**2.2. Morphing blades**

The idea behind morphing blades is to change the aerodynamic characteristics of the blade by a continuous change in its shape. This approach is inspired by the way birds and flying animals are modifying their wings to adapt to the various situations they encounter while flying. Most of these solutions involve a stiff structure that supports the loads and a flexible skin to keep the outer surface of the rotorblade without discontinuities.

#### *2.2.1. Variable droop leading edge*

The concept behind the nose drop is to advance the front part of the profile at an angle. It increases the profile as well as the curvature, as shown in Figure 6. The variable droop leading edge is used to alleviate the dynamic stall by ensuring that the flow passes smoothly over the leading edge for high angles of attack [32]. Although the lift is increased during the downward motion of the leading edge [32], the maximum lift is reduced by 10% [13, 21, 37]. More significantly, the drag and pitching moments are reduced by 50% [13]. The variable droop leading edge concept provides a decrease in helicopter vibrations and loads due to the suppression of dynamic stall within the retreating blade region. However, the helicopter maximum speed is reduced due to a decrease in lift when the droop nose is deployed. Therefore, the variable droop leading edge is studied in combination with the Gurney flap to negate the lift reduction [14]. This concept can also be applied to reduce the noise generated by a helicopter [9].

**Figure 6.** Sketch of the VR-12 profile used for wind tunnel testing at NASA Research Center from Lee paper [32].

#### *2.2.2. Camber change*

6 Will-be-set-by-IN-TECH

68]. Although the Gurney flap induces pitching moment, it provides a beneficial improvement of the efficiency of the rotorblade profile for the hovering situation [68]. In forward flight, the Gurney flap provides the blade with additional lift on the retreating side to balance the lift distribution [59]. For large forward velocity, the Gurney flap improves the airfoil behaviour in light stall conditions, which increases directly the flight envelope of a helicopter [68]. The behaviour of the Gurney flap is related to its length and placement. Studies about the length of the Gurney flap show an increase in drag and pitching moments with increasing lengths [38, 53, 63, 68]. Depending on the application, the Gurney flap length is limited to the point

Furthermore, the Gurney flap can have a positive effect on blade-vortex interaction. Similarly to a trailing edge flap, the Gurney flap acts on the blade mechanical behaviour [68]. Actuating the Gurney flap at 2/rev with suitable control would lead to a decrease in vibration and noise

The idea behind morphing blades is to change the aerodynamic characteristics of the blade by a continuous change in its shape. This approach is inspired by the way birds and flying animals are modifying their wings to adapt to the various situations they encounter while flying. Most of these solutions involve a stiff structure that supports the loads and a flexible

The concept behind the nose drop is to advance the front part of the profile at an angle. It increases the profile as well as the curvature, as shown in Figure 6. The variable droop leading edge is used to alleviate the dynamic stall by ensuring that the flow passes smoothly over the leading edge for high angles of attack [32]. Although the lift is increased during the downward motion of the leading edge [32], the maximum lift is reduced by 10% [13, 21, 37]. More significantly, the drag and pitching moments are reduced by 50% [13]. The variable droop leading edge concept provides a decrease in helicopter vibrations and loads due to the suppression of dynamic stall within the retreating blade region. However, the helicopter maximum speed is reduced due to a decrease in lift when the droop nose is deployed. Therefore, the variable droop leading edge is studied in combination with the Gurney flap to negate the lift reduction [14]. This concept can also be applied to reduce the noise generated

Chord length

Chord line

where these disadvantages outweigh its benefits in lift and stall characteristics.

skin to keep the outer surface of the rotorblade without discontinuities.

**Figure 5.** Sketch of a Naca 23012 profile with a 2% Gurney flap.

in a similar way than active trailing edge flaps [69].

**2.2. Morphing blades**

*2.2.1. Variable droop leading edge*

by a helicopter [9].

Gurney flap

Changing the camber of a profile increases its lift for the same chord length [54]. The benefit is a larger flight envelope of the helicopter by improving the lift on the retreating side of the rotorcraft in the same way than the Gurney flap concept. Once again harmonic actuation at 2/rev could reduce the noise and the vibratory loads on the rotor, improving the rotor performance [18, 20]. Most studies on this concept consider the plane as the main application, envisioning morphing flaps as a main control surface [39].

#### *2.2.3. Active-twist*

Among early helicopter prototypes, some developer considered cyclic twist control [43] for changing the lift of the rotating blades. The idea behind active twist is to modify the twist and the torsional stiffness of rotating blade not only to improve the lift and the global helicopter performance but also to actively damp vibrations. Early experiments on active twist involved changing the twist of the helicopter blade at the root of the blade [2]. Later experiments used a distributed actuation actuation system to modify the blade twist [46, 50, 58, 66]. In a similar manner to the active trailing edge, the placement and the number of actuators modify the amount of vibration that are reduced. Thakkar study shows that up to 69% of reduction in vibrations can be achieved with the actuation of two sections [58]. Wind tunnel tests on a helicopter model demonstrated a 95% reduction in vibrations [66]. In the tests, each of the four blades mounted on the helicopter model was equipped with 24 actuators bonded onto the skin of the blade. Although only up to 1.4 degree of change in the pitching angle of the blade was achieved, torsional vibrations at 3/rev and 5/rev were successfully damp. In addition, the noise generated by the blade-vortex interaction can be reduced by up to 90% using an appropriate control of the blade twist [5].

#### *2.2.4. Extended trailing edge*

The amount of lift a profile can deliver depends on its chord length. For the same geometry, the lift is proportional to the blade surface area as shown in equation 1. Therefore, extending the chord length of a profile increases the lift generated. Studies on an extended trailing edge active blade have shown an increase in the lift without significant increase in the lift-to-drag ratio [36].

## **2.3. Active flow control**

Active flow control devices take another approach to improve the lift on a profile. Instead of modifying the airfoil geometry to act on the flow, they directly modify the air flow by re-energising the boundary layer on the top of the profile with a high speed jet. Such a flow is called a synthetic jet. The objective is to bring the separation point closer to the trailing-edge and therefore improve laminar flow over a larger portion of the airfoil [23]. Actuators for this application are placed inside a cavity which has a tiny opening [26] or a full slot perpendicular to the flow direction on the top part of the profile [67]. Figures 7 and 8 show these two types of synthetic jet actuator.

**Figure 7.** Sketch of a slot synthetic jet system.

**Figure 8.** Sketch of a synthetic jet system with a circular orifice.

Wind tunnel experimentations have shown that synthetic jets improve the aerodynamic performance when driven at a specific frequency [23]. Much better performance is obtained when the actuation mechanism is combined with sensors arrays before and after the position of the synthetic jet system [57]. The sensors monitor the instabilities that will trigger the flow transition and actuate the synthetic jet system so that it damped the instabilities delaying further the transition. The actuation frequencies are in the kHz range and are related to the airflow speed [10]. Most of the literature focuses on fixed wind tunnel test [10] but simulations show a potential increase in the maximum lift of an airfoil by 34% with an increase in the maximum stall angle of a profile [17]. These characteristics make synthetic jets system very promising for improving the characteristics of a profile for helicopter applications.
