Flutter
 

What causes flutter? Can it be fixed?
it happened to me yesterday, both ailerons, half throttle, just after taking off. Third flight, previous two were flawless.
i managed to land her safely and there was no slack in the control horns, they were tight!!
Receiver battery is LiFe and it was fully charged, winds were mild (8), straight down the runway.
As soon as I iddle the engine it stopped doing it.
Ailerons are on separate channels to the receiver.

RE: Flutter
 

Are you sure it was flutter? I had a Tiger 120, still have it as a matter of fact, anyway, it made a flutter noise. It scared the he!! out of me. I landed it and checked everything and put it back in the air. The noise came back.

Fortunately, my mentor/instructor from back a few years - ;) - was there. We looked everything over and found nothing wrong, except, a piece of trim monocoat did not stick to the wing covering just behind the leading edge on the top of the wing. It was tight but not stuck to the monocoat underneath it and the length of the loose piece was about a foot long and parallel to the leading edge. We put a piece of "scotch" tape on the length of where it was loose and put the plane back in the air. The noise was gone.

CGr.

RE: Flutter
 

Aeroelastic flutter. Same principal that causes a flag to flutter in the wind. Caused when the center of lift is ahead of the center of gravity of the control surface.

You can fix it (or reduce it) several ways.

Slow down.

Add a counter-weight to bring the C.G forward.

Tape the hinge line to seal out turbulance (sometimes helps).

Use beefier hinges, horns and connectors.

Make the aileron thicker than the trailing edge of the wing it attaches to.

Note that there are some planes - 3D models especially - that have so much control surface in relation to the wing that they might tear off if you fly too fast.

RE: Flutter
 

Other items to look at are the slop in the control system - clevis to control horn, clevis to servo arm, and even flex in the pushrod itself. I'm working through that issue right now in a Sig Kavalier where at above 3/4 throttle it'll get a loud vibrating sound that sounds like a very mild flutter. I've just about isolated it to the old style elevator pushrod which appears to be made of either hard balsa or very soft pine (the rudder pushrod is spruce). I'm going to replace both of them with a graphite arrow shaft style push rod besides sealing the control surface gaps with covering material and using high-end clevises.

Hogflyer

RE: Flutter
 

Very helpful, thx guys

RE: Flutter
 

It was in fact flutter, both ailerons were flapping like a flag. I was afraid she was going to fall from the sky

RE: Flutter
 

Well then, I guess it was.

CGr.

RE: Flutter
 

G'day

My friend was recently flying his new Balsa USA Student Trainer when it suddenly started to make flutter noises. Friends of his told him to land NOW as they could see the elevator fluttering madly.

He managed a pretty normal landing on half an elevator.

On inspection he found that the join between the two halves of the elevator had failed and also that part of the built up horizontal stabiliser had also suffered damage. Did the flutter cause the damage? Did the failure of the joiner cause the flutter? We are not sure but I think the model would have been better built with either a more substantial joiner or one servo for each elevator half. They are quite large.

Cheers

Mike in Oz

RE: Flutter
 

Could we establish what kind of airplane is being discussed?

RE: Flutter
 

It is a 80" WS 30cc Sbach.

RE: Flutter
 

Thank you Lopflyers. I really have mixed emotions about this one and the reason is you said the onset occurred just after a power reduction in a climb. Now its almost impossible to characterize when flutter will start to occur but more often than not it seems to happen with our models at median to high speed perhaps with some load being applied and at high throttle settings. That cannot rule out aerodynamic flutter in you case however.

Now if we can rule out some sort of weird vibration induced problem going on with any of the electronics and the only way I can think of duplicating that on the ground is with a thourghly anchored airplane and a high power static run up.

If that proves clean, to be honest I would if it were my airplane be mass balancing the ailerons immediately before any further flights.
mass balance contrary to popular mis conception does not need to completely balance the full weight of the surface (in this case the aileron) not anywhere near it and only a tiny portion of the weight will do.

There are many ways to secure a small fishing weight attached to the aileron forward of the hingeline If you are interested then we can explore.

I am not any kind of expert only a survivor;)

G'Day Mike, Concerning the torque structure on the elevator halfs of you friends new ship. I'd bet a burned out glow plug aginst a stale Gazzed Donut that that damage was precipitated by just one of two things, either hanger rash in other words banging a elevator into a a door jam or a tailgate and hear that small crack sound but just saying or hopping it will be alright - Or -the evil F word flutter that is.

Flutter can be an insidious thing and not all surface flutter is auditable. One day a local fellow landed his new 40 sized three D ship and found an aileron servo stripped. Yup a cheap one he thought and bought a more expensive one. Low and behold this one soon did this agine. I suggested flutter and mass balance, nope more better servo yet agine, This time since it was metal gears all the servo screws came out instead This went even another cycle with predictable results and fortunately the dangling servo did not cause a crash. At that time he used one of my simple little balance widgets and there was never a recurance agine.

John

RE: Flutter
 

One of my attempts at a counter-weight. It worked - but caught on grass and was generally a nuisance.

Ultimate solution was ball& socket comtrol linkages and a re-prop to a lower pitch/higher torque prop.

http://img.photobucket.com/albums/v1...psb3a0dcec.jpg

http://img.photobucket.com/albums/v1...psf3daca64.jpg

http://img.photobucket.com/albums/v1...ps3c5ce271.jpg

RE: Flutter
 

Now I must confess to not having known exactly what an SBach is other than a large scale aerobat I looked it up and noticed and correct me if I am wrong but it appears the aileron hinge goes all the way to the wingtip and this is a good thing as it makes for convienent mass balance attachment. While normally I would prefer to put the balance somewhere closer to the middle of aileron span but that's not a hugh consideration.

This type of tip makes it quite simple to make a tip plate on the aileron to carry the balance weight forward

John

RE: Flutter
 

We have got to say that Charlie P. is loaded with some good stuff. I see a lot of paying attention to the flight line on his part. Love your stuff Charlie.

RE: Flutter
 

Yes he does. He often presents his information with humor which is always welcome.

Good stuff, Charlie. John, you're not to shabby either!!! ;)

CGr

RE: Flutter
 

Wow, we are getting technical, balance the ailerons, like you balance a prop or spinner?<div>LOL does Dubro sells ailerons balancer?</div>

RE: Flutter
 

Charlie, I like the weights. What would they look like if you drilled in the boattail?, should be more aerodynamic:)

RE: Flutter
 

Thanky.

It was just some emperical testing. Those ailerons are built up - not solid - with a 1/16" balsa sheet covering over ribsunder the Monokoteand I didn't want to violate the middle of the "spar" that the hinges are anchored in.</p>

RE: Flutter
 

Hi!
An easy fix if you don't ant to mass balance the ailerons is to cut off the outher part of each aileron (10-15cm)at the tips and glue that part to the wing.
Flutter occurs at the wing tip and as such the ailerons should never, never go as far as the tip of the wing!

RE: Flutter
 

That's not right. Flutter is the result of turbulence coming off of the airfoil and can happen anywhere.

RE: Flutter
 

Flutter can and will occur anytime that an external input occurs at the natural frequency of any structure. Now all structures will resonate at some frequency, even an iron bar or a concrete bridge. If that structure is excited by a force at that frequency of resonance, flutter can result. The only thing that will not flutter is something having infinite rigidity and no mass, a physical impossibility. You eliminate flutter in your model by changing the natural frequency of a structure on one that is not ever excited by changing stiffness and/or mass which shifts the frequency to one not excited by flight conditions. This can be done by an almost infinite number of ways, the simplest being to either stiffen the structure or change it's mass or its plan-form. Closing the gap with tape increases the stiffness, adding counterweights changes it's mass, changing shape (removing or adding area or redistribution of area) are all possible solutions. If you know the source of the stimulus, of course removing that will also solve the problem (like changing or reducing speed which changes turbulent patterns).

RE: Flutter
 

Awesome, I'm planning to change the receiver battery, close the gaps and try again. Will test fly her in 3 days

RE: Flutter
 

To Charlie's ( excellent ) post I'll add...

You can also add bracing or supports, ala "flying wires".


Check out some in flight YouTube videos taken with on-board cameras on RC planes, particularly those that look over the wing or elevators.

It is scary how much flex there is on most RC model wings and particularly elevators.

It is quite simple to affix a carbon rod to each elevator half that angles off onto the fuselage, much like a guy wire. This adds a LOT of stiffness to the tail and rudder.

RE: Flutter
 

Tjek this out
http://www.rcuniverse.com/forum/js/f...what_smile.gif
Flutter in the tailplane,:
[youtube]http://www.youtube.com/watch?v=46dAvQiJhio[/youtube]

Glider Aileron flutter:
[youtube]http://www.youtube.com/watch?v=oUusUh3ItQs[/youtube]

Glider Crash due to elevator flutter,
The death of my Vector lll, flutter in the elevator in a mega dive and no chance to pull up:
[youtube]http://www.youtube.com/watch?v=l8uQL7IdONs[/youtube]

Full scala wing flutter:
[youtube]http://www.youtube.com/watch?v=kQI3AWpTWhM&amp;feature=endscreen&amp;NR=1[/youtube]

Flutter is nothing to joke about...


Cheers
S

RE: Flutter
 

Hi!
Most of the time the aileron is not that ridgid,at least not as ridgid as the wing it self and as such the aileron has more tendencies to move= flutter.

RE: Flutter
 

Nice videos SpeedsterDEN, thx.

RE: Flutter
 

I don't recall the OP stating what size airplane, servo brand and model number or a picture of the setup showing us mechanical advantage. While most of what I read regarding the cause of flutter is correct, the most common cause on an airplane like the Sbach is improper setup. Things to check is whether the servo has the power for the job, the servo arm should be at least 1/4" shorter then the control horn and there should be no slop in the linkage. Being that the first few flights on a fresh RX pack went well clues me in a a little . With slightly higher voltage the setup had enough power to not flutter. Look to make sure the servo has enough power and you have good mechanical advantage. Pictures would be a big help.

RE: Flutter
 

<blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;">Yes I did mention is a 73" WS30cc  Sbach.</blockquote><div>Servoes are Hitec, I will post model # and pics of the set up later.</div><div>I just bought a new LiFe 2100 mAh 6.6 v to change the existing one.</div>

RE: Flutter
 

I did skim back over the thread and noticed it was a 30cc airplane. What model # are those Hitecs? How long of a servo arm are you using? Are you using ball links? Are you using the stock servo arms?The LIFE batteries you are using will top out at close to 7V so the additional power gained from that may have been what allowed your setup to work until the battery dropped down to nominal voltage. Do not do any mods to the airframe just yet such as counterbalances or cutting on the ailerons. Things like that are not what is causing your issue. I have built many 35% to 45% airplanes that had the ailerons all the way to the tip. Some of those airplanes had over 200 sq. in. of aileron per side. The more information you can give me the faster we can hone in on the actual cause.

RE: Flutter
 

Here is a short clip of flutter on a twin turbine RC Jet. The aircraft was a total loss.

http://youtu.be/Unwq7IpNLIQ

RE: Flutter
 

They are JR DS 8231 Digital, the arm is 1" the rod is 2".
The rod is metal, ball bearing at the end. Here are two pics. Tomorrow ill take out the receiver battery and measure the volts. I'm changing to a new one anyhow.

RE: Flutter
 

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RE: Flutter
 

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RE: Flutter
 

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RE: Flutter
 

The pictures didn't come through. The servo torque seems sufficient provided the mechanical advantage is there. Some would advise against nylon gears but I have run them in a 70cc airplane without issues. I would not do so today though. A 1 inch servo arm would be a good length but what is the arm made of? Ball bearing at the end of the rod? I am going to assume you mean ball link. If so ball links are not a good idea on plastic arms. Under load it will allow the arm to twist and that can be your cause of flutter. So far you have not mentioned how long your control horn is. That would be the part of the control system that is secured to the aileron. You will want to measure from the hinge to the point where the pushrod connects. With a 1" servo arm you would want that dimension to be at least 1 1/4", 1 1/2 would be even better.

bottom line is that to avoid flutter is to have servos that you think are a good 20% more power then you need, have good mechanical advantage, use ball links only on metal arms, tight hinge lines and make sure the servos get enough voltage to make their max torque. The latter means all wiring should be 20 ga and minimize connections whenever possible.<br type="_moz" />

RE: Flutter
 

The servo arm is plastic. Control horn is 1 1/4, I'm not surprised the pics didnt load. I'm still seeing them from my end.<div>Good advise, thx. I'm planning to fly her wed with the new battery. I think that is where the problem is.</div><div>Out of laziness I always charged this batt with one port instead of balance- charge. </div><div>I'll let you guys know if it was the battery.</div><div>That's why I love RCU, you get tons of advise from the gurus.</div>

RE: Flutter
 

Hope this link works great article by Mike

http://www.rcuniverse.com/magazine/a...rticle_id=1289

RE: Flutter
 

+1 :) I remember helping out with that article :)
ANY hinged flight surface WILL flutter... the trick is determining at what speed ... (preferably above or below the normal flight envelope :) )

RE: Flutter
 

you must be Mikes son ,He helped me out many times [8D]

RE: Flutter
 

hehe... well, you're close at least. I'm Mikey's 'little' brother. His son is 20 years younger than me :)
yup, I miss the old bugger too!