**1. Introduction**

656 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

Shindo, Y., Sasakura, T. & Narita, F. (2012). Dynamic electromechanical response of multilayered piezoelectric composites from room to cryogenic temperatures for fuel injector applications, *ASME Journal of Engineering Materials and Technology*, in press.

> The first helicopter to successfully achieve stable hovering flight and decent forward flight performances was demonstrated in 1935 and is attributed to Louis Breguet and René Dorand [22] as shown in Figure 1. Their patent details the two coaxial counter-rotating blades which resulted in an unprecedented level of performance, stability and safety for a rotorcraft [7]. This success was soon matched by other helicopter pioneers. Henrich Focke at the Focke Wulf Company and Juan de la Cierva within the Weir company demonstrated the hovering capabilities of a side by side rotor configuration in 1936 and 1938 respectivelly [3, 22, 47]. In 1940 Sikorsky flew a single rotor helicopter configuration with three auxiliary tail rotors to negate the counter-torque effect [22, 51] as shown in Figure 2. Sikorsky refined his design and produced a significant number of helicopters during the war, some of which were used during World War II in the Pacific [22]. After the war, this configuration was widely adopted by the emerging industry. Today almost every helicopter uses this single-rotor configuration.

**Figure 1.** Picture of the Breguet-Dorand helicopter.

These successes and the birth of the modern helicopter are the results of the convergence of technology, knowledge and experience. Before Breguet and Sirkorsky flights, many aircraft enthusiasts and pioneers built contraptions that merely hopped a few meters. Successful machines came when mature engine and mechanical technologies met scientific

©2012 Paternoster et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 2.** Picture of Sikorsky VS300 prototype.

study and a good understanding of the specificity of helicopter aerodynamics. In the 1930s, engines were refined by the booming aircraft industry. They were delivering unprecedented power-to-weight ratios [47], enabling helicopters to sustain more efficiently hovering flights. The counter-torque effect was tackled in many ingenious ways. Breguet used two counter-rotating shafts on the same axis to balance the torques. Other concepts used two and even quad-rotor configurations to balance this effect [3, 22, 34]. Patents show the level of engineering that was achieved to overcome the complexity of the various designs. Nevertheless, compelling forward flight performance came when the airflow asymmetry on the rotorblades was balanced.

When hovering, each blade experiences the same distribution of incident airflow velocity. This distribution is linear and proportional to the blade radius and the blade rotation. The lift generated by each blade can be estimated using the lift formula

$$L = \frac{1}{2}\rho v^2 A C\_l \tag{1}$$

where *L* is the lift force on the rotorblade profile, *ρ* is the density of the air, *v* is the velocity of the airflow on the profile, *A* is the surface of the profile considered and *Cl* is the lift coefficient. The lift coefficient is function of the pitch angle of the blade. Assuming the helicopter is hovering, the pitch angle and the airspeed distribution are the same regardless of the position of the blade relative to the helicopter. The lift force of each blade is obtained from the integration of the lift formula along the length of a helicopter blade *R*

$$L = \int\_0^R \frac{1}{2} \rho \mathbf{C}\_l (\omega r)^2 c dr \tag{2}$$

where *c* is the chord length of the blade profile and *ω*, the rotational velocity of the blade. After integration, we obtain

$$L = \frac{1}{6} \rho c \mathbf{C}\_l \omega^2 R^3 \tag{3}$$

As soon as the helicopter goes forward an extra velocity component is added to the velocity profile [6, 48]. We can distinguish the retreating side where the blade motion points in the opposite direction of the helicopter motion and the advancing side where the blade motion is in the same direction as the helicopter motion alike shown in Figure 3. Therefore, the incident airflow speed is increased on the advancing side and reduced in the retreating side. This asymmetry causes a difference in the lifting capabilities of the two helicopter sides. The lift for a blade in the retreating side becomes

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study and a good understanding of the specificity of helicopter aerodynamics. In the 1930s, engines were refined by the booming aircraft industry. They were delivering unprecedented power-to-weight ratios [47], enabling helicopters to sustain more efficiently hovering flights. The counter-torque effect was tackled in many ingenious ways. Breguet used two counter-rotating shafts on the same axis to balance the torques. Other concepts used two and even quad-rotor configurations to balance this effect [3, 22, 34]. Patents show the level of engineering that was achieved to overcome the complexity of the various designs. Nevertheless, compelling forward flight performance came when the airflow asymmetry on

When hovering, each blade experiences the same distribution of incident airflow velocity. This distribution is linear and proportional to the blade radius and the blade rotation. The lift

where *L* is the lift force on the rotorblade profile, *ρ* is the density of the air, *v* is the velocity of the airflow on the profile, *A* is the surface of the profile considered and *Cl* is the lift coefficient. The lift coefficient is function of the pitch angle of the blade. Assuming the helicopter is hovering, the pitch angle and the airspeed distribution are the same regardless of the position of the blade relative to the helicopter. The lift force of each blade is obtained from

*ρCl*(*ωr*)

where *c* is the chord length of the blade profile and *ω*, the rotational velocity of the blade.

As soon as the helicopter goes forward an extra velocity component is added to the velocity profile [6, 48]. We can distinguish the retreating side where the blade motion points in the opposite direction of the helicopter motion and the advancing side where the blade motion is in the same direction as the helicopter motion alike shown in Figure 3. Therefore, the incident airflow speed is increased on the advancing side and reduced in the retreating side. This

*<sup>L</sup>* <sup>=</sup> <sup>1</sup> 6 *ρv*<sup>2</sup>*ACl* (1)

<sup>2</sup>*cdr* (2)

*ρcClω*2*R*<sup>3</sup> (3)

*<sup>L</sup>* <sup>=</sup> <sup>1</sup> 2

generated by each blade can be estimated using the lift formula

the integration of the lift formula along the length of a helicopter blade *R*

*L* = *R* 0 1 2

**Figure 2.** Picture of Sikorsky VS300 prototype.

the rotorblades was balanced.

After integration, we obtain

$$L = \int\_0^R \frac{1}{2} \rho \mathbb{C}\_l (\omega r - v\_n)^2 c dr \tag{4}$$

where *vn* is the component of the helicopter velocity normal to the blade and *ω* is the rotational velocity of the blade. After integration, we obtain

$$L = \frac{1}{2}\rho c \mathbf{C}\_l \left(\frac{1}{3}\omega^2 \mathbf{R}^3 - \omega v\_n \mathbf{R}^2 + v\_n^2 \mathbf{R}\right) \tag{5}$$

The difference between equations 3 and 5 gives the loss of lift Δ on the reatreating side due to the helicopter overall motion

$$
\Delta L = \frac{1}{2} \rho c \mathbf{C}\_l \left( v\_n^2 \mathbf{R} - \omega v\_n \mathbf{R}^2 \right) \tag{6}
$$

The quadratic relation between the loss of lift on the retreating side and the helicopter forward motion velocity shows the importance of this phenomenon. Reverse flow is another important consequence of the forward motion of the helicopter. It happens where the helicopter speed is larger than the velocity of the blade due to its rotation. At high speeds, this region can cover a significant portion of the blade, meaning most of the lift is generated by the outer part of the blade. A cyclic control input was the key to balance the lift. Breguet-Dorand aircraft as well as Cierva and Sikorsky helicopters used a swashplate to vary the pitch of each blade during its revolution [8, 22, 51, 52]. Modifying the pitch of the blade changes the angle of attack and thus the lift for various positions of the blade around the helicopter. The angle of attack is increased on the retreating side and decreased on the advancing side. The lift is therefore evened on the two sides of the helicopter. Other early rotorcrafts pioneer considered a change of the twist of the blade or the deployment of flaps at the trailing edge of the blade to control on the lift [22, 43].

Today, all helicopters use cyclic pitch control for tuning the lift as the blades rotate. But lift can only be maintained by improving the angle of attack up to the stalling point of the blade profile. The maximal speed of a rotorcraft is therefore limited to the amount of lift the rotorblade can develop on the retreating side. In the case of rotorblade, the stall is dynamic, due to the unsteady nature of the flow. The vertical motion of the blade along with time-dependent pitching moments allows the angle of attack of the blade to exceed the quasi-static stalling angle of the profile. This favourable effect is followed by the development of vortexes close to the leading edge which can move towards the trailing edge causing large downward pitching moments [6, 22, 48]. Consequently, the rotor performance and the stability of the aircraft are reduced.

To further improve the helicopter blade performance, adaptive blade concepts are studied. The aim is to adapt the aerodynamic characteristics of the blade to maximise performance on both the advancing and the retreating side of the blade and improve the stall performance for large angles of attack. These systems range from morphing and changing the shape of a full blade profile to smaller devices acting on the boundary layer of the airflow to control its separation.

**Figure 3.** Helicopter in forward flight.
