Engine vibration testing

Citation

(2006), "Engine vibration testing", Aircraft Engineering and Aerospace Technology, Vol. 78 No. 2. https://doi.org/10.1108/aeat.2006.12778bad.009

Engine vibration testing

Equipment

Engine vibration testing

Keywords: Aircraft engines, Vibration measurement

Snecma Moteurs and Bruel & Kjaer have been co-operating for more than three years on a project to design a PULSE-based, state-of-the-art portable system to perform ground vibration tests on Snecma M53 and Larzac gas- turbine engines.

The goal of the design team was to develop a customised system and user interface that was almost automatic in use, and would provide diagnostic data. The Snecma Logistic Integration Support Department and Armée de l'Air (the French Air Force) together determined the parameters that should be visible on the user interface.

The test uses order analysis data in the range from 0 to 25 kHz acquired during an engine run-up/run-down. The simple user interface provides a simple pass or fail indication. If unexpected vibration is monitored, the service technician is prompted to follow a series of further investigation checks. The last resort is to remove an aircraft from operational service and to replace the engine, a time consuming and expensive procedure.

The number of input signals associated with the two engine types makes the use of the rugged and compact 5-channel PULSE 3560B front-end ideal for data acquisition and analysis. The PULSE hardware and power supply are each mounted on special shock resisting damping material.

The system is housed in three waterproof rugged carrying cases. One contains the PULSE system, another houses the PC and a third transports the cables and accessories. Each carrying case weighs about 8 kg. The connectors and fittings used are to fully approved military standards. Each carrying case has been subjected to a full range of MIL tests specified by the Logistic Integration Support Department and based on published standards, including: shock, vibration, CE Mark, drop test, humidity, temperature, EMC compliance, immersion and a salt-spray test.

The data acquisition unit communicates with the PC either using a wireless LAN (range up to 40 m) or direct cables. The complete test system can either be powered from its internal batteries, from a 110/240 V AC mains supply or from an external 12-32V DC supply, allowing it to be powered from the aircraft's own 28 V supply, if required.

To set up the system on an aircraft takes ten minutes. The transducers are not installed permanently on the engine but are mounted for each test using screw fixings on permanently installed special brackets in predetermined positions.

The number of tacho and vibration signals used depends on the type of engine being tested. With the M53, one tacho and three vibration signals are monitored. The Larzac engine test uses two tacho and two vibration signals. The accelerometers are piezoelectric high temperature single axis types with balanced differential output. They are calibrated once per year and the user interface displays the system calibration parameters. Brüel & Kjær is a major supplier of transducers to the Snecma group.

The actual test takes about five minutes. A mechanic in the aircraft cockpit starts the engine and it is run up to a predefined speed and then run down. Monitoring green or red indictors on the display, the test technician can immediately see if the engine is performing normally, and the aircraft is safe to fly, or if unexpected vibration is present.

The test data is recorded on the PC's hard disk for standard post-processing analysis in the laboratory, or detailed investigation by a specialist. The test data also forms an important part of the test and performance record of the individual engine.

Details available from: Bruel & Kjaer UK Limited. Tel: +44 (0)1438 739000; Fax: +44 (0)1438 739099.