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Monday, November 1, 2004

Tool Crib

Eurocopter Offers Smart Rotor-Tuning Tool

Eurocopter is now offering a new tool called Steadycontrol, a maintenance tool that optimizes the tuning of rotors and anti-vibration systems. Steadycontrol is available on the manufacturer's entire range of products, except the EC145, which is not yet certificated. Thanks to the so-called "neural" technique, Steadycontrol should greatly ease rotor tuning after maintenance.

On every helicopter, vibration reduction work focuses on the "b times omega" frequency, where b is the number of blades and omega the rotation frequency of the main rotor. This is the main vibration mode. On the EC 225 Super Puma medium twin, for example, "b omega" is close to 22 Hertz. A new generation of active anti-vibration control systems (AVCS) has been developed for this five-blade-rotor helicopter.

However, "once the main vibration frequency has been attenuated, aircraft occupants can feel the other ones, which are proportional to the rotor's rotation frequency: 1 times omega, 2 times omega, ... (b-1) times omega where b is the number of blades," Tomasz Krysinski, dynamics manager, explained to Aviation Maintenance during a visit to the European manufacturer's headquarters in Marignane, near Marseilles, France early in September.

One-omega vibrations bring significant discomfort as it is generally close to the human body organs' natural frequency. For example, that of the liver is somewhere between 5 and 10 Hertz. One-omega vibrations (4.4 Hertz on the EC 225) result from tiny differences between the blades. The manufacturing process involves stricter and stricter checks but this "non-isotropy" still exists. Current tracking and balancing work minimizes one-omega vibrations, but only this one. Moreover, it involves a stroboscope, a camera, and multiple check flights.

According to Krysinski, Steadycontrol is able to minimize one-omega vibrations and two-omega, three-omega, etc. as well. Thanks to the neural network technique, the new tool can learn how to optimize rotor tuning. It then can optimize the balance using tabs, pitch rod adjustments, and weights on rotor sleeves.

Although the word "neural" seems to hint at a similarity with the human brain, this technique is even superior. It can determine quickly how much of each method needs to be used to get the optimal result, although each can influence one or several vibration modes. In a single flight, the system measures the vibrations and learns the effects that tabs, pitch rods, and weights have on the helicopter's track and balance. This is done in four well-defined aircraft configurations: on the ground, in hover, at cruise speed level flight, and at high cruise speed. Right after the flight, the tool provides the maintenance technician with the data for optimal settings. "Instead of six flights, only one is necessary," Krysinski said.

Steadycontrol can be used when the helicopter is rolled out of the factory and every time a modification is made that can influence balancing. Only two hours are needed to have a maintenance technician trained. Early in September, some 20 Steadycontrol tools were in service.

-- By Thierry Dubois

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