This paper concentrates on the MESA Sine on Random control per MIL STD 810H, method 514.8, annex D, 2.3 category 14. This category governs Sine on Random control for helicopters.

Crystal Instruments designed and released the Spider-80M Multiple-Exciter Single-Axis (MESA) vibration controller in 2018. Further details regarding the MESA testing system are available on the Sentek Dynamics website. Detailed product specifications may be requested from the Crystal Instruments team.

MIL STD 810H
Method 514.8, annex D, 2.3 category 14 discusses the vibration testing of a rotary wing aircraft – helicopter in detail. Any materiel included as part of the helicopter’s physical structure is required to be tested according to this method.

Helicopter vibration is characterized by dominant peaks superimposed on a broadband background, as depicted in Figure 514.8D-4. The peaks are sinusoids produced by the major rotating components (main rotor, tail rotor, engine, gearboxes, shafting, etc.). The peaks occur at the rotation speed (frequency) of each component (i.e., 1P for main rotor, 1T for tail rotor, and 1S where S designates a locally predominate rotating element) and harmonics of these speeds (e.g., 2P, 3P, 4P). The broadband background is a mixture of lower amplitude sinusoids and random vibrations due to sources such as aerodynamic flow noise (see paragraph 2.1). Vibration levels and spectrum shapes vary widely between helicopter types and throughout each helicopter, depending on strength and location of sources and the geometry and stiffness of the structure. Thus, the need for measured data is acute.

The preceding graph defines the broadband random profile and sine tones. Meanwhile, the rotation speed of the main rotor and tail rotor are dependent on the model of the helicopter. The dominant sinusoids are generated by rotating components of the helicopter, primarily the main rotor(s), but also tail rotor, engine(s), drive shafts, and gear meshing. The normal operating speeds of these components are generally constant, varying less than five percent.

The following table lists these rotation speeds.

Table 514.8D-IIIa - Category 14 - Helicopter parameters. (Same as Annex C, Table 514.8C XIVa.)

The vibration exposure levels are defined in the following table.

Table 514.8D-IIIb - Category 14 - Helicopter vibration exposure. (Same as Annex C, Table 514.8C-XIVb.)

Overall, the upper frequency for the broad band random is 500 Hz; with the exception for the drive system elements, which is 2000 Hz.

The preceding drawing illustrates the vibration zones on the helicopter. In cases where the materiel goes to the body, the main rotor source frequencies are predominating.

Test Configuration and Setup
The following photo depicts a dual shaker vertical configuration, which is controlled using MESA vibration control.

Note that the UUT is rigidly mounted on top of both shaker heads.

Another dual shaker vertical case is depicted in the following photo.

The UUT is mounted on top of double spherical joints to physically allow out of phase motion.

Dual shaker systems are commonly configured as vertical, horizontal push-push, or horizontal push-pull.

MESA Sine on Random VCS Solutions
MESA Sine on Random vibration control utilizes a defined reference profile. A weighted average control strategy is typically associated with it.

The following options are available for control mode:

Same mag/phase;
Different phase;
Different mag/phase.

Underneath the MESA vibration control engine, the implemented minimum energy method manages multiple drives to achieve control on the weighted averaged control. In addition, the phase optimization helps to further fine tune the drives and achieve ultimate control.

Based on the helicopter model, the corresponding testing parameters can be determined following the aforementioned MIL STD 810H method 514.8D, Category 14. When considering the general materiel used in an AH-64 helicopter, the broadband random and sine tones are determined as described below.

For broadband random, the upper frequency is 500 Hz, while the levels are 0.001 and 0.01 g^2/Hz.

With a rotor speed of 4.86 Hz, the sine tones are as follows:

Frequency: f1= 4.86 Hz, f2 = 19.44 Hz, f3 = 38.88 Hz, f4 = 58.32 Hz
Level: A1 = 0.136 g, A2 = 0.486 g, A3 = 2.5 g, A4 = 1.5 g

The following screenshot depicts sine tones and broadband random under great control on the dual shaker vertical setup. Users can also execute this test with a different mag/phase mode.

For details regarding the CI Spider-80M MESA vibration controller, visit Crystal Instruments website: https://www.crystalinstruments.com/mesa-vibration-control-overview

Reference:
Mil Std 810H 31 January 2019

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