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Fan /Model match ?

Brushless Fan with Aermachi MB 339 EPP Jet

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Dave Cooper 315/12/2019 00:03:49
65 forum posts

Hi, I have recently acquired a brushless EDF fan unit and a PSS Aermachi model at an auction.

I think the fan unit is an '8-blader' and the jet model is about 51" span. I was intending to fly it PSS to start with and, later, convert it to EDF.

What data would we need to decide whether it was suitable for conversion or not ? I have in mind :

Model weight, KV rating of the motor, fan diameter, fuselage width /depth at the CG for starters.....anything else ?

Thanks for any assistance (only previous 'jet' experience is with "Jetex" models as a young lad !)

Dave

Engine Doctor15/12/2019 11:17:15
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2370 forum posts
29 photos

Hi a 56 inch model will probably need an 80 or possibly a 90 mm fan , Do you know what the recommended battery is for the fan i e 4s or 6s and capacity then you will have a starting point to work out the balance/CoG and weight etc . It will also pay dividends to balance the fan, see you-tube , as a smooth running fan not only sounds much better but produces more thrust due less power being absorbed by the vibrations of an out of balance fan .

Try running the fan up before stripping etc as it might be a dynamically balanced unit.

Hopefully there will be more knowledgeable EDF build designers along to help. I can only offer what I have gleaned from messing about with fan models .

Good luck and let us know how you get on .

E.D.

Dave Cooper 315/12/2019 14:59:45
65 forum posts

Thanks for the reply E.D.

A trip to the workshop has now revealed :

Fan: 6 Blades - 75mm diameter

Motor - 3500KV (QF 2827)

Fuselage width - 70mm and depth 100mm. (measured at approx. C of G). Width could be made wider with air intakes to accommodate the fan unit

Weight : approx. 2lbs (without LiPo and without fan unit). I would imagine a 6s may be needed to give reasonable flight times ?

The model was designed as a PSS glider, so, an EDF version would need some mods ! I will bench test the fan soon to see how it sounds /performs...

Dave

John Tee15/12/2019 15:20:27
805 forum posts
65 photos

I know nothing about EDF but our club had a noise problem with some EDF fliers and it was reccommended to us that 12 blade fans seem to create less noise,. Just an observation.

John

Dave Cooper 315/12/2019 20:50:46
65 forum posts

Yes, thanks for that John - quite a few of our sites are noise-sensitive. I will test the unit and see if the neighbours complain first !

A '12-blader' may be more appropriate anyway - more efficient ? extra thrust perhaps ? A real learning curve for me...

Dave

Engine Doctor16/12/2019 10:59:44
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2370 forum posts
29 photos

Hello Dave you will find that your unit will give excellent thrust but as said will probably be noisy . If it has a out-runner motor it might be a bit quieter .Balancing will help but the higher the blade count they are usually quieter . Have a look here at this 12 blade unit from Bangood . There are quite a few fan units available if you scroll down below the add . I fitted a similar one into a HK venom that was given to me without a fan unit . It was a bit noisy but after a bit of balancing gives a good jet sound and flys the venom well . Not sure if it will be powerful enough for you 56 in span model though . Its always a balance between going for a 4s set up to save weight or the 6s set up for the added power ? with the correct motor a good balance between weight and power is achievable without spending mega bucks .

E.D.

 

Edited By Engine Doctor on 16/12/2019 11:00:30

Dave Cooper 317/12/2019 10:43:27
65 forum posts

Good to hear the thrust should be OK. The "Banggood" 12 blader looks good - couldn't see a price on the ad' though.

As my model is EPP I was just wondering if this would have some sound absorbing qualities. Presumably, I'll have to conjure up some form of thrust pipe as well ? Rolled ply /balsa sheet ?

I'll have a look at the Youtube videos on balancing as well...

Dave

Engine Doctor17/12/2019 11:44:31
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2370 forum posts
29 photos

Hello Dave . The price is £26 -20. Not sure if there is any postage .The last one I bought was on ebay , came from china and took a bit of balancing as it run horribly out of the box .As already said once balanced it runs really well.wink I think they are the rejects from the units fitted into RTF models .

My Habu Mk1 is made from EPP and the motor is quite noisy at full throttle but throttle back a little and its a lot quieter.. That is fitted with a 6 blade 70mm fan running on 6s and gives plenty of power for easy launching and reasonably fast flight. Flights only last about 4 to 5 min though as fan is greedy for power. I would say that the power from this 6 Blade unit is just as good as the !2 blade unit I have in my Viper but the 12 blade gives a smoother quieter exhaust note and no vibes through the air-frame 

Thin ply or balsa should be fine for an eflux tube . As Christmas is coming look out for Christmas Cracker boxes with acetate lids . Its light-ish and fine for an eflux tube and far cheaper than thin ply . A 10% reduction from inlet area to outlet area should work fine . Any EDF officianardos out there will advise if I'm wrong . It might be worth posting here on the EDF forum . .

E.D.

 

 

 

 

Edited By Engine Doctor on 17/12/2019 11:55:22

Dave Cooper 317/12/2019 23:05:24
65 forum posts

Hi E.D. That price sounds more than reasonable if it's a good unit.

Just been on Jet Hangar's site (jethangar.com) and they advise 80-100% of Fan Swept Area (FSA) for inlets and 70-80% for outlet.

They do warn about cheap Chinese clones of their product, a number of which have failed in service....You pays your money....etc.

There's a formula for calculating FSA under the "EDF Rules" heading. It does look like it's a compromise between take-off power and top speed. As I'll have 2 inlets and one outlet I'll have to get the calculator out !

Hope to get bench testing soon...one or two other projects to finish first.

Dave

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