**2. Capabilities of smart systems in rotor blades**

Smart-blades can greatly enhance the performances of modern helicopters. Local modifications of the aerodynamic characteristics of a blade profile provide an optimised performance across the full blade revolution. The control of such systems conditions its capabilities and usage. In the introduction, focus was made on the lift characteristics of the retreating side. Most concepts improve directly the lift of the profile at fixed angle of attack. Other systems increase the efficiency of the helicopter improving the stall behavior of the profile or by reducting the vibration on the rotor. Vibrations decrease an helicopter blade efficiency, influence the dynamic stall and generate noise. The latter is a great concern for helicopter operating in a urban environment.

## **2.1. Flaps**

Flaps on helicopter blades are not designed as a primary control surface like in airplanes. They act as a secondary control to improve the efficiency of the rotorblade by modifying the lift of the profile and by reducing vibrations on the rotor.

### *2.1.1. Active trailing edge flaps*

Active trailing edge flaps are flaps situated at the trailing edge that actively modify the rotorblade performance. A schematic of a trailing edge flap for a helicopter blade taken from Koratkar [28] is shown in Figure 4. Although research has been conducted to study the possibility to use them for control in a swashplateless configuration [49], most of the studies focus on their ability to reduce the vibrations of helicopter blades [1, 15, 18, 27, 28, 35, 62]. The angle of the flap directly relates to a change of the bending of the blade during rotation

**Figure 4.** Schematic of a trailing-edge flap and its actuation system (adapted from Koratkar paper [28]

[18, 28]. The position of the flaps along the rotor has a great influence on the final performances of the mechanism. Although the optimum positioning of the flap depends on the application objective, studies show that multiple flaps achieve a better vibration reduction that a single flap [18, 27, 62]. These flaps only need a few degrees of deflection to affect the system dynamics [15, 28]. Loads on the helicopter rotor are function of the rotational frequency of the blade. The largest loads happens at 1, 2, 3 and 4 times the rotational frequency of the blade [6]. Therefore the flaps need to be actuated at similar frequencies to cancel undesired vibrations. With multiple flaps, the phase between the flaps is another key element. Under a suitable control authority, literature shows that the vibratory loads on the rotor can be reduced up to 80% [15, 18].

The amount of noise generated by helicopters is another important issue, especially because many helicopter missions involve flying over dense populated area. Noise generated by the helicopter blade comes mainly from the interaction between one blade and the vortexes generated by the previous blade [69]. This phenomenon is called blade-vortex interaction (BVI). Decreasing the effects of blade-vortex interaction can not only lead to a reduction in the noise emitted but also to a decrease in the power requirement. Active trailing edge flaps are actively studied to limit this effect by an individual control on each blade [1, 18, 69]. Controlling the trailing edge flap at 2 cycles per revolution (hereafter indicated as 2/rev) shows potential for consequent noise reduction [1].

#### *2.1.2. The Gurney flap*

4 Will-be-set-by-IN-TECH

Smart-blades can greatly enhance the performances of modern helicopters. Local modifications of the aerodynamic characteristics of a blade profile provide an optimised performance across the full blade revolution. The control of such systems conditions its capabilities and usage. In the introduction, focus was made on the lift characteristics of the retreating side. Most concepts improve directly the lift of the profile at fixed angle of attack. Other systems increase the efficiency of the helicopter improving the stall behavior of the profile or by reducting the vibration on the rotor. Vibrations decrease an helicopter blade efficiency, influence the dynamic stall and generate noise. The latter is a great concern for

Flaps on helicopter blades are not designed as a primary control surface like in airplanes. They act as a secondary control to improve the efficiency of the rotorblade by modifying the lift of

Active trailing edge flaps are flaps situated at the trailing edge that actively modify the rotorblade performance. A schematic of a trailing edge flap for a helicopter blade taken from Koratkar [28] is shown in Figure 4. Although research has been conducted to study the possibility to use them for control in a swashplateless configuration [49], most of the studies focus on their ability to reduce the vibrations of helicopter blades [1, 15, 18, 27, 28, 35, 62]. The angle of the flap directly relates to a change of the bending of the blade during rotation

Helicopter motion

Wing speed relative to air

Reverse flow region

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

helicopter operating in a urban environment.

the profile and by reducing vibrations on the rotor.

**2.1. Flaps**

*2.1.1. Active trailing edge flaps*

**2. Capabilities of smart systems in rotor blades**

The Gurney flap is a small flap deployed at 90 degrees at the edge of the trailing edge of the rotorblade, as shown in Figure 5. Typically its length is 2% of the chord length of the blade profile. The Gurney flap modifies the flow at the blade trailing edge and induces a low pressure zone which brings the separation point closer to the trailing edge [53]. The result is an increase of the lift over a large range of angles of attack with a small drag penalty [38, 53, 59,

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

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

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