Detail of GB/Z 41305.7-2023

Introduction of GB/Z 41305.7-2023

GB/Z 41305.7-2023 Environmental conditions - Vibration and shock of electrotechnical equipment - Part 7: Transportation by rotary wing aircraft 1 Scope This document, reviews the available dynamic data relating to the transportation of electrotechnical equipment by rotorcraft (helicopters). The intent is that from all the available data an environmental description will be generated and compared to that set out in IEC 60721 (all parts)[5]1). For each of the sources identified the quality of the data is reviewed and checked for self-consistency. The process used to undertake this check of data quality and that used to intrinsically categorize the various data sources is set out in IEC TR 62131-1[9]. This document primarily addresses data extracted from a number of different sources for which reasonable confidence exist in its quality and validity. This document also reviews some data for which the quality and validity cannot realistically be verified. These data are included to facilitate validation of information from other sources. This document clearly indicates when utilizing information in this latter category. This document addresses data from a number of data gathering exercises. The quantity and quality of data in these exercises varies considerably as does the range of conditions encompassed. Not all of the data reviewed were made available in electronic form. To permit comparison to be made, in this assessment, a quantity of the original (non-electronic) data has been manually digitized. 2 Normative references There are no normative references in this document. 3 Terms and definitions No terms and definitions are listed in this document. 4 Data source and quality 4.1 Vibration of Boeing CH-47 rotorcraft A number of measurement exercises have been undertaken on the Boeing CH-472) rotorcraft, of those the measurements presented in [14] and [15] are typical. Many measurement exercises have focused on the vibration responses of carried goods, passengers and crew. However, the measurements of [14] and [15] were made specifically to characterize the vibration responses of the payload deck area within the rotorcraft. The Boeing CH-47 rotorcraft is a twin rotor, twin engine heavy lift aircraft which first entered service in 1961 . Although it is designed as a military aircraft, a number of commercial variants exists and those versions are widely used for the transportation of large or heavy equipment. They are also typically used to transport items to locations difficult to access by other means. The CH-47 is known by a number of different names including Chinook, Model 234 and Model 414. Also different designations arise indicating variants of the original design. The particular rotorcraft used in the measurement exercise was typical of most Boeing CH-47 variants with twin rotors each comprising three blades. The rotor shaft speed is around 225 rpm (3.75 Hz) giving a rotor blade passing frequency of 11.25 Hz. The Boeing CH-47 was one of the fastest rotorcraft available when it first entered service and even today it is still amongst the fastest rotorcraft in commercial use. As rotorcraft vibration severities are strongly related to aircraft speed, an aspect which will be discussed later, the Boeing CH-47 is often used to set rotorcraft vibration severities for the transportation of equipment. The cargo bay area of the Boeing CH-47 extends from frame 120 which is located just aft of the plane of the forward rotor to frame 482 which is located just forward of the plane of the aft rotor and attachment location of the twin engine. Frame 320 is located approximately in the centre of the length of the cargo bay area. Rotorcraft generate a dominant vibration severity which commonly coincides with sensitivity of the human body to vibration. Indeed prolonged exposure to some rotorcraft vibrations can exceed recommended daily dosage to such vibrations. As a consequence, many rotorcraft vibration measurement exercises are aimed at quantifying human body exposure. However, the sensitivity of the human body to vibrations is predominantly biased towards the low frequencies, which are well below the frequency range normally considered for the testing of electrotechnical equipment. As such, measurement exercises made to quantifying human body exposure are mostly unsuitable for the purpose of this document. Moreover, a rotorcraft of concern from the viewpoint of human body exposure may not necessarily be of concern from the viewpoint of electrotechnical equipment. This is because the sensitivity of the human body is biased towards certain low frequencies. The measurements of [14] and [15] on the Boeing CH-47 rotorcraft comprised twelve piezo-electric accelerometers and associated charge amplifiers. The vibration measurements were recorded on a 14-channel FM recorder. The system provided an effective measurement frequency range of 2.5 Hz to 2500 Hz. The accelerometers were arranged in four mostly tri-axial groups placed on the cargo bay floor, along its length on the starboard side. Separate flights vibration measurements were additionally made on two payloads, each of approximately two tonne, carried within the cargo bay area. All the transducers were internally mounted on relatively stiff airframe locations. Measurements were made during several flights and during a range of different flight conditions. Typically, vibration measurements on rotorcraft are made during a range of different steady state conditions. Such steady state conditions include hover and a variety of straight and level flight speeds at different altitudes. Additionally, vibration measurements are commonly made during a variety of transient flight conditions. Such transient conditions include take-off, landing, transition to hover as well as transition to autorotation. Some of these transient conditions occur at some time on most flights whereas other conditions (such as transition to autorotation) may only be used in emergency or training situations. Transient conditions can be difficult to measure but can give rise to quite severe vibration severities. Steady state and transient vibration conditions can arise due to a number of mechanisms which are addressed in Clause 7. The measurements of [14] and [15] on the Boeing CH-47 rotorcraft were analysed mostly in the form of acceleration power spectral densities (PSDs), although very few of these are presented in the reports referenced. Neither of the two reports indicates the record duration used for the power spectral density analysis. However, the analysis durations, typically used by the agency that made these measurements, is around 30 s for steady state conditions. With that said, durations will be more limited for the transient flight conditions and usually limited to the duration of the events, some of which only occur for a few seconds. The approach used to quantify the vibration amplitudes at the rotor shaft, blade passing frequency and their harmonics, is a particular data analysis issue encountered when addressing rotorcraft vibration data. In this case the frequency analysis bandwidth is around 2.5 Hz. Whilst this is adequate to describe the broadband background vibration induced by rotorcraft, it is generally regarded as inadequate to quantify, in terms of power spectral density amplitude, the tones arising from the rotor blade passing frequency and the associated harmonics. For this reason the tones arising from the rotor blade passing frequency and subsequent associated harmonics, are quantified in terms of root mean square (RMS) values. The usual approach used by this measurement agency, was to compute the tonal component root mean square by integration of the power spectral density amplitudes for each tonal component. Reports [14] and [15] indicate that peak hold spectra were used (rather than the "average" power spectral density values) to estimate the amplitudes at rotor and blade passing frequencies. Reports [14] and [15] present power spectral densities for selected flight conditions only. A number of these are reproduced in Figure 1 to Figure 4. These include straight and level flight at the rotorcraft's typical best sustained flight speed, during hover as well as during transient events of transition to hover and transition to autorotation. The reports mostly present severities in terms of root mean square values at rotor speed (3.75 Hz), the first harmonic of rotor speed (7.5 Hz), rotor blade passing frequency (11.25 Hz) and the next seven harmonics of rotor blade passing frequency (22.5 Hz, 33.75 Hz, 45 Hz, 56.25 Hz, 67.25 Hz, 78.5 Hz, 90 Hz). The reports also present the overall of root mean square values (2.5 Hz to 2000 Hz). Some of this information is presented in this document as Figure 5 to Figure 14. Compared in Figure 5 to Figure 10 are root mean square values for different flight conditions and for three locations along the floor of the cargo bay floor. The figures separately illustrate and compare the values of the overall of root mean square (2.5 Hz to 2000 Hz), at rotor speed, blade passing as well as the second, third and fourth harmonic of blade passing. It should be noted that the overall root mean square value is that with the primary tonal values removed, i.e. it is a measure of the broadband background vibration. Compared in Figure 11 to Figure 14 are root mean square values for different cargo bay floor locations and axes. The comparisons are made for the same four selected flight conditions for which power spectral densities are presented in Figure 1 to Figure 4. Although the information in this document is limited, the quality of the information is reasonable and meets the required validation criteria for data quality (single data item). 4.2 Set down of underslung cargo from a Boeing CH-47 rotorcraft Although the Boeing CH-47 rotorcraft has a significant sized internal cargo area, it is not uncommon to transport bulky items as underslung loads. In such cases the load may be attached by cables or nets to release hooks on the underside of the rotorcraft. Although the Boeing CH-47 rotorcraft has several such release hooks, it is common to utilize a single hook for most single items. It should be noted that when underslung loads are carried by smaller rotorcraft they may use two, three or even four point attachments. In all cases, it is possible for the dynamic responses of the cargo and its suspension arrangement to interact with the dynamics of the rotorcraft inducing increased dynamic loads in the attachment arrangement and hence the cargo/rotorcraft. It is not unknown for failure of such arrangements to occur during flight. Following the measurement exercise described in 4.1, further work [15] was undertaken to establish set down shock conditions of underslung loads. For the purpose of this work an air portable ISO container was suspended (at its upper attachments) by four cables to a single point payload release hook under the rotorcraft fuselage. The air portable ISO container was a 10- foot-(3 m) long unit (i.e. half the length of a standard TFU container) holding a two-tonne payload. The pilot instructions were to perform representative set downs of the container. It was set down onto a hard concrete surface, as far as practicable, on its four corner stacking points using "realistic" rotorcraft decent velocities. The suspended load was instrumented using the same equipment as described in 4.1. However, in this case the two trial-axial accelerometers were located at each of the lower four corners of the air portable ISO container. The system provided an effective measurement frequency range of 2 Hz to 250 Hz with a subsequent acquisition rate of 1000 samples per second (sps). Measurements were made throughout the set down and the specific event subsequently extracted for shock analysis. The analysis was in the form of time histories (which are not suitable for reproduction here) and shock response spectra (SRS). The time histories used for the shock response spectrum calculations were of approximately 1 s duration and adopted a resonant gain or Q of 16.66 to facilitate comparison with some historic US data. Although the measurement exercise encompassed twelve separate set downs of the underslung payload, not all of these provided data of suitable quality for subsequent analysis. Figure 15 show the shock response spectra for six set downs. The figure includes the vertical responses from both instrumented lower corners of the air transportable ISO container. The shock response spectra for six set downs imply that the set down velocities were in the range of approximately 0.2 m/s to 1 m/s. This broadly aligns with broader experience of setting down underslung loads on land or stationary vehicles. The set down velocities would typically be greater when setting down underslung loads onto ships. Although the information in this document is limited in quantity and frequency range, the quality of the information is reasonable and meets the required validation criteria for data quality (single data item). 4.3 Supplementary data The supplementary data, detailed below, comprises information arising from reputable sources, but for which the data quality could not be adequately verified. A study, [16] and [17], was undertaken to identify parameters and methodologies by which estimates could be established of the vibration severities experienced by weapons carried on rotorcraft. Although the objective of this work related to externally carried weapons, it did first need to consider the vibration severities of the rotorcraft itself. The work did not present measured rotorcraft data directly, hence its consideration as supplementary data. However, the work did present information related to parameters which influence vibration severity for several rotorcraft types. Figure 16 to Figure 18 show the relative vibration amplitude at blade passing frequency for different airspeeds (referenced to 100 kn) and for three different rotorcraft i.e. the Lynx, Seaking and Chinook (CH-47). These are respectively small, medium and large rotorcraft. This information indicates that whilst the most severe vibrations occur at the highest speed for the CH-47, that is not necessarily the case for the other types. The study also presented (see Figure 19) aircraft to aircraft variations that occur between different airframes of the Lynx rotorcraft. Early editions of UK Defence Standard 00-35[18] presented some of the information from the study [16] and [17] addressed in the preceding paragraph. Later editions have replaced the rotorcraft to rotorcraft information shown here in Figure 19 with more extensive information from a more modern rotorcraft.

Contents of GB/Z 41305.7-2023