?

2. To fly a plane (Vampire?) with scale inlets sized to an equivalent to 47 mm diam but using a commercial EDF. The "aero-modeller" approach?

First a cheap commercial 55 mm out runner EDF.

Interestingly if you subtract the motor housing the actual fan area is almost exactly equivalent to a 47 mm duct.

The EDF placed over the fuselage of a Vampire sized to give '47 mm duct' inlets. It would have a span of 48" (1230 mm)

The first step will be to build a static duct rig to find out how much thrust is lost with the scale bifurcated inlet.

[Simon Chaddock]

A first run albeit at only 1/3 power. (The only ESC I have spare is 30A whereas the motor really requires a 40 atleast)

Simple and crude it has no diffuser the object being to maximise thrust rather than pressure so the impeller is being run in the opposite direction to the vacuum cleaner giving swept forward blades. This should in theory give a higher exit velocity to the mass flow at the expense of pressure.

A long way to go yet but at least it runs pretty smoothly.

Edited By Simon Chaddock on 26/02/2012 17:21:09

[Erfolg]

OMG, that was disturbing, all that was missing was the wife saying, I am vacuuming this room now.

[Steve W-O]

I think reversing the blades, swept forward, is going to reduce airflow all round, there will be two conflicting directions of travel.

[Doug Ireland]

Sounded like a bag of nails... and that was only 1/3 power!

[Simon Chaddock]

Now have a 60A ESC.

Centrifugal fan test from Quorneng on Vimeo.

Its absorbing about 400W and produces a few ounces thrust.

Revs significantly higher running with the blades swept back but gives about the same thrust.

Will try some other diffuser configurations.

[Simon Chaddock]

Next experiment.

A conventional single outlet volute or "snail" diffuser.

The outlet area is roughly 1/3 that of the inlet. Nothing scientific in this just to see what happens when a bit of compression is used. The direction of rotation is set to have swept back blades which is theoretically more efficient at creating pressure.

A short run. It does indeed push out the air quite fast and certainly generates some thrust.

Centrifugal fan with a snail diffuser from Quorneng on Vimeo.

The automatic volume control on the camera does rather mask just how incredibly noisy it is. The sharp edge at the start of the volute, close to the fan edge, makes a good siren as each blade goes by.

The ESC has to be in the outlet efflux as it gets uncomfortably warm , as does the motor, drawing over 40A.

 

Edited By Simon Chaddock on 09/03/2012 15:18:59

[Erfolg]

Simon

I am not sure that you are proving anything beyond, a centrifugal fan is not the best shape for a model aeroplane, a lot of current is drawn.

How much thrust is produced?