Flutter SUCKS!!!

[Biff]

Guys, I've been reading some of the post crash reports some pilots have shared and feel like we should share some experiences about this terrible menace! For those of you who have experienced flutter, it's a sight and sound like nothing I can think of. For those of you lucky enough not to have experienced it yet,I hope the golden horse shoe residing in your lower-lower intestine stays firmly in place!

Flutter can be caused by a bunch of things; lack of mechanical advantage on your install, excessive speed, weak structure, fuel soaked surfaces, gaps in surfaces etc. The most common issue is speed related. Being as we all want more and more travel, mechanical dis-advantage is something we need to take a pretty good look at.

I flew racers for years. Flutter at well over 150 mph is something that happens so fast there's really nothing that can be done. I've had a couple formula 1 birds turn into high speed confetti in a nano second. I've also witnessed a buddy flying a twin turbine all composite airframe shake the plane apart in 2 seconds. Flutter is a very real thing my friends!

Flutter can be mild or catastrophic. Ailerons and elevator are the usual culprits, but rudder and the entire wing or stab can be included as well. Seeing a composite wing flutter is a real scary thing, but stabs will do it as well. If you're lucky, there will only be a sound like a pheasant taking off and a blurr where your control surface(s) are. Slow down and everything goes back to normal mode. Worst case scenario is a crazy noise then pieces floating down that resemble shooting a Hefty trash can liner full of popcorn with a 12 guage!

How to prevent? Use the strongest servos you can get your hands on. Sacrifice some of the 60 degrees of throw we all want for mechanical advantage on your servos. Make sure your hard points in your surfaces are hard. Inspect your plane often for any slop in your linkages. During construction, make sure you get a minimum gap on all surfaces. Seal the gaps in your surfaces. Exercise good throttle management. Fly your plane as it was intended. I have a long time flying buddy who LOVES to do speed runs. He is flying a big Godfrey Extra with a 150 and thinks nothing of full power diving turns and blasting the runway. It KILLS me to see him do this as it's just a matter of time. He hasn't been blasted by flutter in 16 years of flying. It will happen!! If you want to really haul ass, buy a racer!

There is a very valid argument that our 3D setups with not a bunch of mechanical advantage aren't a problem as we spend most of our flights at post stall conditions. That simply can't be argued. It's very true. As long as we don't let the speed build up. I have a 50cc Yak that is pushing it from how I usually set things up. But I'm using top of the line, big dog servos on every surface and never let it get fast. I also don't let too many people fly it as with the power to weight we now enjoy, the speed could get pretty high in a very short period of time!

QuiQue's deflections are MONSTER but he is also running 2 servos per elevator half,3 on ailerons and God only knows how many on the rudder. You can have copious amounts of throw,but you'll sacrifice weight and $ to do it properly. Aviation is all about trade off's!

I fully understand that I'm snake bit when it comes to flutter and this thread could (should probably!) be 5 pages long to fully explain causes and prevention, but I just felt compelled to start a thread about flutter as the flying season is on us! Please share your thoughts as this is something we don't talk about enough!

I've helped a bunch of guys get into giant scale. The first thing I talk about with our new to gasser crowd is safety. Knowing never to start a gasser by one's self, don't stand in front of a running motor or in it's arc etc. The next thing is throttle management. Nose down power down is my mantra! Also to constantly think about what to do if you hear/see flutter. My flutter response when I'm flying is first go to idle, then positive snap. You need to slow the plane down as fast as you can! This response also lets the pilot know if he has control of the plane and what isn't working properly.

My thought is this; you need to slow the plane because it could greatly reduce the potential for a big airplane going in carrying enormous energy. It also gives you a fighting chance to save your plane. Left unchecked, flutter can reduce your plane to scraps in a second. Better to get the airplane slow so you can fly the parts still working. I've landed more than one airplane who's ailerons have fluttered off by slowing it quickly and just using rudder. I've also landed a plane with only half the elevator functioning. It wasn't pretty, but it still looked like an airplane and nobody got hurt!

I've heard from folks who thought my idle/snap idea was crazy because a snap with a full head of steam could break the plane. A snap could break the plane, flutter WILL break the plane. Simple math for me! We all have go to plans for emergency situations. Over the years I've seen some tremendous saves by guys who think about what to do. I've seen a pilot mid-aired lose half a wing. He simply rolled knife edge (short wing up!) made an approach and rolled level just before touch down (TEXX IS THE MAN!!!) I've seen Animal land a plane with no ailerons,half an elevator (flutter!) No problem. Keep a running thought about how to handle these situations and it will pay off. Also learn to fly using the rudder and don't touch the ailerons. This will save a plane someday, I promise! Another great trick to have in your bag is side slipping. Dead stick,fast and running out of runway? High drag CONTROLLED flight will save your bacon!!

I look forward to hearing from you guys. Not just about flutter, but your "go to" maneuvers for unexpected situations.

I'd also like to pat you guys on the back for sharing your crash stories with us. It takes a full on duck walking man to share such a painful thing. Including pics for cause/prevention is such an awesome tool! Having to learn what could happen the hard way SUCKS. By sharing your terrible experience, you are not only saving airplanes, you might just save someone's life.A Benji,, check out Giant Jargon!

You guys are the best!! Biff

[Temptation]

Biff,

This is a post that has been needed for some time. I guess most people don't think about flutter until it happens. I have experienced it two times, and luckily saved the plane both times. I am one of the lucky ones and had an instructor that was a competition pilot and awesome builder. He tought me a whole lot more than just how to take of and land three times in a row, and then sign me off. Education is the key here. This is a great thread. I think you have covered all of the bases. I will put together a good write up on how to prevent flutter during the building process then post it here. I believe strongly that it all starts with the build. I will post how I set up my pattern planes, and how I construct my control surfaces and hardpoints. What I use for pushrods, servos, geometry, etc. I will get to work on it. I will also post some pics of my hard point construction.

Stay posted for more. I hope to learn something from this thread also. There is a huge talent base here, and I look forward to seeing how some of the other guys do things also. It may lead me to revise how I do things, or learn new things all together. I hope what I share saves someones plane or maybe even a life someday.

I am looking forward to seeing where this thread goes.

Mike Darr

[Jaybird]

Ive only seen it happen twice and both times it wasn't pretty! one was a 25 size Extra WAY over powered and on a high speed pass. The other was a Dave Patrick Edge 540 with nylon geared servos. That bitch was broke in half before it even hit the ground!
Brian

[king160woody]

I was flying my dads "Custom one off" plane on its second flight. I took it off for him and got it couple mistakes high and right when I started passing the tx to him.. flutter!! I backed off the throttle and tried to pull up slightly but elevator wasn't working.. so i was gonna try to knife edge the plane level.. no ailerons! rudder maybe... uhh .. no.. throttle and thats it :'( .. it started into a flat spin and kept going lower and lower. 10 ft off the deck I gave it full power and it flattened the flat spin and hit the ground level .. broke the nose off and landing gear. The wierd thing about it, i was only 1/4 throttle just puttin' around when it did it.

[Donatas]

Biff glad you write this topic, im sure some suggestions here will save many planes !!!

I did not think flutter is something real in RC models, until it happened to my last year. I made first flight with my new 170cm pattern plane. When after 3 minutes of flying, in speedy exit from loop i heard that ugly sound and bam no ailerons... Lucky i reduce throttle quickly and plane wing was strong enough to hold overloading. I must say 2minutes without ailerons last much much longer

I landed plane pretty hard and almost without any damage. Problem was that it was old style plane and had only one 11kg servo for both ailerons, so control horns did not handle that pressure, and my flying style After that accident i use one 7kg servo on each aileron, and fly with that plane until now, made ~100 flights.

This year im going to fly my first gasser 2x2m ExtraFun from 3W, rtf weight ~6kg, and i have 2x11kg servos for elevator, and 3x7kg servos for ailerons and ruder (maybe for ruder i will find something about 10kg). I'm fried this is not enough for plane of that size. So Giant gassers what would be your suggestions?

[Temptation]

Control Surface Flutter

Some causes, and some preventions

What is flutter? Flutter is a dynamic aeroelastic phenomenon that involves the interactions of elastic and inertia forces of the structure with the aerodynamic forces produced by the airflow over the vehicle. It is a self-excited oscillation of the aircraft structure, where energy is absorbed from the airstream. When the elastic structure of the aircraft is disturbed at speeds below the flutter speed, the resulting oscillatory motions decay. However, when the structure is disturbed at speeds above the flutter speed, the oscillatory motions will abruptly increase in amplitude and can ultimately lead to catastrophic failure of the structure. In some instances, flutter oscillations are limited to just a single airplane component such as the wing, whereas in other instances the oscillations may be considerably more complex and involve coupling of wing, fuselage, and empennage vibrations.
The process of ensuring that an airplane is flutter free, commonly referred to as “flutter clearance,” involves carefully planned analytical and experimental studies of the complex aeroelastic interactions that the aircraft will experience throughout its flight envelope with sufficient margins beyond the expected flight conditions. If a significant flutter tendency is discovered within the operational envelope, modifications to the aircraft structure must be made. The most common modification is to increase the most critical structural stiffness, which usually results in an increase in structural weight. This increase in weight is commonly referred to as the “flutter weight penalty.”

Here is a list of things that cause flutter

1. Your airplane is overpowered and the construction of the wing and tail are insufficient to keep the control surface centered due to the very high speed flow of air over and under the surfaces.
2. The servo attached to the surface is not strong enough to hold the control surface at neutral.
3. The control rod coming out of the servo output arm is too far from the center of the servo thereby reducing any leverage.
4. The control rod is too thin and flexible.
5. The control rod is installed too close to the surface of the control surface, again eliminating the leverage that should exist.
6. The hinging is loose and/or weak with too much gap from the TE of the flying surface.
7. The hinge gap is not sealed.
8. The control surface is not balanced.
9. The servo output arm and or control horn are too flimsy for your application.

Here is a list of some of the things that will help to prevent flutter

1. Position your control rod in the closest hole to the center of the servo control arm as possible. This provides greatest leverage and servo strength.
2. Position your control rod in the furthest hole in the control arm from the surface as possible. Again, this provides the greatest amount of leverage.
3. Keep the control rod as short and rigid as possible. Never use all threaded rods as they are weak. Never place excessive bends in the control rod as this adds weak spots in the rod.
4. Make sure all pieces in the linkage are heavy duty for your needs.
5. Choose a high quality servo that can hold the control serface in the desired position. I recommend buying the best servos you can afford.
6. Keep the hinge gap as tight as possible and use a sufficient number of quality hinges.
7. Using your covering material or a tape designed for this purpose, completely seal all gaps on the control surfaces. When sealing hinge gaps that have multiple colors in the scheme I like to use clear Monokote, or Ultracote to seal the gap.
8. Use motor sizes that are recommended by the manufacturer. Extra speed places extra stress on your airframes. If you want to go real fast, build a Racer or a Jet.

Now keep in mind, any control surface, no matter how well it is constructed can flutter if the aircraft is allowed to travel to fast for it’s design. It all boils down to flying the aircraft in the manner it was designed for. When flying a 3D aircraft we need to keep in mind they are not to be flown at high speeds. Even if you have the low rates turned on, most likely you have a mechanical disadvantage that was needed to get the 3D throws when you activate the high rates.

Mike Darr

[Biff]

Good work Mike! I need to see if I can find the vid of flutter I once had. It was a classic! I was doing a low high speeder when it happened. Right infront of the camera so sound and visual is great for those who haven't seen/heard it. The plane was blown to bits!

[Temptation]

	Quote: Originally Posted by Biff
	Good work Mike! I need to see if I can find the vid of flutter I once had. It was a classic! I was doing a low high speeder when it happened. Right infront of the camera so sound and visual is great for those who haven't seen/heard it. The plane was blown to bits!

Yes, Yes, Flutter is a sound, once heard will never be forgotten. It is very unique, that's for sure.

Mike Darr

[3Dkid330]

Great thread guys, I also hope to larn something from here. I have had flutter before on my Edge, it was a moderate speed knife pass, the left horizontal stab was vibrating, followed by the ugly sound of flutter. I knew I had to reduce speed, so I just rolled it inverted and pushed it to vertical. Was caused by a tiny amount of slop when one of the control horn holes had worn out. Flutter, if left alone, will destroy things unfortunately. I have seen elevators depart on speed airplanes, covering torn off at the fluttering surface, and many stripped servo gears. I have even had flutter on the whole vertical stabilizor of an airplane; it neatly broke the wood in the root of the stab, but it did not completely pull it out.

I definitely think that it would be a good thing to teach new pilots about these situations, what causes them, and how to get out of them.

Brandon

[aviti]

The only good thing about flutter is once it happens to you, you never forget it and you do all the things to prevent it in the future. I had it happen with an elevator half in a Goldberg Extra like 14 years ago. Slop in the pushrod caused it. One elevator half pulled out completely. I was able to land with just a bent gear and cracked cowl. Fixed up the elevator properly and have never had flutter with any plane since (knock on balsa). If you can experience it and come out safely, its a great lesson learned.

[Kiwi]

I think I have to be one of the luckiest guys in the hobby when it comes to flutter.

Back when I was learning the basics of the hobby I purchased a 40 sized low wing trainer second hand. Being a new recruit I was busting to break the the limits. I had no idea on weight management, no idea on control throws, gap sealing or for that matter very much else. I could get it off the ground, go round and round and land most times where I was aiming.

Well this new low wing trainer was about to change all that. I fitted a whopping big 46 OS in the thing looking for speed. I mean thats normal when your fresh and excited in this hobby. A need for speed is normal. So I fitted this thing and took it flying. Hells bells it was fast. I was king of of the field right??? no one could come near me. Until the famous fast low diving pass and BRRRRRRRRRRRRR geez??? Whats that. Backed of and it slowed and the noise stopped. Another not so fast and it was evident but not bad.

Well after a few weeks of this I could make it flutter on demand but thank god it was a draggy plane so all it needed was cut the power and it stopped.

So I set out to fix it.

Seal the gap I was told. I did it took a lot more to get it to flutter but it still could on demand.
Stronger servo, I did but it made no difference.
Change the mechanical advantage, short servo arm to long surface control horn. No change.

Long story short in the end it went in so I got to do a full post mortem.

What I found I would swear today was impossble but true as I stand here this is fair dinkum.

The plane was a tower hobbies kaos, orange and white, single servo on the ailerons. It was ailerons because you could see them flutter. Well the wire contol rod from the servo to one of the ailerons was like chewing gum. I mean it looked perfect but you could twist one end 45 degrees and the other end would not move. Of course with no airloads on the control surface and on the ground it tested fine. But with load the wire flexed enormously and the control surface went where it liked. It was really like flying with one aileron disconnected.

The ability of that plane to flutter on demand tought me more than I can describe. For instance today when I do a ground check I hold the surfaces to ensure they move when commanded with load. The learning process of having a control system that worked with no load but could not move with any air loads tought me heaps.

Todays servos are miles ahead of those times but it can still happen.

And yes, the noise is unmistakeable. Sort of like putting a carboard stick in the spokes of a bike wheel.

As has been said before my post you cant spend too much time making your plane safe enough to fly. I agree whole heartedly with Bob about the snap response to the flutter. You will break it if you dont and maybe save it if you do. Your choice, the maths aint too hard to figure.

And if all else fails and you cant get it back then be a gentleman and park it in the trees or the likes.

It would be great if Bogan or IMAC KIWI could come in here as they had a real happening in front of 60,000 spectators last weekend. Only either one of the two should tell the story as I have it third hand. It is a real good save as well.

Kiwi

[onecan]

Hi Kiwi

Bogan did not suffer from flutter at Warbirds over Wanaka last weekend. The rudder on the Comp-ARF 3M 330 he was flying locked about half way left. The cause of this is still to be determined and IMAC Kiwi maybe able to pitch in here, as he is the owner of the plane. However 3 of the 4 JR 8511 servos are locked solid and the 4th is still working but has massive gear slop. There have been many ideas to what may have caused the problem, including powerbox failure etc. However the powerbox tested ok the day after. So some how those servos have been fighting each other big time and the motors have been fried and locked solid. There is no damage to the gears in these servos. They were setup in a SWB tray with solid links.

Sorry for taking over this post, just wanted to clarify it wasn't flutter which effected Bogan's last flight at the airshow.

Cheers
1CAN