Authors: Potito Cordisco, Senior Project Manager; Mauro Terraneo, Chief Technical Officer.
Vicoter has performed in flight load measurements on the ESCAPE is an ultra-light helicopter produced by LH-Lamanna Helicopter.
General aviation regulations for helicopters requires the applicant to verify the loads used in design and static/fatigue tests of the critical parts by in-flight load measurements.
Critical parts include:
- The rotor, including the blades and their attachments.
- The rotor head.
- The control system, from the control stick/pedals to the rotor.
- The tail boom.
Vicoter, after conducting tests on the rotor and control rods (see the other our article: In-flight load measurements on helicopter blades), has completed the test campaign by also measuring the loads acting on the tail, in order to validate the values to be used in static and fatigue tests.
The two bending moments, My and Mz, near the junction with the cabin, were acquired using two Wheatstone bridges in half-bridge configuration. Each half-bridge consists of two strain gauges properly connected to be sensitive to the desired moment and to compensate for axial and temperature effects. In total, four Micro-Measurements CEA-13-125-UWA-350 strain gauges were installed on the ESCAPE’s tail. The strain gauges were installed with the reading-sensitive direction along the tail span, at the ends of the vertical and horizontal section diameters, to maximize the distance from the neutral axes and signal quality. They were placed about 110 mm from the beginning of the tail (on the fuselage side) to avoid edge effects introduced by proximity to the constraint with the rest of the helicopter.
In this case, since it was necessary to measure not only the strain state at certain points but also the internal actions, namely the moments, a calibration procedure for the bridges was required. Through calibration, it was possible to experimentally evaluate the relationship between the bridge’s voltage imbalance—the value actually read by the instrumentation during flight—and the load causing it.
The calibration was performed by applying known forces at known distances from the strain gauges: an Fz force to generate the My bending moment and an Fy force to generate the Mz bending moment. More than one weight was used along each load direction, and the sensitivity coefficients, including cross-sensitivity terms, were evaluated by solving the equation in a least squares manner.
The flight was performed at the helicopter’s maximum takeoff weight. During this test, 16 manoeuvres, previously agreed upon with the authorities were carried out in order to measure the load values under the most severe conditions. The approximate duration of the flight was 20 minutes.
The unbalancing of the two Wheatstone bridges was measured using a Micro-Measurement 7000-32-SM scanner equipped with four strain gauge input cards 7003-8-SG, each with eight channels, positioned on the co-pilot’s seat. It is important to emphasize that during the flight, the acquisition unit was powered by a 12 V sealed lead acid battery to avoid any connection to the aircraft’s electronics that could cause malfunctions in the helicopter itself.
Synchronization of the instrumentation with the flight manoeuvres was done using a chronometer.
Once on the ground, the required moments were obtained through post-processing. Mean, maximum, minimum, and RMS values of the signals, as well as their trends, were calculated for each flight phase and used for the validation of the numerical analyses performed. Thanks to the high sampling rate used, 2048 Hz, it was also possible to visualize the load variation during each rotor revolution and appreciate the contribution of the harmonics at 1x and 2x.
Vicoter warmly thanks Roberto Lamanna and Alessandro Berion for their suggestions and support during all the activity.
