bliebo

Does any one know how to stop it , i have never had a problem with it but there is always a first time. I saw it happen the other day first a strange noise then a death spiral. Scary!

adrian-RCU

yep- slow down !!

adrian-RCU

on the serious side
1. slow down fast
2. land
3. check over all push rods, clevises etc.
4. ask for help from your club expert they would 99% of the time show or tell you the problem.

Unstable

there has been alot of talk about flutter lately. I am thinking about doing an article on it for RCU magazine as soon as I get a chance to talk to marc about it.

I want to get a old plane and video tape some flutter so people know what it sounds like. and what it can do to your plane.

RCKen

My Feedback: (9)

I've been flying for 6 years and have known about flutter for a long time. It is definately a killer. I don't know how many times I have seen somebody show up at the field and has no idea what that "strange noise" coming from their plane is. Sometimes they are lucky enough to get back to the ground in one piece, sometimes not. A push by RCU to educate about flutter would really help.

I've experience flutter a few times, and most times I do what adrian said above : cut throttle and get back to the ground as fast as possible. But as the photo below shows, it doesn't always work. I was flying a Lanier 15 size Shrike (ok, ok, ok, I had it hot rodded up and was flying it with a 25 OS on it. Yes, it was a rocket with wings) and was doing a high speed pass over the runway. I heard some serious flutter as I made this pass. I immediately cut the throttle and turn to make a landing approach back to the runway. Nope, the gravity gods decided they wanted a sacrifice. The plane went into a death spiral into the ground. They funny thing was that Murphy really took a hand in this. I had to wade through waist high grass to find the plane. In fact, there was waist high grass at least 100' in every direction to where the plane came down, which was an approximately 2'-3' area of clear ground. Totaled the plane!!!

I hope that anybody reading this that never considered flutter before will take a real interest in it, and how to prevent it.

adrian-RCU

great- you only understand flutter when you've lost at least two or three planes.

vinnie

My Feedback: (1)

bliebo,

1. control linkage slop is the principal cause of flutter.

2. Excess/unsealed hinge gaps are a contributing factor, although there is some dispute on this.

3. Check all your servo arm and control horn connections to ensure the holes are not too large.

4.Eliminate as best you can any bends in pushrods. Goldenrod flex type linkages benefit greatly from some mid-length support to minimze flexing.

5. Don't power a 40 size airplane with a 1.20 size engine.

6. See #1.

adrian-RCU

and then enjoy your plane a while longer

Rodney

I agree that sloppy push rod connections can contribute to flutter. Flutter will only occur when you hit a resonant point in the structure. How do you determine what that is, very difficult!!!
The best way to stop it is to not let it start. Make sure all control rods are very stiff, that the structure is stiff and no slop in any connection. All structures will resonate at some frequency, no way to prevent it. You can only design the item so that the resonant frequency is out of the normal use spectrum. I do not know of any simple way to calculate this, even in the real world, we approached these resonant points experimentally and changed structure if the resonant points occurred in the usable range of operation, usually by designing to raise the point of natural frequency. Sealing hinge gaps does nothing to eliminate resonance although it may delay the onset. Once that resonant frequency is hit, it is almost impossible to slow it or stop it before damage occurs.

BalsaOvercast

My Feedback: (16)

Click here:

Flutter

You need QuickTime viewer, but really interesting

bliebo

Thanks for all the info, very helpful.

Unstable

I started writing up the article, will post a link to the rough draft when its ready.

A.T.

Best response seen previously =
FLUTTER
By Ed Moorman
We have all seen a flag fluttering in a breeze so we all know what flutter is. The control surfaces on your RC plane can do the same thing. Actually, the whole wing can flutter, too. Flutter is caused by 1. speed, 2. poorly designed or constructed control surfaces and 3. loose, sloppy control hook-ups. Flutter is bad. Flutter is destructive. You may not have seen it on an RC plane, but it is waiting out there to get you. Be aware it can happen.
The onset of flutter is heard as a buzzing or vibrating sound. It is a distinctive sound and virtually everyone at the flying field who hears it will look up. When you fly faster than the buzzing speed, things start breaking or falling off. Clevices, solder joints and servo arms will fail. The servo shaft the arm attaches to will shear off. Hinges will fail and control surfaces will fall off. Whole wings and fuselages will break. All it takes is speed and a little slop in the controls.
Here are my words on flutter:
NEVER, NEVER, NEVER ROUND OFF ANY TRAILING EDGES!
I don't care if the kit plans show it, they are WRONG! If a so-called "expert" tells you to round off any control surface trailing edge, he is an idiot. If the "expert" tells you he rounds his trailing edges off and has never had flutter he is still an idiot, but one who has the skills to build a tight system and who is lucky he has never flown past the flutter threshold airspeed for his planes. These people are doing a great disservice to the modeling community, especially the newcomers who they influence and who, though lack of skills like the "expert," will probably get flutter and may destroy an airplane. You can quote me on that!
Go look in any basic aerodynamics book and look up flutter. It will agree with me. When it comes to control surfaces, STRAIGHT LINES AND SHARP CORNERS DELAY FLUTTER, while CURVED LINES AND ROUNDED CORNERS PROMOTE FLUTTER. Go ask a pylon racer. He'll tell you about inlaying 1/64 ply in the trailing edge of the ailerons so he can sand it to a sharp edge. If you can't make the control surface a straight like to a sharp trailing edge, leave it square and sharp on the corners. Sharp corners preclude flutter while rounded edges promote flutter. NEVER ROUND OFF ANY TRAILING EDGES.
Look at a flag pole. They are rounded and every flag flutters. It's not because cloth naturally flutters, it's the pole.
As for the numerous ARF kits which are sold with rounded of trailing edges, let's look at the economics of the situation. Most people who are not knowledgeable about aerodynamics and flutter automatically think a squared off trailing edge is bad. They erroneously think, "Curves are nice, curves are sexy, curves have to be better." Just looking at a squared off trailing edge, you would have to think it would produce a lot of drag and slow your plane down several miles per hour. I think many people believe a plane with squared off trailing edges won't even fly so they certainly don't want one. It is my opinion the marketing staff insists no one will buy the plane if the ailerons have squared off trailing edges, so good aerodynamic practice is over ruled and the trailing edges are rounded off. Hooray for marketing!
To compensate for this, the building instructions may say, "Keep speed down," "Do not dive at full power," or the sneakier one, "Use long control horns and put the clevice in the furthest out hole." This is to minimize the effect of any slop in your controls. But many fliers will not do this and will end up blowing ailerons off the wing or shaking the tail off the plane.
For trailing edges, here are the choices, and I speak as an engineer and a long time modeler who has sadly and personally verified each of these.
1. Best: A straight line to a sharp trailing edge. This is best and has the least tendency to flutter. It's like commercial aileron stock or trailing edge stock. Don't get me wrong here, these can still flutter, but they will do so at a higher speed than the others. You must still eliminate springy pushrods and sloppy connections.
2. Second best: Straight lines on both sides to a square, flat trailing edge. Squared off trailing edges , too, are very flutter resistant. Think of the elevators of a giant which are made from 3/8 square sticks. DO NOT ROUND OFF THE TRAILING EDGE. Leave the corners square and sharp. I do all mine this way. I recall seeing a magazine article where the designer of a certain Extra 300 kit which specifies rounded trailing edges spoke about the plane. He said to be sure to use the longest control horn and have tight controls. All this does is hold the flutter off until a higher speed. Leave 'em square. It's easier and it works. If you already have the TE rounded off, glue some 1/64 ply vertically on the trailing edge, fill in the gap with Model Magic and Monokote over it. This will give you a sharp, square corner.
3. Next to worst: Sharp trailing edge, but a curved surface. Not very good from a flutter standpoint. These will hold up better than just rounded trailing edge. You probably had to carve some to get this shape and you actually wasted your time. Use long control horns and as tight a connection as you can get. Next time, leave it square or use a sanding block or a plane and carve in a straight line to the trailing edge.
4. The very worst: Completely rounded trailing edges. Think back to the flag pole. If you build up a giant tail surface and round off the trailing edges, you are looking for flutter. If you don't get flutter, it is because you aren't fast enough and you have a really tight control setup. DO NOT DO THIS.
OTHER THINGS YOU CAN DO
Use long control horns and long servo arms, and use the outer most holes. If you have any slop in your control system, the longer arms minimize the angular movement which can allow flutter to start. If you need more control throw, get one of the extra longer servo arms available.
Use stiff pushrods or pull-pull cables, especially on rudder. Tight pull-pull cables take all the slop out of a system. Very good for flutter prevention. Brace the pushrods inside the fuselage if you can.
As a last resort, you can counter balance the control surface."
regards
Alan T.

Unstable

got a quick rough draft done of my "article", I will be sending it to the "proofreaders" soon then I need to get some ilistrations drawn up for it. then flesh it out a bit more.

banktoturn

Speaking somewhat imprecisely, there are two things that are required for flutter. The first is a structure which can flutter. The second is a force to make it flutter. There are two ways that a structure (our structures, anyway) can flutter. If it has no hinges or joints, then it has to deform ( bend, twist, compress, stretch, etc. ). If it has hinges/joints, then there can be undesireable motion as the result of slop in the joints or deformation ( usually buckling ) in the control linkages. I will generalize a little bit here and suggest that most of the fluttering seen in models is the result of the hinged surfaces having some slop or easily deformed linkages. Many of the posts here correctly recommend that all the slop needs to be eliminated. Just think how much easier it is to make an aileron snap back and forth with slop in the system than it is to twist the wing back and forth at the same frequency. If you do a good job of tightening up your linkages, then flutter requires that your actual airframe structure has to deform quite a bit, which takes a lot more force than it does to wiggle a sloppily connected control surface. If you get flutter in the actual structure, you can 1) make it stiffer, 2) try to shift the "resonant" frequency, or 3) add damping to the structure. 2 and 3 are very hard for us to do with our models, so making the structure stiffer is the best bet. I am willing to bet the virtually all the flutter that occurs in our models is in the linkage systems of the control surfaces. If you find that your wings/stabilizers are fluttering, I would bet that sheeting the surfaces with balsa or ply would give enough stiffness to fix the problem.

The other part of the problem is the force, or excitation. For us, the excitation is usually some kind of vortex shedding airflow ( turbulence ), or other unsteady, "periodic" flow. At least one kind of unsteady flow is more likely if you have a big gap at a control surface (the air "can't decide" whether to go through the gap or under the surface, so it flip-flops), which is why we are always advised to make these gaps as small as possible, and/or seal them. Vortex shedding happens at any kind of blunt trailing edge, or any time the flow separates before a trailing edge ( as in stall ). A rounded trailing edge is blunt, and a square trailing edge is blunt. Neither is appropriate if you want to minimize the chance of flutter. A square trailing edge may be a little better, if the width of the actual edge is small, but this is a questionable assumption. Sharp is your best bet.

Flutter tends to occur at higher speed because more energy is available to deform the structure, and possibly because a "resonant" excitation frequency occurs. The people that fly fast have discovered this, and you will notice that their models are stiffer, and more carefully built. Often, some of the control surfaces are balanced, both in the sense of having some mass on both sides of the hinge, and being hinged closer to their aerodynamic centers. This is one way of avoiding some of the fundamental aerodynamic instabilities that can cause control surface flutter.

My guess is that most models are safe enough from flutter if the linkages are tight and the control surface gaps are small. Sealing gaps is a good idea from the drag perspective anyway, and anything besides sharp trailing edges is an abomination.

banktoturn