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VIOLENT VIBES Uncontrolled flutter can shake an aircraft apart
in seconds. Abridged version Flutter
is a violent vibration caused by the interaction of structural
flexibility, mass and aerodynamic forces.
It can shake an aircraft apart in seconds.
As a simplified example, visualize what happens to a control surface when the aerofoil ahead of it flexes slightly, perhaps due to a small gust. We are talking here of wing flexure interacting with aileron rotation, horizontal tail with elevator or fin with rudder. Assume also that the mass of the control surface is behind its hinge-line (as is usual.).
If
the aerofoil moves slightly upward, the mass of the control surface
causes the control to lag behind and rotate with its trailing edge down.
Because of structural flexibility this tends to happen even if
the control system is held firmly. The deflected configuration momentarily increases lift on the
aerofoil and acts to increase its initial displacement. Eventually,
structural stiffness overcomes the aerodynamic forces and the aerofoil
starts to return to its normal position. As the aerofoil moves down, the
control surface again lags behind, but now rotates trailing edge up and
so again accentuates the displacement, this time downward. Above
a certain speed, there is sufficient aerodynamic energy for successive
vibrations like this to build up progressively.
This is flutter. This
simple type of flutter can be suppressed by adding balance weights to
the control surface ahead of its hinge line. Doing this eliminates the
tendency for the control surface to lag behind the vertical flexure of
wing or tail and consequently the two motions no longer interact. However
full mass balancing is too heavy, often impractical and cannot
counteract all flutter tendencies. Apart
from speed, the extent of a flutter interaction is determined by
resonance and phase relationships between the two interacting
deflections. Pushing a
swing illustrates this. A
swing goes high with very gentle pushes provided the pushes are applied
exactly in time with the frequency at which the swing oscillates
backwards and forwards - each time reinforcing the amplitude. A
swing has just one normal frequency whereas an airframe has very many
structural frequencies at which it bends, twists and rotates.
Many of those frequencies vary with speed, as do aerodynamic
loads. The extent to which
two vibrations such as wing bending and aileron rotation can interact
and cause flutter depends on their predominant frequencies, and how
close together those frequencies come within the flight envelope.
During
aircraft design specialist flutter engineers evaluate all possible
interactions over a wide range of frequencies.
Their art is to ensure that critical frequencies do not merge
together and that dangerous interactions never occur. Once
the aircraft is in service, maintenance engineers must keep structural
frequencies where they belong - well separated. Crucial to this are mass-balance of controls, security of
balance weights, control circuit stiffness and control surface free-play.
Even an excessively thick coat of paint or a small repair can
upset the proper balance of a control surface and cause flutter. The
term “flutter” sounds innocuous, but it is not. It is violently
destructive. Avoidance depends on meticulous design, maintenance and
strict compliance with speed limits. Adapted by the author from an article first
published in Flight Safety Australia, September 1997 Martin Aubury lectures in aircraft design and
flutter
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