Active Pitch Link

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The Active Pitch Link

The Active Pitch Link (APL) is the first novel, patented, and only piezoactuator-based, all-electric, high frequency active pitch link in the world. The APL offers a reduction of vibration up to 60% in a fail-safe manner and its easy scalability and diverse applicability opens a wide range of applications. The APL is the only existing active control system designed deliberately to alter the blade root stiffness for vibration reduction in helicopters.

The Active Pitch Link, which is based on the smart spring concept, is designed to replace the conventional pitch link in helicopters and aimed to be used for four purposes:

  1. To control pitch link vibrations.
  2. To control the root stiffness of the blade for reducing rotor (and not just pitch link) vibrations.Read more…
  3. To enhance the effectiveness of a blade-based flow control device (such as the Actively Controlled Flap (ACF) or an Active Twist Rotor (ATR) blade) for reducing rotor vibration. (Note that these flow control devices require LOW blade torsional stiffness for enhanced effectiveness when they work in the “aeroservoelastic mode”. A permanently low blade torsional stiffness is undesired but it can now be achieved for short periods of time–when it is desired and safe–via the APL.)
  4. To simultaneously reduce vibration and noise, when combined with a flow controlled device (such as the ACF or ATR). (Note that with one system only–such as the ACF–when vibration is reduced, noise goes up and vice versa. However, a common theorem of Control Theory is that for two control objectives, one needs two independent control systems. Equipping a blade with both the APL as well as the ACF provides two independent control systems and thus promises to reduce vibration and noise simultaneously. In other words, while the flap scheduling is set to reduce vibration, the pitch link schedule can be set to reduce noise.) Read more…

Based on state of the art computational models developed over several years of research, the form factor and performance characteristics of the APL can be easily scaled to meet different sizes and use requirements without any compromise to performance. The APL is designed to be Fail Safe, meaning in case of actuator failure the APL transformes into a conventional solid pitch link and does not increase a risk to operate a helicopter safely.

The most sophisticated and compact 3rd generation APL was designed and manufactured in February 2011. The first closed-loop control tests were conducted with this device shortly after, showing as much as 55% reduction in the pitch link vibratory loads. Experimental non-rotating test results of the 3rd generation APL are presented below (click on graphs to enlarge):

Time history of pitch link loads during Static Testing
Frequency spectrum of pitch link loads during Static Testing

A year later, in February 2012, the SRS Inc.’s team has completed over 5 hours of the APL testing at the Whirl Tower Test Facility and confirmed as much as 80% of vibration reduction of the 2/rev [N+1/rev] vibrations. An implemented fan-system, with 14 m/s upwash with 20 degrees azimuth rotor radius, generated periodic controlled vibrations, in the rotating frame, to be reduced. The test results, presented in the graph below, show that while the 1/rev vibratory load (corresponding to cyclic control in our system) remains largely unchanged, the 2/rev vibration is reduced by as much as 80%.

Frequency spectrum of pitch link loads during Whirl Tower Testing
Frequency spectrum of zoomed in 2/rev pitch link loads during Whirl Tower Testing
3rd generation APL installed in the Whirl Tower Test Facility
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