Authors: Eng. Potito Cordisco, Senior Project Manager, Vicoter
Eng. Mauro Terraneo, Chief Technical Officer, Vicoter
General aviation regulations for helicopters requires the applicant to verify the critical parts of his helicopter under fatigue tests, besides the static ones.
This implies that the loads the helicopter will sustain during the different phases of its life must be known. To know those loads, a simple analytical approach is not enough, so their measurement is mandatory.
While the measurements taken on not rotating parts of the helicopter are pretty easy, measuring loads acting on the rotor hub or on the rotor blades is not that simple because of the rotation.
Generally, these measurements require strain gauges to be installed on the structure and the generation of relationships between the strains and the loads. Anyway, strain gauges require ad-hoc devices to create a Wheatstone bridge, to power supply the bridge and to amplify the readings.
The most common approach consists, therefore, in using a slip-ring, that is an electromechanical device allowing the transmission of power towards the strain gauge bridge (rotating) and the electrical signals from them to the stationary.
However, slip-rings have several problems. First of all, they usually do not rotate smoothly, especially in environments with strong vibration (and the helicopter is just the case), and this can cause damage to the thin-walled bearings in the slip ring, or cracking of the plastic spindle.
Secondly, the electrical interference of the power supply on the signals is usually not negligible, which causes signals to be processed with very low signal/noise ratios.
Finally, slip rings are expensive, being tailored on a specific helicopter, and their installation requires a long time.
To avoid forementioned problems, Vicoter (www.vicoter.it), an Italian company specialized in testing in the aerospace and automotive fields, developed a special system named VR-DAQ (Vicoter Rotating Data AcQuisition system). The device can power supply, gain, acquire and store up to five Wheatstone full bridges. The maximum gain is equal to 125, the power supplied is equal to 4 V on each channel and the maximum sampling frequency is 80 Hz. The device is power supplied by a 9 V battery and able to save data directly in the rotating system on a SD card. It can be easily installed on the mast of any helicopter after the design and manufacture of a dedicated interface plate.
Thanks to this device, Vicoter was able to measure the loads acting on the ESCAPE helicopter manufactured by Lamanna Helicopter (LH – https://www.lamannahelicopter.com/) during the full flight envelope. The request was to measure the loads acting on one of the two main rotor blades, the load in the three command bars and the strain in four points of the main rotor hub.
Essentially the work consisted of four phases:
- Installing the strain gauges on one of the main rotor blades, on the three command bars and on the rotor hub.
- Calibrating the consequent Wheatstone bridges in laboratory conditions in order to obtain the link between the external loads and the bridge unbalancing.
- Performing the flight test campaign.
- Post processing measure data to extrapolate the loads necessary to carry out the fatigue analyses on the structure.
Phase 1 – Strain gauge installation.
Approximating the blade to a beam, a bending moment generates a flexural strain (εf) that is positive (i.e., tension) on the lower surface and negative (i.e., compression) on the upper surface.
Additionally, both the surfaces can be subjected to the same thermal strain (εT). The unbalancing ΔV of a full-bridge is linked the previous strains by the well-known formula:
Where k is the gauge factor of the used strain gauges, V is the supply voltage of the bridge, Mf the bending moment, E the Young’s modulus, y the distance from the neutral axis and J the inertia of the section.
Consequently, a bending moment acting on a rotor blade during the flight can be measured using the unbalancing of a single full-bridge.
On the ESCAPE’s blade, two Wheatstone’s bridges in full-bridge configuration were installed: one for measuring the flapping moment and one to measure the lag moment.
Each full-bridge was composed by four strain gauges opportunely connected, to measure the moment and to compensate temperature effects.
Regarding the command bars, since each bar is hinged at its ends, the only possible force transmitted is the axial load. Consequently, a single T-rosette was placed on each bar, with the 0° direction aligned with the bar main axis: three half bridges were in total installed.
This configuration allows to measure the axial strain (εa) and to compensate the thermal strain (εT).
Indeed, the unbalancing ΔV of the half-bridge is linked to the applied axial load by the formula:
Where k is the gauge factor of the used strain gauges, ν is the Poisson’s module of the bars’ material, V is the supply voltage, L is the load and A is the area of the bar cross section.
Finally, regarding the rotor hub, four uniaxial strain gauges were placed in the maximum stressed area (individuated thanks to FEM analyses performed by Lamanna Helicopters), with the purpose to measure the in-flight strains on the structure. These strain gauges were acquired as quarter of bridge during flight tests, and, from their measurements, the strain was easily obtained according to the formula:
Where Gain is the gain used in the acquisition system, k is the gauge factor of the strain gauge, V is the power supplied to the strain gauge and ΔV is the unbalancing of the strain gauge during flight test.
Before flight tests, all the strain gauges were protected with tape and resin to avoid damages to the bridges.
Phase 2 – Bridge calibration.
After the strain gauge installation, the bridges were calibrated.
The process consisted in:
- Applying a known load (bending moment for the blade and axial load for the command bars) to the structure instrumented with strain gauges.
- Acquiring the output from the Wheatstone’s bridges.
- Obtaining the sensibility matrix linking the applied loads and moments to the bridges unbalancing.
This activity was carried out in laboratory conditions.
Phase 3 – Flight tests.
During flight tests all the ESCAPE’s flight envelope was explored, as prescribed by regulations.
Phase 4 – Measured data post-processing.
At the end of flight tests, data were collected from the VR-DAQ SD card and post-processed with a dedicated software developed in MATLABTM. Example of obtained graphs can be seen hereafter.
Obtained data were used by Lamanna Helicopter to verify under fatigue tests the main rotor hub, the main rotor blades and the command bars.
Vicoter warmly thanks Roberto Lamanna and Alessandro Berion for their support during the activity and Professor Marco Morandini from Politecnico di Milano for the help in the development of the VR-DAQ.
