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It's short. Here it is.............., every builder should have a
set of Tony's books...............
Control Surface Flutter Problems
By Tony Bingelis (originally published in EAA Sport Aviation,
July 1979)
December 01, 1993
July 01, 1993
FLUTTER? WELL, SIR, flutter is what a flag does wildly on the flag
pole on those days when it is too windy for you to fly. But to
better relate it to our subject, it should be described as a
potentially destructive vibration or buffeting of an aircraft due
to an out-of-balance condition of one or more of its control surfaces.
Now, imagine one of your control surfaces acting like a flag in
the breeze . . . in flight at 100 or 200 mph. How long do you
think it would stay with the airplane? Not for long, I'll wager!
Most of us are aware that the flutter problem is a complex one and
it has been around aviation for a long time. So long that flutter
specialists must be wallowing around knee deep in the accumulation
of flutter fodder generated from years of research and testing.
Fortunately, there are a few useful assumptions and certain
recognized `good practices' which have been sifted out and any
builder willing to apply the guidelines can do much to avoid
having a flutter problem. But before I continue, let me discuss a
few terms and phrases.
*Static Balance*
Static balance - A condition that exists when an object (wheel,
propeller, control surface, anything) remains stationary while
supported on, or suspended from its own center of gravity.
Relating this more specifically to our subject, it also means
balancing a control surface while it is at rest (not in flight).
Automobile wheels, as you know, can be balanced statically (while
at rest). A more effective way, however, is dynamic balancing
(spin balancing). The dynamic balance of aircraft surfaces is
similarly effective but homebuilders really have no practical way
of working out the dynamic balance of a control surface subjected
to the stresses of flight. For this reason, they must fall back on
what might be called a `good practices concept' and assume that,
for all practical purposes, when a control surface is properly
mass balanced in its static condition, it should also be in
dynamic balance. If, in principle, the main objective of dynamic
balance is to prevent or minimize torsional stress in flight, we
can accomplish this adequately by evenly distributing the mass
balance weight along the span of the control surface.
Broadly speaking, to attain a static balance state in a control
surface, we add lead weight to the nose until the center of
gravity falls on the hinge axis. Let me expand on this a bit.
For example, if you were to suspend a control surface from its
hinge axis, one of three static balance conditions would become
immediately apparent to you.
1. If the control surface assumes a trailing-edge-low attitude, it
is statically underbalanced and a tail-heavy condition exists. See
*Figure 1*.
Operating
2. If the control surface remains in a level (horizontal)
attitude, it is said to be statically 100% balanced and its center
of gravity (cg) is co-located with the hinge axis.
3. Should the trailing edge of the control surface rise some
position above a horizontal plane, an overbalance condition is
apparent.
Two of the three conditions described above result in a control
surface that will have a fairly predictable flutter-free flight
performance. The one that is 100% balanced to a level attitude
should consistently give good results. The other surface having a
slight nose down attitude is a typical overbalance condition
essential for good results in high-performance aircraft.
Conversely, the static underbalance, or tail heavy condition first
described, is the least desirable as it may result in
unpredictable flight performance.
The conventional flap type (aileron, elevator, rudder) control
surface, as constructed, is typically tail-heavy. That is to say,
most of its structure is distributed behind the hinge axis. It is
this sort of tail-heavy out-of-balance condition that is generally
considered to be the major cause of control surface flutter and
buffeting incidents. True, speed through the air is also a factor
and there is no doubt that flutter is a more frequent occurence in
high performance aircraft than it is with the slower varieties.
However, it would be dangerous to assume that slower homebuilts
are immune from such a propensity. I'll bet you have heard many
times that homebuilts having cruising speeds under 150 mph were
exempt from the flutter problem. Don't you believe it! Any
airplane can experience flutter . . . even your light and slow
VW-powered job under certain conditions.
Does this mean that you must balance the control surfaces of your
project even though the plans don't call for it? Not at all.
Undoubtedly the prototype of the airplane you are building was
built and flown without having exhibited flutter tendencies and
the designer, therefore, found no need to require static balancing
of the control surfaces. However, you should understand that,
although many other examples of this same design may have been
built and flown, there is no assurance that yours will likewise be
free of flutter problems.
The only way you can prove your airplane to be free of flutter
tendencies is to flight test it with that purpose in mind. This is
a potentially dangerous adventure and must be done only under
carefully controlled conditions. You must prove that your airplane
is controllable, free from flutter, and will be safe to fly. No
amount of reassurance derived from theoretical calculations can
substitute for this requirement.
*Other Flutter Provoking Conditions*
Although there is less risk of encountering flutter in slower
aircraft than in high performance types, individual builders can
cause changes, inadvertently, which could introduce flutter
tendencies. For example, a wing lacking torsional rigidity could
induce a bad case of aileron flutter even at the relatively low
airspeeds generally associated with low and medium-performance
aircraft.
A newly constructed aileron or elevator that is excessively heavy
(due to the use of heavier substitute materials or uncalled for
reinforcements) can be flutter-prone. Flutter is most difficult to
suppress in very large or heavy control surfaces and the balance
weight requirement becomes excessive.
Would it surprise you to learn that even time-tested
production-line aircraft are not immune to the flutter phenomenon?
True! The reason being that anytime anything changes the balance
of the control surfaces it may induce flutter in an aircraft that
has had no history of such tendencies. For example, there have
been instances where flutter developed simply because mud adhered
to the control surfaces following muddy field operation.
In an incident reported by the FAA, moisture had collected inside
the ailerons during winter operations and had frozen (seems to
happen every winter) thereby causing an unbalanced condition that
was not detected during the preflight . . . result? In-flight
flutter and an accident.
During the long days and nights in the life of an aircraft many
changes take place. Dirt accumulates inside the control surfaces,
patches are added to repair dings and tears, and in time, the
surfaces are repainted. All of these things cause a cumulative
change in the mass balance of the control surface. At some point,
the amount of change becomes just too much . . . and increases the
risk of flutter if no steps are taken to rebalance the reworked
surface.
Loose balance weights, water absorption in foam structures,
improperly located or clogged drain holes are all elements which
could contribute to an aerodynamic imbalance situation and result
in flutter.
Avoid free play or slack in the control cables. Stiffness in the
control system does have a useful damping effect on the control
surfaces further inhibiting flutter tendencies. However, this
should not be completely relied upon as later, in service, the
wear and occasional lubrication could free the system of much of
its original friction and result in an increased risk of flutter.
Adding a fixed trim tab to an aileron can further upset a marginal
balance condition.
Controllable trim tabs, too, can be a problem. Trim tab control
linkage failures and trim tabs with loose or improperly installed
and adjusted linkages have caused a considerable number of
accidents and near accidents by exciting flutter in the control
surfaces to which they are attached. A recent incident of that
nature has just come to my attention. Involved was a widely built
and proven design . . . the staid ol’ Emeraude. Here's how it
happened.
*Flutter . . . A First Hand Account*
"I knew the trim wasn't working - but who needs trim for a ten
minute every-which-way hop! I'm not one for flying level long, so
we went into a turning dive -somewhere over 140. All hell broke
loose and I about lost control . . . elevator flutter - it was
violent! Honest, each wing and the whole tail was shuddering.
I came off the power . . . leveled my wing . . . very hard to do,
and started looking for a place to dump her. About 90, the flutter
slowed but still bad - at 80 it quit!
I kept my head, let her glide for a few seconds and then added
power - kept my nose high and flew back to the airport but slow -
making shallow turns and a long straight in to a God awful landing
- but safe!
Once on the ground I found the problem - the trim tab. Suddenly I
remembered. Earlier a boy and his dad were visiting with me while
I was working on my brakes. The boy was in back playing with the
elevator. He must have bent the tab control wire - leaving the tab
to start fluttering at high speed - thus causing the elevator to
flutter.
It took two minutes to fix the cable. I checked for other damage -
none. She is an awful strong design, that Emeraude.
It took a little longer to get up enough guts to fly her again,
but I did the same day.
She's fine now but I have a little more respect for small items. I
also preflight a little more carefully now. I don't know if you
have ever experienced flutter or really know what it's like - I'm
afraid of it now."
There are not too many folks around who can tell you, first hand,
how sudden and destructive control flutter can be. We do know it
can happen and does happen all too often. This gent was lucky. He
had a good stout airplane and did just the right thing.
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