Video Course of Flight Dynamics II (Dynamics and Stability) available at http://nptel.iitm.ac.in

*************

Reviews of our recent work on Aircraft Spin

"Autorotational Spin Evolving Towards Chaos"

(accepted for publication in AIAA Journal of Aircraft)

Reviewer: 1

Apparently chaotic post-stall motion has been described by many authors. Also, bifurcation analysis of aircraft post-stall dynamics has shown periodic and quasi-periodic behavior. What this Note has achieved is to link these two observations by bringing in a hitherto unconsidered factor ? external wind. In a series of convincing simulations, the authors have shown that in the presence of a mean external wind, oscillatory spin dynamics may transition to apparently chaotic motion (with increasing wind speed). This is a very interesting linkage and should hopefully open the door to a more detailed future investigation of the effect of wind on aircraft post-stall dynamic response.

Reviewer: 2

Authors mentioned that WT experimental results inspired them to carry out the research presented in this paper. Could you deliver more details if the initial conditions are related to the WT experiments and if not, how they were created? Could you assess how the simulation results are dependant on initial conditions, especially in highly nonlinear, high angles of attack flight conditions.

Reviewer: 3

The note describes a novel yet simple mechanism by which steady oscillatory spin may evolve via period-doubling into what appears to be a chaotic attractor.  The existence of chaos in aircraft spin has been previously demonstrated (as mentioned in the literature cited in this note), and the period-doubling route to chaos is well known.  It is, however,  difficult to demonstrate this phenomenon in complex flight dynamics models.  The novelty of this work is in the use of a parameter variation that reflects a real-world operating scenario - namely variation in mean wind speed - through which the steady oscillations evolve via quasi-periodic motions towards a chaotic attractor.  The work described in the note is therefore original and of potential interest for those carrying out spin studies; it also correlates qualitatively with a set of  experimental findings referenced in the paper.

It would be good to explore the effect of the wind speed changes in terms of the bifurcation diagram, and thus infer the effect on the underlying flight dynamics: this may help to generalise the findings somewhat from the specific aircraft model used here.