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Old Oct 08, 2012, 02:43 AM
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Joined Sep 2011
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Help!
artech su27

Hi, i need help on my su-27. im newbie in EDF, and bought an underpowered EDF last year, and then neglected it. last month i planned to make it fly, because it never flown. I purchase 2x64mm alloy EDF include 4300KV BL at HK, 2x 70a ESC and use 1x1800mah 4s 20-30c..... and the result was, it still cant fly

then i asked a friend to hand-launched it and the 1st and 2nd launch, but it still cant fly. questions:
1. where are the problems? does it need higher cells battery or?
2. how to calculate the CG for su27?

please help...

i recorded it, here are the videos
Sukhoi 27 Maiden 20121007-a (0 min 15 sec)

Sukhoi 27 Maiden 20121007-b (0 min 7 sec)


Thanks
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Old Oct 08, 2012, 07:00 AM
Life begins at transition
Australia, VIC, Sale
Joined May 2007
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Use the wheels! Let it get up to speed.

I used 4s 2200's on mine (RIP), and it pulled around 70A in total. Went OK.

Where is your centre of gravity? If it's constantly pitching up and down, chances are it's too far back.
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Old Oct 08, 2012, 01:01 PM
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PM answered.
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Old Oct 08, 2012, 08:29 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
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You need speed before the EDF's become efficient and generate thrust. So you should launch the plane with a bungee cord (see attached video).
Bungee Launch No2 without frame (1 min 50 sec)

Secondly, looks to me that it's tail heavy. Try to move the batteries forward if you can.
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Old Oct 08, 2012, 08:41 PM
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Quote:
Originally Posted by Odysis View Post
Use the wheels! Let it get up to speed.
I used 4s 2200's on mine (RIP), and it pulled around 70A in total. Went OK.
Where is your centre of gravity? If it's constantly pitching up and down, chances are it's too far back.
thanks for your info, i will try to move the battery forward. you're right seemingly problem on its CG. is it same with rc propeller plane? where the CG is 1/3 of Leading Edge?

Quote:
Originally Posted by ATIS View Post
PM answered.
hi ATIS, thanks for all of you info. I read and will follow you suggestions in PM

Quote:
Originally Posted by djacob7 View Post
You need speed before the EDF's become efficient and generate thrust. So you should launch the plane with a bungee cord (see attached video).
Secondly, looks to me that it's tail heavy. Try to move the batteries forward if you can.
Thanks djacob7, its great... the bungee cord should be the solution for me, because i cant find the longer runway here yet. And you're right, its tail heavy. so how to calculte CG for this kind plane (su 27)? is it 1/3 of Leading Edge?
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Old Oct 08, 2012, 09:09 PM
Fremont, CA
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It's hard to define the leading edge on a fighter type wing because it's so slanted. I built two similar fighter jets so my guess at the CG is shown below.
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Old Oct 08, 2012, 09:16 PM
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Quote:
Originally Posted by djacob7 View Post
It's hard to define the leading edge on a fighter type wing because it's so slanted. I built two similar fighter jets so my guess at the CG is shown below.
djacob7, Thank you very much --- finally you give me the solution for thing i was looking for so long
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Old Oct 08, 2012, 11:52 PM
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Joined Mar 2006
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"I purchase 2x64mm alloy EDF include 4300KV BL at HK, 2x 70a ESC and use (((((1x1800mah 4s 20-30c)))))..... and the result was, it still cant fly"

There's your problem. Or one of em. Plane will pull 70 amps total. Needs two batteries of this size or larger probably, or one 4000 20C approximately.
(Your battery is rated at 36 to 57 amps max....Probably killing this battery just running up the plane. And likely the airframe was designed to have more weight forward so you are running insanely tail heavy)
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Old Oct 09, 2012, 07:07 AM
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Quote:
Originally Posted by Michael Paxton View Post
"I purchase 2x64mm alloy EDF include 4300KV BL at HK, 2x 70a ESC and use (((((1x1800mah 4s 20-30c)))))..... and the result was, it still cant fly"

There's your problem. Or one of em. Plane will pull 70 amps total. Needs two batteries of this size or larger probably, or one 4000 20C approximately.
(Your battery is rated at 36 to 57 amps max....Probably killing this battery just running up the plane. And likely the airframe was designed to have more weight forward so you are running insanely tail heavy)
thanks for your advice, and i am now understand about the pull amp vs battery amp.

i will try to use 2 battery in order to provide more capacity when the ESC pull amps and to have proper CG.

thanks...
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Old Oct 10, 2012, 08:32 PM
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I ran a similar set up in mine but on 3s 2500 (30C) lipo but I was using EFlite 400 (4200kv) inrunners on two 40 amp ESC's. Like I said in the PM, 4s 2500 (40c) lipo is about the smallest you might be able to get away with. A nice 4s 3200 (40c) would be better. Be carefully making room for the lipo as the foam around the intakes and turtle deck gets thin fast and a hard landing will break the nose off just behind the cockpit.
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