Lateral/directional oscillation (Dutch roll) characteristics are documented by first accurately trimming the rotorcraft in level flight at the desired airspeed and altitude. A lateral/directional oscillation is then excited by using any of the following methods: a release from a SHSS; a lateral cyclic, yaw pedal or collective pulse input; a lateral cyclic step input; or a lateral cyclic or yaw pedal doublet. The release from a SHSS is accomplished by returning all controls to trim simultaneously with rapid ramp control inputs. The control pulse and doublet inputs are typically conducted using a 2-3 cm displacement with a period of 1 second for cyclic inputs and 2 seconds for yaw pedal inputs. All controls remain fixed following the control input and the open loop response of the aircraft is documented.
5.4.2 Spiral stability
It has already been shown that spiral stability is dependent on the sign of the expression (LvNr — LrNv), positive being stable. Now consideration of the equations of motion for TO1Cs, both pedal and cyclic, yields:
For a turn to starboard, r is positive and N0tr and Lv are both negative. The sign of 9tr depends on the sign of (LvNr — LrNv): when the bracketed expression is negative, 9tr is positive. This implies that the control deflection and the yaw rate are in opposite directions since positive 9tr implies left pedal. Alternatively, if (LvNr — LrNv) is positive, 9tr will be negative, implying that the control deflection and yaw rate will be in the same direction (to the right). Also, inspection of the equations indicates that, as Nv is positive and LAi is negative, if (LvNr — LrNv) is positive, cyclic deflection and yaw rate are in the same direction Thus, the control deflection required to maintain a steady turn on one control can be used as a direct indication of the helicopter’s spiral stability: if the pedal or cyclic has to be deflected into the turn (right pedal/cyclic for right turn), the aircraft is spirally stable; alternatively if the control has to be deflected out of the turn, the helicopter is spirally unstable. It is worth noticing that, although the derivatives vary with speed, altitude and configuration, Lr is usually small, so (LvNr — LrNv) is often positive and helicopters are usually spirally stable at modest angles of bank.
The spiral stability test is usually conducted by flying a TO1C-C as these are generally easier to perform than TO1C-P Longitudinal inputs are used to hold airspeed constant throughout the test. When the helicopter is stabilized at the desired bank angle, the cyclic is smoothly returned to the level flight trim value and a time history of the ensuing bank angle is recorded. During testing, inputs are made in a manner that minimizes excitation of the lateral/directional oscillation. It is normally of interest to evaluate an IFR bank angle (20°) and also to try and find the angle at which spiral stability becomes neutral or even negative.