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Fundamentals Series: Vibration in Aircraft

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.  Frequency is 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.

Learn More

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.

Our goal is to put the complicated science and mathematical equations of vibration in the analyzer, so you can focus on quickly and easily installing the solution and enjoying a safe and smooth ride in your aircraft.

Check out our Technical Library where you’ll find additional training resources, including manuals, application notes, and videos.  Subscribe to our newsletter for announcements and additional training resources as they are added.

Coming up next in our Fundamentals Series are articles on:
  • When to Balance (signs and symptoms of vibration problems)
  • Overview of balancing process
  • Setting up the analyzer
  • Installation of vibration sensors
  • Installation of tach sensors and tracking device
  • Collecting data
  • Reviewing solutions
  • Making adjustments
  • Generating reports & documenting maintenance

Feel free to post questions and comments below!


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