Experimental Study Of Pitch Damping In A Coaxial Model Helicopter

Small helicopters are known to have poor rotor damping in pitch and roll and mostly need stabilizers to improve the damping to provide sufficient flying stability. An experimental investigation was carried out to study pitch damping in a model coaxial helicopter fixed with a Bell stabilizer bar to elucidate the effects of the design of a fly-bar. High speed photography was used to observe the motion of the tethered helicopter, subjected to a step change in the shaft angle . The motion of the fly-bar, the upper and lower rotor blades and the shaft were analyzed to obtain the response. The inertia of the fly-bar was varied by replacing the weights made of different materials, namely, aluminum, steel and tungsten carbide. The videos were analyzed to obtain relevant response parameters such as shaft rotation velocity (ω) and the relative displacement of rotor tip path plane (ƍ T). It was found that the lag of the top rotor, to which the fly-bar was connected, systematically increased with the increase of the fly-bar moment of inertia and appeared to follow a parabolic relation. For low fly-bar weights, the value of the parameter ƍ /ω matched with the theoretical expectation given by 16/ γ Ω. The experimental method developed can be used to assess the pitch stability of small helicopters under development such as the Helitrike being developed at NAL - National Aerospace Laboratories at Bengaluru, Karnataka, India.