Fan testing

[Myron Beaumont]

I've run out of headache pills too! I didn't need them when I was ic orientated.I just went out and flew !

NO probs

[Paul Williams]

some one said a picture paints a thousand words.

[Erfolg]

From the sought of books I have read in the past, local contractions will increase losses, but if short enough and well designed, the losses can be relatively small compared to the overall losses (that is in a big system).

The fan does have two surfaces to create surface drag, unavoidable though. Given that circular ducts have a smaller surface area to rectangular ducts for a unit area, they obviously are preferable. Can only think of the Buccaneer, Mig, and Skystreak and perhaps EE Lightening with circular intakes though, most seem to have rectangular or oval intakes.

DF models do not seem to be able to suffer to many avoidable losses though.

Erfolg

[Myron Beaumont]

Paul

& I thought i was primitive && I know where you got your ammeter etc from    Brings back many happy memories,

[Paul Williams]

Yeh, next door neighbour is a land rover nut. I have just done a load of static prop testing and have come to the conclusion it is not a true reflextion of what goes on in flight, read prop testing, I was seriously worried bench testing my twister edf when it bubbled up a li-po after 4minutes at 70%. In the air it runs like a dream barely warming its conponenants and my worrying was unfounded. I dont think a true conclusion can be drawn without field tests. Unless you know different. Still its better than watching TV or doing the garden. Are yo going to Cosford tomorrow? Hope the weather stays good

[Myron Beaumont]

Paul   Too far away untortunately I'm at the moment near Newcastle

[Erfolg]

Paul

It was me who uttered the thousand word sentence.

Now I have seen the pictures I fully understand your set up.

I am also pleased to see that a degree of ingenuity has been used on the instrumentation side. We are into modelling not building expensive test rigs so I applaud your endeavours.

Must build my own, it is finding the time and fixing on a concept which takes little storage space.

I have models in every room at present, I have been told that they will all have to go into my work room in the future, so space is and is becoming more of a premium.

Erfolg

[Tim Mackey]

yes...but not sure of the accuracy of that old car ammeter.

[Former Member]

[This posting has been removed]

[Paul Williams]

Knew you was going to say that Tim, I do have a digital meter which I use on the AC side past the ESC, if you Know where a cheap DC one is that will do more that 10 amp please let me know.

Space is also a premium for me also Erfolg. the cradle that you see on top detaches and bolts on top of my plastic tool box hey presto instant flight box. It then gets moved to my catapult amazing what an old wardrobe makes

[Erfolg]

Paul

I am not sre about the kick launch. The side arm launch beloved of Zaggy pilots works well, also unorthodox.

For the sake of safety do assure our ever vigilent "Helth and Savety" aware readers that you are wearing "toe tector" style shoes.

As for instrumentation, as long as you are able to tell the watts to  approx. +or - 5w, it does not really matter what you use. The bog question for me how much typically do motors unload when flying. I guess those like Timbo with onboard data loggers will be building up a data base of info.

Erfolg

[Paul Williams]

Whats a side arm launch? and who is Zaggy?

Just got back from Coswoth fantastic day out boy are we only playing at in comparison with those guys.

[Tim Mackey]

Coswoth ??? I take it you mean Cosford!

[Paul Williams]

oh yeah, thats the wine at dinner. did you go? I hope you enjoyed it as much as I did.

[Erfolg]

Read Zaggi for Zaggy, good whisky does it for me, to ensure good health of course.

Erfolg

[Richard Sharman]

Myron Beaumont wrote (see)

Doctor Richard   When you arrive at about 70% exit area do you mean as a proportion of the ACTUAL swept fan area covered or the simple area of the front of the entire unit .I'm thinking that the motor/casing in the centre obviously obstructs the flow -probably to the extent of about 30%. So am I to deduce that all we are doing by "restricting " the exit is actually keeping the effective cross-sectional area constant ?

Apologies for slow response - I made the pilgramage to Cosford and back over the weekend, so haven't been doing anything else for a while.

  The 70% I referred to meant the the exit of the tailpipe should be 70% of the entry to the tailpipe - which is (usually) the same as the ducted fan unit exit.  I know that the centre of the fan is obscured by the motor, and some fans have a large central boss, but as a first approximation you can just ignore this.  The volume of air moved is proportional to the area of the fan, and the area size is dominated by the area of the outer part of the swept circle (pi r squared if you remember).  

It's not the case that by restricting the exit you are keeping the effctive cross section constant as the central area accounts for much less of the total than you would normally think.  Example: my powerfan 400 is 65mm diameter, so has an area of  about 3320 sq mm.  The central motor housing is 30mm diameter, which seems huge, but has an area of only about 700 sq mm or 20% of the total, so if this was the effect the restriction would be to 80% which is not optimal.

A word of caution: my experiment was for a duct of one particular length only - different length ducts would have different optimal solutions.

Richard 

[Richard Sharman]

Paul Williams wrote (see)

Enough I have run out of headache pills, the fan dia for my hawk is 89mm what lenth and exit dia should I fabricate the tube? Just the 2 dimentions would be great. Thank you thank you thank you

This is a very good question, and the sort of thing that we (all) need a good feel for.  The simplest thing would be to do a few experiments with your fan, using ducts of the same diameter, but different lengths.  Then combine this with the same length/different cross section data to home in on a "ideal".  I might do this experiment sometime, but in the meantime (or unless anyone else comes up with a good empirical relationship) just use the SHORTEST tailpipe you can get away with, and restrict it only a little.  

Richard

[Richard Sharman]

Paul Williams wrote (see)

..... I have just done a load of static prop testing and have come to the conclusion it is not a true reflextion of what goes on in flight, .......

yes! of course! STATIC testing is not a real guide to DYNAMIC performance because it is ignoring a number factors (commonly referred to as propellor-unloading in flight  although it is more complicated than that).  However, in our case it's all we've got, so it's the best we can do.  In my case, I am trying to estimate how a new airframe, previously unflown, will perform on a given motor fan set-up, so I am desparate for anything that will help. In your case it looks like you have an airframe that flies, but you need to know how different battery/motor/prop combinations will play. I would have thought the figures you get should be pretty meaningful - would you like to share them?

I thought your rig was very good, but I'd use a digital ammeter/watt meter if I were you! 

regards, Richard

[Paul Williams]

I will put together some figures by all means but will they mean anything to you unless you have the same electric set up, batteries, model weight.

[Richard Sharman]

I assume in your prop testing you have tried a selection of prop sizes, with different batteries, motors, etc so you should have a table of  prop rpm/thrust at given volts/amps/watts settings, or similar. It would be interesting to see if any general trends emerge -- although it would be even more interesting if these were fan tests, rather than prop tests!  

[Myron Beaumont]

Richard   Thanks from myself and others for informative reply .An extremely interesting newish subject  & lots to learn (as you go) .My latest project on the drawing board so to speak (along with others) is a slow flying EDF powered m/c -Rather like a single engined A 10 / V1 ! since at my age / reflex response I find a small high speed swept wing jet design a little daunting  plus I want the unit easily removeable/changeable  to come up with what suits me. If only batteries were a lot lighter eh ?

Regards  Myron .

[Erfolg]

It would be interesting to know the difference in thrust from a propeller set up from a DF, drawing similar watts.

Recognising that there will be differences in flight against the rig.

Erfolg

[Richard Sharman]

Dear Erfolg,

well, it so happens that I have been doing a test of that kind just recently. Here is a sample graph which shows thrust (y-axis) plotted against watts(x-axis) for a prop and for a fan. Specifically, the plots are:

• blue 950 maH 3s lipo driving KMS 380/20 with 6x5 prop
• pink 2100mah 3s lipo driving KMS 380/30 with 6x5 prop
• yellow 950mah 3s lipo driving KMS 380/20 with powerfan/400
• cyan 2100mah 3s lipo driving KMS 380/20 with powerfan/400

So you see that the prop produces more thrust that the fan by quite a margin. In fact the fan is quite shockingly bad, and almost makes you want to give up EDF altogether. It shows why pusher prop models are so popular for jet lookalikes, they work a lot better for a small blemish.

[Erfolg]

Richard

Your results are very interesting.

They seem to indicate that a propeller produces twice the thrust of a fan, for the same energy in.

That is a major handicap for a DF.

I think I will stick with propellers for the time being, the handicap is just so great, it indicates that major sacrifices have to be made, to get a DF to work well. Although like so many modellers, I find  DFs so attractive as a concept.

Thanks Richard

Erfolg

[Tim Mackey]

It has always been accepted that fans are way less efficient than props...but some peeps just cant stand to hang a prop on a jet - front or back !

Nice testing Richard....just how I like it, simple and easy to read and fathom.