Bell says its new anti-torque system will offer directional control, reductions in main rotor speed and noise and higher cruise speeds–up to 170 knots. Bell will evaluate the Propulsive Anti-Torque System (PATS) next year using a full-size gas turbine engine, inlet fan, bifurcated ducting and a series of test thrusters. “It would look like the tail end of a real helicopter,” said Dan McIlroy, research and development vice president at Bell’s XworX.

McIlroy expects the new system to be significantly more efficient than no-tail-rotor solutions flying today. “Current fan-fed devices consume more engine power, as altitude increases and air gets thinner. To maximize thermodynamic efficiency at this point, cold air at the front of the engine is drawn across it to mix it with hot exhaust. The fan doubles as a supercharger to increase the pressure boost to the compressor inlet and improve overall engine efficiency.”

PATS is still expected to draw more power than a conventional anti-torque tail rotor but dramatically improve center-of-gravity margins. In military applications, PATS could reduce radar and infrared signatures as well as noise.

Like existing no-tail-rotor systems, PATS uses air flowing over the tailboom to help counter torque. “We use the Coanda Effect approach (the tendency of a stream of fluid to stay attached to a convex surface) as we increase speed,” said McIlroy, “but the main benefits result from mixing the cold air with the exhaust.” Bell has also begun conceptual work on a twin-engine version.

The test rig planned for an 18-month program will couple a representative turboshaft to a real helicopter transmission to apply torque loads to system. Bell is evaluating different configurations, one with pro- and anti-torque thruster ducts, another with louvers, and another with a one-piece moving duct. PATS will require developments in composite materials to build tailbooms compatible with hot exhaust gases. It will also require sophisticated control arrangements to optimize the mix of fan air and exhaust for different flight conditions.

“One of the bigger concerns is integrating all the control laws,” said McIlroy. “We’ve a bit of engineering work to do.” In an effort to preserve the weight savings PATS offers, the test rig will also provide the means to evaluate fly-by-wire controls.