Topic: Scale EDF Design Considerations

Fans Wemo versus HET. They are both very good fans. The HET tends to have a slightly higher amp draw at the same rpm as the Wemo but has a has a slightly higher efflux velocity and its static thrust is very comparable. However the Wemotech can stand higher rpms and will stand up to a smaller diameter exhaust and inlet slightly better so is a better all round fan in my opinion but only by a little bit.

So a good Wemo clone might be, cost for performance, the best way to go. Is the GAD one a good clone... I don't know as I haven't used it.

I would consider the area of the surfaces above and below the intakes to be part of the wing area.

I find that very interesting as I'm not really sure how to approach the build of this. I think you are right in that whichever way it is looked at the Draken is going to need quite a few formers. This sits nicely with the idea of the 1/16 sheet to keep weight down. Its a while off yet but in terms of wing design I was going to incorporate the fuselage and inner wing panel formers as one and borrow (if Dizz doesn't mind!) the use of a stub spar and doubled rib to attach the outer panels.

I was thinking today that as I have increased the inlet size by a given %age I could increase the span by a similar to keep the leading edge angles correct. I'll probably do this in Sketchup first to see if it looks any good.

With regards to the fan, I really don't know of the quality of the wemotec clone until I have it in my hands and can run it up (not until after Christmas I would have thought). But I spoke to the designer of the lightning for some time over choosing a fan and he seemed very happy with the combo he offers.

The following data is given for the fan:
On 4S
Volts=14.8 V
Amps=40 A
Thrust=950g
Watts=735 W
for a 3500kV motor.

To me they don't seem enough for the potential weight of this model. I'm sure however it will power the lightning well.

Hmmm.

No real update here just though this was kind of handy.

I was considering curving the ducting to help it fit into the fus profile better, particularly the  ducting immediately aft of the intakes. To aid in this i managed to splice the 3-view into the middle of the model.

Its helped visualise the model better and also pointed out places where my 3-views disagree with each other 

That's rather clever how did you do that splice of the three view into the Sketch Up model?

It was surprisingly simple to be honest.

I got the initial shapes by importing the 3-view and scaling until the fus was 1m before drawing over the top (see attached photo). That was in 2D. I just copied the scaled 3-view and pasted it into my 3D model then rotated and moved it so it fitted just right.

It was all lined up using the known distance from the first former to the nose. Sketchup allows you to move objects in only one dimension by holding an arrow key whilst dragging, a feature which has proved invaluable!

Now when I have tried to do something like that I have failed miserably. I will give it another go some time.

The clone wars seem to have got replicating the originals down to a fine art now, but I think they still skimp on the material quality.  I have seen some nice looking rotors that could barely tke 20k rpm without failing - stretching and blade tips welding to the shroud.
Those numbers didn't look right, so I checked - 14.8v at 40 Amps is only 592 Watts, so something doesn't add up with those claims.  Have attached my test result spreadsheet which may be of use - been posted before but this is the latest update with the WM400 stuff.  NB majority of data taken on the test stand, but you can certainly identify trends and make comparisons.
All I did with my stub spars was copy what is used with the full size sometimes, so crack on - all fairly easy to draw acurately in CAD.
Does Sketchup allow you to save as a .dxf file?

Thanks for the spreadsheet. Some interesting information in there. Yeah I will be taking a good look at the lightning fan to see what its like. Will be testing it before fitting (glueing!) it into the model.

Thanks, will give the stub spars a go.

Sketchup will allow you to save as a .dxf or .dwg... IF you buy the Sketchup Pro version. Luckily my flatmate has said pro version so when I need to I can send it to him and he can export it for me.

Things may go a little quiet now as I have a lot of work over the holidays and my finals in January. After that I should have some more spare time.

The clone wars seem to have got replicating the originals down to a fine art now, but I think they still skimp on the material quality.  I have seen some nice looking rotors that could barely tke 20k rpm without failing - stretching and blade tips welding to the shroud.

Dizz is spot on as regards the rotors. They are the area of concern. Fortunately you can often buy the genuine rotor and fit it in the fake shroud. Puffin do the 480 rotor and spinner for £13.50 for example.

Just to update this slightly;

Having had a look at the fan (wemotec copy) that came with my GAD Lightning and a quick run up I have observed it is not balanced and the rotor centre is not in the centre (slightly). This gives vibration at 1/3 and 3/4 throttle.

However, Mike and the team at GAD have given good support and I will be getting in touch later today about a replacement rotor, good news! I'm writing this here as it was a possible fan for the Draken, depending on how it went in the lightning.

As an aside I ordered one of the HET fans and a 4S 2200mAh 55C Gens Ace lipo from Giant Cod and have just ran it up with the motor from the lightning...

 

...thats a lot of power! It peaked at;
907 Watts
57 Amps
around 1kg of thrust (I am skeptical of my thrust measurement, it read 1.2kg but I don't believe it). This was done without the intake ring and a fresh pack.

Problem is this was done with a 40A esc so the peak was very brief! As soon as I looked at the amp readout then the 40A continuous label on the esc I had visions of the 'magic smoke' wafting from the esc.

I think I need a bigger esc to continue testing, but this bodes well for the draken, especially if I can step it up to 5S possibly.

Happy Christmas

Rich

Did a little bit more testing, letting the pack voltage stabilise,
Giving 740Watts and 53 Amps. The 40A ESC has a burst current of 55 amps for 10 seconds, so still think using the esc will be pushing my luck a little bit!

That is part of the issue with those sort of impellers tsr and I were talking about.  I have spent 3 hours trying to balance a 12 bladed job (because it will look good in the model it is destined for (P1154)) and it still vibrates like mad - time to give up and use a spare wemo 480 rotor instead I think.
I usually work on a 80% rule - ie run ESC at 80% of quoted capacity and leave 20% in the packs.
The thrust figure you got (1.2kg) is about right, but you will smoke that ESC. The other problem with a 2200mAh pack will be with endurance: you will only get about 2:30 out of it, but of course you might be happy with that.
Pete
   

Yes I think the ESC is going to be upgraded to a 60A job. The use of a 2200 pack was to keep the weight down (and is what I was advised to get) for the lightning.

Hopefully when it is flying it will give me some much needed experience to apply to the draken. I don't expect the lightning will be pulling 50A all the time, probably 40 (though I don't tend to fly slowly  ) so I may be able to stretch to 4 minutes.

I am much happier with the HET fan to be honest, it its 3x the price but it worked straight away.

I was looking towards a slightly bigger pack, maybe 3300, possibly even 5S. I am about to go have a read back through your FD2 thread Pete to glean some info on your battery setup.

Dont forget that the FD2 has been specifically built to go fast with that WM400 fan.  The number of cells depends on what your motor can take.
To get 4 minutes from the 2200 pack (assuming 20% capactiy remaining) the average current draw would have to be 26.4Amps.
Pete
 

Good point, I had forgotten to give the 20% left in the pack.

I am a bit irritated at the minute, I have just looked up the specs for the (unbranded and with no documentation) motor I bought as part of this power set and it is capable of a continuous current of 28A and burst of 38A.

So now I have a 40A esc and motor that I was recommended and a 4S 55C 2200mah battery I was recommended and I can't use them.

I could buy a HET 2W-18 and 60A esc, but then what was the point of spending £45 on motor/esc/fan if I won't use any of them. (there is a lesson in here I'm sure) 

Not sure what to do at the minute.

What was it running at in terms of amps in the GAD fan which was the one I understood the motor was intended for. HET fans tend to run higher amps for a given RPM when compared to Wemotec fans. So the Lightning motor and fan may well work at the right numbers with the battery you have. Personally I would run a bigger battery but that would just be probably a vain effort on my part to get the battery to last for a good number of flights. 

In the GAD fan I was running it on 3S 2200mAh 30C as it was all I had. It was pulling around 260Watts and 20ish Amps and making a horrible noise (wobbly rotor) in the process on a 75% full pack.

I'll try the 4S on the GAD fan when I get a new rotor, though I suspect it will quickly exceed 28A. I don't really want to toast this new gear. Yes the pack did seem quite small to me, though if it isn't going to pulling the same Amps on the GAD fan it may well last longer.

If anything the flight time on the lightning may be just about scale (i.e. about 2 minutes) as its not likely I'm going to fly it slowly! I'm reading 'The Lightning Boys' at the minute which is a compilation of stories about the lightning, seems like they mostly spent they're time going very fast and then running out of fuel 


This has highlighted some good opportunities for learning I guess. For something like the Draken would a 4S 3300mah 50C, 60Amp Esc and Het 2W-18 suffice I wonder? or should I look at an extra cell?

I don't want to get into the more power, more weight, more power spiral of doom though!

Well I am not sure what to say really. As power and weight all come into it as you say. I am now regularly running 4000-4500 mAh lipos where I originally started with 2500 mAh batteries partly because batteries have got lighter. So the original 2500 was about 250grams and a new 4000mAh Gens Ace for example is 350grams give or take. 100 grams extra on a reasonable sized airframe is not going to hurt too badly. Stall speed might go from say 20mph to 24mph
I haven't had as much success with high C ratings as some seem to have had. Although I did try them thoroughly last year but found one 30C-40C gave better performance than some 40C-85C batteries that I bought, this year. I will try some Gens Ace I think as there seem to be some good reviews about and how they perform well approaching their theoretical maximum. This is down to internal resistance or so I am told, which in theory should go hand in hand with C ratings but doesn't always follow quite so closely. 

This entire fuss need not be a concern as square inches is all that is the real consideration for
managing an AUW with a given static thrust. The inlets equaling total FSA is all that is required.
As an example the Dynamax EDF at what can be 18# to 22# static.



We are bifurcating to achieve 40# in our B-47 projects.

Can i ask? What sort of wing loadings do you fellers feel is acceptable?

My He162 , converting to oz/ft² is about 40. The figure itself does not seem too big but am wondering over the practicalities. Any comments?

Can i ask? What sort of wing loadings do you fellers feel is acceptable?

My He162 , converting to oz/ft² is about 40. The figure itself does not seem too big but am wondering over the practicalities. Any comments?

I have been back through the models I have figures for with me now: all 70mm fans and quite powerful set-ups.  I use metric values and I make your 40 oz/ft² = 122 g/dm²
Stock Phase 3 EF16 (56mm fan)   55 g/dm²
M52 (56mm fan & Don's wicked)  100 g/dm²
Modified Phase 3 EF16                76 g/dm²
P1091                                         62 g/dm²
FD2                                             89 g/dm²
P1121                                         100 g/dm²
 
Some other info I have gathered as part of planning a bigger projest
90mm EDF Hawk      117 g/dm²
90mm EDF Hunter   129 g/dm²
Turbine Habu32      132 g/dm²
Turbine Viper         133 g/dm²
Turbine Hawk         127 g/dm²
 
The trend is for W/L to increase with the physical size of the aircraft and for models fitted with an undercarriage.  Just means that they have a higher stalling speed so need to fly faster    I don't see any problems with your He162.
 
Pete

   I don't see any problems with your He162.
 
Pete


Time will tell but thanks very much for the detail info.
[/quote]

Just to bring this thread back up to date.

I finished my finals yesterday (finally!) so hopefully I won't have to do any exams again. I also have managed to get myself a graduate job, just have to do my dissertation now and I can start paying my loan off  :bang head:

I've been itching to get back to this project for a while now and I've spent a couple of hours today working on the ducting again.

I've decided to base the design around the HET fan I have (though I think I'm right in saying its identical to the minifan?)

I've come to the point mentioned on the first page about moving from the 100%FSA of the inlet to the 100%FSA plus spinner. I am going to make the transition as smooth as possible, this means I need to know where the fan is going to be.

Is there a 'proper' place to locate the fan? I guess to counter balance the battery, just how far back is the question? Or is it a case of 'that looks about right'?

Rich

In my case this is always a bit of a compromise. As you say where it will make battery location useful and also be not more than 5 fan diameters from the efflux tube exit, as well as trying to suit the ducting path to make it all as smooth as possible and fit the space available.

Thanks for that. I have put it at 3.7 fan diameters for the minute, seems to look about right. I've drawn up the tube in Sketchup.

I've had to angle it slightly (3.5 degrees) and raised the  fan position form the centreline by 8mm in order to get the outlet in the right place. I'm using 85%FSA too.

It doesn't look like much I know but it was surprisingly difficult to manipulate the tubes to get the right angle.

I was thinking of using acetate sheet formed by the formers, is there any drawback to this on 500-1000W systems? My other EDFs use acetate but they are much lower wattage.

what have you angled if you angle the thrust tube then you are adding upthrust or downthrust depending on which way you angle it unless you correct it with the exhaust cone.

what have you angled if you angle the thrust tube then you are adding upthrust or downthrust depending on which way you angle it unless you correct it with the exhaust cone.

Ah, that might be a problem. The thrust tube (portion from rear of fan to outlet?)  is a cylinder that tapers from 100%FSA+Rotor to 85% FSA concentrically. The fan is mounted along the thrust line but 8mm above it. I have angled the thrust tube by 3.5 degrees to get it to fit correctly.

Never occurred to me that may affect the thrust! I can probably change it back so the thrust tube is straight but it will mean having to add more of a bend to the inlet, not much though I guess. I would assume adjusting the path of the inlet is not going to affect up or down thrust?

As an aside I was thinking about the airflow earlier, am I right in assuming the air flow should be laminar rather than turbulent? Can't imagine the rotor would appreciate turbulence in the flow.

For something like this, the exhaust duct should be ok so long as there are plenty formers. The intake ducts are the ones which will need to be nice and stiff. If it were mine, I'd be inclined to use ply or Proskin, which I've just received for the Canberra ducts, either of which glues better than acetate and if neccessary can be stiffened with carbon tows on the outside in between the formers. Acetate tapes ok but I've never been very successful at gluing it! YMMV!

Congratulations on employment by the way. What do you/will you be doing for a career? 

I did think of thin ply but didn't want to make it too heavy? Though maybe 1/64 would be okay.

The other thought was vacuum forming but I guess if its thin enough to form its too thin for the ducting to be strong enough.

Whats the proskin like to work? I've heard a lot about it but never used it.

A thought that I keep considering is glass cloth? I've never moulded anything before and would quite like to give it a go, though again I don't want un-needed weight.

Congratulations on employment by the way. What do you/will you be doing for a career?

Thanks  I've got 4 months to finish my Chemical Engineering degree and then onto a Process Control Engineer grad program on Teeside. Looking forward to working on these bits of equipment that I'm sick of looking at in textbooks!

The 0.2mm Proskin I've got is a lot like mylar to handle. It's very floppy but only in one direction. It sounds like Rolf Harris' wobble board. There's not way it'll do compound curves, but to roll into a tube I think it's ideal. The bit I bought cost £17 for a 3' x 2' sheet, so I think it's cheaper than 1/64" ply. It also doesn't need any finishing work.

It's just like plastic sheet except it adheres properly.

Re. your career - if you don't enjoy Process Control very much, or want in on Oil & Gas design engineering rates then I'll give your CV to a mate of mine. He's the supervising Process Engineer for us at Aker Solutions. Of course you'd have to move to Aberdeen, but there is a graduate programme and process engineering pays very handsomely 

I might have a look at using some proskin then, sounds good stuff.

I've realised I have a reasonable amount of 1/32 balsa, does anyone have any experience of using balsa for ducting? I'd like to think using aliphatic glue a strong enough duct can be made?

I'm in the process of adjusting the fan/thrust tube to get rid of the up/downthrust issues.

I did originally set out to get a process engineering program but after an assessment centre was offered a process control role. Seeing as the control team were all process engineers and the current graduate employment rate I jumped at the chance. Plus it means I can stay on at my current flying club   I'll keep you in mind Jamie, thats very kind. 

Hi Rich
Very interesting thread glad your staying on the dark side with the jets! You've chosen a great subject albeit complicated but that's what modelling is all about...if I can be of any use re the moulding I've done alot in the past also been playing with the idea of making a vac forming machine as per Bob TV (if you haven't seen this it's a great resource of building tips primarily turbine aircraft) well done on the degree look forward to catching up soon..come over and see the progress on the Lightning
Mark

Hi Mark,

Yeah really been enjoying flying my Sabre recently and having repainted the Skyhawk in New Zealand Airforce colours fancied looking at another jet. Fancied a step up in size too. This one could be a long time in the making though, as you say to me its pretty complicated, I've got a lot of learning to go before I cut any balsa.

Your enthusiasm for the lightning has certainly rubbed off by the way, ended up popping to the sunderland air museum to see their lightning (and vulcan). In turn I ended up ordering 'the lightning boys' book and the GAD 70mm lightning kit. It'd be great to see  your lightning progress, I've still got your number so I'll give you a text.

Just spent a good hour looking over the Bob TV site, some good stuff. Like the flip over vac forming machine, only problem I can see is getting hold of the PET at a reasonable cost?

Nice Pc-21 by the way!

Cheers,
Rich

Good point, I had forgotten to give the 20% left in the pack.

I am a bit irritated at the minute, I have just looked up the specs for the (unbranded and with no documentation) motor I bought as part of this power set and it is capable of a continuous current of 28A and burst of 38A.

So now I have a 40A esc and motor that I was recommended and a 4S 55C 2200mah battery I was recommended and I can't use them.

That set up would suit a 55mm EDF or a 64mm edf so I reckon it is an excuse to go out and buy a little something or design and build a quick depron or foam EDF

That set up would suit a 55mm EDF or a 64mm edf so I reckon it is an excuse to go out and buy a little something or design and build a quick depron or foam EDF

I like this idea! Any ideas suggestions?

Well not specifically as there are so mny models out there for this sort of system. GAD Green Air Designs do some 64mm jets like the Mystere and of course there is the Phase 3 F16 but really the world is your oyster and if you want to design and build something it is really only a matter of what you fancy doing. Then a quick Saab Viggen or Gripen would fit the bill as it would fit Swedish element of where you are already.

Good point, I saw the GAD F100D was on offer, a model I've aways fancied. Though I am tempted to have a go at a quick depron Saab Tunnan maybe. At least I have a use for the motor/esc. I need to look into if HET do a 64mm fan.

I've had chance to do a little more. The ducting is almost complete so I may start on the wings soon.

Is it accepted to use the wing section for the full-size Draken? Or would a simpler model-type section suffice? After all, can't scale down the air I guess.

Rich

Just mirrored the ducting to give an idea of the final product. A little adjustment will be done when a plug/formers (havnt decided how to make it yet) has been made, I've got a bit sick of making small adjustments in sketchup.

Most people disagree with me on this, but what the hell...

On a model like this with so much wing area and indeed so much wing blending into the fuselage, use the real section or something that looks very similar. Feel free to diverge to something else at the tips if you must, so long as it looks passable at all angles. Scale models are supposed to look like the real thing first of all - if they fly like a Thunder Tiger Trainer 40 then great as a secondary consideration.

My rationale is that in side elevation the choice of a drastically different section can really mess up fairings and the overall lines of an aeroplane, particularly one like this where the wings cover 2/3 the length of the fuselage. Like it or not, you've already modelled the root section and modelled the ducting to suit. Say, for arguement's sake, you chose a Clark Y in the hope of achieving trainer-like flying qualities from a small, heavy delta winged jet - how are you going to reconcile a completely non-scale wing section into the curves of the fuselage without some very peculiar contours which stick out like the proverbial sore thumb?

To me, what's really important is finding a section which has the correct depth (or else it looks ridiculous head on), and max thickness at approximately the same % chord as the fullsize. Lastly, try to find one with a leading edge similar to the fullsize. The rest doesn't matter too much as far as looking relatively convincing goes. Nice blunt leading edges generally fly better and have more docile stalls etc, but whenever I've seen someone try to use one in lieu of a sharp leading edge on a modern(ish) jet model, the result has not looked very good at all IMO!

Scale models will never fly as well as ultra lightweight 3D types, so personally I see no point in sacrificing the looks (too much) in pursuit of flying qualities. Of course, it has to fly - but as I said - with a fast delta jet so long as you keep the taps open it would fly fine with a flat plate wing.