L2 is the deliberate bridge between PID and the more general control theory that follows. It exists for two reasons: it’s the natural jump from transfer functions to state-space representation, and it’s where the second airframe — a fixed-wing platform — enters the curriculum.
Two controllers, one theory
Students design the same LQR framework for both the quadrotor (hover-centric, symmetric) and FT-1, the fixed-wing testbed (trim-centric, longitudinal/lateral-directional split). Seeing one control theory applied to two physically different vehicles is worth more than any amount of correct answers memorised in isolation.
Where the fault story goes next
The same standard fault library returns — motor thrust loss, sensor noise — but now the discussion is different: LQR’s multi-loop tuning can outperform PID on the primary axis while quietly degrading performance on cross-axes the single-loop PID never touched. That model-dependence is exactly the weakness L3’s sliding mode control is built to answer.