An analysis of the efficiency of Coanda - NOTAR anti-torque systems for small helicopters Anniversary Session “Centennial celebration of the first jet aircraft invented by Henri Coanda”, organized by INCAS, COMOTI and Henri Coanda Association, 14 December 2010, Bucharest, Romania Ionic ă CÎRCIU*, Mircea BOŞCOIANU* *Corresponding author “Henri Coanda” Air Force Academy, Braş ov, Romania circiuionica@yahoo.co.uk, boscoianu_mircea@yahoo.co.uk Abstract: The use of jet thrust for anti-torque for monorotor small helicopters is based on the circulation control concept, which result in a distributed side force along the entire tail boom assembly. High velocity jets of air from a pressurized tail boom is blown tangential to the surface out of narrow slots that run lengthwise on the side of the tail boom. In combination with the downstream velocity produced by the main rotor, Coanda jets cause the flow to remain attached to the tail boom surface. The anti-torque NOTAR system is in fact the result of Coanda effect and the interest is to analyze the efficiency of replacing the tail rotor on a small monorotor helicopter. Keywords: Coanda effect, NOTAR, slot, attached jet, jet thruster 1. COANDA EFFECT AERIAL VEHICLES Coanda Effect is a classic phenomenon in fluid mechanics and one of the fundamental discoveries of the Romanian inventor Henri Marie Coanda (1886 - 1972). Henri Coanda was a Romanian inventor, aerodynamics pioneer and the designer and the builder of the world's first jet powered aircraft, the Coandă-1910, a revolutionary plane of the beginning of the 20 th century. Fig. 1 Henri Coanda propelling device 0 As a natural phenomenon, Coanda effect describes the tendency of a fluid jet to be attracted to a nearby surface (flaps or airfoils), consecutively its profile being characterized by a significant asymmetry (Fig. 1). In free surroundings, a jet of fluid entrains and mixes with its surroundings as it flows away from a nozzle. When a surface or another stream is placed close to the jet, this restricts the entrained air flow from surroundings into that region. As flow accelerates trying to INCAS BULLETIN, Volume 2, Number 4/ 2010, pp. 81 - 88 DOI: 10.13111/2066-8201.2010.2.4.12