UAV DYNAMICS

UAVs could perform tasks that are difficult for manned aircraft, e. g., chemical or biological warfare missions. UAVs can also be used to test and verify technologies for UAV and for general aviation aircraft. The utilization and operation of UAV entail less risk to humans in combat. UAVs can also be freed from the limitation of human operators. This would improve the performance of UAVs. UAVs utilize various advanced technologies: (1) data/signal processing, (2) off-board on-board sensors, (3) communications links, and (4) integrated avionics and flight control. UAVs could be remotely piloted or they could be autonomous with an autopilot. They can operate at altitudes above 70,000 ft, have higher maneuverability and longer endurance, and can carry optical sensors and radars. However, they need large communications bandwidths. UAVs could also be equipped with aircraft-like con­trols: elevators, ailerons, rudders, flaps, extensions of wings, and undercarriage.

In an autonomous UAV, humans have no direct control over the vehicle. One form of autonomy is the radio-controlled model airplane. The most complicated UAV would be the one that uses a rule-based fuzzy logic (see Chapters 1 and 9) to detect, identify, and attack a mobile target. Flight dynamics modeling of UAVs presents some unique challenges. UAVs are lighter than manned aircraft and hence they have higher natural frequencies. To simulate the dynamic motion, the nonlinear fully coupled EOMs of an aircraft, discussed in Chapter 3, can be used. Simplified EOMs discussed in the present chapter can also be used to carry out preliminary studies relating to UAV performance evaluation. Any one of the following model forms can be selected for UAV dynamic analysis

1. Nonlinear fully coupled equations as described in Equations 3.22, 3.23, and 3.28

2. Linear coupled, e. g., Equation 5.5

3. Nonlinear decoupled, e. g., Equations 5.6 and 5.23

4. Linear decoupled, e. g., Equations 5.16, 5.20, and 5.25

Working with the fully coupled EOMs in option (1) can be time consuming and complex. Therefore, for control system design, designers usually make use of linearized or decoupled models given in options (2), (3), and (4). Of course, one has to keep in mind the limitations of these models. The linearized EOMs are valid in the small range around the trim point when excursions from the reference flight condition are small. Decoupling is also valid when there is negligible interaction between the longitudinal and lateral-directional motion.

In using the above set of EOMs for UAVs, the following assumptions are made:

1. UAVs are rigid bodies and flexibility effects are not considered.

2. They have a conventional configuration (i. e., aft tail).

3. Symmetry about the XZ plane is assumed.

4. The effect of thrust on lateral-directional motion is neglected (Equation 5.25). Reference [13] gives more details about the dynamic modeling of UAVs.