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Old Aug 16, 2009, 02:40 PM
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Quote:
Originally Posted by CSI
This short tail is going to make CG settings very sensitive and just a slight change will make a huge difference. Same for the elevator throws. Just a bit too much and, with that short couple, the wing is going to be very sensitive to snapping and/or tip stalling. There is always going to be some slight difference between the wings...shape, dihedral, twist, incidence, etc, so very common for one to stall slightly ahead of the other.
All of this combined is making the wings prone to snapping out, one wing accelerated stall, causing violent roll.
I have seen this on so many planes in the past. Almost always too much elevator throw, and/or aft cg.
The problem can be helped with moving the cg forward slightly, reducing the elevator throw, etc, BUT,...unfortunately, may never go away entirely. It's not usually a speed issue either.
Ken
Thanks for the advice. I had mine at the forward most CG. The comments about sensitivity have gotten me to thinking that this 1mm discrepancy between the wings could be the source. I'm going to shim in a 1/16" balsa on the left wing panel and see if makes a difference.
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Old Aug 16, 2009, 07:58 PM
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On a 3d plane can you have to much elevater throw??? I run 50 or more of throw on the elevater on all my 3d planes. I would have to say check the CG and make sure its spot on. Now I have flown sport planes and YES to much throw is going to make it snappy but a 3d plane should not dio this.
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Old Aug 17, 2009, 01:44 PM
CSI
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You can definitely have too much elevator throw on a "3d type plane". It really depends on the mode of flight you are in.
Ton's of elevator throw is great in 3d flight, but...you have to get the plane INTO high alpha first. At higher speeds, that large elevator throw is just going to abruptly stall the wings and usually cause a snap roll. The large throws are meant for slower speeds and not while flying on the wing.
I guess the idea is something like....you fly level at 50 knots, then pull a ton of up elevator. That airplane isn't going to instantly stop in place with the nose straight up. It's going to skid along forward a bit as the nose comes up, and unless pefectly setup, those wings are going to stall slightly different, causing a roll, corkscrew, etc.

This is why a sport plane like this is usually flown with dual rates. A tamer setup for regular flying, then a 3d setup for aggressive, high alpha flight.
Can I fly my 3d flat foamie on 3d rates? Sure, but it's a handful and I have to be ver careful on the control inputs. Just a bit too much and it's out of control, that or the wings shred off?!
Of course there are exceptions to everything, but..........
I still love my SU...flying beautifully.
Ken
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Old Aug 17, 2009, 01:52 PM
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Not always true, If you plane is set up right and you are at full speed and you yank your elevater it should go into a wall and not stall out. At any speed I can pull full 50 of elevater and the plane will just flip over on its self. It has NEVER dropped a wing.
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Old Aug 17, 2009, 08:47 PM
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Did some more testing today. Figuring that after 15 packs of shakedown flights with no fix in sight, I decided to start seeing how the plane would handle CG being shifted back. I was expecting an unholy mess, but amazingly at 0.25" back from recommended CG, the SU suddenly was tracking true through mild maneuvers.

It was dusk, so I didn't test the limits fully. What an odd behavior. What conclusion can I draw now that the plane is behaving better with CG back???
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Old Aug 18, 2009, 01:06 AM
CSI
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Wow, that's great news! I can't imagine what might be going on. I look forward to hearing about some more flight testing for sure. Keep us posted please!

losi...I sure wish I had the skills to be able to have my planes flying like that! Definitely cool. I am a sport/pattern flyer, have been for over 40 yrs. I dabble with 3d stuff, but outside of the occasional hover, I don't know much. I guess the well designed and built 3d planes can do some pretty wicked moves. I can't see this little SU doing that type of flying without coming apart.
I'm just happy she's flying so well. I had put one of the EP Reactor planes together the smaller one) and it went crazy on the maiden and I lost it. I had no control after the first downwind turn.
Happy flying to all of you,
Ken
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Old Aug 18, 2009, 11:36 AM
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Anti-rotation pins

Sure wish I had seen the post on the anti-rotation pins before flying mine. They looked OK when I put them in and I went ahead and flew without any further checks. The maiden flight was nearly uncontrolable; it took all available aileron trim to get it to fly straight and level. I was fortunate enough to get it down in one piece. A little checking with the calipers showed a mismatch in the alignment of the two wings. My problems are to figure out which wing is right, so I get the wings properly aligned to the stabilizer, and to "un-epoxy" one pin. Any suggestions for either of those issues?

Thanks,

Bob
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Old Aug 18, 2009, 11:41 AM
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Quote:
Originally Posted by bob93447
My problems are to figure out which wing is right, so I get the wings properly aligned to the stabilizer, and to "un-epoxy" one pin. Any suggestions for either of those issues?

Thanks,

Bob
On mine it was easy to see that one pin was crooked, so I un-CA'd that one.
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Old Aug 19, 2009, 01:14 AM
CSI
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Bob,
Sorry to hear about your troubles. As for the pin....I bet it might be easiest to just cut the old one off flush with the rib, then drill a new hole down through the remaining part. Start with a smaller bit and work up.
Good luck on getting the wings straightened out, not sure how to figure out which one is best?
Ken
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Old Aug 20, 2009, 05:11 PM
Zebro
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Canada, QC, Blainville
Joined Sep 2007
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what about this...

Hi everyone, I am a few days from getting my SU-31, from the tracking number it is now at the customs.

I was reading the manual and reading this thread and I think I have an idea about the wings anti-rotation pins...

Instead of putting one pin in each wing, why not put one pin through the fuselage, sticking out on each side (like the dowels holding the wings rubber bands on trainers)

assuming the fuselage is built straight this would mean the two wing panels would line up perfectly.

what do you think?
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Old Aug 20, 2009, 08:18 PM
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Quote:
Originally Posted by yanickr1973
Instead of putting one pin in each wing, why not put one pin through the fuselage, sticking out on each side (like the dowels holding the wings rubber bands on trainers)

assuming the fuselage is built straight this would mean the two wing panels would line up perfectly.

what do you think?
Novel idea. Let us know how it works out for you.

In the meantime, for those in the process of building, I would suggest setting the included pins into the wings (per the manual) and instead use 5 or 15 minute epoxy to align the wings with the fuselage as the epoxy sets.

Just make sure you don't epoxy the wing to the fuselage.
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Old Aug 21, 2009, 12:50 PM
Zebro
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well, I received the plane today but unfortunately, I will be out of town for the week-end.

I did try my idea, i had a carbon rod just the right side lying around and so far it works, I will let you know more when I start building the plane.

Have a nice day!

P.S. this is my first ARF and my first Great planes product and so far I am really happy with the quality of the wood and the quality of the build

I know some people complained about the quality of the wood but this does not apply to my kit.
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Old Aug 21, 2009, 01:12 PM
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great planes makes really good quality products. i'm on my 2nd edge 540 41" and i love this plane.
once i got my ail differential setup, and a little elevator to rudder mixing it KE's sooo sweet.
this sukhoi looks very nice too, and i think i'm going to have to buy one soon.
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Old Aug 21, 2009, 06:24 PM
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Well, after about 20-30 flights with mine, I'm definitely getting more comfortable with it. I'm having a very hard time trying to get the plane into a hover......which I've only attempted at higher altitude (I'm a novice flyer....just graduated from trainer and have only been doing 3D on the sim). I've got the general aerobatics down, but can't get the plane to settle with the nose vertical. Thinking about moving the battery back about .25 inch, and see what happens? Thoughts??

Now that the subject has been brought up, I too have noticed problems maintaining a nice track in a loop or vertical climb. The plane, ever so slightly, seems to fall off track and I need to correct with the rudder/ailerons. I just assumed it was winds aloft. I'll try again during a calmer day and see what happens.

Still having a blast though! I did manage to flip the thing over on landing. Only broke the cowl loose from the mount ring......no other damage. Fixed in an hour with some epoxy.
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Old Aug 21, 2009, 07:28 PM
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Tis is some of the best set up info I have seen. Check it out. I bet it will really help

http://www.nampamodelaviators.org/im...0Goldsmith.pdf
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