Helicopters with counter-rotating rotors

Counter-rotating rotor helicopters constitute another important family of helicopters. Sev-eral configurations can be found such as coaxial rotor helicopters, tandem rotor heli-copters, intermeshing rotor heliheli-copters, and quadrotor heliheli-copters, etc.. For these vehi-cles, a separate antitorque system (like e.g. a tail rotor) is not required because the rotor systems rotate in opposite directions and thereby neutralize or eliminate each other’s torque. Compared to single rotor helicopters, a principal disadvantage of most of counter-rotating rotor helicopters, like coaxial rotor helicopters, tandem rotor helicopters, and intermeshing rotor helicopters, is the increased mechanical complexity which, in an ele-mentary engineering sense, is more prone to failure. However, the great advantage of these vehicles is the full participation of the available engine power to lift and thrust.

Coaxial rotor helicopters

Figure 1.6: Coaxial rotor helicopters Kamov Ka-26 and Ka-50.

This configuration have been developed since the late 1950’s by the Russian Kamov helicopter design bureau (see Fig. 1.6). Coaxial rotor configuration consists of a pair of rotors mounted in a mast one above the other, and turning in opposite directions about the same axis of rotation. Compared to conventional helicopters, coaxial rotor helicopters are normally more compact thanks to the absence of the tail rotor, and thus less sensitive to wind gusts. Flying controls act on the coaxial rotors with cyclic pitch, and collective

Rotary-Wing VTOL (RWVTOL) vehicles 11 pitch controls similarly to a single rotor helicopter. The yaw control is achieved through differential collective pitch. An automatic mixer box ensures that the rotors’ total lift remains constant during yaw manoeuvres by increasing the blades’ pitch angles of one rotor while decreasing the blades’ pitch angles of another rotor.

Tandem rotor helicopters

Figure 1.7: Tandem rotor helicopters Piasecki HRP Rescuer and Boeing CH-47 Chinook.

The first tandem rotor helicopter was designed by Piasecki and built by Piasecki Heli-copter in 1945 (see Fig. 1.7). Especially used on large cargo heliHeli-copters, the tandem rotor configuration has two large horizontal rotors (i.e. front and aft rotors) mounted on each end of the fuselage and turning in opposite directions. The two rotors are synchronized by a transmission mechanism ensuring that they do not hit each other even during an engine failure. Each rotor operates similarly to the main rotor of a single rotor helicopter, except for the direction of rotation of the aft rotor and the method of keeping directional control. The tandem rotor design achieves yaw control by applying opposite left and right cyclic pitch to each rotor. To achieve pitch, opposite collective pitch is applied to each rotor, allowing to decrease the lift at one end and increase lift at the opposite end and thereby tilt the helicopter forward or backward. Alike coaxial rotor helicopters, because there is no antitorque rotor full engine power can be applied to lift. However, disadvan-tages of tandem rotor helicopters are a complex transmission and more drag due to its large shape.

Intermeshing rotor helicopters

The first concept of this family was developed in Germany by Flettner in 1938. Nowa-days, this concept is used and further developed by Kaman Aircraft Corporation (see Fig. 1.8). Intermeshing rotor helicopters are highly stable and have powerful lift capabil-ities. Intermeshing rotor configuration consists of a set of two rotors turning at the same speed and in opposite directions, with each rotor mast is mounted on the helicopter with a slight angle to the other. The two rotors are synchronized by a transmission mechanism

ensuring that the blades intermesh without colliding. To achieve translational or lateral or vertical flight, a same cyclic pitch or collective pitch control is applied to both rotors, similarly to a single rotor helicopter. In turn, the yaw control is achieved by applying asymmetric control on the flapping hinge of the two rotors.

Figure 1.8: Intermeshing rotor helicopter Kaman K-MAX.

Quadrotor helicopters

Figure 1.9: De Bothezat quadrotor helicopter.

Quadrotor helicopters were developed during the early days of the helicopter history, as exemplified by the Breguet’s gigantic quadrotor (1907), the De Bothezat quadrotor (1921) (see Fig. 1.9), the Oemichen (1922), but rapidly fell into disuse due to their impracticality compared to other helicopters. However, this configuration has recently received increasing interests in the robotics field because many robotic applications such as surveillance, inspection, cartography, etc. do not require the vehicle to carry heavy load or to have a large size (see Fig. 1.10). Many studies on aerodynamics, modeling, and control design have been carried out recently for reduced scale quadrotor helicopters (e.g.(Hamel et al., 2002), (Guenard, 2007), (Pounds et al., 2002), (Pounds et al., 2006), (Defara et al., 2006), (Tayebi and McGilvray, 2006), (Bristeau et al., 2009), (Huang et al., 2009)). Quadrotor helicopters are mechanically less complex than other helicopters due to the absence of the swashplate and transmission mechanisms. A quadrotor helicopter is lifted and propelled by four rotors, with two of them turning in the opposite directions of the others to neutralize the yaw torque. Unlike conventional helicopters for which lift can be regulated via the rotor speed and the collective pitch, the lift of quadrotor helicopters is solely controlled via the rotor speed. Pitch, roll, and yaw are achieved by solely controlling the differential

Rotary-Wing VTOL (RWVTOL) vehicles 13 speed of the four rotors. More precisely, the differential lift of the rotors generate pitch and roll torques, while the yaw control is achieved by the differential rotor torques induced by drag forces acting on the four rotors. Mathematical details will be given in Section 1.4.3.

Figure 1.10: Reduced scale quadrotor helicopters developed by Australian National Uni-versity, by the Centre d’Energie Atomique of France, and by Draganfly Innovations Inc.