Up Spectrum Analysis Reports

The Vibration Spectrum Analysis report and what it means:

The reading of vertical movement (vibration) is in ips (inches per second.


Norcal Aero Services
=================== 'Dave Smith' - Propeller Balance ===================
Customer: Dave Smith
Started: 02/24/2006 13:43.46 Finished: Yes
A/C Reg: 126V A/C Total TM: 5240.3
Eng S/N: TSO: 0.0
Eng Pos: 17733 TSN: 0.0

-----------------------------------------------------------------------
*** Single-engine balance job ***
Prop Balance for 5 Runs ( Engine 1 )
------------------------------------------------------------
Run|Speed| Vib ( p ) | Influence | Solution |Installation|
|    | rpm | mag deg | g/vib deg | gram deg | gram deg |
------------------------------------------------------------
| 1 |2419 | 0.37 165 | 38.5 330 | 9.8 30     | 10.2 40   |Front
|    |         | 0.50 290 |                |                |                |Rear
| 2 |2414 | 0.27 143 | 45.9 309 | 25.2 275 | 25.4 270 |front
|    |         | 0.49 287 |                |                |                |rear
| 3 |2422 | 0.12 195 | 62.5 300 | 38.6 268 | 38.4 275 |front
|    |         | 0.27 282 |                |                |                |rear
| 4 |2413 | 0.08 292 | 66.1 303 | 44.3 270 | 44.3 275 |front
|    |         | 0.20 271 |                |                |                |rear
| 5 |2424 | 0.04 305 | 65.8 303 |                |                |front
|    |         | 0.18 256 |                |                |                |rear
------------------------------------------------------------
Final weight to install:
Engine 1: 44.3 Grams at 275 Degrees

The initial vibration is in red, the final vibration is in green and is in IPS (inches per second). 
These results represent a significant improvement. This is not unsual !!

Here is a basic guide to these values:

DANGER 1.25 Inches Per Second (IPS) Peak Amplitude. The propeller should be
removed and a static balance performed.
VERY ROUGH   1.00 Inches Per Second (IPS) Peak Amplitude. Propeller can be dynamically balanced; however a large amount of weight will be required. 
A propeller
static balance is recommended at a propeller shop.  Operation at this vibration level could cause damage.
ROUGH --- 0.50 Inches Per Second (IPS) Peak Amplitude. Propeller definitely requires dynamic balance. 
  Long term operation at this vibration level could cause
excessive wear.
SLIGHTLY ROUGH --- 0.25 Inches Per Second (IPS) Peak Amplitude. Dynamic balance will improve passenger comfort.
FAIR---  0.20 Inches or less Per Second (IPS) Peak Amplitude. 
This is the maximum
vibration level we aim for during dynamic balancing, although we can usually reduce the level to far less.
GOOD ---  0.07 Inches Per Second (IPS) Peak Amplitude. 
Vibration levels below 0.07 will not be detectable by pilots or passengers but can still be improved upon in most cases.
EXCELLENT ---  0.04 Inches Per Second (IPS) Peak Amplitude. 
This is the level we always aim for.

 

Each engine and propeller combination, when healthy, will produce a normal spectrum display that is characteristic of that combination. The trick is to determine when this spectral display is not normal. When the spectral data is gathered, you have a digital display of these vibration readings, sometimes called a "signature," which then has to be interpreted. Each one of the spikes or peaks shown in the graph are representative of a rotating component or a multiple thereof. These multiples of the fundamental RPM are referred to as "harmonics" or "orders."

Below is a typical spectrum survey, showing the various vibration levels and their harmonics.

Some of the major causes of the various peaks are identified below. 

As well as vibration occurring at the actual engine rpm (2000 in this case), often other information can be extracted from this graph, including misfiring, faulty engine mounts, internal imbalance, misalignment, out-of-time balance weights, component looseness, particularly if this report is compared with those taken from a healthy version of the engine or from the same engine at an earlier time. 
As prop balancers we eliminate first-order  vibration - the only vibration that can be removed externally to the engine. 
However, our  survey is extremely accurate at pinpointing the other areas of concern. Your mechanic will find this information very useful in helping to pinpoint the causes of misfire, worn mounts etc and it will save having to guess or use trial and error to troubleshoot an engine problem.

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Copyright 2006 SPX Communications Inc.
Last modified: October 30, 2009