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Brushed EDF

HET 6904 fan and super 400 cobalt

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Ash27/12/2008 03:07:00
51 forum posts
13 photos

i am not really an electric person but i am a fan of old jets, so i recently decided to build my own 800mm span budget edf dh vampire jet. the wings are being made out of foam and the body is made out of a aseries of formers and stringers. i have calculated the final weight at 500g with the single heaviest object being the nimh which i will change to a lipo if a fair number of the resopnses to this thread think that my idea is plasuible. my main question is do you think that it will work and what size esc do i need to drive the 27 turn cobalt super 400 motor using an 8.4v 1000mah battery.

thanks

Ash

Bert27/12/2008 10:13:00
514 forum posts
10 photos

Ash

The motor I think you are describing is the Wattage 27 turn Cobalt. I would go for a 30A controller, but you need to measure the amps drawn in your set up.

These Wattage motors don't last long if you pull more than 20A for too long.

To be honest it's not a very good motor for your application. They were somewhat over-rated by the distributor.

HTH

Bert

nasa_steve27/12/2008 12:55:00
457 forum posts
21 photos
4 articles

hi ash

    to be honest i think it will be gasping with that motor as bert said those motors were highly overated and to be honest i think you will end up with a pile of balsa much better you buy a budget inrunner brushless than use that motor to be honest it is rather heavy and they did cause a few glitches in their time as well

nasa

Frank Skilbeck27/12/2008 20:22:00
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4727 forum posts
101 photos

I wouldn't waste your money on a brushed ESC, get a brushless motor, ESC and Lipo. You'll get much more power for the same weight.

What fan are you planning to use.

nasa_steve27/12/2008 22:46:00
457 forum posts
21 photos
4 articles

frank

   it says in the tagline to the thread a HET6904

nasa

Ash28/12/2008 08:00:00
51 forum posts
13 photos

thanks for the advice guys. if brushless inrunners are really that much better than i will just go with that. the problem is that i can find a hi mark 3600 kv inrunner made for 69mm edf for AU$47 but unless i buy from china the only esc that i can find in Australia is $90 and above which is a bit more than i would like to pay but then i might just go with it in the end if thats what is best. one final problem is that i am reducing the size of the exaust from the fan of 69mm to 52mm is this alright.

thanks

Frank Skilbeck28/12/2008 08:58:00
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4727 forum posts
101 photos

Sorry didn't realise you were in Aus (lucky you), but i've bought a couple of Turnigy motors and ESC's from Hobbycity and they've been pretty good and excellent value.

Note it's actually good to reduce the size of the eflux (exhaust) as this increases the air velocity and hence thrust, not sure if 69mm to 52mm is too much, but give it a go.

Ash29/12/2008 09:38:00
51 forum posts
13 photos

thanks for the advice Frank, it's good to find new websites and sources that can be trusted espically with low prices.

 Asher

Terry Whiting29/12/2008 17:46:00
387 forum posts

Ash, 

I do not want to dampen your spirit, but  the DH Vampire is not the easiest of jets for a first trial . The air ducting in the wing root will have to be quite large for an 800mm span model  especially if you use a 69/70mmEDF, and the thrust tube exit would be about 58mm. using a 55mmEDF will enable smaller air inlets, and thrust exit of about 44mm.

For first time scratch building  EDF models I suggest a model with straight through airducting, ie, the Sabre, Fury, BA Hawk,.

Tim Mackey29/12/2008 18:03:00
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20920 forum posts
304 photos
15 articles
brushes are for floors, and definately not EDFs in the 21st century. Another vote for Hobby king here too
Ash30/12/2008 22:32:00
51 forum posts
13 photos

well it looks like i'd better get my self an brushless setup and hobby king looks like the place to get it from. and terry thanks for the advice on first time edf's, i did realise this problem myself, after i had started building it. i guse it pays to plan ahead eh.

thanks

Tim Mackey30/12/2008 23:20:00
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20920 forum posts
304 photos
15 articles
With the £ V $ rate , you might be better off looking at some of the UK budget suppliers like Giant Cod, and foamyjets etc....
Terry Whiting31/12/2008 09:01:00
387 forum posts

Ash,

If you haven't started on the fuselage I would advise doing the long nose version.

 My first own design EDF was a Vamp of 1m span, I  made it  the early short nose version, and had problems when balancing. By removing the battery and with the aid of elastic bands I attached the battery under the nose and rebalanced, I realised the nose  required being an extra 45mm longer.          A complete new fuselage forward section was made..... She looked much nicer, and flies like a dream.

Ash31/12/2008 11:15:00
51 forum posts
13 photos

Good news Terry, the fuselage is built out of foam and balsa with the nose being foam extending it should not be a problem at all. it's also good to know that someone else has boilt this plane and had sucess. how did you make yours.

Thanks for pointing out the exchange rates Timbo i must admit i did not consider it.

Asher

Terry Whiting31/12/2008 15:36:00
387 forum posts

Hi Ash,

Being an old 'Balsa Basher', mine is an all balsa model with liteply fuselage formers, balsa wing ribs sheeted with 1.5mm sheeting.The fuselage is all planked with strips cut from 2mm sheet 6mm in width., might sound labourious, but  it creates very strong compound curved airframe.                            

My Vamp's  AUW  ready for flight is 2lb3oz (1klo 28g)           servos 3  9g.....battery 3s 2500ma  25/30c   EDF,  Air Power 700,  motor  an EDF  Mega

           My plan was from a scaled up  3 view line drawing obtained from. .www.richard.ferriere            type that into Google Search   and another one  www.planephoto.co.uk  

These two sites have always been a great help to me   

      

Simon Chaddock01/01/2009 22:42:00
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5709 forum posts
3034 photos

Ash

Advice as above but I like to give an explanation as to "why". (My apologies if you know it already)

Fans have to be turned fast to get the required thrust. Brushed motors have carbon brushes pressing on the commutator to feed (and switch) the current to the armature coils. So the higher the motor speed the more the fiction, wear and the shorter the motor life. Brush-less motors use electronics to do the same thing, so higher speeds (particularly in-runners), greater efficiency, a longer life and they are likely to be lighter as well.

The same really goes for LiPo batteries. Yes they need a bit more care and must use specific chargers but compared to NiMH for the same weight they have about 50% more capacity and can deliver a higher current. Useful characteristics in any electric plane but even more so in an EDF.

Ash02/01/2009 02:50:00
51 forum posts
13 photos

Great minds must think alike Terry, i scaled up my vampire from the plans on www.richard.ferriere  as well. Sounds like you put some serious work into your vamp. i must admit i do like working with balsa and i get a lot of satisfaction from an uncovened air frame (or maybe it's tha balsa glue).

Thanks for the extra insite Simon, thats why i started this thread to find out more.

Ash

Terry Whiting02/01/2009 07:23:00
387 forum posts

Ash,  

One great thing about this hobby is it's one continuous learning curve which has given me satisfaction for many, many years. I sold off all my I/C models and switched to electric ( no pun intended) just three years ago, mainly due to the EDfans which were appearing on the scene. I loved classic jets, but disliked I/C ducted fans. The EDF, ESC, and new battery  technology has opened a new chapter for me which ten years ago I would have never thought possible, and will only improve.

Happy New Year to you

Terry      

Tim Mackey02/01/2009 09:44:00
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20920 forum posts
304 photos
15 articles
Well said Terry, an excellent example of electric power triumphing over IC - Ducted fans.
Ash04/01/2009 01:32:00
51 forum posts
13 photos

Even though i am a petrol head i must admit that from what i have seen in terms of ducted fans, electric is better than IC which is fair enough because we can't all own and run model turbines.

Ash

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