# Modeling and Control of an Articulated Multibody Aircraft
## 1 Introduction
### Related Work
#### 1.1.2 Moving-Mass Control in Spacecraft and Aircraft
Momentum generated by moving-mass actuation has been observed to counteract the effects of these disturbances and stabilize the spacecraft [79]. Moving masses are attractive for exo-atmospheric situations, where aerodynamic forces are not available. Spacecraft can also suffer from wobbling. A study carried out by [PERSON] [80] identified the movement of astronauts (moving mass) inside the space station as the main source of wobbling motion. Accordingly, [PERSON] designed and implemented a momentum exchange mechanism for attitude stabilization with a single moving mass [81; 82]. The mechanism was comprised of two cylindrical rigid bodies axially connected by cables that was effective in generating torque to eliminate wobbling. When angular velocity is high, docking and reentry of spaceships is unsafe for astronauts. Accidental collisions can cause uncontrolled tumbling, which is also dangerous. [PERSON] and [PERSON] using a movable mass control concept, designed a mechanism to convert tumbling motion into simple spin and demonstrated it through simulation. Results from the study showed that a large space station tumbling after collision was able to stabilize using a movable mass of about 1% of its total mass. Due to the low density of the upper atmosphere, aerodynamic controllers such as a rudder may be inadequate for reentry vehicles [83]. Because aerodynamic control surfaces are prone to ablation and degradation by hypersonic flow, internal moving-mass actuation has been used to control reentry vehicles for decades [84]. [PERSON] et al. [85] investigated the combined use of moving mass and aileron control. Pitch and yaw were controlled by moving masses along two orthogonal rails, while roll was controlled by ailerons. The study demonstrated the benefits of combining these control methods. [PERSON] et al. [86] also demonstrated roll control for a non-axisymmetric reentry vehicle using a moving mass. The moving mass was able to provide robust stability and performance throughout the flight without the need to modify controller gains. For other applications of moving mass in air and spacecraft, see [36]. Fluid transfer is another mass-shifting mechanism that has been used for control. For instance, because the Concorde could fly at supersonic speeds, changing the internal mass distribution by moving fuel helps with trimming the aircraft in transonic conditions [87; 88; 89]. For aircraft, adjustment of balance has by far been the preferred moving-mass application. The choice of mechanism for moving the masses varies with application. Internal solid mass actuation is done with linear motion in most applications [65; 68; 70; 72; 86; 90; 91; 92; 93; 94] and along a circular path in some other applications [76; 77; 95]. From the literature on aircraft control, it is clear that any moving-mass control is an exception rather than the norm. For automated systems, mass movement has usually been slow (apart from gyroscope reaction torque-based control), generally dedicated to the function, usually linear, and typically internal.