The paper considers a boundary control formulation for PDEs with a system output given by a spatial integral of weighted functions of the state. This formulation is directly applicable to the control of an aircraft with articulated flexible wings, in which case the output of interest is a net aerodynamic force or moment. Flexible wings can be controlled via actuation at the root or the tip. The problem of beam twist is analysed in detail to illustrate the formulation, and it shown that the control law ensures that the error between the desired output signal and the actual output signal decreases exponentially to an uniform ultimate bound. Stability of the closed loop system is proved by Lyapunov techniques. The formulation is demonstrated by simulations.