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Reducing
the amount of transmitted engine and propeller vibration to the airframe
of an aircraft is one of the most challenging tasks of an aircraft
engineer. Vibration is indeed the enemy. But, just how much vibration
is too much vibration? Most would answer that question with, the
lesser the better. It's true, the less vibration the less
component fatigue and the more comfortable the flight experience.
 The
dilemma facing the aircraft designer is that of building an aircraft
with the component strengths necessary for a rigid airframe, yet with
the flexibility to dissipate vibration. The more rigid the
airframe, the harder it is to dampen vibration. Why? Because
firm structures are better transmitters of movement. Vibration is
repetitive movement. Traditionally, vibration is controlled by the
engine mounting system. The engine mounting plate is isolated from
the airframe by rubber dampeners. The dampeners reduce the
conductance of vibration from the engine to adjacent components by having
a layer of rubber in between any part of the contact area. This
rubber intermediate compresses and expands to absorb the engine
movement, therefore preventing the continuation of that movement to the
airframe. The types of mounts vary from
manufacturer
to manufacturer, but they all serve the same purpose; there is a single
vibrating media, the engine, which is isolated by the rubber
intermediate component. Different densities of rubber (durometer)
can be chosen for the mechanical and frequency characteristics of the
application. Have you ever grabbed the ring of a Blue Heron PPC
and given it a shake, or picked it up off the ground? It's
solid! This quality, which we all admire, has further challenged
us to a new level of engineering accomplishment which has reversed the
odds of vibration control. The recently developed Dual Media
Isolation System is revolutionizing the mechanics by which we reduce
vibration.
 The
Dual Media Isolation System uses a two-tier approach to vibration
dampening. First, the engine plate (media #1) is rubber-shock
mounted to the cross mount components of the airframe. This allows
the engine to move, reducing the amount of vibration transmission to the
cross mount. Although in this configuration the cross mount does
not vibrate nearly as much as the engine, it continues to transmit a
certain percentage of the vibration energy produced. Next, the
cross mount component (media #2) is shock-mounted at its attachment
point to the main airframe. This final isolation allows the cross
mount to move as an intermediate media, taking the previously reduced
amount of vibration energy, and diminishing it even further.
Use
of the Dual Media Isolation System results in a flight experience
that is unprecedented in comfort. The difference is so dramatic
from that of any other aircraft that we had to do a comparative
test. The test we performed compared the relative vibration energy
transmission to the pylon tubes of the blue Heron airframe at all
frequency levels. The results are plotted below, and the advantage
of this new system is visibly obvious. As the traditional single
media method shows a steady increase in vibration energy from 3000 rpm
to full throttle, the dual media method is distinguished in its ability
to actually reduce the amount of transmitted energy as the vibration frequencies
increase with engine speed.
The
Best Of Both Worlds. Blue Heron pilots can now have the
strongest airframe and the smoothest ride. 2001 model aircraft come
with the Dual Media Isolation System as standard equipment.
Current Blue Heron owners can upgrade their mounting system as
well by contacting their Blue Heron dealer.
Oscilloscope Displays and
Comparative Vibration Measurements
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