Fan testing

[Erfolg]

There were certainly commercially available tandem fans available in the ventilation industry. The main reason I understood was either a lack of space to get a larger single fan in a duct run, but more often to be able to develop a higher staic pressure.

I am not at all certain that the two tandem fan set up is more efficient, than the single larger unit. The trouble that we have is that the fan curve is not available to us so as to evaluate (guess) the optimum mode of operation (speed I guess).

Yey the idea is not without virtue

Perhaps talking to a fan expert such as "Woods of Clolchester" or looking through the IHVE (Institute of Heating and Ventillation) "ventillation guide" may provide a better in sight as to what may or not work for us in DF. I would think that all their data is with a zero velocity inlet and a defused outlet. Althogh there will be useful data/discussion on the difference to static pressures and dynamic pressure and its relevance

Erfolg

[Simon Chaddock]

The problem with fans in series is the effect the first has on the second. The airflow leaving a fan rotates and is rather turbulent which reduces the efficiency of the downstream fan. Contra rotating fans can help but normally multiple stage compressors use fixed blades between each stage to straighten the flow. These "stator" blades also create losses so its quite tricky to ensure an overall gain. Note the second fan should also be a bit smaller diameter.

However in terms of thrust/watt I am sure a single larger diameter fan will ALWAYS beat a smaller multi stage one - unless of course you use the increased pressure available from a multi stage to burn fuel, raise the temperature and exit the increased volume through a nozzle. Here we go again!

[Erfolg]

Simon

I agree that in principal that a smaller second fan would theoretically be better, but given the small size, low compression, I am not sure it would matter much to us.

I totally agree about the intermediate diagram blades, the gains will be almost certainly not worth the effort. If the built by me, the inaccuracies would totally wipe out any gains.

In the May 1955 Aeromodeller, there is a reference to a Henry Pytuik of English Electric with photos, who built a two stage blower with reheat (still think the term is wrong in this context) for a model aircraft. I have a far more comprehensive article again published in Aeromodeller by some guy in the Fens or Cambridgeshire way, I will see if I can dig those out, out of curiousity. I guess none worked very well.

Erfolg

[Erfolg]

An 8 page article was written by W Benson-Ball on what he called his cold and hot jet engines in the Aeromodeller December 1960. There are many photographs and sketches of his set ups. All  similar to the ideas being put foreword.

I guess the results were less sucsessful than the claims. 

Erfolgs

[Simon Chaddock]

I recall the article in Aeromodeller but things have moved on a bit since then, not least the power and efficiency of the modern EDF units. Maybe the results might be better now.

After all the first full size ducted fan aeroplane (with after fan burning) was built in 1910 - by Henri Coanda (he of the Coanda Effect). It briefly flew but surprise, surprise, caught fire.

[Paul Williams]

http://www.modelbatteries.com/catalog/df90-high-power-unit-8blade500-size-p-17465.html

Would you like to have a look at the above fan unit, I am considering this for my hawk.

I stayed up very late reading your thoughts and theories and although very interesting I cant help but think why? I purchased a multiplex twister the other day for a grand total of £110 wired up 2 x 3s 2200mah 20c (cheap brand) in parallel and got 20mins plus flight time. The jet takes me as a moderate pilot to my limits, its fast, cheap, simple, light, works great and is clean enough to put on the back seat of my car. If it aint broke why fix it.

thanks for your answer on the extended wires to esc. I see your point but mounted in front of the fan (short wires to motor) gives the benefit of excellent cooling. I wonder if this is the reason they have designed it this way?

Are you saying I would get a better performance if I adapted the model to fit the inlet ring on the fan? 

[Tim Mackey]

"stayed up very late reading your thoughts and theories + although very interesting I cant help but think why?"

Because when trying to power the sort of model that this particular fan is going into, squeezing every last ounce of thrust out of it is very important. The model is not a proven and established simple little thing like the twister, and besides,once the fan unit is installed and buried within the fuselage it will be very difficult to gain access to it - again, unlike the twister. If people didn't experiment and test and compare things..your twister would not even exist today. Many people have successfully extended the battery wires to motors or even both motor AND esc wires - with little or no problem. Indeed the large 1000 Watt+ model I refer to above shows VERY long wires to the battery, and people have fitted it like this without incident. This doesn't mean its correct - we often do things in life which are not procedurally correct and get away with them - usually.

As the article was aimed at beginners, I felt it was my duty to explain the correct procedure.

I am sure the Twister will be fine, most people have done it the way it shows in the manual, again with no ill effect. I cannot say for sure whether your particular fan unit would benefit from an inlet lip - the purpose of which is to smooth out the airflow and "feed" the fan better.

Without knowing your unit, ( and testing it - see above ) - it is impossible to be sure - it should improve things, but again, what happens in static testing is diifferent to flight. The main purpose of this particular test was to compare different fans against each other in reference to thrust / power consumption / RPM etc.

Likewise, I can't offer much comment about the unit you link to, other than, the specs are obviously wrong, which may perhaps indicate a lack of real information about the unit by the supplier - had they checked their advert more thoroughly, they should have spotted that to achieve 3lbs of thrust from a 90 mm fan unit doing 2400 RPM would require some sort of miracle ! Why on earth they chose to describe it as a "500" size unit is also a mystery to me as no-one would really use a "550 - 600" motor in an EDF.  It is obviously meant to read 24000 RPM but as they mention motors from 1500 to 2000Kv one would not really know which to use - and that depends almost entirely on the intended battery. For the money, I would be inclined to stick with a proven and well known unit such as wemotec or airpower etc.

[Paul Williams]

I agree the testing you are doing is beneficial I was referring to afterburners and such like. I found a cheapo fan unit last night at a grand cost of 10 quid, would you like to run it on your rig?

[Erfolg]

I agree with you Paul, use the KISS principal. Forget burning fuel in the duct.

Yet I do applaud those who try such things.

The work that Timbo does is exemplary, these and other tests do help people like me to have an idea what might work. Yet I am just if  not more interested in what will not work.

If you are not careful and knowledgeable you may buy silly things. Yes I bought a 75mm EDF, with an outrunner, which draws 43 amps, yet the thrust is quite low. Caveat Emptier.

Erfolg

[Tim Mackey]

Paul Williams wrote (see)

I agree the testing you are doing is beneficial I was referring to afterburners and such like. I found a cheapo fan unit last night at a grand cost of 10 quid, would you like to run it on your rig?

No probleemo....just send it to me !

I didnt really have a lot of input to the whole "afterburner novelty" part of this thread

[Paul Williams]

its on its way from China the country of cheap and sometimes nasty, email me the address to send it to. Will just the fan and motor do or would you like esc and batteries? Will laugh if it performs as well as dearer models.

pjwilliams at sky.com

[Tim Mackey]

fan and motor only is fine. - better tell me what battery it is supposed to be used with.

[Paul Williams]

Motor Type Brushless outrunner motor D2217

Shaft size3.175mm

KV3500kv

Max drain current35A

voltage11.1v ~ 14.8v(3s ~ 4s lipo)

I intended to try it on a 3s 2200 25c. What connectors are you using for fan to esc?  I could fit 3mm gold (male or female)  

[Tim Mackey]

4mm gold..but I have converters

[Paul Williams]

A word to the wise, 90mm EDF Fan arrived today from the land of the sun and I decided I may as well build my own rig save troubling Timbo for testing. (thanks for your kind offer)I I copied his design except I fancied it to be more versatile IE; stick a plane on it instead of just fan or motor. I had a load cell that's measures pull, so to save buying digital scales I fitted that behind the pivot arm. Anyway 4 hours of sawing and swearing later I wired up the fan and run it up. I had to shut down at 25% and spent the next 10 minutes picking up all the tools it had rattled off the bench. It was suffering serious vibration. I pulled it apart and discovered the impeller was running out .020"! That's .5 of a millimeter. Far from being just a balance problem. I should have thrown it in the bin but being a tight ar++ I drilled it out clearance and set it on the shaft using a finger clock managed to get it within 002".

Back on the rig it was more than happy to rev up 100% in fact very smooth. I did one run and the result was a max 2.7lbs. I am more than pleased it should fly the foamy (AUW no more than 2lbs). The test was done on a 3s 2200amh 25c li-po.No meter readings as yet will try to find time tomorrow.

The point here is unless you have some good engineering skills and equipment the £24 fan would have been useless. It can not have been tested before selling. You get what you pay for. Still back on schedule for a flying hawk under £75.

[Tim Mackey]

Yep, badly balanced and off centre propshaft holes are common I am afraid. It is always wise to balance ANY fan unit before use also, even the very best and most expensive ones.

Well done for sorting it mind....and with more than a 1:1 PWR should fly well

[Richard Sharman]

Timbo - forum moderator wrote (see)

Test rig Info.

........ The distance from the pivot point to fan blade centre point is exactly the same as the distance from pivot to pressure point on the scales in order to avoid unequal leverage results.........

general side view

Timbo - a question.... 

are you sure that the fan centre to pivot distance  IS the same as the pivot to pressure point?  In the picture they don't look quite equal?

I only mention this because I have just built a simplified version of your excellent rig, and noticing that these distances were not exactly equal, decided to apply a correction coefficient instead.  In fact, there is nothing wrong with having them unequal, provided you apply the correction, and this can be used to increase the accuracy of the device, especially considering that many scales round measurements anyway.

[Tim Mackey]

Yep...sure...to the mm actually, remember the contact point is NOT the rubber mat you see in the picture, but is a small threaded bolt - but as you say, a clever bod will arrange compensation systems

[Paul Williams]

If you had the rig and scales on a board you could turn the lot 90 degrees and calibrate with a known wieght. Just a thought.

[Tim Mackey]

Yep...exactly what i did with a spring balance too.

[Richard Sharman]

Must be my eyes, then? On my screen the pivot-point distance is 50mm and the pivot-fan distance is 60mm to the arrow, and 64mm to the end of the wood where I think you mount the fan lugs.  Perhaps I am getting distortion on my screen?

Here is my very simplified version of the test rig :

It is intended only as an experiment, and then only for small fans.  In this case the powerfan 400 does not have any lugs, hence the strap arrangment to hold it on.  Under the kitchen scales is a small table(not visible) stuck to the bench so it can't move.  The fan arm is in its "back" position as it's not in use in the picture, and the scale zero can be set.  When the fan fires up the arm moves to exactly vertical/horizontal position and presses on the scales giving a reading. The arm doesn't move perceptually after that (just presses harder as the fan increases) so I only fitted a plain bearing at the pivot point, not anything fancier.  And since the arm doesn't move I don't think I need to worry about the cables providing any drag.  The pressure  point is central on the scales, and adjustable as per Timbo.

I tried calibrating the device at 90 degress, but, amazingly, my kitchen weighing scales only work when the right way up, and not when on its side!  It's quite accurate but sensitive to orientation. 

As observed, the  pivot-point and pivot-fan distances are different, giving the arm a mechanical advantage of 1.6 : 1 in this case, so it's easy to apply a correction when the figures are put in a spreadsheet. A different fan would have its centreline at a different point and so would need a diferent correction anyway.

I've done some simple tests of this fan, so I have some figures to compare to the Jetsome-style model test results.  So I now have comparative thrust figures for propellor model, fan model, and open fan which I'll post when I've checked them. 

[Tim Mackey]

Well done there Richard

[Paul Williams]

Timbo, did you try different length tubing attached to the fan rear? I am holding off building the hawk until I get some information on how this will fare. I was going to get some cardboard tube and try differing lengths and cone shapes. Perhaps even set in some ribs to create swirl. The twister is fantastic as my confidence grows flying it I am really impressed with its performance I flew it yesterday on full aileron movement the roll rate is ballistic around three full turns per sec. The point being the internal shape of the exit airflow is quite different to the hawk. I wonder if its makes a big difference to the power output.

One other thing I quite fancy building a larger scale version with twin fan, have you seen any fans that run in opposite direction? I was thinking of counter acting the torque. Why are most edf models under 1m wing span?

[Tim Mackey]

No I never got around to doing that, as I decided I had better get on with actually building the damn model

I dont thing torque reaction will be  problem with a twin EDF - after all the spiralling airflow is not hitting the fins, and  pure torque effect is pretty minimal on multiblade EDFs in my experience. 

I think the reason most EDFs are on the small side is purely down to cost.....the efficiency and thrust from a fan unit is poor compared to propo, so big models mean big and high revving fans, which mean big batteries....ESCs , and then the weight goes up....and you know the rest

[Erfolg]

I am considering building a test stand of my own.

From your experiences have you any pointers regarding both the design and materials of construction.

The reason is I built a model and I am getting indifferent performance from the motor propeller combination. The supplied BRC sheet suggests I should be pulling near to 200w yet the meter is saying less than 125w a substantial difference. More importantly the model is performing lesds well than when I used a geared 480 motor. I am therefore interested how much thrust I am getting as well as watts pulled.

Has any one built a test stand to take the whole model, so that I do not have to take the motors out of my models, so as produce a data table of performance.

Erfolg