I recently flew a 27% YAK 54 (great planes). I set up my plane with a Fuji 43 cc gas, with a 20x10 zinger prop, hobbico cs-70 metal gear high torque servos.
The first flight went very well. After a couple flights I started to try higher speed upon trying a pass at half throttle the plane experienced a sevre wing flutter and Ilost complete control of the plane and it crashed.
Some of my friends with 30 years experience in the hobby could not believe the violent action from the wing.
All contol surfaces were set to the manufactures spec and the plane was inspected by club members with experience in large aircraft. All club members agreed that the plane was throughly built.
Any input on this issue would be greatly appreciated.

The first place to look would be the attachment of the control horn to one of the ailerons.
Security of the fit to the surface.. pushrod attachment.. servo arm....Something in the control system let go and let the aileron flutter, most likely.

Ok here go's in laymens terms:
Any wing can flutter. To get flutter you must have a few things:
1. A wing that is not stiff enough. The wing or surface (vertical or horozontal tail) will help dampen the flutter (oscillatory motion): Now this is a tradeoff between weight and stiffness. A wing build out of solid wood would be very stiff...a 3D airplane is built Extremely light but with less stiffness...a lot of 3d airplanes can not be flown very quickly without flutter. (Full size also)

2. Something to "drive" the surface...now this could be unattached flow on the back of the wing....but more than likely the aileron drove the wing into flutter...now when this happens the flow starts the trailing edge of the surface one direction then the other...if there is any slop in the controls the aileron will start moving back and forth. this will put a "back and forth" force on the wing trailing edge. The small force will build up (It's kinda like pushing your little sister in a swing a small force at the right time will make the sister go higher and higher)....

So, make sure all the surfaces are tight...hinges are tight...linkage is tight...etc....and don't fly the plane faster than it was designed for which is slow for a 3d plane..faster for a sport model and as fast as it will go for a pylon racer.

Since the plane is greatly damaged or written off due to the crash investigating the cause is going to be tough. It may have been something you did that was missed by the club "inspectors" or it may have been a set of joints in the wing that were not glued well at the factory or it may well be that Doc316 hit the nail on the head and you just flew it too fast for the type of airplane. If a control horn busted away or one of the pushrod ends of the servo to aileron surface let go the resulting chaos could easily drive the wing into major flutter and quick destruction. The fact that it survived and did well in the previous flights and only did this during your flight where you were visiting the high speed region suggests that this is likely the case. The weakening may not even have occured right at that moment. If you were doing some higher speed dives earlier and then did a moderate but still higher speed dive or pass but did something like commanded a flick roll or whatever it could be the straw that broke the camel's back.

Sadly this newer class of 3D model does come with a very definite Vne and it would appear at first glance from your admission of playing with higher speeds that you found the limit.

The amazing thing about flutter is that by the time you realize you have it ....it is generally too late. This happens on full scale airplanes also....it is very dangerous testing.

I actually got wing flutter this weekend on a model...but luckly got it slowed down before it went divergent (wing break). It was a 1/4 scale Balsa USA DeHavland (a great looking model). We installed a bit big motor for this model (and it is a Huge Model)..a DA 85...it is supposed to have a 45 or 50 in it...well I knew it was going to be overpowered..so we took off 1/8 throttle...and we inched up on the speed over two flights untill we got to half throttle...then the wing started to oscillate...well we are going to stiffen things up a bit and maybe borrow a restrictor plate from Nascar to limit the power and speed.

Another point about flutter and the cause and manifestation:

Mass balance and mass distribution.
When building a kit or from an old drawing someone might be tempted to change a "heavy" spruce/birch wing leading edge spar to balsa that is strong enough. Later on when covering, heat shrink plastic is substituted for doped on paper.

On the first flight the wing flutter and the modell crash.

First, the shrink wrap covering is less stiff than doped on paper, making the wing more flexible in twisting motion.

Second, with a lighter leading edge the centre off mass moved aft, closer to or aft of the polar centre in the wing. (Polar centre, the point the wing will naturally twist about.) With the centre of mass moving aft there is less damping in the system untill it goes aft of the polar centre and the system become increasingly unstable. This make it easier for any disturbance to set the wing in oscillatory motion and the amplitude of the motion will increase faster.

The combination of these two changes are really bad, but the mass part might be as important as the stiffness part.

Very good point, Himat.

However, that was not the case for the OP.
I believe he did nothing wrong while flying horizontally at HALF throttle.

He did remain within the engine specifications for that airplane:

Engine: 1.6-1.8 cu in (25-30 cc) 2 stroke
OR 1.8-2.1 cu in (30-36 cc) 4 stroke
OR 1.9-2.6 cu in (32-43 cc) gasoline

If the OP is certain the linkages and instalation was correct, I believe he should bring the incident up to the seller or to Great Planes.

I appreciate all of the insight into this incident.
The plane began flutter when it was flying level. The only things I had attempted with the plane was level flight no maneuvers were tried.  I would assume that the problem occurred due to the speed after reading the posts on this situation.
I would not have expected the plane to experience flutter at half throttle.
I have a friend in modeling who said that flutter can happen even at low speed.
The left aileron had the control horn ripped out however I don't know if this happened during the crash or if this is what led to the flutter.

That's the darned thing about these things. If it happened in an almost explosive way and unless someone had a video camera on the model at the time it's REALLY hard to determine what the course of events truly were.

However there is no doubt at all that with ailerons that wide and since they would be fairly heavy towards the rear that they would be highly prone to flutter once "let loose" by something like the control horn breaking loose from the mounting. Once that happened the aileron moving in rapid flutter would act like a boost tab to get the wing twisting violently as well and things would lead to failure within a very short period of time or shake with such violence that the model could well lose control or break other items loose from the violence.

....And that leads us here were we can all gather around and guess at what may or could have caused it. Armchair analysis at it's best.... Tytor, aside from being sorry about you losing the model only you are in a decent position to try to choose which possible cause may have the highest possibility of being the real cause. You WERE there after all. Best of luck with a replacement and whatever happened don't do it again....

Flutter is a killer, and airspeed is the cause.

The cure is mass balance. I once had a 60 size sport model and at full power in level flight it was fine.
At full power in a slight dive it suffered aileron flutter.
I rebuilt it and added mass balances, after which I could perform a steep dive at full power without a problem, and I have mass balanced anything fast ever since.

My next prop model is a 30% yak with a 55cc, and I guess I'd better mass balance even though the instructions don't mention it.

As an aside, most of these big light scale aerobats are flown with quite fine props, and that will limit their speed somewhat. Maybe the 20x10 was a contributor. A 22/8 might have contained the speed just enough.

Yes, mass balances help (and are often found on full sized airplanes)...most full sized airplanes use cables which have a lot of "slop" in them so the mass balance is really necessary.

Most model rely on the stiffness of the aileron/linkage/servo to dampen out vibrations. Why? Probably because it is simple to upgrade to really powerfull servos/linkage/etc. where it is harder to get pilots with more torque...

But typically 3D airplanes are built so lightly (so they can hover etc)...that they are not very stiff. I blew a couple of them up before I got it figured it out. But back then they were 40 sized.....
Steve

I ordered the Great Planes Sukhoi SU-31 27% today. I wondered if anyone would like to way in on what I should do to try help decrease the chances of having flutter again?
I decided against the yak simply because I like the Sukhoi's look.
I wonder if stronger servo's or doubling up on the aileron servo's would help.

Only if it wasn't a control horn or pushrod that pulled out or apart. If you're going to explore the higher speed side of the flight envelope you may also want to look at modifying the ailerons to add strut like mass balances at mid span and maybe mass horn balances at the tips to move the balance point of the ailerons closer to the hinge line.

Assemble it with zero slop in the rigging. Use the appropriate size servo's, the ones recommended will work perfectly. Balance the surfaces as laid out by BMatthews. Fly the heck out of it. Lots of Sukhoi's are flying with no problems. Almost none have balanced surfaces. So that's added insurance.

Keep in mind you're buying an ARF so there is no guarantees every joint is glued, nor that properly selected wood was used. They are what they are.

The less mass hanging off the hinges, the less likely there will be flutter.
Changing from a slab elevator to one built up from sticks eliminated flutter on one of my kit fun flies, which fluttered on the first flight.
A good stiff control system is also needed.

Very important to seal the hinge lines and be sure your control rods are attached the farthest away you can from the hinge line, and still get the throw you want.

I learned the hard way that if you aren't using control surface mass dampers (balancers), you are skirting disaster with every flight. The violent kind of flutter described requires an initial deflection to set it off, usually a series of air bumps spaced about right to get the wing or stab into oscillation at the critical amplitude, after which it just takes off, with amplitude increasing rapidly with each cycle, until something breaks, detuning the system.

If you are lucky, an aileron or elevator may simply break off, leaving the rest of the structure in flyable shape. In severe cases, you hear a bang, accompanied by a cloud of hundreds of airplane pieces. Properly designed mass dampers will remove the danger 99 percent of the time.

Removing slop from linkages will seldom make any difference with this type of flutter, and may just raise the airspeed of onset, making the flutter even more destructive.

Unfortunately, there is more than one cause for flutter, and we seldom know what the cause actually is. Good thing is that ever since people made certain to remove slop they almost never saw it again.

I agree with everyone....looking at all the above posts....

1. Tighting up the system will just move the natural frequency of the system (airplane) up to a higher speed...if that speed is higher than the plane can obtain you are good. This is true in full scale airplanes also.
2. Damping the system will increase the natural frequency of the system (airframe)....sealing the gaps, tight control surfaces, rigid control surfaces (use hard balsa), tape on the outer part of the aileron connecting it to the outer wing tip, a silicoln bead (use clear) in the aileron gap then when dry slit it, thick torque rods...I could go on will stiffen the system and raise the natural frequency...and raise the flutter speed.
3. Balancing will make the system will make a self damping system.

Most model airplanes that are not large use 2. to dampen the system. Most full scale airplanes use 3. One full scale LSA uses 2 and have had problems with flutter...the recomendations were to check the rigging on the ailerons to make sure they were in spec on tensions (slop).

Steve

Nothing to add to the excellent elimination advice, but i would add that recognising the early on-set is a good step to saving the aircraft.  One dead givaway that often occurs is a very audible buzzing caused by the rapid high frequency surface oscillations - whether aileron or whole wing. If you hear it decelerate immediately. More violet twisting will actually be visible if you look carefully on a flyby - it will show up as shimmering of the colour of the covering - you might not notice it unless looking for it, Any change in expected control response or unexpected control input not given by you, can be another clue. Sadly, flutter is frequently so quick and violent that by the time you relise whats going on, it too late.  I had a scratch built 9 foot thermal soared years ago that got terminal flutter, i was gently diving her to get better penetration when i heard the buzz, a second later i could see the wing twisting around the right tip, another second and the wing wasoff! The cause was very light building and insufficient torsional rigidity.
It also showed up recently on one of those ultralight foamy shockflyers i had - very wide ailerons of very twisty foam - a bit of excess speed and the buzz set in followed by a rapid partial roll - faster than the normal roll though, more of a jerk really. However, a lightweight foamy worth pennies is nothing to get excited over, but on a big 3D money pot would make me sweat some.........