The results of two in-flight simulation programs on the impact of vertical axis characteristics on rotorcraft handling qualities are presented. The parameters investigated in these studies were heave damping, thrust to weight ratio, and various dynamic response characteristics of the combined engine, governor and rotor system. Flight tasks included hover, hover manoeuvring and nap of the earth flight. Evaluation of 11 configurations where heave damping and thrust to weight ratio values were varied provides the basis to suggest th at the amount of heave damping represented by Zw = -0.20 sec-1 and a thrust to weight ratio of 1.08 are the boundary values for Level 1 helicopter handling qualities. These results are compared with other relevant work on the topic. Stabilization and control of engine torque was found to be a major source of pilot workload for models with higher order engine-rotor system response characteristics of a 3 db resonant peak in the transfer function between collective and engine torque is postulated as an upper limit for Level 1 handling qualities. The benefit of displacing this peak to higher frequencies, corresponding to separating the resonance from the typical pilot's bandwidth, is suggested. The evaluation results tend to discount the use of a vertical velo city shaping parameter for definition of Level 1 attributes when thrust to weight ratio is a dominant factor and suggest that the impact of such dynamic characteristics is highly sensitive to pilot technique and adaptability.