News & Events

World Class Analyser Takes Off - A Case Study at Eurocopter UK

February 2010

A&D Instruments Ltd is proud to announce the purchase of the WCAmini portable FFT analyser by Eurocopter UK Ltd.

Eurocopter UK Ltd is located in Oxfordshire and has over 30 years experience in helicopter sales, bespoke completions, maintenance and spares support. Its in-house engineering capabilities include design, installation, flight-testing and certification. The company is also able to offer a completion service to further adapt or modify helicopters to meet the most demanding specific mission requirements.

The application was for the measurement of Helicopter Payload Vibrations.

External payloads mounted to helicopters, such as a searchlight or cameras are subjected to vibration excitation. The frequency of this vibration is dominated by the blade rotational frequency and the number of blades fitted to the rotor hub. The blade rotational frequency (rotor hub) is typically fixed and around 400-500rpm.

On a 4 bladed rotor operating at 400rpm, this would give a dominant frequency of :
(400/60) x 4 = 26.6Hz

The severity or shock of the vibration (g) will also be influenced by the design of the rotor hub depending on the amount of damping fitted. High agility aircraft have rigid rotor hubs where passenger aircraft have semi-rigid rotor hubs incorporating a vibration damping mount between the blade and rotor hub.

The level of vibration experienced by the external payload will have an effect on the lifetime of operation, such as bulb life in a searchlight or quality of picture from an external camera.

Ground Vibration Test (GVT)

First the FRF, frequency response of the payload is measured when mounted to the helicopter. From this the natural frequency of the payload and its associated frequency of responses can be measured. The aim is to ensure that these frequencies do not coincide with the dominant frequency of vibration or harmonics thereof of the helicopter. Using the 4 channel WCAmini with an impact modal hammer and measuring the vibration response simultaneously in all 3 axis x y z achieve this. The WCApro real time analysis software allows the engineer to quickly verify his measurements are correct and more importantly repeatable.

Flight Testing

The payload vibration performance can then be measured. Using the Throughput to Disc feature of the WCAmini each phase of the flight test envelope can be individually recorded in all 3 axis x y z. This increases the accuracy and reduces the flight time testing requirements as all the measurements are recorded in a single flight. The vibration data can then be post-processed and compared to the required DO160 limits as required.

Field Performance

Airworthiness Engineers at Eurocopter UK Ltd found the WCAmini has met all of their expectations, especially :

• The ease of operation of the WCAmini
• The portability of the equipment when being used in conjunction with PCB Piezotronics ICP modal hammer and tri-axial sensor (The complete equipment is powered by the Laptop USB drive)
• The repeatability of vibration measurements
• Flight data recording with the Throughput to Disc feature
• The time saved in flight testing by using simultaneous 3 axis measurements.

It was commented "The WCAmini has provided us with fast, accurate and repeatable measurements and has also reduced our helicopter operational flight time required for complete flight envelope testing."

The experience of Eurocopter UK Ltd has proven the WCAmini FFT Analyser to be an invaluable tool, saving both time and money, by providing a wealth of data in a very short timeframe. These savings can be transferred across a wide variety of testing and data acquisition scenarios. For further information on the WCAmini and all of the A&D product range please visit www.aandd-eu.net or www.centrateq.com.

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