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Vibration
in Aircraft
Fundamentals Series
By Robert Berlin
Going back to the basics to
answer one of the main questions: What is Aviation Vibration?
Wherever there are spinning
parts, there’s going to be vibration. And in aviation, there are lots of
spinning parts: propellers, gearboxes, rotors, and crankshafts.
But what is vibration?
How do we measure it? When is it bad? What happens when you have
bad vibrations? And how do we get rid of this problem?
These are
good questions, so let’s dig into the subject and see what we can uncover.
What is
vibration?
Vibration
occurs in spinning objects when the object becomes unbalanced, meaning it’s
heavier on one side than on another.
This
additional weight on one side, throws off the center balance and makes the
rotation lopsided. As this lopsided weight travels around the circle, it causes
a wobble in that rotation.
An
engineer, however, might describe it this way: “Vibration
occurs when the weight of a rotating propeller and engine assembly is not
equally distributed, the dominant or heavy point attempts to continue moving in
a straight line. The resulting force attempts to pull the assembly outside its
rotational axis. The assembly then orbits what would be the normal center of
rotation, causing vibration.”
Causes of Vibration
There are varieties of causes, but in aviation, the most common include:
·
Crankshaft unbalance
·
Bearing problems
·
Loose or worn components in the assembly
·
Defective crankshaft counterweights
·
Cracked airframe components
Measuring
Vibration
There are a few key
words to understand when it comes to measuring vibration.
Maybe
the easiest to understand are: frequency and amplitude.
First, let’s take a look at the
sinusoidal wave closer to explain frequency and amplitude.
Frequencyis the time it takes for one complete up-and-down cycle.
Amplitude describes
how high (and how low) a weight travels on its journey around the circle.
So we now have words to describe the
height and width of a wave pattern. Now we need new words to label the
speed at which our part is rotating.
To label the frequency phenomenon, an
engineer would typically use Hertz, which is the vibration frequency in Cycles per SECOND.
We can also use CPM, which is the
vibration frequency in Cycles per MINUTE.
But
as aviation technicians, we’d most likely call it Revolutions per Minute (RPMs).
CPM
and RPM are measured exactly the same and can be used interchangeably.
However,
to convert from Hertz to CPMs and RPMs, you simply multiply the Hertz value by
60 (seconds).
We also need a label for the height
of our wave (which indicates a stronger vibration). Here we can describe
the severity in three ways:
· Displacement (the
physical change of position);
· Velocity (the
rate of change of displacement with time); and
· Acceleration (the
rate of change of velocity with time).
We measure displacement
(the physical change of position) using a unit of measurement called Mils, or 0.001 inches.
This
unit of measurement is more common in slow-speed machines; however, you may
encounter this unit when working on General Electric and large Pratt &
Whitney turbine engines.
We measure velocity using a unit of
measurement called IPS,
or inches per second. This is the most common or universal measurement
unit when describing vibration in aviation.
Basically,
when we use IPS to measure vibration, we are describing how fast a heavy spot
moves through a cycle.
And, finally, we measure acceleration
using a unit of measurement called G’s, or equivalent gravities. This type of
measurement is most commonly used when working with high-speed machines such as
gear boxes.
To capture this information, we mount
vibration sensors to our aircraft at strategic points. These devices
create an electrical pulse which is then read by an analyzer and plotted on a
graph.
When is Vibration Considered
to be Bad?
Engine and propeller manufactures each have
their own specific answers for specific applications; however, in general, you
can use the following graph to determine the severity of vibration in an
aircraft.
0 – .2 IPS = Good
.21 – .4 IPS = Fair
.41 – .6 IPS =
Slightly Rough
.61 –
.8 IPS = Rough
.81 – 1.0 IPS = Very Rough
1.01 – 1.2 IPS = Dangerous
Consult
your aircraft’s manuals for more detailed information.
Effects
of Vibration
Despite
the fact that some vibration is normal and expected in aviation, when aircraft
vibration becomes excessive, you may expect the following complaints:
· Cracked exhaust stacks and
sheet metal
· Higher than normal occurrence
of oil leaks and light bulb failures
· Physical movement of the
airframe (buzz in the seat, yoke and rudder pedals)
· Passenger complaints of noise
in the cabin
Eliminating
Vibration in Aircrafts
Well that’s a bit of a joke. You can’t
actually eliminate vibration from your aircraft, but reducing the vibration as
much as possible will make a noticeable difference in the ride and increase the
longevity of your airframe and avionics.
To determine what your
vibration levels are and how to make adjustments, you’ll need the following
pieces of equipment:
·
A device to collect vibration and phase data;
·
A phase angle or speed-sensing device;
·
A vibration sensing device; and
·
Weights and hand tools.
The
basic idea of aircraft vibration analysis is to locate the heavy spot and place
a counter-weight on the opposite side to balance out the imbalance.
ACES Systems manufactures several analyzers designed to
measure vibration in your aircraft and mathematically compute the best place to
install counterweights on your propeller or rotor.
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