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Old Nov 20, 2014, 02:12 PM
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Andrew u.k.'s Avatar
Berkshire U.K.
Joined Oct 2006
573 Posts
Question
Understanding EDF. The fundamentals explained. Where?

My Christmas project will be a guanli sea harrier.
Before I start the build and buy the power system I'd quite like to understand the fundamentals of EDF. Such as how intake and outlets affect thrust, the difference between fans, are larger better, how many blades. Etc etc.
In the past I've been put off of EDF as there seems to be a bewildering amount of variables.
So is there a thread with the fundamentals explained? I've looked and I can't find one, so if anyone can point me in the right direction I would appreciate it.
Cheers Andrew
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Old Nov 20, 2014, 02:38 PM
EDF Vulcan To The Sky
Radar-Guy's Avatar
Near Koblenz, Germany
Joined Jul 2002
207 Posts
How good is your German?

http://www.schuebeler-jets.de/know-how.html

Cheers
Frank
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Old Nov 20, 2014, 04:09 PM
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Andrew u.k.'s Avatar
Berkshire U.K.
Joined Oct 2006
573 Posts
Danka, we had our summer holiday in Potsdam and the Harz Mountains. We picked the the wet week which was a shame.
Thanks, lots of info and in English.
Cheers Andrew
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Old Nov 21, 2014, 05:19 PM
we can take off without that
green_flyer's Avatar
London, UK
Joined Nov 2008
901 Posts
There is a old thread about that Sea Harrier
http://www.rcgroups.com/forums/showthread.php?t=1304948

Looks like it takes a 70mm fan, so fan size is already chosen for you unless you want to mod it heavily to fit a bigger fan.
I assume Guanli designed the intakes and exhaust to be optimal for that size fan so putting in a bigger fan might cause problems?

I always try to aim for 1:1 thrust to weight, and keep wing loading down, hopefully 40g/dm2 or below if possible.
There are so many choices of 70mm fans out there.
I always try to chose the most light weight edf and motor, which will deliver high thrust, for low amps. These are all conflicting goals
A lighter plane will be easier to fly, so I like to pick the most light weight components possible but also keep an eye on price.
For example I would avoid Dr Mad Thrust 70mm alloy 11-blade fans because they use so much metal they weigh 230g, ok they produce a lot of thrust, but you can build a Change Sun 70mm 12 blade fan to around 150g and get around 1.2kg thrust and also draw less amps and so be able to use a smaller lighter battery.
High blade count fans like the ones above are a bit sensitive to intake and exhaust area compared to fan swept area, they like the exhaust to be around 100% of the fan swept area and the intake probably comfortably bigger. Also I get the impression they don't do so well in long ducts. If this Harrier bifurcates the exhaust into 4 small exhaust ducts perhaps a 10+ blade fan will not be able to produce its maximum thrust in there. Only testing will reveal how it will behave.

The Change Sun 70mm fan is very cheap and is a powerful set up with a nice cheap Hobby King L2855 2800kv motor, but can be fiddly to set up and balance. If you are happy to spend money to avoid messing around matching motor to fan, fitting and balancing, you could buy one of the ready to go units, and something like Wemotec are a really good choice with correct motor choice already installed, fan balanced, very efficient and not so sensitive to ducting. But they cost 100+ euros.
To save money and end up with a good motor/fan combo its going to take a lot of research on rcgroups of fan and motor set ups people already tested which are good, reliable, light, produce a lot of thrust without being amp hogs.
If you don't mind sacrificing the nice 10+ blade fan sound, Freewing fans are usually very decent, cheap, light, low amps, acceptable thrust.
http://rc-castle.com/shop/product_in...roducts_id=779

If you want to build very light, keep wing loading low, you may be able to get away with using a 3s battery. Weight your airframe, and estimate the weight with a given set up.
For example, if bare airframe (with servos, rx) weighs 400g, and you pick a light weight 3s fan and motor, lets say 120g, a light weight 50A esc 45g, a 2000mah 3s battery 200g your all up weight might be 765g so you only need around 760g thrust from the system to have nice easy hand launches, get decent verticals and be able to do some aerobatics. However if you want a really high top speed then a low voltage low amp 3s set up is not the right choice.

Here is another thread with a 4s set up:

http://www.rccanada.ca/rccforum/showthread.php?t=287137

Calculating from Wikipedia this model's wing area should be around 19dm2 so the above thread built it to a nice weight 650g/19dm2 = 36g/dm2 that is nice low wing loading.
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Old Nov 22, 2014, 06:29 AM
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Andrew u.k.'s Avatar
Berkshire U.K.
Joined Oct 2006
573 Posts
Thanks for the reply.
I think the idea of keeping it light and slower is probably the right way to go for me and this plane.
I'm thinking of trying this unit

http://brchobbies.com/catalog/produc...oducts_id=1338

Will this need balancing?
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