Need FLUTTER definition (aero)

"Have you heard a buzzing sound coming from your sloper, especially during a high-speed pass? If your covering isn't torn and flapping the noise is probably control surface flutter. The sound is usually caused by loose ailerons, but can also come from the rudder or elevator. What hap- pens is that alternating high and low pressures on either side of control surfaces can cause them to vibrate like the reed in a wind instrument. The side of the control surface in the windstream is subject to high pressure while the opposite surface is subject to low pressure. If these surfaces are loose they will vibrate back and forth in and out of the windstream causing rapidly alternating high and low pressure build-up. That's why in extreme cases the flutter will sound like a card in the spokes of a bicycle. If you hear flutter it's best to land right away and fix it. The alternative is losing control of your sloper!"

"During the last three years, AFOSR-supported research at Lehigh University has led to a new understanding of limit cycle oscillations (LCOs) that occur as a result of interactions between airframe components and the moving external airflow. Such oscillations are, at the minimum, annoying to flight crews and contribute to pilot fatigue. At the worst, they can threaten the durability and structural integrity of the aircraft. The researchers have identified the key parameters governing this LCO phenomenon and potential passive control techniques that can be applied to supersonic flight conditions.

"Lehigh Professors C.R. Smith and J.D.A. Walker, along with two doctoral candidates, have focused on developing an understanding of the cause and effect relationships of LCOs. Such oscillations, which are characterized by a cyclic nonlinear oscillation of airframe parts, occur frequently in flight and are very expensive to fix. Examples include wing bending oscillations induced in B-lA aircraft, external armament motion, wing tip oscillations, and control surface flutter brought on by free play in the system.

"In an initial set of experiments, Professors Smith and Walker demonstrated that oscillation of an aircraft surface in subsonic flight can be provoked by vortex motion in the air stream, as well as by separation of the surface boundary layer. (Vortices are a common and unavoidable feature of flow around modern airplanes.) Generally, such phenomena constitute a complex interaction between the moving airflow, the viscous boundary layers on the surface, and the flexing airframe surface."

"Flutter is the oscillation of the control surface (normally the ailerons). The aileron starts oscillating up and down as the air traveling over its surface moves it back and forth. It may start as a dull buzz and can easily end up with the aileron flying off the wing within a few seconds. So what creates the opportunity for flutter to occur? Here are the possible choices:

"1. Your airplane is overpowered and the construction of the wing and tail are insufficient to keep the aileron 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 aileron 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 aileron, again eliminating the leverage that should exist.

" 6. The hinging is loose and weak with too much gap from the TE of the wing.

"7. The hinge gap is not sealed.

"8. The aileron is not balanced.

" 9. The servo output arm and or control horn are too flimsy for your application.

" 10. Etc, I am sure there are more. "

Marty

"Flutter is a condition when one or more of a plane's control surfaces start flapping, much like a flag or laundry on a line in a stiff breeze. This flapping motion can be caused by a number of different factors, and can break control linkages or even rip the control surface off the airplane. Either way, it can result in what you see here."

Try the above. Since this is a mall show, you don't want to get too technical. As a former boss of mine told me when I was writing an SOP manual, "Keep it on a third-grade level."

Hope this helps, Morris

Flutter is an oscillation of the control surfaces, usually ailerons, at high speed which occurs when aerodynamic effects excite the natural resonance of the control system and the wing structure. Flutter can be avoided by keeping the control system free of flexibility and play and wing torsion ally stiff.

Paul

TX snipped-for-privacy@webtv.net wrote:

Nicely written by Ed Moorman some time ago and still valid today. =

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. Clevises, 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 modelling 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.

1. 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.

2. 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.

1. 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.

========================================================= =========================================================

Many modelers balance their pattern ships laterally (wing tip to wing tip), by adding weight to the light tip after finishing. Never add weight to top or bottom of wing tip or control surface as bulge will effect flying characteristics. The very best thing that you could do to insure against flutter besides stiffening things up, would be to statically counterbalance the surface with weight ahead of the hinge line. Use enough weight and arm length to at least balance the weight of the surface - another good argument for keeping the control surface weight down

See the very good article in April 1986 R/C Modeler Vol. 23 No. 4 Pg. 16

regards Alan T. Alan's Hobby, Model & RC Web Links

1) I have had an elevator flutter with a square trailing edge. I have also had flutter with round edges. 2) Great Planes kits round everything, and I have never had flutter with them. I will continue to follow their instructions. 3) My main problem has been with fun-fly models (like Morris Hobbies) that have a 1/4 inch open frame stab. I am now making such structures with 3/16 sticks and sheeting with 1/32 balsa. The result is the same thickness as the original and is much stiffer. I have flown a SU-do-KHOI so altered far beyond its original flutter threshold with no sign of flutter.

For my money, the golden rule is KISS (Keep It Stiff, Stupid). This includes tight linkages, stiff control surfaces AND THE SURFACES THEY ATTACH TO. (!)

This does not mean that the square trailing edge has no merit. But, it is not close to being a cure-all.

-- Mike Norton

Thanx Guys, It's appreciated.This should explain the pile of balsa and ply stacked on the table. LOL>

Thanx again Walt

After reading all the preceding replies (at least up thru 6/9 - my last checkin), and generally agreeing with most, I'm surprised no one mentioned the solution I learned ~30 years ago that seemed to work for us. I don't recall who first suggested it, but it was published somewhere way back then. And I reminded a group of it, at least 10 years ago, and again, others remembered it also.

The solution is - make the leading edge of the control surface slightly thicker than the trailing edge of the fixed surface to which it is attached. This is most effective in preventing flutter of the ailerons, but works for rudder and elevator as well. NOTE - this does not attach any significance to whether the trailing edge is sharp or rounded. It's effectiveness results from the disruption of the air flow as it BEGINS its passage over the control surface.

Olin McDaniel, AMA 30932 To reply by email, please remove "abcd" from Return address

----------------------------------------------------- "Ignorance is treatable, Stupidity is incurable. Sometimes the difference is hardly distinguishable, however."

All of the explainations I've read seem acurate and helpful. The one that I haven't seen came back to me as I was going out to help a fellow clubmember pick up what started the flight as a whole Chipmunk. To me flutter is that sound that you hear just before a terminal - rekitting- event for a plane. Hard to describe, but once you've heard it, you'll always know it, even if you aren't looking.

Andy

We can make a box of wood.....FLY!!

Thanx Guys, I did find out what people do at a mall show.Or rather what they are interested in. We had maybe 25 planes on display.All types,all sizes (one 13 ft wingspan).Speed planes,scale planes.You name it. I had put my wrecked Fantasy on a table with the deffintion of flutter and what happens next to the plane.I found that more people stopped at that table to read about it than even the pretty planes.This was the first day that I noticed that. On the second day I decided to put my two speed planes on a table with a description of what plane it was,the size and make of engine,radio use, estimated speed range,run time.These two planes (Viper,Patriot) got a lot of attention.Really surprised me. This was our first mall show and most of you already know this,but people like to read about the plane instead of just looking at it.Even wrecked ones. I found that the youngun's like to see a plane wrecked..LOL>

Thanx a mil' for yalls input.....

Happy flying and Happy Mall Shows.