Tail Rotor - Alternative Technologies

Alternative Technologies

There have been three major alternative designs which attempt to solve the shortcomings of the tail rotor system.

The first is to use a ducted fan rather than an un-ducted fan. This design is referred to as a fantail, or by the trade name Fenestron, a trademark of Eurocopter. Placing the fan within a duct reduces tip vortex losses, shields the tail rotor from damage, shields ground crews from the hazard of a spinning rotor, and is much quieter than a conventional tail rotor. The ducted fan uses shorter and more numerous blades, but otherwise it is very similar to a conventional tail rotor.

McDonnell Douglas developed the NOTAR (NO TAil Rotor) system, which eliminates having any rotating parts out in the open. The NOTAR system uses a variable pitch ducted fan driven by the helicopter's powerplant, but the ducted fan is mounted inside the fuselage ahead of the tail boom, and the exhaust passes through the tail boom to the end, where it is expelled out one side. This creates a boundary layer which causes the downwash from the main rotor to hug the tail boom according to the Coandă effect. This creates a force which cancels out the main rotor torque and provides directional control. The advantages of the system are similar to the Fenestron system discussed above.

There are at least four ways to eliminate the necessity of a tail rotor altogether :

• Tandem / Transverse rotors: to use two non-overlapping main rotors which turn in opposite directions, so that the torque created by one rotor cancels out the torque created by the other. Such a design is commonly seen on heavy lift helicopters like the tandem rotored CH-47 Chinook and effectively with the tiltrotor configuration V-22 Osprey.
• 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 rotate into each other 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 prop in one way or another to drive them forward to begin with.

Recent technology in emergency tail rotor recovery systems has been advanced by the US Army in an SBIR research award to EATTS. The emergency anti-torque thruster system (EATTS) would allow a helicopter that has encountered tail rotor failure, or loss of tail rotor effectiveness, to land safely. The system is designed to counteract the torque from the helicopter's main rotor system during tail rotor failure as a result of mechanical problems as well as loss of the tail rotor from RPG hits.