Measuring aircraft structure using the MSR165 data logger

During the aircraft design process, designers must envision the aircraft's purpose in advance to predict the static and fatigue loads the aircraft will be subjected to. For military aircraft, they often draw on experience from previous aircraft types to develop analog roles or functions to set future roles for the aircraft (Figure 1).

Figure 1, Test aircraft.

When an aircraft enters military service, the structural usage assumptions must be validated to ensure the structural integrity of the aircraft. Validation for the UK Ministry of Defence (UK MOD) is undertaken by the Military Aviation Authority (MAA).

However, traditional verification procedures are very expensive, which continues to increase pressure on defense spending across Western governments, so the lowest cost solution must be developed to meet the requirements. To this end, the Ministry of Defense Aircraft Project Team at the UK Ministry of Defense’s Defense Science and Technology Laboratory (DSTL) worked with Ministry of Defense contractors to conduct a series of laboratory tests and flight tests using the Modular Signal Recorder data recorder produced by Swiss MSR Electronics and provided by Caption Data Co., Ltd. , and has now captured basic structural usage information on several aircraft fleets.

The rationale for choosing the MSR165 was its small size, light weight, battery operation (reducing the aircraft integration task), flexibility, and cost-effectiveness. The MSR165 uses a 3-axis acceleration, pressure, temperature, and relative humidity sensor (Figure 2).

Figure 2. MSR165 data logger.

The normal acceleration (NZ) in the raw data is used to evaluate the structural performance of fixed wing aircraft. In addition, the pressure data is used to evaluate the altitude during flight and help determine takeoff and landing times. The MSR165 is located in the unpressurized area of ​​the aircraft.

The key to the success of this solution is to keep the technicians’ operations of the aircraft to a minimum. Therefore, the MSR165 is configured to automatically record acceleration at 50 samples per second from engine start to engine shut-down, while also continuously recording pressure, temperature and relative humidity data at one sample per second each. This configuration can be adjusted by the technicians on board the aircraft and maintain full autonomy for up to 75 hours or 2 months of flight time.

Typical graphs of normal acceleration and altitude estimates, and pressure data from two different aircraft are shown in Figures 3, 4 and 5 (close-up at 40 seconds). After 2 months or 75 hours of flight time, the technician replaces the MSR, which is returned to the Defense Science and Technology Laboratory (DSTL) for analysis and comparison with design assumptions, fatigue test spectra or previous service use. The 50 sample acceleration data captured per second is passed through a low pass finite impulse response frequency filter (11Hz to 1Hz). This allows the data to be compared with data recorded using a Royal Aeronautical Establishment (RAE) fatigue meter (bandwidth 11Hz). In addition, the 1Hz low pass data is used to determine the maneuver content of the signal.

Above are typical graphs of normal acceleration and altitude estimates from pressure data from two different aircraft, as shown in Figures 3, 4, and 5 (close-up at 40 seconds).

The program has been very successful and to date, flight trials have been carried out on two small UK Ministry of Defence fleets (Islander and Defender) and two historic aircraft platforms (Battle of Britain Memorial Flight Lancaster and Royal Navy Historic Flight Swordfish). During these trials, nearly 100 hours of flight data have been acquired and the data acquired has been used for further investigation. Currently, several fleet-wide adjustments and further trials are being planned (Figure 6).

Figure 6. The programme has been flight tested on two small UK Ministry of Defence fleet and two historic aircraft platforms.