what is ,and what causes flutter and how does tapeing the hinge line prevent it?

The easiest way to describe is look at a flag in a breeze, that is flutter. Taping a hinge joint will reduce flutter because it prevents air from flowing through the joint creating vacuums and pressure zones. causing the surface to be moved back and forth by airpressure. If the joint is to big and not too rigid the hinge taping will help a little but won't solve the problem. Taping also increases the effectiveness of the control surfaces and reinforces the hinges.

Rob pretty much covered what it is...
Basically it is turbulent airrlow.
If the wavelength of the turbulence matches the airframe or control surface resonance the plane will self-destruct.
Worst case this happens.
I seen Rob try for it.

http://www.youtube.com/watch?v=nRit6tcNT4s

back at my home field in MI, I watched the coolest thing... someone was flying his Ucan Do like a Pylon plane......
low pass down the field and then the sound... and poof... a shower of balsa... nothing was left.......... just a debris field...


flutter is turbulent airflow...can be caused by large gaps, bad servos, bad linkages, etc. etc

Besides sealing your hinge gaps, you can try mass balanced control surfaces. The use of pull/pull linkage will also reduce fluter. I have also heard that boost tabs can help if your dealing with a large control surface.
But basically slowing down below the VNE for the airplane is the best choice.

Also, very large control surfaces tend to flutter more easily. Take the GP U-Can-Do for example. It is very prone to aileron flutter with those huge surfaces sticking way back behind the hinge line even if it has good hinge gaps and stiff, tight, linkage. Keep it slow and the work great, go to fast and it is a recipe for disaster.

No, No, No, No!

Please guys do not ever think that sealing your hinge line will prevent, or even reduce flutter.

In fact, to the contrary, sealing the gaps can be a factor in CREATEING flutter.

Flutter is a harmonic vibration - Period.

What is a harmonic vibration? Simple - everything has a frequency at which it will vibrate. Let's look at a guitar string. The second string on a guitar is an "A". A perfectly tuned "A" vibrates at 440 vibrations per second - weather it's a guitar string, a tuning fork or whatever.

If you hold the string down at the 12th fret (Which is the middle of the string) and pluck it, it will play an "A" one octave higher, or A - 880.

With me so far?

Ok, now if you place your finger lightly at the 12th fret and pluck it, but then release your finger, the string will do a "Double Vibration" - both ends of the string will vibrate oposite to each other - EVEN THOUGH YOU ONLY PLUCKED ONE SIDE. To guitars players, this is even known as a "Harmonic" (Am I right, all you guitar players out there?)

Here's how it looks:



Ok, Now here's a really cool experiment to demonstrate harmomnic vibration. Go to a music store - one that has a lot of accoustic guitars hanging on the wall. Take an electric guitar, plug it into an amp, crank up the volume and pluck an "A" - But then right after you pluck the string, mute it with your hand and listen...

All of the guitars on the wall will be humming their "A" string (Provided they are all in tune)

The reason for this is that they want to vibrate at 440 Beats per second and they were just hit with air currents at 440 beats per second - so they are vibrating or "Fluttering".

This is what happens when any part of your airplane is "Tuned" to any incoming vibrations.

Now it is true that IF you are getting flutter, Sealing the gap MIGHT de-tune it to the vibrations, but it is also true that if an aileron with a sealed gap is fluttering, UNsealing the gap might de-tune it.

So the bottom line is, Harmonic Vibrations cause flutter. Sealing a gap and expecting it to stop flutter is like sealing one edge of a flag to the pole and expecting it not to wave.

Guitar music is scored one octave higher than it is played in order to make the music fit closely within the treble clef so a guitar's middle A is really 220 HZ, not 440 HZ, but the second string of a guitar is tuned to 110 HZ, two octaves below concert pitch middle A.

The A string of a violin is 440 HZ.

You still made your point Minn . ENJOY !!! RED

So sue me

My attorney will be in touch.

You really don't need a room full of guitars to demonstrate this, just pluck the high E string of a well tuned guitar and then damp it. You still hear the tone ringing until you also dampen the low E string and the A string. The low E is resonating with the 4th order harmonic and the A string resonates with its third order hamonic.
Did you ever sing a note into a grand piano while holding the sustain pedal down and listen to the piano echo your note?
Sitars, Hardanger fiddles, and hurdy gurdys all had symphonic strings that were only there to resonate with the strings that were being played.

The only practical way to cure flutter on an aircraft that has demonstrated control surface flutter is mass balancing. This is a very simple way to acheve that and it will work every time.

The surface does not have to be balanced 100% or even close. With some fun fly and 3D types that have aerodynamic balance sections forward of the hinge line it is very simple to imbed a little lead to acheve mass balance.

John

What is: Flutter is a rapid vibration of any control surface, which can destroy it very quickly.

What causes flutter: The turbulences and the pressure pulses of the stream of air flowing around the control surface, create forces that push that surface to rotate around the hinges' line, in an harmonic oscillation or vibration. That vibration may grow until reaching resonance conditions, when the frequencies of the disturbing oscillation and the control surface match. The conditions that make flutter appear more easily are: high air speed, loose linkages and big control surfaces.

How to prevent it: Tapeing the hinge line is always good, because it eliminates turbulences created by air leaks between the high and low pressure sides of the wing. However, the only known effective cure for flutter is to move the center of gravity or mass of the control surface to the hinges' line. No mass moving, no reinforced oscillation or resonance is possible. To achieve that in models and real airplanes, balancing weights are installed on the control surface in front of the hinge' line, so the assembly just balances at that line.

Regards!

I'd going to disagree a bit with Minn on the sealing bit. While it is true that a sealed hinge line is not immune to flutter, and there is no guarantee that sealing will not make the issue worse, in general, once source of the turbulent flow is from an excessive hinge gap. In addition, sealing, because of the less chaotic flow, makes for a more effective and precise control surface, so sealing is just good construction technique in my book.

You need to be careful and know what you are doing if you mass balance. Done improperly, the mass balance will tear off the control surface. The mass balance vibration can get 180 degrees out of phase with the control surface, and due to construction methods and materials we use in the models, it will quickly destroy the control surface. I know of one 40% 3D ship that was destroyed becase of the mass balancers.

Sealing the hinge gap is indeed a good construction technique.

But saying it's going to reduce flutter is like saying a covered wing will flutter less than a painted wing. (Note: wings, stabs, fins etc are also subject to flutter, not just control surfaces) The following video is of a wing with no aileron (So no gap)

http://www.youtube.com/watch?v=8D7YCCLGu5Y

What causes most model airplane flutter? Large control surfaces, poorly fitted linkages, sloppy or weak pushrods, weak servos, and most important, the left control stick being pushed too far forward for the type of model that you are flying. The puny wire in tube pushrods are a prime suspect, but since most of you are flying ARF's you don't have too much leeway in fixing the problem without performing some fairly major surgery. Look at your linkages. Hold your servo arm to keep it from moving. Try to move the control surface. If you have any backlash at the control horn/pushrod connection, then you probably will experience flutter. Tighten them up, even if you have to replace some parts. Zero slop is what you want. Then fly the model within the designed speed envelope.

Bill, AMA 4720
WACO Brotherhood #1

It is said that "everything has a speed at which it will flutter, even a bowling ball". And that speed depends on the size and weight of the surface, and if the surface is a hinged one, the strength and rigidity of it's rigging.

With our models, the most effective cure (when you're lucky to have something left to cure) is to take the slop out of and stiffen the rigging.

When our models exhibit flutter, the first question to ask is, "was this model meant to fly that fast". The 3D models really are not. They've got huge surfaces and usually have very poor rigging design. The large servo arms and short surface horns guarantee that the rigging, no matter how slop-free has little chance. Mass balance can help them.

As for sealing the hinge gap, that's another one of our model airplane rules of thumb that needs to be last on the list, if at all. MinnFlyer has TWO (2) very excellent examples that too many people seem not to have understood. Seal a flag to the pole and it still flutters. Wings without ailerons flutter.

When our models exhibit flutter, your first correction (if you're lucky enough to have something to correct) should be to the rigging. Stronger and less slop. Do that and the next step is to consider if the plane was actually supposed to fly fast or not. If it has big surfaces? It wasn't. Stop doing it. Mass balancing will do wonders, but in today's ARF culture, you're probably not a builder, right? Well, it's easy, try it.

Want to know how hard it is to understand flutter and it's solutions?

Do you take R/C Report Magazine? The last issue had an article about a model they reviewed that showed flutter and lived. How did they choose to fix it? I'm not going to repeat exactly what they did because it's exactly backwards to what theory says to do. And they think it worked. It had to do with mass balancing.

The heavier a surface is, the more prone it is to flutter. It works two ways. The heavier the surface, the stronger, stiffer, and slop free the rigging has to be. And the mass of the surface contributes to flutter and lowers the speed at which it starts. And mass balancing works against all of that (except the slop of the rigging).

The way mass balancing works it to balance the mass of the surface across the hingeline. By adding a weight to that surace that is on the other side of the hingeline, the rigging has a lot easier job holding the surface still. And it's worked to solve fluttering surface problems forever. It actually doesn't stop the flutter, just raises the speed at which the flutter is going to happen. You can actually use the same concept when you have solid balsa surfaces. Put lightening holes in them and you're reducing the mass of the surface that the rigging has to stabilize.

With all due respect Bill, I just want to clarify that none of these things CAUSE flutter, but they WILL contribute to allowing the flutter to happen.

I always figured that there was a good reason that the control surfaces of full scale P-51 Mustangs (and other warbirds) were fabric covered.

Also, the blades of a lot of model helicopter rotors have lead wire glued into the leading edge of the blades, to keep the blade's center of mass ahead of the blade's center of lift.

there is alot more to flutter than i ever imagined! its really pretty simple to understand after it is explained ! thanks for the pics, they helped alot............now when i here flutter i will slow down ,dont want anything to happen like what did in the video!

Minni flyer has it pretty well explained. I worked in instrumentation on real aircraft design and flutter was always one of the criteria that had to be checked and tested for. Yes, everything will flutter unless it has zero mass and infinite stiffness (an impossible situation). Tapeing hinge lines does not cure flutter, it may do two things to minimize it; 1; prevent some of the turbulence that might initiate the flutter and 2: changes the mass and stiffness of the member which is fluttering so changes the frequency outside the range at which flutter might be initiated. Also, counter balance does not in itself do anything for flutter although it might delay the onslaught and will definately change the resonant frequency as it has changed the mass. It will be a big help in lessoning the static load on the servo though and if airodynamic balance is done as well will relieve the working load. Main help in models is light weight, very stiff structure and no slop in linkages or hinges. Note that sometimes it is the main structure itself (wing, stab or fin) that is fluttering, not the moveable surface. The same requirement for stiffness applies there.

What causes flutter?

An aerodynamic force pushes or displaces the surface. The surface then moves until it's own stiffness or it's aero shape halts the displacement. At that point, aero force usually contributes to "replace" the surface where it came from. The structure of the surface contributes to the return as well. During this displacement, the mass of the surface contributes to the forces involved. The greater the mass of the surface, the greater the contribution.

OK, the surface has been displaced, it has moved as far as it can, and for several reasons it's starting to return "to center". The mass of it really contributes now. It aids the force and speed of the return motion. So the surface now passes center and repeats the deflection in the opposite direction, but with more energy than at the start. etc etc etc ....... until you either get off the throttle and the model slows down, or POW!!!!

Flutter is an oscillation that is self strengthening (usually). Rodney's post covers it pretty well. And he clarifies the contribution of mass balancing better. But one detail about mass balance is worth mentioning.

Our models are often fairly close to flutter. And sometimes it only takes a couple mph more speed to get flutter. And sometimes our rigging strength or stiffness needs just a bit more strain to fail. Have you noticed that flutter often happens on a pullout? It also happens on really hard turns. Centrifugal force is real and is a significant contributor to the load our rigging sees. When a surface is not mass balanced, the entire weight of that surface bears down on the rigging. Look at the surfaces on an IMAC model when it's parked. Gravity pulls all those surfaces and they droop. Mass balance them and they're less apt to droop. But the purpose of mass balancing isn't just to have the airplane look better when parked. When that sucker is pulling it's guts out around a turn or screaming out of a dive, those surfaces are pitting more than their own weight against the rigging and the servo(s). You know, models often see flutter when the load on the surface is more than the servo can handle. And constantly overstressing the rigging wallows out the fittings. But hang a mass balance across the hingeline from that surface and what happens? The mass balance sees the same force the surface sees but works to lessen the force the rigging sees. It cuts down on the wear and tear on the rigging and the load on the servos. When it helps and what it helps is sometimes it's importance.

Ok, maybe a silly question, what are some signs at the field of flutter, I guess that I have not seen or heard it in person before. Do you usually see it on the airplane? or do you hear it, or does it feel different?

Thanks