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Old Oct 21, 2012, 12:58 AM
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East Bethel, MN USA
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Catastrophic horizontal stab flutter - UMX Sbach 3D on 3s

I decided to try 3s in my AS3X-equipped UMX Sbach 3D, given the success many of us have had with running 3s on the AS3X-equpipped UMX Beast 3D. I also have a non-gyro V1 UMX Sbach that has no problem on 3s. However, when I went to WOT, I heard the sound of flutter. The plane suddenly & violently pitched up. I was able to regain enough control to bring it back around & get it down in one piece. Was it aerodynamic flutter, or gain-induced flutter?

Sbach 3D - touch & go practice (4 min 52 sec)






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Old Oct 22, 2012, 10:56 PM
Thailand
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The whole model is just not stiff enough and flutter is almost certain to occur at certain speeds.
The only thing you can really do with that style of model is to add stiffening braces from the fin to the stab, top and bottom. You can use thin carbon rod.
many full size use this method and a lot of models too.
I would still be careful at high speeds though.
Jim
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Old Oct 23, 2012, 12:41 AM
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East Bethel, MN USA
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Quote:
Originally Posted by Fox-composites View Post
The whole model is just not stiff enough and flutter is almost certain to occur at certain speeds.
The only thing you can really do with that style of model is to add stiffening braces from the fin to the stab, top and bottom. You can use thin carbon rod.
many full size use this method and a lot of models too.
I would still be careful at high speeds though.
Jim
Hi Jim,

I've stiffened-up the tail on a number of large-scale ships when I decided to run on the high-side of the power recommendations. It's just that my V1 Sbach handles 3s just fine, and I didn't need to beef up the tail. The Sbach 3D uses the same airframe & power system. However, it has the AS3X 3-axis damping system, and fixed-gain damping systems are prone to gain-induced oscillation when the airspeed is too high for the gain setting on a particular axis. Hence, why I'm wondering if the oscillation was aerodynamic or gain-based. I added a pair of 1mm cf rods to the underside of the stab that run through the fuse, which should help a lot if it was aerodynamic flutter. I might prop-down from the 5 x 4.3 to a 5 x 3 next time I try 3s, though - just to be sure.

I definitely had my hands full on the downwind & approach! I was nearly fully cross-coupled on ailerons & rudder just to keep it upright.

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Old Nov 04, 2012, 08:47 AM
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I just got the new sbach 3d yesterday , maidened it this morning in lowish wind conditions.
I was running a 300mah MA lipo all the way back and i had some pretty violent tail flutter on full throttle downwind passes!
Now looking at the pics of your plane i think i might strengthen my tail.
Probably just add some tape for a start as i don't think i'll run 3s for a while, not till it gets beat up a bit anyway
Amazed you managed to get it down in one piece with the whole horizontal stab loose like that. Good skills
Cheers
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Old Nov 05, 2012, 09:35 PM
Thailand
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A small amount of mass balancing would make a big difference. i:e some lead in front of the hinge line to make the elevator balanced.
It would need a very small amount.
Make sure the rudder is not fluttering as well.
Stiffening braces would really help the most though.
Jim
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Old Nov 06, 2012, 10:08 AM
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Hi guys,

I repaired the tail & added a cf rod under the horizontal stab. This completely eliminated the horizontal stab/elevator oscillation @ WOT on 3s. Now that the elevator is behaving, I noticed that the rudder oscillates at WOT.

I found that the elevator & rudder pushrods are a sloppy fit in the horns. It appears that the pushrod holes are slightly larger than those on my other UMX planes. I used a drop of ca on each horn to create a bearing - which really tightened things up. Now I just need some decent wx for the test-flight. If that doesn't do it, I'll try mass balancing. Ahh, the fun of pushing the envelope! I love it!

Mygeneration,

Your experience on 2s sounds similar to what I experienced on 3s. The tape might be all you need on 2s. I ended up using a 1mm cf rod for 3s ops. Also - check for slop in the pushrod holes. Excessive control linkage play can cause oscillation problems with stability-augmentation systems. Use a drop of ca + kicker to make a 'ca bearing', if needed. After the ca has cured, break the pushrod loose from the horn by hand so that it can rotate freely in the 'bearing'.

Jim,

Great idea! Although I have used mass balancing on larger planes, I had not thought of doing so at this scale. Thanks for the tip! I'd guess that the rudder itself weighs a few tenths of a gram, at most. I wonder if a small piece of clear packing tape near the LE of the counterbalance would be enough to do the trick.

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Old Nov 06, 2012, 08:00 PM
Thailand
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A few years ago you could buy lead sticky tape.
It was used on helicopter blade leading edges and was quite thin but very heavy..
I have often wondered if it was still manufactured.
jim
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Old Nov 15, 2012, 10:26 PM
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On that plane you could just cut a short piece of solder and push it into the control surface. Tape it on first to get the size right.

Ken
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Old Nov 21, 2012, 01:22 PM
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Guys,

Thanks for the ideas! Bracing the horizontal stab solved the aerodynamic flutter problem.

After solving the horizontal stab issue, I then noticed some tail-wag at high speed. The oscillation frequency was much lower than typical aerodynamic flutter, so I figured it was probably a gyro thing. On closer inspection, I noticed that the rudder linkage geometry was a bit off. The pushrod angled downward toward the rudder horn. Applying left rudder or pushing the rudder to the right caused the pushrod to deflect downward at the horn end to the point where Z-bend was close to pushing through the horn - like they often do in a crash. This caused a significant amount of play when deflecting the rudder to the right by hand. I tweaked the pushrod a bit so that it moves in the same horizontal plane as the rudder horn, which eliminated most of the slop. She's nice & smooth at high speed now.

Here's the test-flight on 3s after doing the above. Ten years ago, I would have never guessed that we'd have scale 2.5 oz planes that feel like large-scale ships, laugh at 20+ MPH winds, and have power systems that can generate ~300W/pound & 50+ MPH top speeds with nothing but a battery change. What a great time it is to be in this hobby!

Eflite UMX Sbach 3D on 3s! (5 min 23 sec)


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