Field of the Invention

This invention relates to rotor blades and is application with particular advantage to helicopter rotor blades.

Background to the Invention

Optical fibre strain sensors are known and patent publication WO 2004/056017 discloses a method of interrogating multiple fibre Bragg grating strain sensors along a single fibre. In the system of WO 2004/056017, Bragg gratings are defined in the optical fibre at spaced locations along the optical fibre. When the optical fibre is put under strain, the relative spacing of the planes of each Bragg grating changes and thus the resonant optical wavelength of the grating changes. By determining the resonant wavelength of each grating, a strain measurement can be derived for the location of each grating along the fibre. Optical strain sensors operating on the principle of back scattering which do not require discrete gratings along the fibre are also known.

It is desirable to monitor the loads on rotor blades, especially helicopter rotor blades, for safety reasons and also to monitor the performance of the blade. Advantageously, the loads on rotor blades due to impacts can be monitored and quantified. However, applying sensors to monitor loads to the outside of the rotor blade may affect the aerodynamic properties of the blade and the sensors themselves are exposed to damage from impacts with foreign bodies. Furthermore, the sensors cannot be contained within the blade, as with hollow wind turbine rotor blades, because helicopter blades are of closed section and getting the sensor out is difficult. Whilst it is possible to embed the sensor within the blade, this may change the structural properties of the blade and a blade with an embedded sensor would significantly increase the recertification cost for the blade. Furthermore, should the sensor become damaged, the whole blade would need replacing.

Summary of the Invention

Accordingly, the present invention comprises a method of manufacturing a helicopter rotor blade comprising the steps of; applying at least one optical fibre strain sensor to the leading edge of the rotor blade body; and fitting an erosion shield to the leading edge of the rotor blade body, over the optical fibre sensor.

Helicopter rotor blades have sacrificial erosion shields that are externally bonded around the leading edge. The erosion shields extend the life of the blade and are routinely replaced once they have experienced a significant amount of wear. Positioning an optical fibre strain sensor between the blade body and the erosion shield does not interfere with the structural or aerodynamic properties of the blade and also allows easy access to the sensor for maintenance purposes. Should the optical fibre break in two or more places, a new fibre can be fitted by removing the erosion shield and bonding a new fibre to the blade body. Should the optical fibre strain sensor break in just a single position, this can be catered for by terminating both ends of the fibre.

Advantageously, a plurality of optical fibre strain sensors is applied to the length of the blade body. This allows the detection of impact forces and for profiling of the aerodynamic forces along the blade.

Preferably, optical strain sensors are positioned to measure strains in the axial direction of the blade. By positioning a plurality of sensors around the leading edge at a single cross- section, measurements of the axial strain can be made to determine the lift and drag forces on the rotor blade.

Advantageously, one or more optical fibre strain sensors are positioned at an angle of the order of 45 degrees, for example substantially 45 degrees to the rotor blade axis. By positioning a sensor at this angle, the torsion in the blade can be measured. Other angles for example, 30 degrees to 60 degrees to the rotor blade axis could also be used.

As the rotor blade is impacted, a variety of the natural nodes of oscillation are excited. In a preferred configuration, the optical fibre strain sensors are positioned at locations other than at the nodes of oscillation. By arranging the optical fibre strain sensors away from the nodes of oscillation, it is possible to identify the impact location from analysis of the signals from the sensors. Furthermore, the magnitude of the impact can be identified from the amplitude of the sensor signals.

The invention includes within its scope a helicopter rotor blade comprising a rotor blade body and an erosion shield, wherein at least one optical fibre strain sensor is provided between the rotor body and the erosion shield.

Brief Description of the Drawings

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is an axial cross-sectional view of a helicopter rotor blade in accordance with the present invention; and Figure 2 is a plan view of the helicopter rotor of Figure 1, wherein the erosion shield has been removed.

Detailed Description of Exemplary Embodiments

Figures 1 and 2 show a helicopter rotor blade 10, comprising a rotor blade body 12 and an optical fibre strain sensor 14 attached to the rotor blade body. An erosion shield 16 is bonded to the leading edge of the rotor blade body 12, which protects the optical fibre strain sensor 14 and rotor blade body 12 from damage from impacts with foreign bodies. The erosion shield 16 can detached from the rotor blade body 12 replaced if necessary. As shown in Figure 2, the optical fibre strain sensors comprise Bragg gratings 18 spaced along the length of the fibre 14. The optical fibre 14 extends the length of the rotor blade body 12 and a plurality of optical fibres 14 are positioned longitudinally about the leading edge of the rotor blade body 12 to allow measurements of the life, drag and torsion of the rotor blade 10.

Whilst the present invention has been described in relation to helicopter rotor blades, it may also be used in other applications, such as in aircraft propellers.

In summary, a method of manufacturing a helicopter rotor blade comprising the steps of applying at least one optical fibre strain sensor to the leading edge of the rotor blade body and fitting an erosion shield to the leading edge of the rotor blade body, over the optical fibre sensor.