Stability and control resume

This Tour would be incomplete without a short discussion on ‘stability and control derivatives’ and a description of typical helicopter stability characteristics. To do this we need to introduce the helicopter model configurations we shall be working with in this book and some basic principles of building the aircraft equations of motion. The three baseline simulation configurations are described in Appendix 4B and rep­resent the Aerospatiale (ECF) SA330 Puma, Westland Lynx and MBB (ECD) Bo105 helicopters. The Puma is a transport helicopter in the 6-ton class, the Lynx is a utility transport/anti-armour helicopter in the 4-ton class and the Bo105 is a light utility/anti – armour helicopter in the 2.5-ton class. Both the Puma and Bo105 operate in civil and military variants throughout the world; the military Lynx operates with both land and sea forces throughout the world. All three helicopters were designed in the 1960s and have been continuously improved in a series of new Marks since that time. The Bo105 and Lynx were the first hingeless rotor helicopters to enter production and service. On these aircraft, both flap and lead-lag blade motion are achieved through elastic bending, with blade pitch varied through rotations at a bearing near the blade root. On the Puma, the blade flap and lead-lag motions largely occur through articulation with the hinges close to the hub centre. The distance of the hinges from the hub centre is a critical parameter in determining the magnitude of the hub moment induced by blade flapping and lagging; the moments are approximately proportional to the hinge offset, up to values of about 10% of the blade radius. Typical values of the flap hinge offset are found between 3 and 5% of the blade radius. Hingeless rotors are often quoted as hav­ing an effective hinge offset, to describe their moment-producing capability, compared with articulated rotor helicopters. The Puma has a flap hinge offset of 3.8%, while the Lynx and Bo105 have effective offsets of about 12.5 and 14% respectively. We can expect the moment capability of the two hingeless rotor aircraft to be about three times
that of the Puma. This translates into higher values of kp and Sp, and hence higher rotor moment derivatives with respect to all variables, not only rates and controls as described in the above analysis.

The simulation model of the three aircraft will be described in Chapter 3 and is based on the DRA Helisim model (Ref. 2.15). The model is generic in form, with two input files, one describing the aircraft configuration data (e. g., geometry, mass proper­ties, aerodynamic and structural characteristics, control system parameters), the other the flight condition parameters (e. g., airspeed, climb/descentrate, sideslip and turn rate) and atmospheric conditions. The datasets for the three Helisim aircraft are located in Chapter 4, Section 4B.1, while Section 4B.2 contains charts of the stability and control derivatives. The derivatives are computed using a numerical perturbation technique applied to the full nonlinear equations of motion and are not generally derived in explicit analytic form. Chapters 3 and 4 will include some analytic formulations to illustrate the physics at work; it should be possible to gain insight into the primary aerodynamic effects for all the important derivatives in this way. The static stability derivative Mw is a good example and allows us to highlight some of the differences between fixed – and rotary-wing aircraft.