When an aircraft is flying and burning fuel the center of gravity (c.g.) of the aircraft shifts slowly. The shift in c.g. is difficult to measure or estimate so the flight control systems need to be robustly designed to cope with this variation. I think Robust MRAC can be suitable for better cope up with change in center of gravity in flight control laws 

That depends entirely on what you mean with 'suitable'. If the goal is just to get something flying with the least effort, then go for a simple (scheduled) PID controller. If you need to cope with large nonlinearities (for example from aggressive maneuvers), then you're better of with a nonlinear controller such as backstepping. For disturbance rejection you can look at Incremental Nonlinear Dynamic Inversion control.

Hi Asad,

Yes, it all depends of what you need. If high performance is not what you are looking for, then try the simplest, even PID.

Otherwise, I am "a bit" Inclined in some direction, so try to getr some idea of what is now called Simpl,e Adaptive Control (SAC). For your specific application, try to get a hold of 

I. Barkana: "Classical and Simple Adaptive Control Design for a Non-Minimum Phase Autopilot," AIAA Journal of Guidance, Control and Dynamics, Vol. 28, No. 4, pp. 631-638, 2005.

The object is exactly UAV  and the paper compares SAC with three otner alternatives, used in previous publications: PID, Classical, and Fuzzy controllers.

Thank you very much everyone. However, the model is nonlinear and has lot of uncertainties. PID might not be a good idea. Backstepping I used in my master thesis a long time ago, Dr. Kampan gave an interesting idea, will see through it. These days, use of backstepping alone is not very popular any more. mixing backstepping with SMC also give some interesting results. Trying to read Prof. Naira's L1 Adaptive control because of its fast robust adaptation. 

Many controllers can deal with  flight control applications in UAV.

I suggest you can use PID control, adaptive MRAC.

Hi Asad,

Have you considered implementing Active Disturbance Rejection Control (ADRC)? The ADRC scheme has been adopted to flight control.

The following is an excerpt from the Preface of the book titled, “Active Disturbance Rejection Control for Nonlinear Systems: An Introduction,” by Bao-Zhu Guo, and Zhi-Liang Zhao.

• Basically, the ADRC consists of three main parts. The first part is the tracking differentiator (TD) that is relatively independent and is actually thoroughly discussed in the control theory. The aim of the TD is to extract the derivatives of the reference signal and is also considered as transient profile for output tracking.
• The second part of the ADRC is the extended state observer (ESO) which is a crucial part of the ADRC. In ESO, both the state and the “total disturbance” are estimated by the output of the system. This remarkable feature makes the ADRC a very different way of dealing with uncertainty. The ESO is the generalization of the traditional state observer where only the state of the system is estimated.
• The final part of the ADRC is the extended state observer-based feedback control. Since the uncertainty is estimated in the ESO and is compensated for in the feedback loop, the barriers between the time invariant and time varying, linear and nonlinear have been broken down by considering the time-varying part and the nonlinear part as uncertainty. At the same time, the control energy is significantly reduced. More importantly, in this way, the closed-loop systems look like linear time-invariant systems, for which a reliable result can be applied.

Conference Paper A Practical Solution to Some Problems in Flight Control

Conference Paper Application of Active Disturbance Rejection Control to Integ...