at the ends of fixed wings. This allows the rotors to serve instead as propellers when flying forward at full speed. Coaxial. Other designs such as the Kamov Ka-50 and Sikorsky X2 use coaxial counter-rotating main rotors, which means that both rotors spin around the same axis but in opposite directions. The complexity of any dual main rotor system almost invariably requires the addition of a fly-by-wire flight control system, which increases costs drastically. Intermeshing rotors also turn in opposite directions, but the blades rotate into the gaps between the opposing blades so the rotors can intersect each other's path without colliding. Invented by Anton Flettner and used in Flettner Fl 282, Kaman HH-43 Huskie, and Kaman K-MAX. Tip jet. Another way to eliminate the effect of torque created by the rotorwing is by mounting the engine on the tips of the rotorwing rather than inside the helicopter itself; this is called a tip jet. One example of a helicopter using such a system is the NHI H-3 Kolibrie, which had a ramjet on each of the two wingtips, and an auxiliary power unit to spin up the rotor before starting the ramjets. Another example would be the Fairey Rotodyne. Also, unpowered rotors used in autogyro, gyrodyne, and derived concepts do not need a tail rotor either, although nearly all models that utilize this concept of propulsion do need a second propeller in one way or another to drive them forward to begin with. See also Piasecki X-49 Loss of tail-rotor effectiveness References Helicopter components