Helicopter Flight Dynamics, 3rd Edition

5Modelling Helicopter Flight Dynamics: Stability Under Constraint and Response Analysis

The German DLR variable stability (fly‐by‐wire/light) Bo105 S3 (Photograph courtesy of DLR Braunschweig)

Everybody's simulation model is guilty until proved innocent.

(Thomas H. Lawrence at the 50th Annual Forum of the AHS, Washington, 1994)

5.1 Introduction and Scope

Continuing the theme of ‘working with models’, this chapter deals with two related topics – stability under constraint and response. The response to controls and atmospheric disturbances is the third in the trilogy of helicopter flight mechanics topics − where Chapter 4 focused on trim and natural stability, response analysis is given prime attention in Section 5.3. Understandably, a helicopter's response characteristics can dominate a pilot's impression of flying qualities in applied flying tasks or mission task elements. A pilot may, for example, be able to compensate for reduced stability provided the response to controls is immediate and sufficiently large. He may also be quite oblivious to the ‘trim‐ability’ of the aircraft when active on the controls. What he will be concerned with is the helicopter's ability to be flown smoothly, and with agility if required, from place to place, and the associated flying workload to compensate for cross‐couplings, atmospheric disturbances, and poor stability. Quantifying the quality ...