**3.4. Failure**

10 Will-be-set-by-IN-TECH

Active concept Actuation frequency Lifetime Retreating side actuation 1/rev 66,460 hours BVI noise 2/rev 33,230 hours Vibrations 4/rev 16,615 hours Flow control 2kHz 278 hours

**Table 1.** Comparison of the life time of an 10<sup>9</sup> cycles actuation mechanism in a 250rpm rotor blade system for various active control concepts. For the Active flow control system, the system is in operation

at the surface of the blade must not exceed the maximum strain of the actuator that will lead

In addition to centrifugal loads, helicopter blades are subjected to large vibrations. In the most common configuration, helicopter blades are attached to the rotor by a joint that allows rotations with three degrees of freedom. The motions of the blade relative to the joint are defined as flapping, leading-lagging and feathering. Each of these motions are associated with one degree of freedom. While the blade is rotating the cyclic loads excite each degree of freedom causing vibrations at frequencies that are multiples of the blade rotational frequency [6, 48]. As a consequence, the design of a mechanism must address the resulting loads. Moreover, these loads constrain the use of mechanical elements like hinges or friction surfaces

Any mechanism built in a commercial aircraft must comply to a set of rules to ensure the reliability of the system after a large number of actuation cycles, as well as the safety and the integrity of the aircraft in the event of a failure. Helicopter blades in a general purpose helicopter are designed for 10,000 flight hours [30]. Although composite blade design can handle even more loading cycles, manufacturers specify helicopter blades to be maintained and replaced on a much shorter basis [25, 30]. Actuation mechanisms for the active blade need to have a design life superior to the blade design life and have to maintain performances through their operational life. In aircrafts, hydraulic and pneumatic mechanisms are widely used due to these concerns. They are especially utilized for moving control surfaces that must satisfy a reliability requirement of 10−<sup>9</sup> failure per hour and their performance is hardly affected even after a large number of cycles. It is only recently that electrical mechanisms have reached equivalent levels of safety and have been used to control control-surfaces in aircraft

The reliability of the actuation system is also depending a lot on the application type. For alleviating the lift asymmetry on the retreating side, the mechanism performs a cycle once per revolution. This figure is to be compared with an actuation system for actively cancelling vibrations that need to operate at 2/rev and 4/rev or even at 5/rev in the case of torsional frequencies [18, 66]. Table 1 shows the various expected life time for a mechanism that has been design for 109 cycles in the case of various active blade concepts for a 250 rpm rotor

Although all the cases satisfy the 10,000 hours of operating life, the vibration damping case shows that a high quality actuation system certified for 10<sup>9</sup> has an operating life close to that

only on the retreating side of the helicopter.

to breaking or debonding.

that will tend to jam and fail.

[4].

system.

**3.3. Reliability and environmental constraints**

In addition to being designed to exceed the life time of the blade, the active blade actuation system must also be developed not to influence the performance of the helicopter in the event of a failure. For distributed systems like the active-twist, if the patch actuators are not working, they will not reduce the performance of the blade profile. On the contrary, for the Gurney flap, the variable droop leading edge and the trailing edge active blade concept, in the event of a jamming, the blade profile will be modified during the full rotation of the rotor blade. Hence, care must be taken to make sure the helicopter is stable and able to be controlled with a modified profile. Furthermore, in the event of a power failure, the mechanism must go back to its initial state. This can be done by prestressing the mechanism or making sure that the aerodynamic loads are sufficient to bring the mechanism back to its inactive state.

## **3.5. Power requirement**

To operate an actuator in a rotorblade, power needs to be transfered from the helicopter to the blades. Electrical, pneumatic and hydraulic power can be provided to a rotating blade by the use of specialized rotor mounts which add to the complexity of the rotor hub [24]. The type and the amount of power that can be drawn for an actuation system is a serious limitation to some active system. Large helicopter blades include a de-icing system for high altitude flight. Such a system draws up to 1kW of electrical power that is transfer to each blade. This gives a good estimation of the power available for an electrical actuation system.

## **3.6. Complexity of the system**

Developing smart systems for helicopters is tremendously complex due to the number and the variety of the domains involved. For designing an active helicopter rotorblade, knowledge is required not only in aerodynamics and mechanics, but also in control, material science and electronics. Simulating for validating a concept, selecting of a suitable actuation technology and defining an application demand skills in all these domains. In order to move from research and laboratory experiments to flying prototypes and commercial products, the European union has created a Consortium within the Clean Sky Joint Technology Initiative to bring various research partners together. The Green Rotorcraft Consortium, among its lines of research, manages the evaluation of the Gurney flap technology to improve helicopter performance and noise reduction with both academic and industrial partners [16].

#### **4. Piezoelectric actuators for smart-rotor blade systems**

Many actuation technologies are available to actuate every smart blade concepts. Among them, piezoelectric actuators have a tremendous potential to meet and exceed the various requirements of these specific applications. This section focuses on piezoelectric actuators and their potential for the actuation of active systems for helicopter blades.

#### **4.1. Piezoelectricity**

Piezoelectric materials are materials that have the property to convert mechanical energy into electrical energy. When such a material is subjected to a strain, an electrical charge is created inside the material. This property is called the direct piezoelectric effect. Additionally, when the material is subjected to an electrical field, it deforms according to the electrical field magnitude. This is called the converse piezoelectric effect. A piezoelectric material is characterized by the piezoelectric strain constant *dij* which relates the strain to the electrical field. The subscript *i* indicates the direction of the applied electrical field and the subscript *j* indicates the direction of the deformation. Prior to be used, the piezoelectric material is poled. Conventionally the poling direction is along the vertical axis (3-axis) as shown in Figure 9. When an electrical field is applied in the poling direction, the material is contracting in that direction and extending in other directions (1- and 2-axis). Changing the direction of the electrical field will result in a contraction along the 1- and 2-axis and extension along the vertical axis. To quantify these piezoelectric effects, the direct and shear strains are related to the electrical field by the following constants: *d*<sup>31</sup> = *d*32, *d*33, *d*<sup>25</sup> = *d*15. Equation 8 is the equilibrium equation that relates the electrical field *E* to the strain and shear components of the material (*ε* and *γ*) when no mechanical constraint is applied on the material.

**Figure 9.** Axis reference system for piezoceramic components.

$$
\begin{pmatrix}
\varepsilon\_1\\ \varepsilon\_2\\ \varepsilon\_3\\ \gamma\_{23}\\ \gamma\_{31}\\ \gamma\_{12}
\end{pmatrix} = \begin{bmatrix}
0 & 0 & d\_{31} \\ 0 & 0 & d\_{32} \\ 0 & 0 & d\_{33} \\ 0 & d\_{25} & 0 \\ d\_{15} & 0 & 0 \\ 0 & 0 & 0
\end{bmatrix} \times \begin{pmatrix}
E\_1 \\ E\_2 \\ E\_3
\end{pmatrix} \tag{8}
$$


**Table 2.** Table of some piezoceramic materials. PZT-SP4 and PZT-5A1 are from Smart-Material Company. PZT-5H is taken from Chopra review on actuators [15]. PZT-PSt-HD and PZT-PSt-HPSt are from Piezomechanik Company. Finally, PZT-PIC-255 and PZT-PIC -151 are from Physic Instrumente Company [45].

Consequently, the knowledge of the 3 constants *d*31, *d*<sup>33</sup> and *d*<sup>15</sup> is sufficient to fully characterize the electromechanical properties of a piezoelectric material. A deformation along the 1- and 2-axis which is characterized by the *d*<sup>31</sup> coefficient is called the *d*<sup>31</sup> effect and a deformation along the 3-axis is called the *d*<sup>33</sup> effect as the *d*<sup>33</sup> coefficient characterizes this deformation as shown in Figure 10.

**Figure 10.** Piezoelectric effects for an electrical field applied in the poling direction.

#### **4.2. Types of piezoelectric actuators**

Although some polymers can exhibit piezoelectric characteristics [61], most piezoelectric actuators are based on piezoceramics. Piezoceramic materials have been widely studied and used since the second world war to manufacture ultrasonic transducers. Table 2 lists some piezoceramics available from manufacturers.
