This paper presents a new robust controller design approach to the yaw control of a small-scale helicopter mounted on an experimental platform. The yaw dynamic system is linearized into a linear system, which is modelled by an affine uncertainty model. We proposed a novel robust H, feedback controller with adaptive mechanisms for the linear system with guaranteed control performances. The feedback gains are obtained by the solutions of a series of linear matrix inequalities (LMIs). The design approach reduces conservatism inherent in robust control with a fixed gain controller and improves performances in time-response. Numerical simulations illustrate the theoretical results.