Model Forum / Radio Controlled / Air Models / June 2004

"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!"

<http://lsss.homestead.com/control_surface_flutter.html>

"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."
<http://www.afosr.af.mil/pages/afrtrh96.htm>

"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.  "

<http://www.giantscaleplanes.com/flutter.htm>

                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

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.

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.

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

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
http://homepages.ihug.co.nz/~atong

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