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Old Dec 12, 2012, 08:26 PM
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Help: Parkzone Sukohi Su-26xp

Hi,

Wondering if anyone here can provide some helpful advice.

I've had the SU-26xp for 6 months or so but it has been sitting most of that time. From the beginning, it has been hyper pitch sensitive; flying it is to experience a series of uncontrolled 45 to 90 degree pitches up or down. Typically, a 90 degree pitch up to vertical at low throttle will soon lead to a stall and then crash.

I've tried moving the cg forward (verified this week that it was 32mm back from the front wing which is 5mm forward of factory) and I took it apart and did a bunch of stuff to take play out of the elevator linkage and add stiffness to it. I also cut off the edges of the elevator to reduce elevator area dramatically.

??What else can I do to make this thing fly somewhat smooth? I've flown 40-60 sized Kaos, 40 sized extra 300, 90 sized Sukohi years ago; recently Habu and pkzone gunfighter mustang. I've never seen anything close to as unstable/sensitve in pitch as this Sukohi; it's just impossible to keep it airborne long and it's not much fun fighting the whole way.

Again...I'm getting uncontrolled pitch movement from 45 to even 90 degrees. It can be flying relatively smooth for a few yards and then BOOM.

Thanks for any advice,

Andy
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Last edited by sothpaw; Dec 12, 2012 at 08:55 PM.
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Old Dec 13, 2012, 10:42 AM
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United States, TX, Caldwell
Joined Oct 2012
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Just a shot in the dark, but try limiting the range the control surfaces move. Instead of 100% throw, set them back to say 50%. That way your inputs will be less sensitive. Now this is using a DX6i. If you are using a RTF that has a tx that came with the plane, there is a way to go from "high" rate to "low" rates. They reccomend low rates for learning.
Hope this helps.
Daryl (cwmoss)
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Old Dec 13, 2012, 11:32 AM
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Quote:
Originally Posted by cwmoss View Post
Just a shot in the dark, but try limiting the range the control surfaces move. Instead of 100% throw, set them back to say 50%. That way your inputs will be less sensitive. Now this is using a DX6i. If you are using a RTF that has a tx that came with the plane, there is a way to go from "high" rate to "low" rates. They reccomend low rates for learning.
Hope this helps.
Daryl (cwmoss)
Daryl,

Thanks! I've tried lower rates and for sure I have not been using full ele throw. I was at 50% with 90% expo; now I will try 35% throw w/ 95% expo.

I re-read the manual last night and the cg range said 29-31 mm. I had found an on-line version that stated 35mm. I added balast to the front to affect this change. I've flown it w/this balast before but it requires much more up trim and did not fix the issue. I'll hope here that less (yet) throw, plus the balast (very fwd cg), plus the reduced linkage play(and added linkage stiffness) will be a winning combo.

I will also fly only in a flat calm. Most days I try it are 5mph wind or less but I will strive for absolute zero to see if that is any part of what I'm experiencing. I don't think it is. I think it might just be a lazy servo that is not fully relaxing when input is released.
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Old Dec 13, 2012, 11:36 AM
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Tons of exponential is almost a necessity on these little models. i really struggled with mine until i got the expo and rates set.

Set up your low rate with maybe 50% or less of the recommended throw and put at least 50% exponential on all surfaces, more like 75% with the full rate setting.

It's certainly not the CG because I fly with mine a fair way back from the recommended position and once the rates and expo settings are sorted it's no problem.

If you don't have a Tx with expo then all you can do is reduce the throws way down, but that will impact on it's aerobatic potential obviously.
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Old Dec 13, 2012, 11:38 AM
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Just saw your last response... I think you may have overdone the expo I don't use anything close to 95%.. the problem with that much is that you get virtually no response around centre then all of a sudden all hell brakes loose!
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Old Dec 13, 2012, 02:28 PM
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Quote:
Originally Posted by JetPlaneFlyer View Post
Just saw your last response... I think you may have overdone the expo I don't use anything close to 95%.. the problem with that much is that you get virtually no response around centre then all of a sudden all hell brakes loose!
Thanks! I know I didn't start w/95% expo. I just keep dialing it up since I don't know what else to do. The ailerons were initially a problem also, but with expo I'm very happy with 100% throw. It's all the elevator/pitch control.

I will be flying along straight and level as desired and then (as if a hit to the electronics or a hit by wind), the nose will shoot up 45 degrees. Now I need to respond quickly with down elev; now nose is down. All my flights look like this w/oscillation. I always give up as I have near misses w/ground. Flights (or fights as I should say!) only last 30-90 sec tops.

BTW...this is MUCH worse at full throttle. I can't fly it at anything past 1/2. Plus, even w/ smooth throttle application, throttle application seems to excite the pitch problem and cause the plane to pitch violently.

I'll try 60% expo w/35% ele throw my next time out...need to replace shaft as it broke.
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Old Dec 13, 2012, 04:22 PM
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Here's my XP, it's got a 2s brushless conversion so goes a fair bit faster than stock but as you will see from the video it's still quite manageable. First time I flew it I had a real shock because it was sooo twitchy on all controls but expo and finesse on the sticks was the fix.

UM Sukhoi 26XP Brushless (7 min 14 sec)
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Old Dec 13, 2012, 11:16 PM
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Thanks! The 2S conversion looks cool. Yours definitely flies straight and level whereas mine would climb or dive (uncontrolled) after 5-10 yards max.

How high of a wind was that? I'm thinking that besides the 35% ele throw, I should try it in absolute zero wind again just to be sure.
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Old Dec 14, 2012, 02:57 AM
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I'm not sure how windy it was that day, it was a while ago. Like most ultra micros the Su26 flies best in pretty calm conditions but can be flown an a fair breeze once you get the feel for it.
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